Scholarly article on topic 'Unraveling the CP phase of top-Higgs coupling in associated production at the LHC'

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Abstract of research paper on Physical sciences, author of scientific article — Saurabh D. Rindani, Pankaj Sharma, Ambresh Shivaji

Abstract We study the sensitivity of top polarization observables to the CP phase ζ t in the top Yukawa coupling in the process p p → t h j at the 14 TeV high-luminosity run of the Large Hadron Collider (HL-LHC). We calculate the top polarization in this process as well as an azimuthal asymmetry of the charged lepton arising from the decay of the top in the lab frame. We find that the dependence of this lab-frame azimuthal asymmetry on the phase ζ t closely resembles the dependence of the top polarization on ζ t . As compared to the cross section, which is sensitive to ζ t for larger values, the lepton azimuthal asymmetry can provide a sensitive measurement of ζ t for smaller values.

Academic research paper on topic "Unraveling the CP phase of top-Higgs coupling in associated production at the LHC"

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Physics Letters B

www.elsevier.com/locate/physletb

Unraveling the CP phase of top-Higgs coupling in associated production at the LHC

Saurabh D. Rindania, Pankaj Sharmab *, Ambresh Shivajic

a Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India

b Center of Excellence in Particle Physics at Tera Scale, The University of Adelaide, 5005 Adelaide, South Australia, Australia c INFN, Sezione di Pavia, Via A. Bassi 6,27100 Pavia, Italy

CrossMark

A R T I C L E I N F 0

Article history:

Received 18 May 2016

Received in revised form 24 July 2016

Accepted 1 August 2016

Available online 4 August 2016

Editor: J. Hisano

A B S T R A C T

We study the sensitivity of top polarization observables to the CP phase Zt in the top Yukawa coupling in the process pp — thj at the 14 TeV high-luminosity run of the Large Hadron Collider (HL-LHC). We calculate the top polarization in this process as well as an azimuthal asymmetry of the charged lepton arising from the decay of the top in the lab frame. We find that the dependence of this lab-frame azimuthal asymmetry on the phase Zt closely resembles the dependence of the top polarization on Zt. As compared to the cross section, which is sensitive to Zt for larger values, the lepton azimuthal asymmetry can provide a sensitive measurement of Zt for smaller values.

© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.

1. Introduction

Particle physics has entered a new era with the discovery at the Large Hadron Collider (LHC) of a spin-0 particle of mass around 125 GeV in its first run [1,2]. The couplings of this particle, presumed to be a Higgs boson, to standard model (SM) fermions and electroweak (EW) gauge bosons have been determined through the measurement of its production and decay properties, albeit with large uncertainties. Thus the current LHC data still permits a lot of leeway for the existence of new physics. Currently the Higgs boson couplings to the EW gauge bosons W, Z point to a spin-0 particle with a purely pseudoscalar boson being ruled out at 95% CL [3]. However a CP mixture with both scalar and pseudoscalar components is still allowed. Thus it would be one of the important goals of the next run of the LHC, which will be a high energy and high luminosity run, to determine the CP composition of the Higgs.

In this context, Higgs boson couplings to the third generation of fermions, particularly the top quark, are important since the corresponding Yukawa couplings are the largest. So far, the information regarding the tth coupling is inferred from loop-induced hgg and hyy couplings, which are deduced from the Higgs boson production and decay at the LHC. However as these processes are loop induced, they may involve contributions from new physics. Thus,

* Corresponding author.

E-mail addresses: saurabh@prl.res.in (S.D. Rindani), pankaj.sharma@adelaide.edu.au (P. Sharma), ambresh.shivaji@pv.infn.it (A. Shivaji).

at the LHC, the top Yukawa coupling can be directly probed only in production associated with a Higgs boson as the decay h — tt is kinematically forbidden. In the SM, there are two associated top-Higgs production processes possible: a) Higgs with a ttt pair and b) Higgs with a single top, the former being the dominant one.

In this letter, we study single-top production in association with a Higgs boson h and a light-quark jet, which we denote as thj production. This process has a low cross section in the SM, around 18 (70) fb at NLO at 8 (14) TeV [4,5]. However, in the presence of anomalous couplings, the cross section can be substantially enhanced [6]. The reason is that in the SM, there is a high degree of destructive interference between the diagrams containing Higgs emission from an internal W line and from a top-quark line. If either the WWh coupling or the tth coupling is anomalous, the cancellation between the two types of diagrams does not take place, and the cross section is high. For example, a change in the sign of the tth coupling results in a cross section of 235 fb, significantly higher than even the tth cross section of 130 fb at 8 TeV [7]. This allows the flipped sign of the top Yukawa coupling to be observed or excluded [4,5,8-12]. The CMS collaboration at the LHC performed searches for this process for a variety of signatures, covering various Higgs decay channels, assuming the top quark to decay semileptonically [13], putting limits on the cross section. Thus, though the process of thj production at the LHC has negligible cross section in the SM, it can become observable when there are anomalous couplings present. In particular, the cross section is sensitive to the phase Zt of the top Yukawa coupling. This phase

http://dx.doi.org/10.1016/j.physletb.2016.08.002

0370-2693/© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.

determines the pseudoscalar admixture to the scalar coupling, and is thus CP violating. It is found that increasing |Zt | reduces the pp ^ tth cross section [14], but enhances the pp ^ thj cross section [9,15].

Mainly because of its large mass mt = 172.99 ± 0.91 GeV [16], the top-quark sector is considered to be one of the few places where new physics could arise. The top-quark life time is very short and the top decays rapidly before any non-perturbative QCD effects can force it into a bound state. Thus, its spin information is preserved in terms of the differential distribution of its decay products. So by studying the kinematical distributions of top decay products, it is, in principle, possible to measure top polarization in any top production process. As a pseudoscalar coupling violates parity, it flips the spin of the top quark when a Higgs boson is emitted. This fact has been utilized in many studies. Top-quark polarization thus depends on the phase Zt [14,17], and may be used to distinguish among various choices of phases. Ellis et al. [14] consider longitudinal as well as transverse polarizations as measured by the forward-backward asymmetry of the decay lepton with respect to the spin-quantization axis in the rest frame of the top. Yue in [17] has analyzed the utility of the h ^ yy channel as a probe of the CP-violating phase Zt in the process pp ^ thj, taking advantage of the fact that in addition to the cross section and the top-quark polarization, also the branching ratio for the diphoton channel increases with |Zt |.

In this work, we focus on thj production in the presence of the CP-violating phase Zt of the top Yukawa coupling at the 14 TeV LHC and examine the possibility of using top polarization and other angular observables constructed from top decay products in the top rest frame as well as the laboratory (lab) frame to measure this phase. Since earlier work has largely focused on measurement of cross sections and of top polarization through decay distributions in the top rest frame to enable the determination of the top Yukawa coupling and its phase Zt, our main emphasis will be to show how lab-frame observables can be used to probe Zt.

The rest of the article is organized as follows. In the next section, we write down the effective top-Yukawa coupling and constraints on the CP violating phase Zt from Higgs production and decay processes. In Sec. 3 we describe the results of the calculation of the cross section for the process, and in Sec. 4 we study the top polarization and its reconstruction through charged-lepton angular distributions in the rest frame as well as in the lab frame. In Sec. 5, we discuss asymmetries in the rest frame of the top quark as well as in the lab frame to study their sensitivities to determine the CP phase. Our conclusions are contained in Sec. 6.

2. Effective top-Yukawa couplings

In an extension of the SM, where there is at least one extra neutral Higgs boson, the mass eigenstates of the scalars will in general be mixtures of the original states. In case CP is not conserved, there can be mixing between CP-even and CP-odd scalars, giving rise to CP-violating couplings of the scalar eigenstates. We analyze the results of such a mixing in a model-independent scenario and parametrize the couplings in a general way.

Thus, assuming that a scalar h is a mass eigenstate, the most general tth coupling, without imposing CP invariance, may be written as

Ltth = -ytt (cos Zt + i Y5 sin Zt)th. (1)

Here Zt is the phase of the Yukawa coupling. Zt = 0 or Zt = n correspond to a pure scalar state while Zt = n/2 to a pure pseudoscalar state. Any intermediate value 0 < Zt < n/2, or n/2 < Zt < n signals CP violation. Zt = n/4 denotes a maximally CP vio-

lating case. In this work, we focus on the effects of Zt, so we will take yt = ySM = mt/v while treating Zt as a free parameter.

Constraints on yt and Zt have been obtained from current LHC data. In Refs. [14,15,18-21], using the limits on hgg and hyy couplings derived from the Higgs boson production and decay respectively, the authors have obtained constraints in the plane of (yt, Zt). Constraints on these parameters are also derived taking into account the unitary violation in gauge boson (W, Z) scattering with the top quark [22,23]. The most stringent constraints on the phase Zt comes from electron dipole-moment (EDM) measurements [24-26]. These analyses are based on certain assumptions about Higgs couplings to other fermions and gauge bosons. However relaxing those assumptions can allow, in principle, a larger values for yt and Zt. For example, in presence of only anomalous top Yukawa coupling, the current bound from electron EDM measurement allows values for the phase Zt in a narrow band around 0 and n [24]. However, if we assume similar anomalous coupling for the electron as well, Zt can take any value between 0 and n and is highly correlated with the phase corresponding to electron Yukawa (Ze). The conclusion remains same for future prospects where the experimental bounds are expected to improve by a factor of 20 resulting into a tighter correlation between electron and top Yukawa phases. The EDM constraints from neutron and mercury atom are also expected to get much relaxed if light quark anomalous couplings are turned on. Note that assuming light fermion Yukawa couplings also anomalous does not affect our collider signal.

On the collider side, with yt = ytSM the global analysis allows Zt in the range [0, 2n/3] at 95% confidence level. Nevertheless, it is clear that the cases of Zt = n/2 and Zt = n are already ruled out by the LHC Higgs data. The forecast for the future sensitivity at 14 TeV LHC with 3000 fb-1 integrated luminosity can push Zt very close to 0.003n. The expected sensitivity at 240 GeV TLEP would be able to rule out values of Zt larger than 0.07n [15,24]. However these limits have been obtained using loop processes while the objective of the present work is to measure the CP violating phase from direct search. The existing limits on top Yukawa from the direct searches in pp ^ tth channel are very poor [18,27].

In what follows, we assume that h is indeed the spin-0 boson with a mass of about 125 GeV discovered at the LHC. Also for the sake of completeness, we vary Zt in the full range between 0 and n. Since the WWh coupling is directly constrained by the Higgs data, we stick to its SM value in our analysis.

3. Signal and backgrounds

Associated production of the top quark with a Higgs and a jet at the LHC proceeds via the partonic process

b + q ^ t + h + q', (2)

where q, q' denote light quarks. The corresponding Feynman diagrams are shown in Fig. 1. As Higgs couplings to the light quarks and the b quark are negligible, the corresponding diagrams are not shown.

We implement the effective tth couplings of Eq. (1) using FeynRules [28] and obtained the cross section for thj production for the 14 TeV LHC at the leading order using Madgraph [29]. In Fig. 2, we show the fractional deviation in the production cross section including anomalous couplings relative to the SM. We find that the cross section is fairly sensitive to the CP phase Zt in tth couplings in the region Zt > n/2 where the interference between the two diagrams becomes constructive. Below Zt <n/2 the interference is still destructive though its degree decreases with Zt, thus increasing the cross section by around 200% at Zt = n/2. On

Fig. 1. Feynman diagrams for the process bq ^ thj at the LHC. The blob denotes the effective tth coupling.

Fig. 2. The fractional deviation of the cross section from the SM value as a function of CP phase Zt in the tth coupling for thj process at LHC14.

the other hand, for Zt = n the cross section can be enhanced by up to 1200%.

Let us now consider the possible signatures of the thj process at the LHC and the corresponding dominant backgrounds. The search strategy for the thj signal relies on the very forward light-flavour jet which opportunely enhances the signal-to-background ratio. For the Higgs of mass of 125 GeV, the dominant decay mode is to a pair of b quarks with branching fraction (BR) around 60%. However the cleanest decay mode is h ^ yy using which the Higgs was first observed at the LHC. Despite its very small BR, viz., 2 x 10-3, it has been shown in Refs. [10,17] that the viability of the pp ^ thj(h ^ yy) signal reaches a sensitivity similar to the one where the Higgs decays to a pair of b quarks. The observability of the pp ^ thj process at the LHC in bb decays of the Higgs has been studied extensively in Refs. [6,15,30,31]. As our lab-frame asymmetry does not depend on the different modes of Higgs decay but only on the charged lepton coming from top decay, we consider both the Higgs decays in our analysis in order to enhance the statistical significance of the observables.

For the case where h decays to a bbt pair and the top decays semi-leptonically, the signal consists of an isolated charged lepton i±, 3 b jets, 1 forward jet and missing transverse energy ET . The irreducible background contribution to such a signal comes from Wbbbj processes. The Wbbbj processes include the contribution from single-top processes, viz., tZj and tbbj. The dominant background comes from top-pair production tt + j where one of the light jets fakes a b jet. Moreover there are other QCD backgrounds resulting from light jets faking b jets as in tbjj and Wbbjj. All these backgrounds have been systematically analyzed in Ref. [6,30] where the authors use some standard cuts to reduce the backgrounds and improve the signal-to-background ratio.

On the other hand, when h decays to a photon pair, the signal consists of an isolated charged lepton i±, one b jet, one forward jet, a pair of photons and missing transverse energy E T. For such a signal, the irreducible background is a tjyy continuum. As

Fig. 3. Top polarization in pp ^ thj at LHC14 as a function of the CP phase Zt of the tth coupling.

this background is non-resonant, it can be efficiently suppressed through a cut on the invariant mass of the photon pair. Other reducible contributions are from ttyy where one of the two tops decays hadronically, b is mistagged as a light jet and two of the light jets do not fall inside the detector, and from Wjjyy where one of the light jets is mistagged as a b jet [10,17].

In the following, we present various angular distributions of the charged lepton coming from top decay both in the top rest frame and in the lab frame. We work at the parton level throughout, and in presenting all distributions, we apply the following standard cuts:

pb/ > 20 GeV, \nb,i| < 2.5, pJT > 25 GeV, \nj| > 2.5, ARjj, ji > 0.4.

Note that the cut \nj\ > 2.5 corresponds to a very forward light jet which is a characteristic signature of thj process and is instrumental in suppressing the background efficiently.

4. Top polarization and angular distributions of the charged lepton

The degree of longitudinal polarization Pt of the top quark is given by

a(+) - a(-) a(+) + a(-),

where a(+) and a(-) denote the cross sections for positive- and negative-helicity top quarks, respectively. The sum of a(+) and a(-) gives the total cross section for the process. We have obtained the polarized cross sections a(+) and a(-) using the helic-ity amplitudes in MadGraph. In Fig. 3 we display the polarization of the top in pp ^ thj at the LHC as a function of the CP phase Zt. One can easily see that the polarization is quite sensitive to low values of Zt, i.e., Zt < n/2. This is because of the pseudoscalar coupling which flips the helicity of the top quark in the production amplitude. As the pseudoscalar component in the Higgs admixture is increased with increase in Zt, it is expected that the polarization of the top quark would also be affected accordingly. Had there been only one diagram where the Higgs is emitted from the top, the polarization curve would be symmetric around Zt = n /2 because we retrieve the same CP admixture as in the range (0, n/2). However, the presence of the second diagram and its interference with the first one results in the flattening of the polarization curve beyond Zt >n/2.

In the rest frame of the top quark, the angular distribution of a decay product f for a top-quark ensemble has the form

Fig. 4. The normalized polar distribution, cos6f, of the charged lepton in the topquark rest frame for pp — thj (upper panel) process for different values of CP phase Zt of the tth couplings and backgrounds (lower panel) at LHC14. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1 dr f 1

— -= - (1 + KfPt COS 6f ).

rf dcos6f 2 1 1

Here 6f is the angle between f and the top spin vector in the top rest frame and Pt (defined in Eq. (4)) is the degree of polarization of the top-quark ensemble. rf is the partial decay width. The standard way to measure top polarization is through the angular distribution of its decay products in the rest frame of the top quark, in particular, through the charged lepton and down-type quark distribution whose spin-analysing powers Ke = Kd ~ 1 are maximum while KVl = Ku = -0.30 and Kb = -kw + = — 0.39.1 A larger kf makes f a more sensitive probe of the top spin. Thus the e+ or d have the largest probability of being emitted in the direction of the top spin and the least probability in the direction opposite to the spin. Since at the LHC, the lepton energy and momentum can be measured with high precision, we focus on lep-tonic decays of the top.

As mentioned earlier, the standard way to determine top polarization is to study the charged-lepton polar distribution in the top-quark rest frame, Eq. (5). However, this needs a full reconstruction of the top momentum which is a difficult task at the LHC. Utilizing the W-boson on-shell condition: (pe± + pV)2 = MW, one can obtain a quadratic equation in the longitudinal component of neutrino momentum pV L. Solving this equation, we determine the missing information about pvL which brings in a two-fold ambiguity and may thus lead to a considerable loss in the number of events. This becomes even more significant for the case of rare processes like the one under consideration.

We show in Fig. 4 (upper panel) the normalized distribution in cos 6f, where 6f is the polar angle of the lepton measured with respect to the top-quark spin direction in the rest frame of the top quark, for pp — thj at LHC14 for two values of anomalous CP phases in ttth couplings. Also shown is the distribution for the case of the SM. It can be seen that the top polarization, as measured by the slope of the cos 6i distribution, is sensitive to the phase Zt of the top Yukawa coupling. We also show, in Fig. 4 (lower panel), the cos 61 distribution for processes ttj and tjyy which are the main backgrounds for pp — thj, (h — bb) and pp — thj, (h — yy) signals respectively. The ttj production is a strong process conserving parity. Hence it leads to vanishing polarization which can be visualized through the flat distribution while tjyy production is mostly electroweak and gives rise to highly polarized tops as evident in the Fig. 4 (lower panel). In order to reconstruct top rest frame, as mentioned earlier, we determine the neutrino longitudinal momentum pvL by imposing the invariant mass constraint

M2v = M W ±:

Pv L =

AwPíL ± EejAW ± 4p2„/fj

where AW = MW± + 2pT ■ /T. If two solutions for pvL are found, the one which gives Mlv closer to the W± mass is adopted. Also, we reject the events with complex solutions.

In order to avoid difficulties associated with the reconstruction of the top rest frame, we consider an observable that can be measured directly in the lab frame, viz., the azimuthal distribution of the charged lepton arising from top decay. To define the azimuthal angle fa, we choose the proton beam direction as the z direction, and the production plane of the top quark as the xz plane. The measurement of fa does not need full reconstruction of the top momentum, but only the transverse momentum of top quark.

The angular distribution, analogous to Eq. (5), in the lab frame in terms of angle 6ti between the top and lepton directions can be written as [33]

1 dre 1 2

— TT^- = 2 (1 — P 2)(1 — Pt P)

Fi d cos 6ti where /) 1 — m2 / Ef, cos 6ti = cos 6t cos 6i + sin 6t sin 6i cos faf,

1 + P^ cos 6te (1 — p cos 6te)3 '

Pt — P 1 — Pt p.

All k values are evaluated at tree level [32].

Thus, the azimuthal distribution not only depends on polarization of top but also on a kinematic effect. According to Eq. (5), the decay lepton is emitted preferentially along the top spin direction in the top rest frame, with k f = 1. The corresponding distributions in the lab frame are given by Eq. (7). The rest-frame forward (backward) peak corresponds to a peak for cos 6ti = ±1, as seen from the factor (1 + P^ff cos6ti) in the numerator of Eq. (7). This is the effect of polarization. The kinematic effect is seen in the factor (1 — pt cos 6ti)3 in the denominator of Eq. (7), which again gives rise to peaking for large cos 6tl. Eq. (8) therefore implies peaking for small fai. This is borne out by the numerical results.

We show in Fig. 5 the normalized azimuthal distribution of the charged lepton in thj production at LHC14 for a few values of Zt, Zt = 0 corresponding to the SM. As expected and as can be seen from the figure, the distribution is sensitive to Zt. We also show in Fig. 5 (bottom), the distribution for processes ttj and tjyy which are the main backgrounds for pp — thj, (h — bb)

h g 0.04

-a - a) 0.03

ti 0.02

a £ H

g 0.02

i i :] Ct y G 0 - o = tt/4 .............. : = tt/2 ................. "

: ^-L-Lj«!

1 1 "■■"''■■'"■"'(r.Vv.&i-.mi 1

7T JT 4 2 <¡>1 3TT 7 4

1 1 tin'- ■ ttj .............. -

Fig. 5. The normalized distribution in the azimuthal angle fal of the charged lepton in thj production (upper panel) for different values of the CP phase Zt in the tth coupling and in the background processes (lower panel) at LHC14. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and pp — thj, (h — yy) signals respectively. The fal distribution in Fig. 5 is symmetric under the interchange of fal with 2n — fal. This is because of the fact that the LHC is a symmetric collider and there is no way to define a unique positive z axis. In Fig. 5 we have shown the distribution only up to n.

The lab-frame charged-lepton azimuthal distribution as a probe of top-quark polarization was first proposed in Ref. [34]. Subsequently, it has been studied extensively in the context of various new-physics scenarios in processes involving top pair production [33,35,36] and (associated) single-top production [37-44] at the LHC.

5. Asymmetries

As seen in the previous section, one can use polar and az-imuthal angular distributions of the charged lepton to discriminate amongst possible values of the top Yukawa phase Zt. However, making a fit to the distributions requires a reasonably large data sample. It is, thus, preferable to compare the data to a single number defined in terms of an integral over the distribution. For this purpose, we define an asymmetry in each of the previous cases, and evaluate it as a function of Zt.

We define a polar asymmetry, which is also the forward-backward asymmetry of the charged lepton in rest frame of top quark, by

A™ =

o (cos 0l> 0) — o (cos 9l < 0)

o (cos 9l > 0) + o (cos 9l < 0)

where, as before 6l is the polar angle of the charged lepton relative to the top spin direction in the top rest frame.

In the production plane of the top-quark, we define an azimuthal asymmetry, which is in fact the "left-right asymmetry"

Fig. 6. Charged-lepton polar asymmetry (A™) (upper panel) and azimuthal asymmetry (Afa) (lower panel) in pp -— thj at LHC14 as a function of the CP phase Zt of the tth coupling. Also shown are the values of the asymmetries for the background processes ttj and tjYY. The different shades of gray regions denote the 1o, 2o and 3o of statistical uncertainty in the measurement of the asymmetries in the SM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of the charged lepton at the LHC defined with respect to the beam direction, with the right hemisphere identified as that in which the top momentum lies, and the left one being the opposite one. In the Fig. 5, it can be easily seen that the fal distribution is highly asymmetric in the two different regions, viz. left (cos fal < 0) and right (cos fa > 0), of the detector. We define the lab frame left-right asymmetry of charged lepton, as follows:

o (cos fal > 0) — o (cos fal < 0)

o (cos fal > 0) + o (cos fal < 0)'

where the denominator is the total cross section.

We also study the sensitivities of these asymmetries as a probe of the CP violating phase at the LHC14 with the full integrated luminosity, viz., 3000 fb—1. For this, we estimate the statistical uncertainty in the measurement of an asymmetry using the formula

1 — A2

where L, ASM and oSM are the integrated luminosity, the value of an asymmetry and the total cross section in the SM respectively.

In Fig. 6, we present the leptonic asymmetries A™ and Afa as functions of CP phase Zt at LHC14. We can see from the figure that the asymmetry Afa reconstructs fairly accurately the behaviour of the top polarization. The top rest-frame polar asymmetry A™ also follows the same behaviour, though to a lesser extent. The advantage of Afa, in addition to having a shape closer to that of the actual polarization, is that it can be measured in the lab frame. Thus we expect better sensitivity to Zt from Afa than A™. In the Fig. 6,

we also show the regions which can be probed with 3000 fb-1 of integrated luminosity at 1a, 2a and 3a of significance at the 14 TeV LHC. In particular, with a total luminosity of about 3 ab-1 likely be available at the end of the HL-LHC run, A^ could be used to determine Zt to within n/8, n/4 and 3n/8 at 1a, 2a and 3a confidence level (CL) respectively.

6. Conclusions

Post the Higgs discovery, the need of the hour is to determine the CP properties of the Higgs boson unambiguously. The fact that a pseudoscalar does not couple to the EW gauge bosons at tree level spurs the idea of studying the CP properties of the Higgs in fermionic Yukawa couplings as they are more democratic to CP even and odd scalars. Moreover the current measurement of the CP phase in the top Yukawa couplings relies on hyy and hgg couplings which are deduced from a loop-level calculation, and thus allow contamination from various new physics effects. This compels us to look for direct determination of such couplings at the LHC. The processes which have the putative couplings have very small cross sections and thus would require a high energy and high luminosity run of the LHC to be completed.

In this letter, we have studied the prospects of measuring the CP phase in the top-Higgs coupling in the associated thj production at the LHC. In this context, we utilize a simpler lab-frame asymmetry A^ of the charged lepton from top decay, which is also the left-right asymmetry of the charged lepton, at the LHC. We find that the left-right asymmetry is quite sensitive to the CP violating phase and can probe it up to n/6 with 3 ab-1 of the integrated luminosity at the LHC. We also study the angular distribution of charged-lepton in the top rest frame. The rest-frame forward-backward asymmetry A™ gives a measure of top-quark polarization in production. However it requires a full reconstruction of top momentum which brings in large systematic uncertainties. Thus the sensitivity of A™ is lesser than A^ which only requires the reconstruction of transverse momentum of top quark.

The asymmetries and their sensitivities have been estimated at the parton level though we have employed all the relevant cuts to suppress the signal-to-background ratio. However, including the detector effects may lead to reduction in the sensitivities of these asymmetries. It is thus needed to perform a full detector level simulation to estimate the realistic efficiencies of these observables. We have left this as a future work.

Acknowledgements

S.D.R. acknowledges support from the Department of Science and Technology, India, under the J.C. Bose National Fellowship programme, Grant No. SR/SB/JCB-42/2009. The work of P.S. was supported by the University of Adelaide and the Australian Research Council through the ARC Center of Excellence in Particle Physics (CoEPP) at the Terascale (CE110001004).

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