LHC上的增强thj信号：CP违反top-Higgs耦合与γγ衰减的探测潜力

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本文主要探讨了在高光度14 TeV LHC（HL-LHC）环境下，对顶夸克-希格斯耦合中的CP违反现象以及其对thj（即t（→ℓνℓb）h（→γγ）j）过程的影响。研究焦点在于非标准顶夸克Yukawa耦合，这是一种可能偏离标准模型（SM）预测的新物理现象。作者Jason Yue来自澳大利亚悉尼大学Terascale粒子物理卓越中心，他在文章中指出，当顶夸克与希格斯粒子的相互作用存在CP违反相ξ不为零时，thj事件中的双光子衰变分支比会有所增强。具体来说，当顶夸克-希格斯相互作用的模量假设为标准模型的值yt = ytSM时，当CP违反相ξ取值0.25π（或者更大的0.5π），信号显著性可以分别达到2.7σ和7.7σ，这意味着这样的事件可能会在高统计意义下被探测到。此外，论文强调了不同顶夸克Yukawa耦合对顶夸克极化的改变，这提供了额外的信息来源。通过分析最终状态轻子与其自旋相关的不对称性，可以进一步区分这些非标准模型的信号与标准模型的预言。相比于之前研究的pp→t（→ℓνℓb）h（→bb′）j通道，thj通道由于其双光子衰变模式，被认为在揭示CP违反和新物理方面更具潜力。这篇文章对于粒子物理学家和实验学家来说具有重要价值，因为它不仅提供了理论上的预测，还可能为未来LHC的实验数据提供新的分析途径，以验证或排除某些新物理模型。同时，它也展示了如何通过精细的理论计算和实验测量，逐步揭示基本粒子间的相互作用及其潜在的CP违反效应。

Physics Letters B 744 (2015) 131–136

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Enhanced th j signal at the LHC with h →γγ decay and CP-violating

top–Higgs coupling

Jason Yue

ARC Centre of Excellence for Particle Physics at the Terascale, School of Physics, The University of Sydney, NSW 2006, Australia

a r t i c l e i n f o a b s t r a c t

Article history:

Received

9 December 2014

Received

in revised form 13 February 2015

Accepted

20 March 2015

Available

online 24 March 2015

Editor:

J. Hisano

We study the observability of non-standard top Yukawa couplings in the pp → t(→ ν



b)h(→ γγ) j

channel

at 14 TeV high-luminosity LHC (HL-LHC). The small diphoton branching ratio is enhanced when

the CP-violating phase, ξ, of the top–Higgs interaction is non-zero. When the modulus of the top–Higgs

interaction assumes the SM value, y

= y

, we ﬁnd that the signal signiﬁcance reaches 2.7σ (7.7σ )

when ξ = 0.25π (0.5π). Furthermore, the different couplings modify the polarisation of the top quark,

and can subsequently be distinguished via asymmetries in spin correlations with the ﬁnal state leptons.

The diphoton decay mode is found to be signiﬁcantly more promising than the previously considered

pp →t(→ν



b)h(→bb) j channel.

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

1. Introduction

Since the discovery of the Higgs resonance [1,2], the measured

properties [3,4] of this boson have been consistent with a minimal

Higgs sector. In particular, there has been compelling evidence that

it carries a spin quantum number of zero [5–8] (for a discussion

on a generic spin-2 impostor, see [9]). The next priority is to estab-

lish

the CP -properties of this Higgs boson, as this is important for

pinning down new physics together with the associated cosmo-

logical

implications. Currently, pure scalar couplings are favoured

over pseudoscalar couplings to the electroweak gauge bosons [7,

8].

However, a non-minimal Yukawa sector containing substantial

pseudoscalar admixture is not yet excluded (cf. [10]) and requires

further investigation.

The top–Higgs Yukawa coupling is the largest in the Standard

Model (SM) and therefore plays an important role in electroweak

symmetry breaking, notably in the context of Higgs vacuum sta-

bility

(see e.g. [11–13]). Also, a CP -violating top–Higgs sector may

provide additional sources of CP -violation that may have impli-

cations

for the electroweak phase transition and baryogenesis [14,

15].

Direct constraints on non-standard top Yukawa couplings must

come from observation of processes where it enters at the tree-

level,

with the leading contribution coming from Higgs production

associated with a top pair, pp →tth (see e.g. [16–21]). Speciﬁcally,

the prospect for the LHC to distinguish the scalar and pseudoscalar

E-mail address: j.yue@physics.usyd.edu.au.

components of the top Yukawa coupling in this channel has been

considered [22–29]. Given the relatively small pp →tth cross sec-

tion

(∼ 130 fb at 8TeV [30]), current luminosity and analysis are

not yet sensitive enough to observe such signal. Nonetheless, an

upper limit on the signal strength, μ

tth

= σ

tth

/σ

< 3.9 has been

set by the ATLAS Collaboration at 95% C.L. limit, using a combina-

tion

of the h → bb and h → γγ channels [31]. A corresponding

limit of μ

tth

∈[0.9, 3.5] is established by the CMS Collaboration

using all search channels [32].

This work focuses on the pp → th j production [33–41], de-

spite its

smaller SM cross section (18 fb at 8TeV [42–45]). The

justiﬁcation is that the interference between the th j Feynman dia-

grams

with tth and WWh couplings [10,25,33,34,42,46–50] makes

it more sensitivity to the CP-violating phase, ξ , of the top Yukawa

coupling, in addition to its modulus, y

. Speciﬁcally, increased |ξ |

values reduce the pp → tth cross section [25], but enhance the

pp →thj cross section [10,46]. Due to this interference, the extrac-

tion

of the top Yukawa coupling will require a suﬃciently precise

measurement of the hW W coupling, which is expected to reach

the 2–5% level at 3000 fb

−1

at the LHC [51]. A further advantage

of the single top channel is that the resulting top quark is polarised

by the left-handed weak interaction through a t-channel virtual W

boson,

but not in the top pair channel. As the t-quark decays be-

fore

it hadronises, spin information is inherited by its decay prod-

ucts

[52–58]. Top quark polarisation induced by the CP -violating

top-Yukawa couplings will therefore manifest in the spin correla-

tion

of the top decay products (see e.g. [25,27,48,59–63]). Polarisa-

http://dx.doi.org/10.1016/j.physletb.2015.03.044

0370-2693/Crown

by SCOAP

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