LHC上τ介子CP振动效应的精确探测与h→ττ衰变角度测量

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本文探讨了在大型强子对撞机（LHC）中探测希格斯玻色子（Higgs boson）与τ介子（tau lepton）之间CP违反效应的一种创新方法。随着新型探测器技术的发展，研究人员得以实现对大型强子对撞机中τ介子所有主要强子衰变事件中产生的中微子进行逐事件精确测量。这一进步使得对每个事件的矩阵元素计算成为可能，从而能够精细地分析Higgs粒子的质量特性，并在此基础上研究h→ττ衰变过程中的CP混合角。作者们提出了一个新算法，通过中微子的完全重建，不仅能够计算出每个事件的物理性质，而且能提高对希格斯玻色子与τ介子相互作用的了解，特别是其CP混合角的测量精度。这一测量对于理解基本粒子物理学中的CP违反现象至关重要，这是标准模型预言中的一项关键参数。根据文中所述，通过在13 TeV的能量下进行详细的探测器模拟，预计使用3 ab$^{-1}$的数据，CP混合角的测量误差将显著降低，达到5.2°。这一结果表明，LHC在测量Higgs-hττ耦合中的CP特性方面已经超越了之前对轻子电偶极矩（Electric Dipole Moment, EDM）搜索的灵敏度，显示出在粒子物理学实验中的前沿地位。此外，文章还强调了这项工作对于验证或排除某些新物理模型，如多Higgs双极子模型或者超对称理论，的重要性。这篇文章提供了对如何利用LHC的数据来深入探究基本粒子物理的CP违反现象以及测量希格斯玻色子性质的有力方法论，预示着未来在粒子物理研究领域将有更精确的CP混合角测量结果。

Physics Letters B 790 (2019) 332–338

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Probing the CP-Violation effects in the hττ coupling at the LHC

Xin Chen

a,b,c,∗

, Yongcheng Wu

Department of Physics, Tsinghua University, Beijing 100084, China

Collaborative Innovation Center of Quantum Matter, Beijing 100084, China

Center for High Energy Physics, Peking University, Beijing 100084, China

Ottawa-Carleton Institute for Physics, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada

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

Article history:

Received

31 August 2018

Received

in revised form 1 January 2019

Accepted

18 January 2019

Available

online 24 January 2019

Editor:

A. Ringwald

Keywords:

Higgs

Tau

lepton

A new method used to calculate the neutrino for all major tau hadronic decay event by event at the LHC

is presented. It is possible because nowadays better detector description is available. With the neutrino

fully reconstructed, matrix element for each event can be calculated, the mass of the Higgs particle can

also be calculated event by event with high precision. Based on these, the prospect of measuring the

Higgs CP mixing angle with h → ττ decays at the LHC is analyzed. It is predicted that, with a detailed

detector simulation, with 3 ab

−1

of data at

√

s = 13 TeV, a signiﬁcant improvement of the measurement

of the CP mixing angle to a precision of 5.2

◦

can be achieved at the LHC, which outperforms the

sensitivity from lepton EDM searches up to date in the hττ coupling.

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

1. Introduction

To account for the large asymmetry between the matter and

anti-matter in our Universe, enough CP violation effects should

be presented in the theory. However, in the Standard Model, the

CP phase in the CKM matrix is not suﬃcient for this purpose.

New physics is therefore needed to introduce more CP violation

sources. Possible candidates are Supersymmetry, Left–Right Sym-

metric

model, etc.

the other hand, the new discovered Higgs boson also opens

a window towards the new physics. The precision measurement

of Higgs properties will be one of the most important targets of

the LHC in the next running periods. Among them, the CP prop-

erty

is an important topic. The pure CP eigenstate assumption has

already been investigated at the LHC experiments [1–3]in the di-

boson

decays, and the pure CP-odd situation is excluded better that

99.9% CL. The h → ZZ

∗

→ 4l is the golden channel for this mea-

surement,

subject to the scale suppression due to dim-6 operators,

whereas for the Yukawa coupling, h → ττ is the best channel we

could use and is widely investigated in the literature [4–23]. How-

ever,

the missing neutrino from the decay of each tau makes it

diﬃcult to achieve a better precision on measuring the CP mix-

Corresponding author.

E-mail

addresses: xin.chen@cern.ch (X. Chen), ycwu@physics.carleton.ca (Y. Wu).

ing angle (φ) of the hττ interaction which we assume to have the

following effective form:

L =−y

τ (cos φ +iγ

sin φ)τh, (1)

In this work, utilizing the mass constraints and impact param-

eters,

the new method in [5]to reconstruct neutrinos with impact

parameters and resolutions taken into account is used, which is

combined with the matrix element in the LHC setting for the ﬁrst

time. With the momentum of the neutrino from the tau decay fully

reconstructed within certain accuracy, an observable based on the

matrix element to retrieve the CP information in the hττ inter-

action

can be calculated, which can achieve higher precision as

it contains more information in the ﬁnal states integrated in the

matrix element. Under this framework, all major tau hadronic de-

cay

modes are included and combined in this work simultaneously,

which gives the important prediction on the best we can do with

the Higgs CP in its fermionic coupling at the LHC. Similar to [24],

the energy-angle correlations of the ﬁnal tau lepton decay prod-

ucts

are used to extract the CP information. A detailed simulation

study shows that a signiﬁcant improvement of the measurement

of the CP mixing angle can be achieved. The method is illustrated

using the measurement of CP-violation effects in hττ interaction

as an example, extension to other situation containing tau decay

should be straightforward and will further improve the precision.

this work, the track trajectory is assumed to follow a he-

lix,

as opposed to an idealized straight line assumed in [18,21].

https://doi.org/10.1016/j.physletb.2019.01.038

0370-2693/

SCOAP

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