Scholarly article on topic 'Measurements of Quarkonium Production Cross-Sections with the CMS Experiment'

Measurements of Quarkonium Production Cross-Sections with the CMS Experiment Academic research paper on "Physical sciences"

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Abstract of research paper on Physical sciences, author of scientific article — Yu Zheng

Abstract Using large data samples of dimuon events, CMS has performed detailed measurements of differential cross sections of J / Ψ , Ψ ( 2 S ) and ϒ ( nS ) states, as a function of transverse momentum in several rapidity bins. The measurements reach significantly higher transverse momentum than previously reported. The quarkonium production cross-sections are compared with recent predictions in the context of NLO nonrelativistic QCD. In this talk, we will present a new measurement on ϒ ( nS ) production cross sections in proton-proton collisions at s = 7 TeV . We report the cross section results integrated over the transverse momentum range p T < 50 GeV/c and rapidity range | y | < 2.4 . We also report the measurement of the ϒ ( nS ) differential cross sections as a function of transverse momentum and rapidity and the cross section ratios ϒ ( 2 S ) / ϒ ( 1 S ) and ϒ ( 3 S ) / ϒ ( 1 S ) . Finally, we report the very recent measurement of χ c 2 / χ c 1 production cross-sections, differentially in p T , extending to p T = 25 GeV/c , thereby providing an important test of QCD calculations at higher p T than previously possible.

Academic research paper on topic "Measurements of Quarkonium Production Cross-Sections with the CMS Experiment"

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Nuclear Physics B (Proc. Suppl.) 233 (2012) 217-222

www.elsevier.com/locate/npbps

Measurements of Quarkonium Production Cross-Sections with the CMS

Experiment

Yu Zheng1

Department of Physics, Purdue University, West Lafayette, IN, USA

Abstract

Using large data samples of dimuon events, CMS has performed detailed measurements of differential cross sections of J/Y, Y(2S) and Y(nS) states, as a function of transverse momentum in several rapidity bins. The measurements reach significantly higher transverse momentum than previously reported. The quarkonium production cross-sections are compared with recent predictions in the context of NLO nonrelativistic QCD.

In this talk, we will present a new measurement on Y(nS) production cross sections in proton-proton collisions at Vs = 7 TeV. We report the cross section results integrated over the transverse momentum range pT < 50 GeV/c and rapidity range |y| < 2.4. We also report the measurement of the Y(nS) differential cross sections as a function of transverse momentum and rapidity and the cross section ratios Y(2S)/Y(1S) and Y(3S)/Y(1S).

Finally, we report the very recent measurement of^c2 /xc1 production cross-sections, differentially in pT, extending to pT = 25 GeV/c, thereby providing an important test of QCD calculations at higher pT than previously possible.

Keywords: quarkonium, charmonium, bottomonium, Xc

1. Introduction

No existing theoretical approach successfully reproduces both the differential cross section and the polarization measurements of the J/Y or Y states [1]. Quarkonium production at hadron colliders provides important tests of calculations in the context of both pertur-bative and non-perturbative quantum chromodynamics (QCD), via measurements of production cross sections and polarizations. Studying quarkonium hadroproduc-tion at higher center-of-mass energies and over a wider rapidity and momentum range will facilitate significant improvements in our understanding of the processes involved.

A detailed description of the CMS detector can be found in [2]. Its central feature is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the field volume are the silicon pixel and strip tracker, the crystal electromagnetic

1for the CMS Collaboration

© CERN for the benefit of the CMS Collaboration.

calorimeter, and the brass/scintillator hadron calorimeter. Muons are measured in gas-ionisation detectors embedded in the steel return yoke. In addition to the barrel and endcap detectors, CMS has extensive forward calorimetry. The muons are measured in the pseudora-pidity window |nl < 2.4, with detection planes made of three technologies: Drift Tubes, Cathode Strip Chambers, and Resistive Plate Chambers.

The paper is organized as follows. Section 2 presents the J/Y and Y(2S) prompt and non-prompt cross section results and comparisons to the theory predictions. In Sec. 3, the Y(nS) cross section results and comparisons to other experiments and to the theory predictions are presented. Finally, Section 4 presents a measurement of the^c2/xc1 cross-section ratio.

2. Charmonia

CMS has recently published the measurement of the J/Y and Y(2S) meson production cross sections in pp

0920-5632/ © 2013 CERN. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.12.080

collisions at ySs =7 TeV. The data correspond to an integrated luminosity of 36.7 pb-1 [3].

Figure 1 shows the measured "inclusive" cross sections as a function of pT for the various rapidity bins, compared with the ones published in [4], which are statistically independent and remain of interest since they cover a lower pT range. A nice agreement is observed between the results published in [3] and the ones published in [4].

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Luminosity and polarization uncertainties not shown

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Figure 1: Measured differential cross section for J/T inclusive production as a function of pT for five rapidity bins, plotted with the results published in [4], which extend to a lower pT range. (The multiplicative factors - showing as additive offsets in log scale - are used to achieve a convenient graphical separation of the measurements from different rapidity bins.)

To estimate the J/T fraction from b-hadron decays, a two-dimensional fit is performed, in which the pdfs and fit procedure are the same as those described in Ref. [4]. The variables used for the two-dimensional fits are the dimuon invariant mass and the "pseudo proper decay length" £j/T, defined as the most probable value of the transverse distance between the dimuon vertex and the primary vertex, corrected by the transverse Lorentz boost of the J/T. The measured b-fraction values for J/T and T(2S) mesons are shown in Fig. 2. The uncertainties shown are statistical and systematic, and the measured values are plotted as a function of pT.

Prompt and non-prompt cross-sections have also been determined, as shown in Fig. 3 and Fig. 4. The nonrelativistic QCD (NRQCD) prediction includes indirect production in the J/T case, so it can be directly compared to the data: good agreement is found in both the J/T and the T(2S) case. For non-prompt

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(b) T(2S )

Figure 3: Measured differential cross section for prompt J/W (a) and W(2S) (b) production as a function of pT for the different rapidity bins. The shaded histograms indicate the theoretical predictions from NRQCD calculations, the lines are added only for illustrative purposes.

pT (GeV/c)

production an overall scale difference is found in the Y(2S) case, possibly due to the large uncertainty of B ^ Y(2S) decay rates: in general, for both states, the observed cross-sections spectrum seems to fall more rapidly than the Fixed Order plus Next-to-Leading Logarithms (FONLL) prediction at high pT. The resulting prompt and non-prompt Y(2S) to J/Y cross section ratios are shown in Fig. 5 as a function of pT.

(b) Y(2S )

Figure 4: Measured differential cross section for non-prompt J/Y (a) and Y(2S) (b) production as a function of pT for the different rapidity bins. The shaded colored histograms indicate the theoretical predictions from FONLL calculations, the lines are added only for illustrative purposes.

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Figure 5: Measured value of the Y(2S) to J/Y differential cross section ratio for prompt (a) and non-prompt (b) production, averaged over rapidity and plotted as a function of pT. The top plot also includes the comparison with the NRQCD prediction, while the bottom plot shows the predictions of the theoretical models used to determine B(B ^ Y(2S)X), after the theoretical curves have been rescaled to the measured value of the inclusive B(B ^ Y(2S)X) branching fraction. The shaded bands show the uncertainties on the theoretical predictions. The error bars give the total uncertainties on the measurements; polarization uncertainties are not included.

3. Bottomonia

The measurements of the Y(nS) differential production cross sections in the dimuon channel in protonproton collisions at ys= 7 TeV [5][6] with the CMS experiment are summarized in this section. The results are

presented in two ways, both in the form of acceptance-corrected cross sections and within the muon fiducial acceptance. The Y(nS) pT differential fiducial cross-section results for the rapidity region [yY| < 2.4 is shown in Fig. 6. The Y(nS) pT-integrated, rapidity-differential fiducial cross-section results compared to PYTHIA [7] are shown in Fig. 7. The ratios of Y(nS) pT-differential fiducial cross sections are shown in Fig. 8. The ratios increase with pY. The Y(3S) is expected not to have a significant feed-down component unlike the Y(2S) and Y(1S), therefore a possible explanation for the pT dependence of the ratios is that the pT dependence of the cross section for direct production of each Y resonance is the same, and the softening of the spectrum is due to feed-down from higher mass states.

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Figure 6: Differential fiducial cross sections of the Y(nS) as a function of pY in the rapidity range |y| < 2.4.

The Y(1S) pT differential cross-section results compared with various theoretical models are shown in Fig. 9. Each of the predictions is for unpolarized Y(nS) production, and both the measurement and the prediction are for [yT| < 2.0. The comparison is made to the CASCADE MC generator in the FONLL framework [8] including feed-down from xt(iP) and Xb(2p), PYTHIA, the Color Evaporation Model (CEM) [9] (no feed-down estimation is available), NRQCD to next-to-leading order (NLO) including feed-down as described in Ref. [10], the Color Singlet Model (CSM) to NLO and to next-to-next-to-leading order (NNLO*) [11] where the feed-down is accounted for by scaling the Y(1S) direct production cross section by a factor 2 for the inclusive Y(1S) cross section. The theory predictions are private communications [12][13][14][15] as no

Figure 7: Differential fiducial cross sections of the Y(nS) as afunction of rapidity, compared to PYTHIA predictions.

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Figure 8: Ratios of differential fiducial cross sections of the Y(nS) as a function of pY in the rapidity range |/r| < 2.4.

predictions for unpolarized Y(nS) production are available in the literature at yS= 7 TeV. Our measured Y(1S) cross section is lower than the CEM prediction at low pT Y. The data agree well with NRQCD above pT = 10 GeV/c for Y(1S). The NLO CSM cannot describe the data while the NNLO* CSM shows better agreement with data within the large uncertainties. The Y(nS) rapidity-differential cross sections measured with CMS and the LHCb results [16] are shown in Fig. 10. The results are complementary in their rapidity coverage and consistent within the uncertainties in the region of overlap.

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4. Xc States

A measurement of the relative prompt production rate of Xci and Xcu with 4.62 fb-1 of data collected at LHC at ys= 7 TeV is presented [17]. The measurement is based on the reconstruction of the radiative decays to J/Y plus photon, with the low transverse momentum photons (in the GeV range) being detected through their conversion in electron-positron pairs. The photon selection is implemented as follows. Photon conversions are characterized by an electron-positron pair originat-

ing from a common vertex. The e+e- invariant mass is consistent with zero and the two tracks are parallel at the conversion vertex and open only in the transverse plane because of the magnetic field generated by the CMS solenoid. Any conversion building a pair whose invariant mass falls in the range between 0.11 and 0.15 GeV/c2, corresponding to a 2< window around the reconstructed n0 mass, is rejected. In order to restrict the measurement to the prompt signal component, the pseudo-proper decay length of the J/Y, defined in [4], is required to be less than 30 mm.

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Figure 11: Xci/XA cross section ratio (circles) and ratio of the cross section multiplied by the branching ratios to J/T+y (squares). The shaded band corresponds to the systematic error and the error bars to the statistical error. For the ratio of cross sections the 10% error derived from the branching fractions is not shown.

The results of the measurement of ^(pp^Xc2+X) x

BR(xc2 ^J/fy) and ^(pp^xci+X)

BR(Xc1 JYy) and <(pp^xc1+X) for the kinematic range

pt(y) > 0.5 GeV/c and |yJ/Y| < 1.0 are shown in Fig. 11. The results are compared to kt-factorization and NLO NRQCD theoretical approaches as shown in Fig. 12 and Fig. 13. As the fcrfactorization model predicts that both ^c1 and xc2 are produced with helicity zero in the he-licity frame, for a proper comparison the ratio was recalculated under this assumption. The ¿(-factorization cannot describe the measurement. The NLO NRQCD predictions were produced without a cut on the photon transverse momentum. A correction factor derived from Monte Carlo is applied to extrapolate the measurements down to zero photon pT.

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Figure 12: Comparison of the measured ratio of cross sections multiplied by the branching ratios with predictions from the kt-factorization.

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Figure 13: Comparison of the measured ratio of cross sections multiplied by the branching ratios with predictions from NLO NRQCD.

5. Summary

In summary, CMS has measured the J/T, T(2S) and Y(nS) meson production cross sections in pp collisions at yS = 7 TeV. The J/T and T(2S) cross sections and their ratio have been measured as a function of the meson transverse momentum pT (up to 70 GeV/c for the J/T and to 30 GeV/c for the T(2S)) in several rapidity ranges assuming null polarization in the prompt production. Results have been compared to NRQCD and FONLL predictions respectively, finding a reasonable agreement except for non-prompt cross-sections at high

pT. The inclusive branching fraction B(B ^ T(2S )X) is extracted from the ratio of the non-prompt cross sections, improving the relative uncertainty on the previous world average by a factor of three. The differential cross sections for each Y(nS) state and the cross-section ratios have been measured as a function of p'Y and rapidity and compared to various theory predictions. The relative prompt production rate of Xd and xa is measured. The result is among the most accurate and precise measurements ofXc production in hadron collisions, and extends the explored pT range of previous works. Possible systematic uncertainties coming from the unknown polarization of the Xc are also investigated.

References

[1] N. Brambilla et al., Heavy quarkonium: progress, puzzles, and opportunities, The European Physical Journal C - Particles and Fields 71 1-178.

[2] The CMS Collaboration, The CMS experiment at the CERN LHC, Journal of Instrumentation 3 (08) S08004.

[3] The CMS Collaboration, J/T and T(2S) production in pp collisions at V? = 7TeV, Journal of High Energy Physics 2012 1-46.

[4] The CMS Collaboration, Prompt and non-prompt J/T production in pp collisions at V? = 7TeV, The European Physical Journal C - Particles and Fields 71 1-26.

[5] The CMS Collaboration, Upsilon production cross section in pp collisions at y? = 7 TeV, Phys. Rev. D 83 (2011) 112004. doi: 10.1103/PhysRevD.83.112004.

[6] The CMS Collaboration, Measurement of the Upsilon(nS) production cross section in pp collisions at V? = 7 TeV, BPH-11-001.

[7] V. M.Bargiotti, Heavy Quarkonia sector in PYTHIA 6.324 : tuning, validation and perspectives at LHC(b), Report No. LHCb-2007-042.

[8] H. Jung, et al., The CCFM Monte Carlo generator CASCADE 2.2.0arXiv:1008.0152, doi:10.1140/epjc/ s10052-010-1507-z.

[9] A. D. Frawley, T. Ullrich, R. Vogt, Heavy flavor in heavy-ion collisions at RHIC and RHIC II, Phys. Rept. 462 (2008) 125. arXiv:0806.1013, doi:10.1016/j.physrep.2008. 04.002.

[10] K. Wang, Y.-Q. Ma, K.-T. Chao, Y(1S) prompt production at the Tevatron and LHC in nonrelativistic QCD, Phys. Rev. D 85 (2012)114003. doi:10.1103/PhysRevD.85.114003.

[11] J.-P. Lansberg, J/| production at V? = 1.96 and 7 TeV: Color-Singlet Model, NNLO* and polarisationarXiv:1107.0292.

[12] K.-T. Chao private communication.

[13] J.-P. Lansberg private communication.

[14] H. Jung private communication.

[15] R. Vogt private communication.

[16] The LHCb Collaboration, Measurement of Upsilon production in pp collisions at y? = 7 TeVarXiv:1202.6579.

[17] The CMS Collaboration, Measurement of ^(xc2)/^(Xrf) at V? = 7 TeV, CMS-PAS-BPH-11-010.

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