Scholarly article on topic 'Neutral pion production at midrapidity in pp and Pb–Pb collisions at $$\sqrt{s_{{\mathrm {NN}}}}= 2.76\,{\mathrm {TeV}}$$ s NN = 2.76 TeV'

Neutral pion production at midrapidity in pp and Pb–Pb collisions at $$\sqrt{s_{{\mathrm {NN}}}}= 2.76\,{\mathrm {TeV}}$$ s NN = 2.76 TeV Academic research paper on "Physical sciences"

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Eur. Phys. J. C
OECD Field of science

Academic research paper on topic "Neutral pion production at midrapidity in pp and Pb–Pb collisions at $$\sqrt{s_{{\mathrm {NN}}}}= 2.76\,{\mathrm {TeV}}$$ s NN = 2.76 TeV"

Eur. Phys. J. C (2014) 74:3108 DOI 10.1140/epjc/s10052-014-3108-8

The European Physical journal C

Regular Article - Experimental Physics

Neutral pion production at midrapidity in pp and Pb-Pb collisions at y^NN = 2.76 TeV

The ALICE Collaboration*

CERN, 1211 Geneva 23, Switzerland

Received: 16 May 2014 / Accepted: 30 September 2014 / Published online: 16 October 2014

© CERN for the benefit of the ALICE collaboration 2014. This article is published with open access at

Abstract Invariant yields of neutral pions at midrapidity in the transverse momentum range 0.6 < pT < 12GeV/cmea-sured in Pb-Pb collisions at ,/sNN = 2.76 TeV are presented for six centrality classes. The pp reference spectrum was measured in the range 0.4 < pT < 10GeV/catthesamecenter-of-mass energy. The nuclear modification factor, RAA, shows a suppression of neutral pions in central Pb-Pb collisions by a factor of up to about 8-10 for 5 < pT < 7GeV/c. The presented measurements are compared with results at lower center-of-mass energies and with theoretical calculations.

1 Introduction

Quantum chromodynamics (QCD) predicts a transition from hadronic matter to a state of deconfined quarks and gluons, i.e., to the quark-gluon plasma (QGP), at a temperature of Tc « 150-160 MeV at vanishing net baryon number [1,2]. Energy densities created in Pb-Pb collisions at the LHC are estimated to be sufficiently large to reach this state [3,4]. At low transverse momenta (roughly pT < 3GeV/c) it is expected that pressure gradients in the QGP produced in an ultrarelativistic collision of two nuclei give rise to a collective, outward-directed velocity profile, resulting in a characteristic modification of hadron spectra [5]. At sufficiently large pT (> 3-8GeV/c), hadrons in pp and Pb-Pb collisions originate from hard scattering as products of jet fragmentation. Hard-scattered quarks and gluons, produced in the initial stage of the heavy-ion collision, must traverse the QGP that is produced around them and lose energy in the process through interactions with that medium. This phenomenon ("jet quenching") leads to a modification of hadron yields at high pT [6,7]. By studying observables related to jet quenching one would like to better understand the mechanism of parton energy loss and to use hard probes as a tool to characterize the QGP.

The modification of the hadron yields for different pT intervals in heavy-ion (A-A) collisions with respect to pp collisions can be quantified with the nuclear modification factor

Raa(Pt ) =

d2 N/d pxd y Iaa (Taa) x d2a/dpTdy|

* e-mail:

where the nuclear overlap function (TAA) is related to the average number of inelastic nucleon-nucleon collisions as (TAA) = (Nou)/o^i. In the factorization approach of a per-turbative QCD calculation of particle production from hard scattering, the overlap function TAA can be interpreted as the increase of the parton flux in going from pp to A-A collisions. Without nuclear effects, RAA will be unity in the hard scattering regime.

Parton energy loss depends on a number of factors including the transport properties of the medium and its space-time evolution, the initial parton energy, and the parton type [812]. The nuclear modification factor, RAA, is also affected by the slope of the initial parton transverse momentum spectrum prior to any interaction with the medium and by initial-state effects like the modifications of the parton distributions in nuclei. An important constraint for modeling these effects comes from the study of p-A collisions [13], but also from the study of A-A collisions at different center-of-mass energies and different centralities. For instance, the increase in from RHIC to LHC energies by about a factor 14 results in larger initial energy densities and less steeply falling initial parton spectra [14]. Moreover, at the LHC, pions with pT < 50 GeV/c are dominantly produced in the fragmentation of gluons [15], whereas the contribution from quark fragmentation in the same pT region is much larger and more strongly varying with pT at RHIC [16]. Therefore, the pion suppression results at the LHC will be dominated by gluon energy loss, and simpler to interpret than the results from RHIC. Compared to measurements of the RAA for inclusive charged hadrons, differences between the baryon and meson RAA provide additional information on the parton energy loss mechanism and/or on hadronization in A-A collisions

[17,18]. Experimentally, neutral pions are ideally suited for this as they can be cleanly identified (on a statistical basis) via the decay n0 ^ yy.

The suppression of neutral pions and charged hadrons at large transverse momentum [19-23] and the disappearance of azimuthal back-to-back correlations of charged hadrons in central Au-Au collision at RHIC [24,25] (see also [2629]) were interpreted in terms of parton energy loss in hot QCD matter. Neutral pions in central Au-Au collisions at = 200 GeV were found to be suppressed by a factor of 4-5 for pT > 4 GeV/c [30,31]. The rather weak dependence of Raa on pT was described by a large number of jet quenching models [32]. The ^/sNN and system size dependence was studied in Cu-Cu collisions at ^sNN = 19.4,62.4, and 200 GeV [33] and in Au-Au collisions at ^sNN = 39, 62.4, and 200 GeV [22,34]. In central Cu-Cu collisions the onset of Raa < 1 was found to occur between ^JsNN = 19.4 and 62.4 GeV. For unidentified charged hadrons in central Pb-Pb collisions at the LHC, RAA was found to increase from Raa < 0.2 at Pt « 7GeV/c to Raa ^ 0.5 for pT > 50 GeV/c, in line with a decrease of the relative energy loss with increasing parton pT [35-37].

The dependence of the neutral pion RAA on ^/sNN and pT in Au-Au collisions at RHIC energies for 2 < pT < 7GeV/c is not fully reproduced by jet quenching calculations in the GLV framework which is based on perturbative QCD [34,38,39]. This may indicate that, especially for this intermediate pT range, jet quenching calculations do not yet fully capture the relevant physics processes. With the large increase in ^/sNN the measurement of RAA at the LHC provides a large lever arm to further constrain parton energy loss models. Phenomena affecting pion production in the pT range 0.6 < pT < 12 GeV/c of this measurement include collective radial flow at low pT and parton energy loss at high pT. The data are therefore well suited to test models aiming at a description of particle production over the full transverse momentum range, including the potentially complicated interplay between jets and the evolving medium.

2 Detector description

Neutral pions were reconstructed via the two-photon decay channel n0 ^ yy which has a branching ratio of 98.8% [40]. Two independent methods of photon detection were employed: with the photon spectrometer (PHOS) which is an electromagnetic calorimeter [41], and with photon conversions measured in the central tracking system using the inner tracking system (ITS) [42] and the time projection chamber (TPC) [43]. In the latter method, referred to as photon conversion method (PCM), conversions out to the middle of the TPC were reconstructed (radial distance R « 180 cm). The material in this range amounts to (11.4 ± 0.5) % of a radia-

tion length X0 for | n | < 0.9 corresponding to a plateau value of the photon conversion probability of (8.6 ± 0.4) %. The measurement of neutral pions with two independent methods with different systematics and with momentum resolutions having opposite dependence on momentum provides a valuable check of the systematic uncertainties and facilitates the measurements of neutral pions in a wide momentum range with small systematic uncertainty.

PHOS consists of three modules installed at a distance of 4.6 m from the interaction point. PHOS subtends 260° < y < 320° in azimuth and |nl < 0.13 in pseudorapidity. Each module has 3584 detection channels in a matrix of 64 x 56 cells made of lead tungstate (PbWO4) crystals each of size 2.2 x 2.2 x 18 cm3. The transverse dimensions of the cells are slightly larger than the PbWO4 Molière radius of 2 cm. The signals from the cells are measured by avalanche photodiodes with a low-noise charge-sensitive preamplifier. In order to increase the light yield and thus to improve energy resolution, PHOS crystals are cooled down to a temperature of -25 °C. The PHOS cells were calibrated in pp collisions by equalizing the n0 peak position for all cell combinations registering a hit by a decay photon.

The inner tracking system (ITS) [44] consists of two layers of silicon pixel detectors (SPD) positioned at a radial distance of 3.9 and 7.6 cm, two layers of silicon drift detectors (SDD) at 15.0 and 23.9 cm, and two layers of silicon strip detectors (SSD) at 38.0 and 43.0 cm. The two SPD layers cover a pseudorapidity range of | n l < 2 and | n l < 1.4, respectively. The SDD and the SSD subtend |n| < 0.9 and | n | < 1.0, respectively.

The time projection chamber (TPC) [43] is a large (85 m3) cylindrical drift detector filled with a Ne/CO2/N2 (85.7/9.5/4.8%) gas mixture. It covers a pseudorapidity range of |n| < 0.9 over the full azimuthal angle for the maximum track length of 159 reconstructed space points. With the magnetic field of B = 0.5 T, electron and positron tracks were reconstructed down to transverse momenta of about 50 MeV/c. In addition, the TPC provides particle identification via the measurement of the specific energy loss (dE/dx) with a resolution of 5.5 % [43]. The ITS and the TPC were aligned with respect to each other to a precision better than 100 |im using tracks from cosmic rays and proton-proton collisions [42].

Two forward scintillator hodoscopes (VZERO-A and VZERO-C) [45] subtending 2.8 <n < 5.1 and -3.7 <n < -1.7, respectively, were used in the minimum bias trigger in the pp and in the Pb-Pb run. The sum of the amplitudes of VZERO-A and VZERO-C served as a measure of centrality in Pb-Pb collisions [46]. Spectator (non-interacting) protons and neutrons were measured with zero degree calorimeters (ZDCs), located close to the beam pipe, 114 m away from the interaction point on either side of the ALICE detector [44].

3 Data processing

3.1 Event selection

The pp sample at = 2.76 TeV was collected in the 2011 LHC run. The minimum bias trigger (MBOR) in the pp run required a hit in either VZERO hodoscope or a hit in the SPD. Based on a van der Meer scan the cross section for inelastic pp

collisions was determined to be ainel = (62.8-4.0 ± 1.2) mb and the MBOR trigger had an efficiency of ctMBor /aiael = 0.881+0'°55 [47]. The results were obtained from samples of 34.7 x 106 (PHOS) and 58 x 106 (PCM) minimum bias pp collisions corresponding to an integrated luminosity Lint = 0.63 nb-1 and Lint = 1.05 nb-1, respectively. PHOS and the central tracking detectors used in the PCM were in different readout partitions of the ALICE experiment which resulted in the different integrated luminosities.

The Pb-Pb data at sNN = 2.76 TeV were recorded in the 2010 LHC run. At the ALICE interaction region up to 114 bunches, each containing about 7 x 107 208Pb ions, were collided. The rate ofhadronic interactions was about 100 Hz, corresponding to a luminosity of about 1.3 x 1025 cm-2s-1. The detector readout was triggered by the LHC bunch-crossing signal and a minimum bias interaction trigger based on trigger signals from VZERO-A, VZERO-C, and SPD [46]. The efficiency for triggering on a hadronic Pb-Pb collision ranged between 98.4 and 99.7%, depending on the minimum bias trigger configuration. For the centrality range 080 % studied in the Pb-Pb analyses 16.1 x 106 events in the PHOS analysis and 13.2 x 106 events in the PCM analysis passed the offline event selection.

In both pp and Pb-Pb analyses, the event selection was based on VZERO timing information and on the correlation between TPC tracks and hits in the SPD to reject background events coming from parasitic beam interactions. In addition, an energy deposit in the ZDCs of at least three standard deviations above the single-neutron peak was required for Pb-Pb collisions to further suppress electromagnetic interactions [46]. Only events with a reconstructed vertex in |zvtx | < 10 cm with respect to the nominal interaction vertex position along the beam direction were used.

3.2 Neutral pion reconstruction

The PHOS and PCM analyses presented here are based on methods previously used in pp collisions at = 0.9 and 7 TeV [48]. Neutral pions were reconstructed using the n0 ^ yy decay channel either with both photon candidates detected in PHOS or both photons converted into e+e- pairs and reconstructed in the central tracking system. For the photon measurement with PHOS adjacent lead tungstate cells with energy signals above a threshold

(12MeV) were grouped into clusters [49]. The energies of the cells in a cluster were summed up to determine the photon energy. The selection of the photon candidates in PHOS was different for pp and Pb-Pb collisions due to the large difference in detector occupancy. For pp collisions cluster overlap is negligible and combinatorial background small. Therefore, only relatively loose photon identification cuts on the cluster parameters were used in order to maximize the n0 reconstruction efficiency: the cluster energy for pp collisions was required to be above the minimum ionizing energy Ecluster > 0.3 GeV and the number of cells in a cluster was required to be greater than two to reduce the contribution of hadronic clusters. In the case of the most central Pb-Pb collisions about 80 clusters are reconstructed in PHOS, resulting in an occupancy of up to 1/5 of the 10,752 PHOS cells. This leads to a sizable probability of cluster overlap and to a high combinatorial background in the two-cluster invariant mass spectra. A local cluster maximum was defined as a cell with a signal at least 30 MeV higher than the signal in each surrounding cell. A cluster with more than one local maximum was unfolded to several contributing clusters [49]. As the lateral width of showers resulting from hadrons is typically larger than the one of photon showers, non-photonic background was reduced by a pT dependent shower shape cut. This cut is based on the eigenvalues X0, X1 of the covariance matrix built from the cell coordinates and weights Wi = max[0, W0 + log(Ei /Ecluster)], W) = 4.5 where Ei is the energy measured in cell i . In the Pb-Pb case only cells with a distance to the cluster center of Rdisp = 4.5 cm were used in the dispersion calculation. A 2D pT-dependent cut in the X0-X1 plane was tuned to have an efficiency of ~0.95 using pp data. In addition, clusters associated with a charged particle were rejected by application of a cut on the minimum distance from a PHOS cluster to the extrapolation of reconstructed tracks to the PHOS surface [49]. This distance cut depended on track momentum and was tuned by using real data to minimize false rejection of photon clusters. The corresponding loss of the n0 yield was about 1 % in pp collisions (independent of pT). In Pb-Pb collisions the n0 inefficiency due to the charged particle rejection is about 1 % in peripheral and increases to about 7 % in central Pb-Pb collisions. In addition, to reduce the effect of cluster overlap, the cluster energy was taken as the core energy of the cluster, summing over cells with centers within a radius Rcore = 3.5 cm of the cluster center of gravity, rather than summing over all cells of the cluster. By using the core energy, the central-ity dependence of the width and position of the n0 peak is reduced, due to a reduction of overlap effects. The use of the core energy leads to an additional non-linearity due to energy leakage outside Rcore: the difference between full and core energy is negligible at Ecluster < 1 GeV and reaches ~4% at Ecluster ~ 10 GeV. This non-linearity, however, is well reproduced in the GEANT3 Monte Carlo simulations [50]

of the PHOS detector response (compare pT dependences of peak positions in data and Monte Carlo in Fig. 2) and is corrected for in the final spectra.

PHOS is sensitive to pile-up from multiple events that occur within the 6 |xs readout interval of the PHOS front-end electronics. The shortest time interval between two bunch crossings in pp collisions was 525 ns. To suppress photons produced in other bunch crossings, a cut on arrival time \t| < 265 ns was applied to reconstructed clusters which removed 16% of the clusters. In the Pb-Pb collisions, the shortest time interval between bunch crossing was 500 ns, but the interaction probability per bunch crossing was much smaller than in pp collisions. To check for a contribution from other bunch crossings to the measured spectra, a timing cut was applied, and the pile-up contribution was found to be negligible in all centrality classes. Therefore, a timing cut was not applied in the final PHOS Pb-Pb analysis.

The starting point of the conversion analysis is a sample of photon candidates corresponding to track pairs reconstructed by a secondary vertex (V0) finding algorithm [49,51]. In this step, no constraints on the reconstructed invariant mass and pointing of the momentum vector to the collision vertex were applied. Both tracks of a V0 were required to contain reconstructed clusters (i.e., space points) in the TPC. V0's were accepted as photon candidates if the ratio of the number of reconstructed TPC clusters over the number of findable clusters (taking into account track length, spatial location, and momentum) was larger than 0.6 for both tracks. In order to reject K°, A, and A decays, electron selection and pion rejection cuts were applied. V0's used as photon candidates were required to have tracks with a specific energy loss in the TPC within a band of [-3o, 5o] around the average electron dE/dx, and of more than 3o above the average pion dE/dx (where the second condition was only applied for tracks with measured momenta p > 0.4GeV/c). Moreover, tracks with an associated signal in the TOF detector were only accepted as photon candidates if they were consistent with the electron hypothesis within a ±5o band. A generic particle decay model based on the Kalman filter method [52] was fitted to a reconstructed V0 assuming that the particle originated from the primary vertex and had a mass MV0 = 0. Remaining contamination in the photon sample was reduced by cutting on the x2 of this fit. Furthermore, the transverse momentum qT = pe sin 0V0e [53] of the electron, pe, with respect to the V0 momentum was restricted to qT < 0.05GeV/c. As the photon is massless, the difference A0 = \0e- - 0e+ \ of the polar angles of the electron and the positron from a photon conversion is small and the bending of the tracks in the magnetic field only results in a difference A<p = \<e- - <e+ \ of the azimuthal angles of the two momentum vectors. Therefore, remaining random track combinations, reconstructed as a V0, were suppressed further by a cut on the ratio of AO to the total opening angle of the e+e- pair calculated

after propagating both the electron and the positron 50 cm from the conversion point in the radial direction. In order to reject e+e- pairs from Dalitz decays the distance between the nominal interaction point and the reconstructed conversion point of a photon candidate had to be larger than 5 cm in radial direction. The maximum allowed radial distance for reconstructed V0's was 180 cm.

Pile-up of neutral pions coming from bunch crossings other than the triggered one also has an effect on the PCM measurement. At the level of reconstructed photons, this background is largest for photons for which both the electron and the positron were reconstructed with the TPC alone without tracking information from the ITS. These photons, which typically converted at large radii R, constitute a significant fraction of the total PCM photon sample, which is about 67 % in case of the pp analysis. This sample is affected because the TPC drift velocity of 2.7 cm/^s corresponds to a drift distance of 1.41 cm between two bunch crossings in the pp run which is a relatively short distance compared to the width of oz & 5 cm of the distribution of the primary vertex in the z direction. The distribution of the distance of closest approach in the z direction (DCAz) of the straight line defined by the reconstructed photon momentum is wider for photons from bunch crossings other than the triggered one. The DCAz distribution of photons which had an invariant mass in the n0 mass range along with a second photon was measured for each pT interval. Entries in the tails at large DCAz were used to determine the background distribution and to correct the neutral pion yields for inter bunch pile-up. For the pp analysis, this was a 5-7 % correction for pT > 2GeV/c and a correction of up to 15 % at lower pT (pT & 1 GeV/c). In the Pb-Pb case the correction at low pT was about 10%, and became smaller for higher pT and for more central collisions. For the 20-40 % centrality class and more central classes the pile-up contribution was negligible and no pile-up correction was applied. In the PCM as well as in the PHOS analysis, events for which two or more pp or Pb-Pb interactions occurred in the same bunch crossing were rejected based on the number of primary vertices reconstructed with the SPD [49] which has an integration time of less than 200 ns.

In the PHOS as well as in the PCM analysis, the neutral pion yield was extracted from a peak above a combinatorial background in the two-photon invariant mass spectrum. Examples of invariant mass spectra, in the n0 mass region, are shown in Fig. 1 for selected pT bins for pp collisions, and peripheral and central Pb-Pb collisions. The combinatorial background was determined by mixing photon candidates from different events. In the PCM measurement the combinatorial background was reduced by cutting on the energy asymmetry a = \EY1 - EY2\/(EY1 + EY2), where a < 0.65 was required for the central classes (0-5, 5-10, 10-20,20-40%) and a < 0.8 for the two peripheral classes

0.15 Mn (GeV/c2)

0.8 < p™< 1.0 GeV/c : Pb-Pb ^sNN = 2.76TeV 60-80%

0.15 Afn (GeV/c2)


0.8 <p™< 1.0 GeV/c Pb-Pb \sNN = 2.76 TeV 0-10%

Signal x 15

0.15 M„ (GeV/c2)

■p 400

_PHOS 2.0 < p™< 2.5 GeV/c

pp lis = 2.76 TeV


PHOS 2.0 <' p^< 2.5 GeV/c '

_ Pb-Pb ^ = 2.76 TeV _

- 60-80% -

0.15 M„ (GeV/c2)


PHOS 2.0 <p™< 2.5 GeV/c Pb-Pb (SN„ = 2.76 TeV 0-10%

0.15 fli™ (GeV/c2)

Fig. 1 (Color online) Invariant mass spectra in selected pT slices for PCM (upper row) and PHOS (lower row) in the n0 mass region for pp (left column), 60-80% (middle column) and 0-10% (right column) Pb-Pb collisions. The histogram and the filled points show the data before and after background subtraction, respectively. For the 0-10 %

class the invariant mass distributions after background subtraction were scaled by a factor 15 and 5 for PCM and PHOS, respectively, for better visibility of the peak. The positions and widths of the n0 peaks were determined from the fits, shown as blue curves, to the invariant mass spectra after background subtraction

(40-60, 60-80 %). In both analyses the mixed-event background distributions were normalized to the right and left sides of the n0 peak. A residual correlated background was taken into account using a linear or second order polynomial fit. The n0 peak parameters were obtained by fitting a function, Gaussian or a Crystal Ball function [54] in the PHOS case or a Gaussian combined with an exponential low mass tail to account for bremsstrahlung [55] in the PCM case, to the background-subtracted invariant mass distribution, see Fig. 1. The Crystal Ball function was used in the PHOS analysis of pp data. A Gaussian was used alternatively to determine systematic uncertainties of the peak parameters. In the Pb-Pb case with worse resolution and smaller signal/background ratios, the difference between Crystal Ball and Gaussian fits disappeared and only the latter were used in the PHOS analysis. In the case of PHOS the number of reconstructed n0's was obtained in each pT bin by integrating the background subtracted peak within 3 standard deviations around the mean value of the n0 peak position. In the PCM analysis, the integration window was chosen to be asymmetric (mn0 - 0.035 GeV/c2, mn0 + 0.010 GeV/c2) to take into account the left side tail of the n0 peak due to bremsstrahlung energy loss of electrons and positrons from photon conversions. In both analyses the normalization and integration windows were varied to estimate the related sys-

tematic uncertainties. The peak positions and widths from the two analyses are compared to GEANT3 Monte Carlo simulations in Fig. 2 as a function of pT. The input for the GEANT3 simulation came from the event generators PYTHIA 8 [56] and PHOJET [57] in the case of pp collisions (with roughly equal number of events) and from HIJING [58] in the case of Pb-Pb collisions. For the PCM analysis the full width at half maximum (FWHM) divided by 2^j2 ln 2 & 2.35 is shown. Note the decrease of the measured peak position with pT in Pb-Pb collisions for PHOS. This is due to the use of the core energy instead of the full cluster energy. At low pT in central Pb-Pb collisions, shower overlaps can increase the cluster energy thereby resulting in peak positions above the nominal n0 mass. A good agreement in peak position and width between data and simulation is observed in both analyses. The remaining small deviations in the case of PHOS were taken into account as a systematic uncertainty related to the global energy scale.

The correction factor e(pT) for the PHOS detector response and the acceptance A(pT) were calculated with GEANT3 Monte Carlo simulations tuned to reproduce the detector response. The factor e(pT) takes the loss of neutral pions due to analysis cuts, effects of the finite energy resolution and, in case of Pb-Pb collisions, effects of shower overlaps into account. The shape of the n 0 input spectrum needed

2 15 u

<0 <u a

142 140 138

£1 132

<g 130 a

: pp {s = 2.76 TeV (a); r Data MC T PCM • FWHM/2.35 - PHOS c - « , « s-9t- ; 60-80% Pb-Pb ^sN = 2.76 TeV ' (b) : + : - ft _ ««a« Y- ; ■»«««» >1' • ~T" : : 0-5% Pb-Pb = 2.76 TeV : -1 '^Hflftil (c) II " D -

r (d)i 7 S 7 (e) (f)i

r + i L "Lq h > J ■ -< 1 Vo ] T Q : .........*

PT (GeV/c)

1 10 PT (GeV/c)

1 10 PT (GeV/c)

Fig. 2 (Color online) Reconstructed n peak width (upper row) and position (lower row) as a function of px in pp collisions at -Js = 2.76 TeV (a, d), peripheral (b, e) and central (c, f) Pb-Pb collisions

at snn = 2.76 TeV in PHOS and in the photon conversion method (PCM) compared to Monte Carlo (MC) simulations. The horizontal line in (d-f) indicates the nominal n0 mass

for the calculation of e(pT) was determined iteratively by using a fit of the corrected spectrum of a given pass as input to the next. In the case of Pb-Pb collisions the embedding technique was used in the PHOS analysis: the PHOS response to single n0's was simulated, the simulated n0 event was added to a real Pb-Pb event on the cell signal level, after which the standard reconstruction procedure was performed. The correction factor e( px) = (Nf* (px)-Nf (px))/NSim (px) was defined as the ratio of the difference of the number of reconstructed n0's after and before the embedding to the number of simulated n0's. In the pp case, the PHOS occupancy was so low that embedding was not needed and e(pT) was obtained from the n0 simulations alone. Both in the Pb-Pb and the pp analysis, an additional 2 % channel-by-channel decalibration was introduced to the Monte Carlo simulations, as well as an energy non-linearity observed in real data at low energies which is not reproduced by the GEANT simulations. This non-linearity is equal to 2.2 % at pT = 1 GeV/c and decreases rapidly with pT (less than 0.5 % at pT > 3 GeV/c). For PHOS, the n0 acceptance A is zero for pT < 0.4 GeV/c. The product e ■ A increases with pT and saturates at about 1.4 x 10-2 for a neutral pion with pT > 15 GeV/c. At high transverse momenta (pT > 25 GeV/c) e decreases because of merging of clusters of n 0 decay photons due to the decreasing average opening angle of the n0 decay photons. The correction factor e does not show a centrality dependence for events in the 20-80 % class, but in the most central bin it increases by ~ 10 % due to an increase in cluster energies caused by cluster overlap.

In the PCM, the photon conversion probability of about 8.6 % is compensated by the large TPC acceptance. Neutral pions were reconstructed in the rapidity interval |y| < 0.6 and the decay photons were required to satisfy < 0.65. The n0 efficiency increases with pT below pT « 4GeV/c and remains approximately constant for higher pT at values between 1.0 x 10-3 in central collisions (0-5 %, energy asymmetry cut a < 0.65) and 1.5 x 10-3 in peripheral collisions (60-80 %, a < 0.8). For the centrality classes 0-5, 5-10, 10-20, 20-40 %, for which a < 0.65 was used, the n0 efficiency varies between 1.0 x 10-3 and 1.2 x 10-3. This small centrality dependence is dominated by the cen-trality dependence of the V0 finding efficiency. Further information on the PHOS and PCM efficiency corrections can be found in [49].

The invariant differential neutral pion yield was calculated

1 1 111 Nn

d3 p 2n Nevents pT e A Br AyApT:

where Nevents is the number of events; pT is the transverse

momentum within the bin to which the cross section has been

assigned after the correction for the finite bin width ApT, Br

0 n 0 is the branching ratio of the decay n0 ^ yy, and Nn is

the number of reconstructed n0's in a given Ay and ApT bin. Finally, the invariant yields were corrected for the finite pT bin width following the prescription in [59], i.e., by plotting the measured average yield at a pT position for which the differential invariant yield coincides with the bin average.

Table 1 Summary of the relative systematic uncertainties in percent for selected pt bins for the PHOS and the PCM analyses

PP Pb-Pb, 60- -80% Pb-Pb, 0-5%

1.1 GeV/c 7.5 GeV/c 3 GeV/c 10 GeV/c 3 GeV/c 10 GeV/c

Yield extraction 8 2.3 0.8 6.8 3.7 5.7

Photon identification - - 1.7 1.7 4.4 4.4

Global E scale 4 6.2 4.1 5.3 6.1 7.8

Non-linearity 9 1.5 1.5 1.5 1.5 1.5

Conversion 3.5 3.5 3.5 3.5 3.5 3.5

Module alignment 4.1 4.1 4.1 4.1 4.1 4.1

Other 2 1.4 2.4 2.4 3.1 3.4

Total 13.9 8.8 7.6 10.7 10.7 12.7

PP Pb-Pb, 60- -80% Pb-Pb, 0-5%

1.1 GeV/c 5.0 GeV/c 1.1 GeV/c 5.0 GeV/c 1.1 GeV/c 5.0 GeV/c

Material budget 9.0 9.0

Yield extraction 0.6 2.6

e+ /e- identification 0.7 1.4

Photon identification (x2(y)) 2.4 0.9

n0 reconstruction efficiency 0.5 3.6

Pile-up correction 1.8 1.8

Total 9.5 10.3

9.0 9.0 9.0 9.0

3.3 5.9 10.6 5.0

2.9 5.3 9.0 10.5

3.7 4.6 4.0 6.7

3.5 4.1 6.7 8.4

2.0 2.0 - -

11.4 13.6 18.3 18.2

Secondary n0's from weak decays or hadronic interactions in the detector material were subtracted using Monte Carlo simulations. The contribution of n0's from K° as obtained from the used event generators was scaled in order to reproduce the measured K0 yields [60]. The correction for secondary n0's was smaller than 2 % (5 %) for pT > 2 GeV/c in the pp as well as in the Pb-Pb analysis for PCM (PHOS).

A summary of the systematic uncertainties for two representative pT values in pp, peripheral and central Pb-Pb collisions is shown in Table 1. In PHOS, one of the largest sources of the systematic uncertainty both at low and high pT is the raw yield extraction. It was estimated by varying the fitting range and the assumption about the shape of the background under the peak. In central collisions, major contributions to the systematic uncertainty are due to the efficiency of photon identification and the global energy scale. The former was evaluated by comparing efficiency-corrected n0 yields, calculated with different identification criteria. The latter was estimated by varying the global energy scale within the tolerance which would still allow to reproduce the peak position in central and peripheral collisions. The uncertainty related to the non-linearity of the PHOS energy response was estimated by introducing different non-linearities into the Monte Carlo simulations under the condition that the simulated pi-dependence of the n0 peak position and peak width was still

consistent with the data. The uncertainty of the PHOS measurement coming from the uncertainty of the fraction of photons lost due to conversion was estimated by comparing measurements without magnetic field to the measurements with magnetic field.

In the PCM measurement, the main sources of systematic uncertainties include the knowledge of the material budget, raw yield extraction, electron identification (PID), the additional photon identification cuts, and n0 reconstruction efficiency. The uncertainty related to the pile-up correction is only relevant in pp and peripheral Pb-Pb collisions. The contribution from the raw n0 yield extraction was estimated by changing the normalization range, the integration window, and the combinatorial background evaluation. Uncertainties related to the electron and photon identification cuts, and to the photon reconstruction efficiency were estimated by evaluating the stability of the results for different cuts. The total systematic uncertainties of the PCM and the PHOS results were calculated by adding the individual contributions in quadrature.

The comparisons of the fully corrected n0 spectra measured by PHOS and PCM in pp and Pb-Pb collisions are presented in Figs. 3 and 4, respectively. For a better comparison the PCM and PHOS data points were divided by a function which was fitted to the combined spectrum. In all

2.0 ~ 1.5

pp is = 2.76 TeV

stat. syst. PCM • □ PHOS ■ m

PT (GeV/c)

Fig. 3 (Color online) Ratio of the fully corrected n0 spectra in pp collisions at -Js = 2.76 TeV measured with PHOS and PCM methods to the fit of the combined spectrum. Vertical lines represent statistical uncertainties, the boxes systematic uncertainties

cases, agreement between the two measurements is found. The PHOS and PCM spectra were combined by calculating the average yields together with their statistical and systematic uncertainties by using the inverse squares of the total uncertainties of the PHOS and PCM measurements for a given pT bin as respective weights [40].

4 Results

The invariant neutral pion spectra measured in pp and Pb-Pb collisions are shown in Fig. 5. The pT range

0.6-12 GeV/c covered by the measurements includes the region pT & 7GeV/c where the charged hadron Raa exhibits the strongest suppression [35-37]. The invariant neutral pion yield in inelastic pp collisions shown in Fig. 5 is related to the invariant cross section as E d3o/d3 p = E d3 N/d3 p x oinel. Above pT & 3 GeV/c the pp spectrum is well described by a power law E d3 N/d3 p a 1/ pT .A fit to pT > 3GeV/c yields an exponent n = 6.0 ± 0.1 with x2/ndf = 3.8/4, which is significantly smaller than the value n = 8.22 ± 0.09 observed in pp collisions at V? = 200 GeV [31].

Neutral pion production from hard scattering is dominated by the fragmentation of gluon jets in the pT range of the measurement. The presented n0 spectrum in pp collisions can therefore help constrain the gluon-to-pion fragmentation function [61]. A next-to-leading-order (NLO) perturbative QCD calculation employing the DSS fragmentation function [62] agrees reasonably well with the measured neutral pion spectrum at = 0.9TeV. At = 7TeV, however, the predicted invariant cross sections are larger than the measured ones [48]. The comparison to a NLO perturbative QCD calculation using the CTEQ6M5 parton distributions [63] and the DSS fragmentation functions in Fig. 6 shows that the calculation overpredicts the data already at V? = 2.76 TeV by a similar factor as in pp collisions at = 7 TeV. The data are furthermore compared to a PYTHIA 8.176 (tune 4C) [56,64] calculation which reproduces the shape of the spectrum with an overall offset of about 20 %. It will be interesting to see whether calculations in the framework of the color glass condensate [65], which describe the neutral pion spectrum in pp collisions at s/s =

- 1.5 i! ■¡S

re 1.0

2.0 1.5

(0 1.0 Q

0-5% Pb-Pb ifsNN = 2.76 TeV

5-10% Pb-Pb (SN = 2.76 TeV

10-20% Pb-Pb ^SNN = 2.76 TeV

20-40% Pb-Pb tfSNN = 2.76 TeV

stat. syst. PCM • □ PHOS ■ □

40-60% Pb-Pb IfsNN = 2.76 TeV

60-80% Pb-Pb = 2.76 TeV

1 10 PT (GeV/c)

1 10 PT (GeV/c)

1 10 PT (GeV/c)

Fig. 4 (Color online) Ratio of the fully corrected n0 spectra in Pb-Pb collisions at ^/snn = 2.76 TeV in six centrality bins measured with PHOS and PCM to the fits to the combined result in each bin. Vertical lines represent statistical uncertainties, the boxes the systematic uncertainties

i-a. ■o

10 10 10 10 1

10 10 10 10 10 10 10-7 10-8 10-9

®pp (s = 2.76 TeV

- - Tsallis fit

— power law fit

H 0- 5% Pb-Pb fsNN = 2.76 TeV x 27 "a 5-10% Pb-Pb fsNN = 2.76 TeV x 25 [] 10-20% Pb-Pb fsNN = 2.76 TeV x 23 " 0 20-40% Pb-Pb fsNN = 2.76 TeV x 22 H 40-60% Pb-Pb fsNN = 2.76 TeV x 21 " [■] 60-80% Pb-Pb fsNN = 2.76 TeV x 20 — fits to Pb-Pb

PT (GeV/c)

Fig. 5 (Color online) Invariant differential yields of neutral pions produced in Pb-Pb and inelastic pp collisions at .J?nn = 2.76 TeV. The spectra are the weighted average of the PHOS and the PCM results. The vertical lines show the statistical uncertainties, systematic uncertainties are shown as boxes. Horizontal lines indicate the bin width. The horizontal position of the data points within a bin was determined by the procedure described in [59]. For the pp spectrum a fit with a power law function 1/pT for pt > 3 GeV/c and a Tsallis function (also used in [48]) are shown. The extrapolation of the pp spectrum provided by the Tsallis fit is used in the raa calculation for pt > 8 GeV/c

7 TeV, will also provide a good description of the data at V? = 2.76 TeV.

The nuclear modification factor, RAA, was calculated according to Eq. 1. For pT > 8GeV/c the extrapolation of the pp spectrum provided by the power law fit shown in Fig. 5 was used as a reference. The systematic uncertainty of the extrapolation was estimated based on the variation of the fit range (pT > 2, 3, 4 GeV/c) and the systematic uncertainty in the bin from pT = 6-8GeV/c. The average values of the nuclear overlap function TAA for each centrality class were taken from [46] and are given in Table 2. They were determined with a Glauber Monte Carlo calculation [66,67] by defining percentiles with respect to the simulated impact parameter b and therefore represent purely geometric quantities.

The combined RAA was calculated as a weighted average of the individual RAA measured with PHOS and PCM.

; pp is = 2.76 TeV ALICE -- NLO | = 0.5 p --- NLO | = p T

.....NLO | = 2 Tp

" Pythia 8, Tune 4C

3 4 5 6 7 8 910

PT (GeV/c)

Fig. 6 (Color online) Ratio of data or theory calculations to a fit of the neutral pion spectrum in pp collisions at .J?nn = 2.76 TeV. The renormalization, factorization, and fragmentation scale of the next-to-leading order QCD calculation were varied simultaneously (^ = 0.5pt, pT, 2pt). The calculation employed the CTEQ6M5 [63] parton distribution functions and the DSS fragmentation function [62]. The solid red line is a comparison to the PYTHIA 8.176 (tune 4C) event generator [56,64]

Table 2 Values for the overlap function (Taa) for the centrality bins used in this analysis

Centrality class (%) (Taa) (1/mb) Rel. syst. uncert. (%)

0-5 26.32 3.2

5-10 20.56 3.3

10-20 14.39 3.1

20-40 6.85 3.3

40-60 1.996 4.9

60-80 0.4174 6.2

This has the advantage of reduced systematic uncertainties of the combined result. In particular, the dominant uncertainty in the PCM, related to the material budget, cancels this way. The results for the combined RAA are shown in Fig. 7. In all centrality classes the measured RAA exhibits a maximum around pT « 1-2 GeV/c, a decrease in the range 2 < pT < 3-6GeV/c, and an approximately constant value in the measured pT range for higher pT. For pT > 6 GeV/c, where particle production is expected to be dominated by fragmentation of hard-scattered partons, RAA decreases with centrality from about 0.5 - 0.7 in the 60-80 % class to about 0.1 in the 0-5% class. The RAA measurements for neutral pions and charged pions [68] agree with each other over the entire pT range for all centrality classes. Agreement between the neutral pion and charged particle RAA [37] is observed for pt > 6GeV/c.

It is instructive to study the ^sNN dependence of the neutral pion Raa. Figure 8 shows that for central collisions the Raa at the LHC for pT > 2 GeV/c lies below the data points at lower ^sNN. This indicates that the decrease of RAA resulting from the higher initial energy densities created at larger

B 0 - 5% n0 C*J 20 - 40% n0 B 60 - 80% n0

PT (GeV/c)

Fig. 7 (Color online) Neutral pion nuclear modification factor Raa for three different centralities (0-5, 20-40, 60-80 %) in Pb-Pb collisions at .Jsnn = 2.76 TeV. Vertical error bars reflect statistical uncertainties, boxes systematic uncertainties. Horizontal bars reflect the bin width. The boxes around unity reflect the uncertainty of the average nuclear overlap function (Taa) and the normalization uncertainty of the pp spectrum added in quadrature

^/snn dominates over the increase of RAA expected from the harder initial parton pT spectra. To illustrated this point, one can consider a somewhat oversimplified model with a pT independent fractional energy loss e in conjunction with pT spectra described by a power law [70]. In this model e = 0.2 corresponds to Raaic ^ 0.25 at ^sNN = 0.2 TeV. The same fractional energy loss in conjunction with the flatter spectra at ySNN = 2.76 TeV, however, yield RAHC ^ 0.4. The shape of Raa (pT) in central collisions at ^sNN = 200 GeV and ysNN = 2.76 TeV appears to be similar. Considering the data for all shown energies one observes that the value of pT with the maximum RAA value appears to shift towards lower pT with increasing ^JsNN. The centrality dependence of RAA at pT = 7GeV/c is shown in Fig. 9 for nuclear collisions at ^SNN = 39, 62.4, 200 [22,34], and 2,760GeV. At this transverse momentum soft particle production from the bulk should be negligible and parton energy loss is expected to be the dominant effect. It can be seen that the suppression in Pb-Pb collisions at the LHC is stronger than in Au-Au collisions at ^/sNN = 200 GeV for all centralities. In particular, the most peripheral class of the LHC data already shows a sizable suppression whereas at the lower energies the suppression appears to develop less abruptly as a function of the number of participating nucleons (Npart).

In Fig. 10 the measured RAA is compared with a GLV model calculation [38,39] and with theoretical predictions from the WHDG model [71]. These models describe the interaction of a hard-scattered parton with the medium of high color charge density within perturbative QCD [11].

n0 ALICE 0-10% Pb-Pb \SNN = 2.76 TeV

n0 PHENIX 0-10% Au-Au □ \SNN = 200 GeV o vSNN = 62.4 GeV o = 39 GeV

n0 WA98 0-13% Pb-Pb = 17.3 GeV

6 8 10 12 14

PT (GeV/c)

Fig. 8 (Color online) Neutral pion nuclear modification factor, Raa , in Pb-Pb collisions at snn = 2.76 TeV for the 0-10 % class in comparison to results at lower energies. The box around unity reflects the uncertainty of the average nuclear overlap function (Taa) and the normalization uncertainty of the pp spectrum added in quadrature. Horizontal bars reflect the bin width. The center-of-mass energy dependence of the neutral pion RAA is shown with results from Au-Au collisions at V5NN = 39, 62.4 [34], and 200GeV [31] as well as the result from the CERN SPS [69] (using scaled p-C data as reference) along with the results for Pb-Pb at .Jsnn = 2.76 TeV. The scale uncertainties of the measurements at lower energies of the order of 10-15 % are not shown

n0 ALICE Pb-Pb \SNN = 2.76 TeV

n0 PHENIX Au-Au \SNN = 200 GeV

\SNN = 62.4 GeV \SNN = 39 GeV

• •

100 150

250 300 350 400

Fig. 9 (Color online) Centrality dependence of the n0 nuclear modification factor Raa at pt = 7 GeV/c in Au-Au and Pb-Pb collisions at ^SNN = 39, 62.4, 200 [22,34], and 2,760GeV

Both calculations assume that the hadronization of the hard-scattered parton occurs in the vacuum and is not affected by the medium. They model the energy loss of the parton but not the corresponding response of the medium. Their appli-

Pb-Pb ysNN = 2.76 TeV

- 0-5% Pb-Pb VsNN = 2.76 TeV - - M]n0 ALICE E3 GLV gg WHDG - 1 fU— 5-10% Pb-Pb tfsNN = 2.76 TeV -■ n0 ALICE " 1 10-20% Pb-Pb VsNN = 2.76 TeV ■ n0 ALICE 1

-' 20-40% Pb-Pb 1 list=2.76 1 Tev1111111- Bin0 ALICE 1 - 40-60% Pb-Pb VsNN = 2.76 TeV H]n0 ALICE ;-h " 1 LyBsssssssssfe ....................................... - 60-80% Pb-Pb VsNN = 2.76 TeV -[Hn0 ALICE 1 sss .......................................

0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18

p (GeV/c)

p (GeV/c)

p (GeV/c)

Fig. 10 (Color online) Comparison of the measured nuclear modifi- lines indicate the bin width. The boxes around unity reflect the scale

cation factor Raa with a GLV calculation [38,39] and with a WHDG uncertainties of data related to Taa and the normalization of the pp [71] parton energy loss calculations. Vertical lines show the statistical spectrum uncertainties, systematic uncertainties are shown as boxes. Horizontal

cability is limited to transverse momenta above 2-4GeV/c as soft particle production from the bulk is not taken into account. The Pb-Pb n0 spectra are therefore also compared to two models which aim at a description of the full pT range: an EPOS calculation [72] and a calculation by Nemchik et al. based on the combination of a hydrodynamic description at low pT and the absorption of color dipoles at higher pT [73,74]. These comparisons are presented in Fig. 11.

The GLV calculation takes final-state radiative energy loss into account. It includes the broadening of the transverse momenta of the incoming partons in cold nuclear matter ("nuclear broadening" or "Cronin effect"). The main parameter of this model, the initial gluon density, was tuned to describe the neutral pion suppression observed in Au-Au collisions at RHIC. For the calculation of the parton energy loss in Pb-Pb collisions at the LHC the initial gluon density was constrained by the measured charged-particle multiplicities. The model can approximately reproduce the centrality and pT dependence of the n0 RAA.

The WHDG model takes into account collisional and radiative parton energy loss and geometrical path length fluctuations. The color charge density of the medium is assumed to be proportional to the number of participating nucleons from a Glauber model, and hard parton-parton scatterings are proportional to the number of binary nucleon-nucleon collisions. Parameters of the model were constrained by the neutral pion RAA measured at RHIC. Like in the case of the GLV calculation, the neutral pion RAA at the LHC is then predicted by translating the measured charged-particle mul-

tiplicity dNch/dn in Pb-Pb collisions into an initial gluon density which is the free parameter of the model. For central collisions this yielded an increase in the gluon density from dNg/dy « 1400 at RHIC to dNg/dy « 3,000 at the LHC. The WHDG model reproduces the n0 RAA in central collisions reasonably well, but predicts too strong suppression for more peripheral classes.

The two model predictions for the full pT range are compared to the measured spectra in Fig. 11. EPOS is based on the hadronization of flux tubes produced early in the collision. Hard scattering in this model produces strings with transversely moving parts. String segments with low energies are assumed to be part of the bulk whose space-time evolution is modeled within hydrodynamics. String segments with sufficiently large energy fragment in the vacuum. A third class of string segments with intermediate energies is considered to have enough energy to leave the medium accompanied by quark pick-up from the bulk during the fragmentation process. In EPOS particle production is determined by hydrody-namic flow at low pT (<4 GeV/c), followed at higher pT by energy loss of high-pT string segments. In central collisions the EPOS calculation describes the measured n0 spectrum rather well. Towards more peripheral collisions a discrepancy develops for 1 < pT < 5GeV/c which may possibly be attributed to underestimating the contribution of hydro-dynamic flow in peripheral collisions.

The calculation by Nemchik et al. also combines a model for hadron suppression at high pT with a hydrodynamic description of bulk particle production at low pT. Hadron

<0 1.5

^ 0 1.0

H 0.5

cr 0 H" 1.0

H 0.5

cr 0 »¡s 1.0

H 0.5

En0 ALICE —EPOS - - Nemchik hydro

- - - Nemchik dipole abs. - Nemchik sum , -

^0-5% Pb-Pb vSNN = 2.76 TeV

20-40% Pb-Pb = 2.76 TeV

0.0 P60-80% Pb-Pb

SNN = 2.76 'eV--2 3

PT (GeV/c)

4 5 6 7 8910

Fig. 11 (Color online) Comparison of the measured n0 spectra for three centrality classes (0-5, 20-40, 60-80 %) with two calculations which make predictions for the full pT range of the measurement. The calculated spectra and the data points were divided by a fit of the measured n 0 spectra. For the data points the error bars represent the statistical uncertainties and the boxes the systematic uncertainties. Calculations with the EPOS event generator [72] are shown by the solid line. The fluctuations of the EPOS lines at high pT are due to limited statistics in the number of generated events. The calculations by Nemchik et al. [73,74] combine a hydrodynamical model at low pi with a color dipole absorption model for pi > 3 GeV/c. The two components and the sum (for pi > 3 GeV/c) are shown separately

suppression in this model results from the absorption of pre-hadrons, i.e., of color dipoles which are already formed in the medium by hard-scattered partons during the production of hadrons with large z = phadron/pparton. As the model, at high pT, predicts only RAA, the calculated RAA values

were scaled by (TAA) x E d3ameas/d3p and then added to the calculated n0 invariant yields from the hydrodynamic model in order to compare to the measured n0 spectra. The hydrodynamic calculation dominates the total n0 yield up to pT = 2GeV/c and remains a significant contribution up to 5 GeV/c. From about 3 GeV/c the contribution from hard scattering becomes larger than the one from the hydrody-namic calculation. The spectrum in central Pb-Pb collisions (0-5 %) is approximately described except for the transition region between the hydrodynamic and the hard contribution. In the 20-40 % class the hydrodynamic calculation overpre-dicts the data up to pT = 2 GeV/c.

5 Conclusions

Measurements of neutral pion production at midrapidity in pp and Pb-Pb collisions at ^sNN = 2.76 TeV were presented. The measurements were performed with two independent techniques, by measuring the photons with the electromagnetic calorimeter PHOS, and by measuring converted photons with the ALICE tracking system. The two independent measurements were found to give consistent results, and were combined for the final results.

The neutral pion spectrum in pp collisions was compared to a NLO perturbative QCD calculation using the DSS fragmentation functions. This calculation, which describes the pion spectrum in pp collisions at = 0.9 TeV rather well, tends to overpredict the n0 cross section already at Vs = 2.76 TeV. Along with a similar observation in pp collision at = 7 TeV this indicates the likely need for improvements in the gluon-to-pion fragmentation function. A similar observation was made for transverse momentum spectra of charged particles in proton-proton and protonantiproton collisons at 1.96 < < 7TeV [61,75].

The neutral pion nuclear suppression factor RAA was calculated from the measured neutral pion spectra, and was compared to measurements at lower energies and to theoretical predictions. The n0 suppression in the most central class (0-5%) reaches values of up to 8-10 for 5 < pT < 7GeV/c. The suppression in Pb-Pb collisions at Vsnn = 2.76 TeV is stronger than in Au-Au collisions at Vsnn = 200 GeV (and lower energies) at RHIC for all cen-tralities.

The general features of the centrality and pT dependence of the Raa for pT > 2GeV/c are approximately reproduced by GLV and WHDG parton energy loss calculations, although the WHDG calculation performs less well in peripheral collisions. For both calculations the main free parameter, the initial gluon density, was chosen to describe the neutral pion suppression at RHIC and then scaled to LHC energies based on the measured charged-particle multiplicities. The measured n0 spectra were also compared to calculations with the EPOS event generator and a calculation by Nemchik et al. By combining soft particle production from a hydrodynamically evolving medium with a model for hadron suppression these models are capable of making predictions for the entire pT range. An important task on the theoretical side will be to establish whether the observed deviations from the data simply indicate a suboptimal adjustment of parameters or hint at important physical phenomena missing in the models. Future analyses based on runs with higher integrated luminosities, e.g. the 2011LHC Pb-Pb run, will also include the ALICE lead-scintillator electromagnetic calorimeter (EMCal) and will allow us to extend the neutral pion measurement to higher transverse momenta. The role of initial-state effects on the particle production in Pb-Pb

collisions will be investigated by measurements of particle production in p-Pb collisions.

Acknowledgments We would like to thank Jan Nemchik, William A. Horowitz, Ivan Vitev, and Klaus Werner for providing the model calculations shown in this paper. This work was supported by the grants RFBR 10-02-91052 and RFBR12-02-91527. The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: State Committee of Science, World Federation of Scientists (WFS) and Swiss Fonds Kidagan, Armenia, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (FINEP), Fundafáo de Amparo á Pesquisa do Estado de Sáo Paulo (FAPESP); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and the Ministry of Science and Technology of China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Foundation; The European Research Council under the European Community's Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the 'Region Pays de Loire', 'Region Alsace', 'Region Auvergne' and CEA, France; German BMBF and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian OTKA and National Office for Research and Technology (NKTH); Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche "Enrico Fermi", Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan;

Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF); CONACYT, DGAPA, México, ALFA-EC and the EPLANET Program (European Particle Physics Latin American Network) Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), The Netherlands; Research Council of Norway (NFR); Polish Ministry of Science and Higher Education; National Science Centre, Poland; Ministry of National Education/Institute for Atomic Physics and CNCS-UEFISCDI - Romania; Ministry of Education and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research; Ministry of Education of Slovakia; Department of Science and Technology, South Africa; CIEMAT, EELA, Ministerio de Economía y Competitivi-dad (MINECO) of Spain, Xunta de Galicia (Consellería de Educación), CEADEN, Cubaenergía, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); The United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio.

Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

Funded by SCOAP3 / License Version CC BY 4.0.

6 Appendix

For the calculation of the RAA above pT > 8GeV/c an extrapolation of the measured transverse momentum spectrum in pp collisions at s/s = 2.76 TeV based on the Tsallis functional form

1 d2 N 2npT dpTdy

A (n - 1)(n - 2) 1 2n nC [nC + m(n - 2)] c2

Í ypT + m2 - m

was used (where m is the mass of the neutral pion and c the speed of light). The parameters are given in Table 3.

In order to compare the individual PCM and PHOS measurements to the combined results in Pb-Pb collisions the parameterization

(b+c/( pT+e))

2npT dpTdy

with pT in GeV/c was used to fit the combined spectrum for each centrality class. The corresponding parameters are given

Table 3 Parameters of the fits of the Tsallis parameterization (Eq. 3) to the combined invariant production yields for n0 mesons in inelastic collisions at ^/s = 2.76 TeV

System A C (MeV/c2) n

PP 1.7 ± 0.7 135 ± 29 7.1 ± 0.7

60-80 % Pb-Pb 31.7 142 7.4

The uncertainties (statistical and systematic added in quadrature) were used to evaluate the uncertainty of the extrapolation used in the calculation of Raa for pt > 8 GeV/c. The uncertainty on the parameter A due to the spectra normalization of 3.9% at -Js = 2.76 TeV is not included. For the measurment in 60-80% Pb-Pb collisions the fit parameters are given without uncertainties as the parameterization is only used to facilitate the comparison with model calculations

Table 4 Parameters of the fits to the combined invariant yields of n0 mesons in Pb-Pb collisions in different centrality classes with the functional form given in Eq. 4

Centrality (%) a (c2/GeV2) b c d e

0-5 28.96 5.85 -199.17 4.64 95.30

5-10 21.97 5.79 -33.54 2.96 10.84

0-10 25.53 5.84 -49.95 3.35 18.49

10-20 18.91 5.71 -44.76 3.37 19.66

20-40 11.54 5.74 -18.43 2.62 7.37

40-60 4.18 5.67 -9.43 2.00 3.39

The spectra were fitted taking into account the combined statistical and systematic errors

in Table 4. For the most peripheral centrality class the Tsallis parameterization Eq. 3 was used for which the parameters are given in Table 3. These parameterizations describe the data well in the measured momentum range.


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The ALICE Collaboration

B. Abelev69, J. Adam37, D. Adamová77, M. M. Aggarwal81, M. Agnello88,105, A. Agostinelli26, N. Agrawal44, Z. Ahammed124, N . Ahmad18,1 . Ahmed15, S. U . Ahn62, S . A . Ahn62,1 . Aimo88,105, S . Aiola129,M . Ajaz15, A . Akindinov53, S . N . Alam124, D. Aleksandrov94, B. Alessandro105, D. Alexandre96, A. Alici12,99, A. Alkin3, J. Alme35, T. Alt39, S. Altinpinar17,1. Altsybeev123, C. Alves Garcia Prado113, C. Andrei72, A. Andronic91, V. Anguelov87,J. Anielski49, T. AntiCiC92, F. Antinori102, P. Antonioli99, L. Aphecetche107, H. Appelshäuser48,S. Arcelli26,N. Armesto16, R. Arnaldi105,T. Aronsson129,I. C. Arsene91, M. Arslandok48, A. Augustinus34, R. Averbeck91, T. C. Awes78, M. D. Azmi83, M. Bach39, A. Badala101, Y. W. Baek40,64, S. Bagnasco105, R. Bailhache48, R. Bala84, A. Baldisseri14, F. Baltasar Dos Santos Pedrosa34, R. C. Baral56, R. Barbera27,

F. Barile31, G. G. Barnaföldi128, L. S. Barnby96, V. Barret64, J. Bartke110, M. Basile26, N. Bastid64, S. Basu124, B. Bathen49,

G. Batigne107, B. Batyunya61, P. C. Batzing21, C. Baumann48, I. G. Bearden74, H. Beck48, C. Bedda88, N. K. Behera44, I. Belikov50, F. Bellini26, R. Bellwied115, E. Belmont-Moreno59, R. Belmont III127, V. Belyaev70, G. Bencedi128, S. Beole25, I. Berceanu72, A. Bercuci72, Y. Berdnikov79,b, D. Berenyi128, M. E. Berger86, R. A. Bertens52, D. Berzano25, L. Betev34, A. Bhasin84, I. R. Bhat84, A. K. Bhati81, B. Bhattacharjee41,J. Bhom120, L. Bianchi25, N. Bianchi66, C. Bianchin52, J. Bielcík37,J. Bielcíková77, A. Bilandzic74, S. Bjelogrlic52, F. Blanco10, D. Blau94, C. Blume48, F. Bock68,87, A. Bogdanov70, H. B0ggild74, M. Bogolyubsky106, F. V. Böhmer86, L. Boldizsár128, M. Bombara38, J. Book48, H. Borel14, A. Borissov90,127, F. Bossú60, M. Botje75, E. Botta25, S. Böttger47, P. Braun-Munzinger91, M. Bregant113, T. Breitner47, T. A. Broker48, T. A. Browning89, M. Broz37, E. Bruna105, G. E. Bruno31, D. Budnikov93, H. Buesching48, S. Bufalino105, P. Buncic34,0. Busch87, Z. Buthelezi60, D. Caffarri28, X. Cai7, H. Caines129, L. Calero Diaz66, A. Caliva52, E. Calvo Villar97, P. Camerini24, F. Carena34, W. Carena34, J. Castillo Castellanos14, E. A. R. Casula23, V. Catanescu72, C. Cavicchioli34, C. Ceballos Sanchez9, J. Cepila37, P. Cerello105, B. Chang116, S. Chapeland34, J. L. Charvet14, S. Chattopadhyay124, S. Chattopadhyay95, V. Chelnokov3, M. Cherney80, C. Cheshkov122, B. Cheynis122, V. Chibante Barroso34, D. D. Chinellato115, P. Chochula34, M. Chojnacki74, S. Choudhury124, P. Christakoglou75, C. H. Christensen74, P. Christiansen32, T. Chujo120, S. U. Chung90, C. Cicalo100, L. Cifarelli12,26, F. Cindolo99, J. Cleymans83, F. Colamaria31, D. Colella31, A. Collu23, M. Colocci26, G. Conesa Balbastre65, Z. Conesa del Valle46, M. E. Connors129, J. G. Contreras11, T. M. Cormier127, Y. Corrales Morales25, P. Cortese30, I. Cortés Maldonado2, M. R. Cosentino113, F. Costa34, P. Crochet64, R. Cruz Albino11, E. Cuautle58, L. Cunqueiro66, A. Dainese102, R. Dang7, A. Danu57, D . Das95, I . Das46, K. Das95, S . Das4, A . Dash114, S . Dash44, S . De124, H . Delagrange107,a, A . Deloff71, E. Dénes128, G. D'Erasmo31, A. De Caro12,29, G. de Cataldo98, J. de Cuveland39, A. De Falco23, D. De Gruttola12,29, N. De Marco105, S. De Pasquale29, R. de Rooij52, M. A. Diaz Corchero10, T. Dietel49, P. Dillenseger48, R. Divia34, D. Di Bari31, S. Di Liberto103, A. Di Mauro34, P. Di Nezza66,0. Djuvsland17, A. Dobrin52, T. Dobrowolski71, D. Domenicis Gimenez113, B. Dönigus48, O. Dordic21, S. D0rheim86, A. K. Dubey124, A. Dubla52, L. Ducroux122, P. Dupieux64, A. K. Dutta Majumdar95, T. E. Hilden42, R. J. Ehlers129, D. Elia98, H. Engel47, B. Erazmus34,107, H. A. Erdal35, D. Eschweiler39, B. Espagnon46, M. Esposito34, M. Estienne107, S. Esumi120, D. Evans96, S. Evdokimov106, D. Fabris102, J. Faivre65, D. Falchieri26, A. Fantoni66, M. Fasel87, D. Fehlker17, L. Feldkamp49, D. Felea57, A. Feliciello105, G. Feofilov123, J. Ferencei77, A. Fernández Téllez2, E. G. Ferreiro16, A. Ferretti25, A. Festanti28, J. Figiel110, M. A. S. Figueredo117, S. Filchagin93, D. Finogeev51, F. M. Fionda31, E. M. Fiore31,

E. Floratos82, M. Floris34, S. Foertsch60, P. Foka91, S. Fokin94, E. Fragiacomo104, A. Francescon28'34, U. Frankenfeld91, U. Fuchs34, C. Furget65, M. Fusco Girard29, J. J. Gaardh0je74, M. Gagliardi25, A. M. Gago97, M. Gallio25, D. R. Gangadharan19, P. Ganoti78, C. Garabatos91, E. Garcia-Solis13, C. Gargiulo34,1. Garishvili69, J. Gerhard39, M. Germain107, A. Gheata34, M. Gheata34,57, B. Ghidini31, P. Ghosh124, S. K. Ghosh4, P. Gianotti66, P. Giubellino34, E. Gladysz-Dziadus110, P. Glässel87, A. Gomez Ramirez47, P. González-Zamora10, S. Gorbunov39, L. Görlich110, S. Gotovac109, L. K. Graczykowski126, A. Grelli52, A. Grigoras34, C. Grigoras34, V. Grigoriev70, A. Grigoryan1, S. Grigoryan61, B. Grinyov3, N. Grion104, J. F. Grosse-Oetringhaus34, J.-Y. Grossiord122, R. Grosso34, F. Guber51, R. Guernane65, B. Guerzoni26, M. Guilbaud122, K. Gulbrandsen74, H. Gulkanyan1, M. Gumbo83, T. Gunji119, A. Gupta84, R. Gupta84, K. H. Khan15, R. Haake49,0. Haaland17, C. Hadjidakis46, M. Haiduc57, H. Hamagaki119, G. Hamar128, L. D. Hanratty96, A. Hansen74, J. W. Harris129, H. Hartmann39, A. Harton13, D. Hatzifotiadou99, S. Hayashi119, S. T. Heckel48, M. Heide49, H. Helstrup35, A. Herghelegiu72, G. Herrera Corral11, B. A. Hess33, K. F. Hetland35, B. Hippolyte50, J. Hladky55, P. Hristov34, M. Huang17, T. J. Humanic19, N. Hussain41, D. Hutter39,

D. S. Hwang20, R. Ilkaev93,1. Ilkiv71, M. Inaba120, G. M. Innocenti25, C. Ionita34, M. Ippolitov94, M. Irfan18, M. Ivanov91, V. Ivanov79, A. Jacholkowski27, P. M. Jacobs68, C. Jahnke113, H. J. Jang62, M. A. Janik126, P. H. S. Y. Jayarathna115, C. Jena28, S. Jena115, R. T. Jimenez Bustamante58, P. G. Jones96, H. Jung40, A. Jusko96, V. Kadyshevskiy61, S. Kalcher39, P. Kalinak54, A. Kalweit34, J. Kamin48, J. H. Kang130, V. Kaplin70, S. Kar124, A. Karasu Uysal63, O. Karavichev51, T. Karavicheva51, E. Karpechev51, U. Kebschull47, R. Keidel131, D. L. D. Keijdener52, M. M. Khan18,c, P. Khan95, S. A. Khan124, A. Khanzadeev79, Y. Kharlov106, B. Kileng35, B. Kim130, D. W. Kim40,62, D. J. Kim116, J. S. Kim40, M. Kim40, M. Kim130, S. Kim20, T. Kim130, S. Kirsch39, I. Kisel39, S. Kiselev53, A. Kisiel126, G. Kiss128, J. L. Klay6, J. Klein87, C. Klein-Bösing49, A. Kluge34, M. L. Knichel91, A. G. Knospe111, C. Kobdaj34,108, M. Kofarago34, M. K. Köhler91, T. Kollegger39, A. Kolojvari123, V. Kondratiev123, N. Kondratyeva70, A. Konevskikh51, V. Kovalenko123, M. Kowalski110, S. Kox65, G. Koyithatta Meethaleveedu44, J. Kral116, I. Králik54, F. Kramer48, A. Kravcáková38, M. Krelina37, M. Kretz39, M. Krivda54,96, F. Krizek77, E. Kryshen34, M. Krzewicki91, V. Kucera77, Y. Kucheriaev94,a, T. Kugathasan34, C. Kuhn50, P. G. Kuijer75, I. Kulakov48, J. Kumar44, P. Kurashvili71, A. Kurepin51, A. B. Kurepin51, A. Kuryakin93, S. Kushpil77, M. J. Kweon87, Y. Kwon130, P. Ladron de Guevara58, C. Lagana Fernandes113, I. Lakomov46, R. Langoy125, C. Lara47, A. Lardeux107, A. Lattuca25, S. L. La Pointe52, P. La Rocca27, R. Lea24, L. Leardini87, G. R. Lee96, I. Legrand34, J. Lehnert48, R. C. Lemmon76, V. Lenti98, E. Leogrande52, M. Leoncino25, I. León Monzón112, P. Lévai128, S. Li7,64, J. Lien125s, R. Lietava96, S. Lindal21, V. Lindenstruth39, C. Lippmann91, M. A. Lisa19, H. M. Ljunggren32, D. F. Lodato52, P. I. Loenne17, V. R. Loggins127, V. Loginov70, D. Lohner87, C. Loizides68, X. Lopez64, E. López Torres9, X.-G. Lu87, P. Luettig48, M. Lunardon28, G. Luparello52, C. Luzzi34, R. Ma129, A. Maevskaya51, M. Mager34, D. P. Mahapatra56, S. M. Mahmood21, A. Maire87, R. D. Majka129, M. Malaev79, I. Maldonado Cervantes58, L. Malinina61,d, D. Mal'Kevich53, P. Malzacher91, A. Mamonov93, L. Manceau105, V. Manko94, F. Manso64s, V. Manzari98, M. Marchisone25,64, J. Mares55, G. V. Margagliotti24, A. Margotti99, A. Marín91, C. Markert111, M. Marquard48, I. Martashvili118, N. A. Martin91, P. Martinengo34, M. I. Martínez2, G. Martínez García107, J. Martin Blanco107, Y. Martynov3, A. Mas107, S. Masciocchi91, M. Masera25, A. Masoni100, L. Massacrier107, A. Mastroserio31, A. Matyja110, C. Mayer110, J. Mazer118, M. A. Mazzoni103, F. Meddi22, A. Menchaca-Rocha59, J. Mercado Pérez87, M. Meres36, Y. Miake120, K. Mikhaylov53,61, L. Milano34, J. Milosevic21,e, A. Mischke52, A. N. Mishra45, D. Miskowiec91, J. Mitra124, C. M. Mitu57, J. Mlynarz127, N. Mohammadi52, B. Mohanty73,124, L. Molnar50, L. Montano Zetina11, E. Montes10, M. Morando28, D. A. Moreira De Godoy113, S. Moretto28, A. Morsch34, V. Muccifora66,

E. Mudnic109, D. Mühlheim49, S. Muhuri124, M. Mukherjee124, H. Müller34, M. G. Munhoz113, S. Murray83, L. Musa34, J. Musinsky54, B. K. Nandi44, R. Nania99, E. Nappi98, C. Nattrass118, K. Nayak73, T. K. Nayak124, S. Nazarenko93, A. Nedosekin54, M. Nicassio91, M. Niculescu34,57, B. S. Nielsen74, S. Nikolaev94, S. Nikulin94, V. Nikulin79, B. S. Nilsen80,

F. Noferini12,99, P. Nomokonov61, G. Nooren52, J. Norman117, A. Nyanin94, J. Nystrand17, H. Oeschler87, S. Oh129, S. K. Oh40,f, A. Okatan63, L. Olah128, J. Oleniacz126, A. C. Oliveira Da Silva113, J. Onderwaater91, C. Oppedisano105, A. Ortiz Velasquez32, A. Oskarsson32, J. Otwinowski91, K. Oyama87, P. Sahoo45, Y. Pachmayer87, M. Pachr37, P. Pagano29, G. Paic58, F. Painke39, C. Pajares16, S. K. Pal124, A. Palmeri101, D. Pant44, V. Papikyan1, G. S. Pappalardo101, P. Pareek45, W. J. Park91, S. Parmar81, A. Passfeld49, D. I. Patalakha106, V. Paticchio106, B. Paul95, T. Pawlak126, T. Peitzmann52, H. PereiraDa Costa14, E. Pereira De Oliveira Filho113, D. Peresunko94, C. E. Pérez Lara75, A. Pesci99, V. Peskov48, Y. Pestov5, V. Petrácek37, M. Petran37, M. Petris72, M. Petrovici72, C. Petta27, S. Piano104, M. Pikna36, P. Pillot107, O. Pinazza34,99, L. Pinsky115, D. B. Piyarathna115, M. Ploskons68, M. Planinic92,121, J. Pluta126, S. Pochybova128, P. L. M. Podesta-Lerma112, M. G. Poghosyan34, E. H. O. Pohjoisaho42, B. Polichtchouk106, N. Poljak92, A. Pop72, S. Porteboeuf-Houssais64, J. Porter68, B. Potukuchi84, S. K. Prasad127, R. Preghenella12,99, F. Prino105, C. A. Pruneau127, I. Pshenichnov51, G. Puddu23, P. Pujahari127, V. Punin93, J. Putschke127, H. Qvigstad21, A. Rachevski106, S. Raha4, J. Rak116, A. Rakotozafindrabe14, L. Ramello30, R. Raniwala85, S. Raniwala85, S. S. Räsänen42, B. T. Rascanu48, D. Rathee81, A. W. Rauf15, V. Razazi23, K. F. Read118, J. S. Real65, K. Redlich71,g, R. J. Reed129, A. Rehman17, P. Reichelt48, M. Reicher52, F. Reidt34, R. Renfordt48, A. R. Reolon66, A. Reshetin51,

F. Rettig39, J.-P. Revol34, K. Reygers87, V. Riabov79, R. A. Ricci67, T. Richert32, M. Richter21, P. Riedler34, W. Riegler34, F. Riggi27,A. Rivetti105, E. Rocco52, M. Rodríguez Cahuantzi2, A. Rodriguez Manso75, K. R0ed21, E. Rogochaya61, S. Rohni84,

D. Rohr39, D. Röhrich17, R. Romita76, F. Ronchetti66, L. Ronflette107, P. Rosnet64, A. Rossi34, F. Roukoutakis82, A. Roy45, C. Roy50, P. Roy95, A. J. Rubio Montero10, R. Rui24, R. Russo25, E. Ryabinkin94, Y. Ryabov79, A. Rybicki110, S. Sadovsky106, K. Safank34, B. Sahlmuller48, R. Sahoo45, P. K. Sahu56, J. Saini124, S. Sakai68, C. A. Salgado16, J. Salzwedel19, S. Sambyal84, V. Samsonov79, X. Sanchez Castro50, F. J. Sánchez Rodríguez112, L. Sándor54, A. Sandoval59, M. Sano120, G. Santagati27, D. Sarkar124, E. Scapparone99, F. Scarlassara28, R. P. Scharenberg89, C. Schiaua72, R. Schicker87, C. Schmidt91, H. R. Schmidt33, S. Schuchmann48, J. Schukraft34, M. Schulc37, T. Schuster129, Y. Schutz34,107, K. Schwarz91, K. Schweda91, G. Scioli26,

E. Scomparin105, R. Scott118, G. Segato28, J. E. Seger80, Y. Sekiguchi119, I. Selyuzhenkov91, J. Seo90, E. Serradilla10,59, A. Sevcenco57, A. Shabetai107, G. Shabratova61, R. Shahoyan34, A. Shangaraev106, N. Sharma118, S. Sharma84, K. Shigaki43, K. Shtejer25, Y. Sibiriak94, S. Siddhanta100, T. Siemiarczuk71, D. Silvermyr78, C. Silvestre65, G. Simatovic121, R. Singaraju124, R. Singh84, S. Singha73,124, V. Singhal124, B. C. Sinha124, T. Sinha95, B. Sitar36, M. Sitta30, T. B. Skaali21, K. Skjerdal17, M. Slupecki116, N. Smirnov129, R. J. M. Snellings52, C. S0gaard32, R. Soltz69, J. Song90, M. Song130, F. Soramel28, S. Sorensen118, M. Spacek37, E. Spiriti66, I. Sputowska110, M. Spyropoulou-Stassinaki82, B. K. Srivastava89, J. Stachel87, I. Stan57, G. Stefanek71, M. Steinpreis19, E. Stenlund32, G. Steyn60, J. H. Stiller87, D. Stocco107, M. Stolpovskiy106, P. Strmen36, A. A. P. Suaide113, T. Sugitate43, C. Suire46, M. Suleymanov15, R. Sultanov53, M. Sumbera77, T. Susa92, T. J. M. Symons68, A. Szabo36, A. Szanto de Toledo113, I. Szarka36, A. Szczepankiewicz34, M. Szymanski126, J. Takahashi114, M. A. Tangaro31, J. D. Tapia Takaki46,h, A. Tarantola Peloni48, A. Tarazona Martinez34, M. G. Tarzila72, A. Tauro34, G. Tejeda Muñoz2, A. Telesca34, J. Thäder91, D. Thomas52, R. Tieulent122, A. R. Timmins115, A. Toia102, V. Trubnikov3, W. H. Trzaska116, T. Tsuji119, A. Tumkin93, R. Turrisi102, T. S. Tveter21, K. Ullaland17, A. Uras122, G. L. Usai23, M. Vajzer77, M. Vala54,61, L. Valencia Palomo64, S. Vallero87, P. Vande Vyvre34, J. Van Der Maarel52, J. W. Van Hoorne34, M. van Leeuwen52, A. Vargas2, M. Vargyas116, R. Varma44, M. Vasileiou82, A. Vasiliev94, V. Vechernin123, M. Veldhoen52, A. Velure17, M. Venaruzzo24,67, E. Vercellin25, S. Vergara Limón2, R. Vernet8, M. Verweij127, L. Vickovic109, G. Viesti28, J. Viinikainen116, Z. Vilakazi60, O. Villalobos Baillie96, A. Vinogradov94, L. Vinogradov123, Y. Vinogradov93, T. Virgili29, Y. P. Viyogi124, A. Vodopyanov61, M. A. Völkl87, K. Voloshin53, S. A. Voloshin127, G. Volpe34, B. von Haller34, I. Vorobyev123, D. Vranic34,91, J. Vrláková38, B. Vulpescu64, A. Vyushin93, B. Wagner17, J. Wagner91, V. Wagner37, M. Wang7,107, Y. Wang87, D. Watanabe120, M. Weber115, J. P. Wessels49, U. Westerhoff49, J. Wiechula33, J. Wikne21, M. Wilde49, G. Wilk71, J. Wilkinson87, M. C. S. Williams99, B. Windelband87, M. Winn87, C. G. Yaldo127, Y. Yamaguchi119, H. Yang52, P. Yang7, S. Yang17, S. Yano43, S. Yasnopolskiy94, J. Yi90, Z. Yin7, I.-K. Yoo90, I. Yushmanov94, V. Zaccolo74, C. Zach37, A. Zaman15, C. Zampolli99, S. Zaporozhets61, A. Zarochentsev123, P. Závada55, N. Zaviyalov93, H. Zbroszczyk126, I. S. Zgura57, M. Zhalov79, H. Zhang7, X. Zhang7,68, Y. Zhang7, C. Zhao21, N. Zhigareva53,D. Zhou7, F. Zhou7, Y. Zhou52, Zhou Zhuo17, H. Zhu7, J. Zhu7, X. Zhu7, A. Zichichi12,26, A. Zimmermann87, M. B. Zimmermann34,49, G. Zinovjev3, Y. Zoccarato122, M. Zyzak48

1 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia

2 Benemérita Universidad Autónoma de Puebla, Puebla, Mexico

3 Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine

4 Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Bose Institute, Kolkata, India

5 Budker Institute for Nuclear Physics, Novosibirsk, Russia

6 California Polytechnic State University, San Luis Obispo, CA, USA

7 Central China Normal University, Wuhan, China

8 Centre de Calcul de l'IN2P3, Villeurbanne, France

9 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba

10 Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain

11 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico

12 Centro Fermi-Museo Storico della Fisica e Centro Studi e Ricerche "Enrico Fermi", Rome, Italy

13 Chicago State University, Chicago, USA

14 Commissariat a l'Energie Atomique, IRFU, Saclay, France

15 COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan

16 Departamento de Física de Partículas and IGFAE, Universidad de Santiago de Compostela, Santiago de Compostela, Spain

17 Department of Physics and Technology, University of Bergen, Bergen, Norway

18 Department of Physics, Aligarh Muslim University, Aligarh, India

19 Department of Physics, Ohio State University, Columbus, OH, USA

20 Department of Physics, Sejong University, Seoul, South Korea

21 Department of Physics, University of Oslo, Oslo, Norway

22 Dipartimento di Fisica dell'Università 'La Sapienza' and Sezione INFN, Rome, Italy

23 Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy

24 Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy

25 Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy

26 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy

27 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy

28 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padua, Italy

29 Dipartimento di Fisica 'E.R. Caianiello' dell'Università and Gruppo Collegato INFN, Salerno, Italy

30 Dipartimento di Scienze e Innovazione Tecnologica dell'Università del Piemonte Orientale and Gruppo Collegato INFN, Alessandria, Italy

31 Dipartimento Interateneo di Fisica 'M. Merlin' and Sezione INFN, Bari, Italy

32 Division of Experimental High Energy Physics, University of Lund, Lund, Sweden

33 Eberhard Karls Universität Tübingen, Tübingen, Germany

34 European Organization for Nuclear Research (CERN), Geneva, Switzerland

35 Faculty of Engineering, Bergen University College, Bergen, Norway

36 Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia

37 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic

38 Faculty of Science, P.J. Safárik University, Kosice, Slovakia

39 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany

40 Gangneung-Wonju National University, Gangneung, South Korea

41 Department of Physics, Gauhati University, Guwahati, India

42 Helsinki Institute of Physics (HIP), Helsinki, Finland

43 Hiroshima University, Hiroshima, Japan

44 Indian Institute of Technology Bombay (IIT), Mumbai, India

45 Indian Institute of Technology Indore (IITI), Indore, India

46 Institut de Physique Nucléaire d'Orsay (IPNO), Université Paris-Sud, CNRS-IN2P3, Orsay, France

47 Institut für Informatik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany

48 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany

49 Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, Münster, Germany

50 Institut Pluridisciplinaire Hubert Curien (IPHC), Université de Strasbourg, CNRS-IN2P3, Strasbourg, France

51 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia

52 Institute for Subatomic Physics of Utrecht University, Utrecht, The Netherlands

53 Institute for Theoretical and Experimental Physics, Moscow, Russia

54 Institute of Experimental Physics, Slovak Academy of Sciences, Kosice, Slovakia

55 Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic

56 Institute of Physics, Bhubaneswar, India

57 Institute of Space Science (ISS), Bucharest, Romania

58 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico

59 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico

60 iThemba LABS, National Research Foundation, Somerset West, South Africa

61 Joint Institute for Nuclear Research (JINR), Dubna, Russia

62 Korea Institute of Science and Technology Information, Taejeon, South Korea

63 KTO Karatay University, Konya, Turkey

64 Laboratoire de Physique Corpusculaire (LPC), Clermont Université, Université Blaise Pascal, CNRS-IN2P3, Clermont-Ferrand, France

65 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France

66 Laboratori Nazionali di Frascati, INFN, Frascati, Italy

67 Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy

68 Lawrence Berkeley National Laboratory, Berkeley, CA, USA

69 Lawrence Livermore National Laboratory, Livermore, CA, USA

70 Moscow Engineering Physics Institute, Moscow, Russia

71 National Centre for Nuclear Studies, Warsaw, Poland

72 National Institute for Physics and Nuclear Engineering, Bucharest, Romania

73 National Institute of Science Education and Research, Bhubaneswar, India

74 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

75 Nikhef, National Institute for Subatomic Physics, Amsterdam, The Netherlands

76 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, UK

77 Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Rezu Prahy, Czech Republic

78 Oak Ridge National Laboratory, Oak Ridge, TN, USA

79 Petersburg Nuclear Physics Institute, Gatchina, Russia

80 Physics Department, Creighton University, Omaha, NE, USA

81 Physics Department, Panjab University, Chandigarh, India

82 Physics Department, University of Athens, Athens, Greece

83 Physics Department, University of Cape Town, Cape Town, South Africa

84 Physics Department, University of Jammu, Jammu, India

85 Physics Department, University of Rajasthan, Jaipur, India

86 Physik Department, Technische Universität München, Munich, Germany

87 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany

88 Politecnico di Torino, Turin, Italy

89 Purdue University, West Lafayette, IN, USA

90 Pusan National University, Pusan, South Korea

91 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany

92 Rudjer Boskovic Institute, Zagreb, Croatia

93 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia

94 Russian Research Centre Kurchatov Institute, Moscow, Russia

95 Saha Institute of Nuclear Physics, Kolkata, India

96 School of Physics and Astronomy, University of Birmingham, Birmingham, UK

97 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru

98 Sezione INFN, Bari, Italy

99 Sezione INFN, Bologna, Italy

100 Sezione INFN, Cagliari, Italy

101 Sezione INFN, Catania, Italy

102 Sezione INFN, Padua, Italy

103 Sezione INFN, Rome, Italy

104 Sezione INFN, Trieste, Italy

105 Sezione INFN, Turin, Italy

106 SSC IHEP of NRC Kurchatov institute, Protvino, Russia

107 SUBATECH, Ecole des Mines de Nantes, Université de Nantes, CNRS-IN2P3, Nantes, France

108 Suranaree University of Technology, Nakhon Ratchasima, Thailand

109 Technical University of Split FESB, Split, Croatia

110 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Kracow, Poland

111 Physics Department, The University of Texas at Austin, Austin, TX, USA

112 Universidad Autónoma de Sinaloa, Culiacán, Mexico

113 Universidade de Sao Paulo (USP), Sao Paulo, Brazil

114 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil

115 University of Houston, Houston, TX, USA

116 University of Jyväskylä, Jyväskylä, Finland

117 University of Liverpool, Liverpool, UK

118 University of Tennessee, Knoxville, TN, USA

119 University of Tokyo, Tokyo, Japan

120 University of Tsukuba, Tsukuba, Japan

121 University of Zagreb, Zagreb, Croatia

122 Université de Lyon, Université Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, France

123 V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia

124 Variable Energy Cyclotron Centre, Kolkata, India

125 Vestfold University College, Tonsberg, Norway

126 Warsaw University of Technology, Warsaw, Poland

127 Wayne State University, Detroit, MI, USA

128 Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary

129 Yale University, New Haven, CT, USA

130 Yonsei University, Seoul, South Korea

131 Zentrum für Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms, Germany a Deceased

b Also at: St. Petersburg State Polytechnical University, St. Petersburg, Russia c Also at: Department of Applied Physics, Aligarh Muslim University, Aligarh, India

d Also at: M.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of Nuclear Physics, Moscow, Russia e Also at: University of Belgrade, Faculty of Physics and "Vinca" Institute of Nuclear Sciences, Belgrade, Serbia f Permanent Address: Konkuk University, Seoul, Korea

g Also at: Institute of Theoretical Physics, University of Wroclaw, Wroclaw, Poland h Also at: University of Kansas, Lawrence, KS, USA