未来LHC探测顶希格斯CP性质：揭示精确度与强子对撞机灵敏度提升

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在未来大型强子对撞机(Large Hadron Electron Collider, LHeC)的研究中，一项关键任务是精确测量顶级希格斯玻色子与顶夸克（top quark）的相互作用的CP性质。CP性质，即宇称（Charge conjugation）和宇称时间反转（Parity）的联合对称性，对于理解粒子物理学标准模型(SM)的基础参数至关重要。在本研究中，科学家们利用电子-质子(e-p)碰撞作为探针，通过分析特定的过程pe-→t¯hνe来考察CP相ζt的影响。在这个过程中，研究者特别关注了顶夸克的衰变模式，即h→bb（希格斯玻色子衰变为两个底夸克）和t→轻子(lνt)通道。他们通过观测一系列可测量的因素来揭示ζt的特性，包括顶夸克的横截面(σ)，极化状态，以及h和t之间的速度差异，以及诸如角度不对称性这样的物理效应。这些观测有助于区分CP偶(CP-even)和CP奇(CP-odd)的贡献，从而揭示ζt的潜在CP行为。为了评估对ζt的敏感度，研究者根据不同σ水平下的基准横截面，建立了与积累的亮度(L)相关的排除限。当考虑60 GeV和7 TeV的电子和质子束能量，以及在L = 100 fb^-1的数据集时，他们在2σ置信水平下排除了π/5 < ζt ≤ π的区间，这表明LHeC在高能环境下具有更高的CP敏感性。进一步的分析预测，随着数据积累至最终的L = 1 ab^-1，即1 abisi^-1的亮度，研究者有望在s = 1.3 TeV和1.8 TeV的中心-of-mass能量下，达到测量SM top-Higgs耦合κ（强度比）的精度，即κ=1.00±0.17（0.08）。这表明，LHeC不仅能够探测CP性质，还可能提供关于顶夸克-希格斯相互作用精细结构的重要信息，有助于检验标准模型的预言，并可能揭示超出标准模型的新物理现象。这项工作对于理解粒子物理学的基本原理以及可能的理论突破具有重要意义。

Physics Letters B 770 (2017) 335–341

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measuring CP nature of top-Higgs couplings at the future

Large Hadron electron Collider

Baradhwaj Coleppa

, Mukesh Kumar

b,∗

, Satendra Kumar

, Bruce Mellado

Department of Physics, Indian Institute of Technology Gandhinagar, Gandhinagar 382 355, India

National Institute for Theoretical Physics, School of Physics and Mandelstam Institute for Theoretical Physics, University of the Witwatersrand, Johannesburg,

Wits 2050, South Africa

University of the Witwatersrand, School of Physics, Private Bag 3, Wits 2050, South Africa

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

Article history:

Received

25 February 2017

Received

in revised form 27 April 2017

Accepted

1 May 2017

Available

online 4 May 2017

Editor:

G.F. Giudice

Keywords:

Electron–proton

collision

Top-Higgs

coupling

Top

polarisation

We investigate the sensitivity of top-Higgs coupling by considering the associated vertex as CP phase

(ζ

) dependent through the process p e

−

→

th ν

in the future Large Hadron electron Collider. In par-

ticular

the decay modes are taken to be h → b

b and

t → leptonic mode. Several distinct ζ

dependent

features are demonstrated by considering observables like cross sections, top-quark polarisation, rapidity

difference between h and

t and different angular asymmetries. Luminosity (L) dependent exclusion limits

are obtained for ζ

by considering signiﬁcance based on ﬁducial cross sections at different σ -levels. For

electron and proton beam-energies of 60 GeV and 7TeV respectively, at L = 100 fb

−1

, the regions above

π/5 <ζ

≤ π are excluded at 2σ conﬁdence level, which reﬂects better sensitivity expected at the Large

Hadron Collider. With appropriate error ﬁtting methodology we ﬁnd that the accuracy of SM top-Higgs

coupling could be measured to be κ = 1.00 ±0.17(0.08) at

√

s = 1.3(1.8) TeV for an ultimate L = 1 ab

−1

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

1. Introduction

The recent discovery of the Higgs boson at the Large Hadron

Collider (LHC) serves as the last step in establishing the particle

content of the Standard Model (SM). The next step that has been

undertaken is the characterisation of its properties regarding spin,

CP-nature and the nature of interaction with other particles. While

the spin-0 nature of the Higgs boson has been established by the

experiments [1–5] and a complete CP-odd nature excluded at a

99.98% conﬁdence limit (C.L.) [6,7], the possibility remains that the

Higgs boson could still be an admixture of CP-odd and even states.

Investigation of this possibility in a future Large Hadron electron

Collider (LHeC) is the goal of this article via a detailed analysis

of the associated production of the Higgs boson with an anti-top

quark.

Since

in the SM the Higgs boson coupling to fermions is directly

proportional to the mass of the fermions, the Yukawa coupling as-

sociated

with the third generation is important in the context of

Corresponding author.

E-mail

addresses: baradhwaj@iitgn.ac.in (B. Coleppa), mukesh.kumar@cern.ch

(M. Kumar),

satendrak@iitgn.ac.in (S. Kumar), bruce.mellado@wits.ac.za

(B. Mellado).

investigating the properties of the Higgs boson. Deviations in the

top-Higgs coupling directly affect the production cross section of

Higgs boson at the colliders, while changes in the bottom-Higgs

coupling affect the total branching ratios.

Here we study the associated production of the Higgs boson

with an anti-top quark at the future e

−

p collider which employs a

7TeV proton beam from a circular pp collider, and electrons from

an Energy Recovery Linac (ERL) being developed for the LHeC [8,9].

The choice of an ERL energy of electron of E

= 60 to 120 GeV,

with available proton beam energy E

= 7TeV provides centre

of mass energy of

√

s ≈ 1.3to1.8TeV. While the LHC is clearly

energetically superior, the LHeC conﬁguration is advantageous for

the following reasons: (i) since initial states are asymmetric, back-

ward

and forward scattering can be disentangled, (ii) it provides

a clean environment with suppressed backgrounds from strong

interaction processes and free from issues like pile-ups, multiple

interactions etc. (iii) such machines are known for high precision

measurements of the dynamical properties of the proton allowing

simultaneous tests of electroweak and QCD effects. A detailed re-

port

on the physics and detector design concepts of the LHeC can

be found in the Ref. [8]. A distinguishing feature of the e

−

p col-

lider

is that the production of the Higgs is only due to electroweak

processes [10,11] and as noted above, since the e

−

and p energies

are different, the machine can also produce interesting patterns of

http://dx.doi.org/10.1016/j.physletb.2017.05.006

0370-2693/

SCOAP

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