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JHEP05(2017)128

Published for SISSA by Springer

Received: December 21, 2016

Revised: April 14, 2017

Accepted: May 15, 2017

Published: May 23, 2017

Measuring the CP property of Higgs coupling to tau

leptons in the VBF channel at the LHC

Tao Han,

a,c

Satyanarayan Mukhopadhyay,

Biswarup Mukhopadhyaya

and Yongcheng Wu

c,a

PITT-PACC, Department of Physics and Astronomy, University of Pittsburgh,

Pittsburgh, PA 15260, U.S.A.

Regional Centre for Accelerator-based Particle Physics, Harish-Chandra Research Institute, HBNI,

Chhatnag Road, Jhusi, Allahabad 211019, India

Department of Physics, Tsinghua University,

Collaborative Innovation Center of Quantum Matter,

Beijing 100086, China

E-mail: than@pitt.edu, satya@pitt.edu, biswarup@hri.res.in,

wuyongcheng12@mails.tsinghua.edu.cn

Abstract: We study the prospects of measuring the CP property of the Higgs (h) coupling

to tau leptons using the vector boson fusion (VBF) production mode at the high-luminosity

LHC. Utilizing the previously proposed angle between the planes spanned by the mo-

mentum vectors of the (π

) and (π

−

) pairs originating in τ

decays as the CP-odd

observable, we perform a detailed Monte Carlo analysis, taking into account the relevant

standard model backgrounds, as well as detector resolution eﬀects. We ﬁnd that excluding

a pure CP-odd coupling hypothesis requires O(400 fb

−1

) luminosity at the 14 TeV LHC,

and values of the CP-mixing angle larger than about 25

◦

can be excluded at 95% conﬁ-

dence level using 3 ab

−1

data. It is observed that the uncertainty in the angular resolution

of the neutral pion momenta does not constitute a signiﬁcant hurdle. Achieving a signal

to background ratio (S/B) close to one, while keeping a high enough signal yield required

to study the angular distributions selects out VBF as a promising mode to probe the CP

nature of the hττ coupling, with gluon fusion suﬀering from a low S/B, and the W

h/Zh

mode (with leptonically decaying W

/Z) having a much smaller signal rate.

Keywords: Higgs Physics, CP violation

ArXiv ePrint: 1612.00413

Open Access,

 The Authors.

Article funded by SCOAP

doi:10.1007/JHEP05(2017)128

JHEP05(2017)128

Contents

1 Introduction 1

2 Analysis setup 3

2.1 Eﬀective interaction Lagrangian 3

2.2 Observable sensitive to the CP structure of hτ

−

coupling 4

2.3 Monte Carlo simulation of signal and background processes 5

3 Results 6

3.1 Kinematic selection of signal region: signal and background rates 6

3.2 CP-odd correlations and measurement reach at HL-LHC 8

4 Summary 12

A Validation of MC simulation with ATLAS 8 TeV analysis 14

1 Introduction

Measurement of the properties of the 125 GeV scalar boson [1, 2] constitutes one of the

most important experimental programs in the coming decades, at the ongoing 13 TeV run

of the LHC and its high-luminosity upgrade [3], as well as at future planned lepton and

higher-energy hadron colliders [4–7]. In addition to a precise determination of the coupling

strengths of the Higgs boson (h) to standard model (SM) gauge bosons and fermions [8–10],

future large statistics data sets would allow for the measurement of several diﬀerential

distributions of interest, which are sensitive to the Lorentz structure and CP property of

Higgs couplings [11, 12]. As is well known, extensions of the standard model, motivated

by a range of arguments, predict observable deviations in the Higgs properties from their

corresponding SM predictions [4, 6, 7, 13]. In particular, considerations on successful

electroweak baryogenesis often require an extended Higgs sector, with potentially new

sources of CP violation in Higgs couplings [14–16].

ATLAS and CMS collaborations have utilized the h → ZZ

∗

→ (`

−

)(`

0−

) decay

mode (with `, `

= e, µ) to probe the CP nature of the Higgs coupling to Z-bosons, and

using the 8 TeV LHC data, a pure CP-odd hypothesis has been excluded at 99% conﬁdence

level [17–21]. This, however, leaves open the possibility that h is not an eigenstate of

CP, but an admixture of CP-even and CP-odd states, and the present measurements do

not constrain the amount of such a mixing signiﬁcantly. CP violation in the h → ZZ

∗

decay process is not expected to be large as well, since the CP-even part of the amplitude

originates from the hZ

term, which appears at dimension four, while the CP-odd

coupling structure, 

µνρσ

µν

ρσ

, can only be generated from dimension six SM gauge

invariant operators. On the other hand, the Higgs coupling to SM fermions can include

both CP-even and CP-odd Lorentz structures with similar strength, for example, via the

– 1 –

JHEP05(2017)128

mixing of CP-even and CP-odd scalars in a two-Higgs doublet model, and the CP-odd

couplings can therefore be potentially large. Furthermore, the eﬀective operators leading to

the CP-odd Lorentz structures in the Higgs coupling to gauge bosons and to SM fermions

(including diﬀerent fermion generations) may have separate origin, thereby making the

corresponding coupling strengths uncorrelated.

Among the third generation fermions, extensive studies have been performed to de-

termine the LHC sensitivity of the structure of the Higgs coupling to top quarks [22–28],

and a realistic analysis including the eﬀect of the relevant SM backgrounds shows that the

high-luminosity (HL) LHC run with 3 ab

−1

of data can probe a CP-mixing angle larger

than O(60

◦

) at 95% C.L. [28]. For the Higgs coupling to b-quarks, a measurement would be

challenging, as no information of the b polarization is retained in the angular distribution

of the lightest B meson’s decay products, while only a fraction of the lightest baryon, Λ

is expected to retain the polarization information [29–31].

The fact that tau polarization information is retained in the angular distribution of its

decay products [32, 33] has led to a number of studies on the prospects of measuring the CP

property of the hτ

−

coupling at the LHC and at e

−

colliders, utilizing both one and

three prong τ decay modes [34–45]. The existing phenomenological analyses have mostly

focussed on the formulation of suitable observables that are sensitive to the CP nature of

the hτ

−

coupling, with the restriction that due to the presence of missing neutrinos, it is

not possible to accurately determine the τ

momentum vectors or the Higgs rest frame at

the LHC. The one prong decay τ

∓

→ ρ

∓

(−)

, with ρ

∓

→ π

∓

is found to be promising in

this regard [35, 43], as the angle between the decay planes spanned by (π

) and (π

−

)

can be utilized to deﬁne a CP-odd variable carrying the spin correlation information of the

decaying taus. However, in order to properly access the viability of such a measurement

at the LHC, it is important to study speciﬁc Higgs production modes, and determine to

what extent such correlations can be extracted in the presence of large SM backgrounds.

The Higgs coupling to τ leptons has already been established with a combined signiﬁ-

cance of more than 5σ [46] by the ATLAS [47] and CMS [48] collaborations, using the 7 and

8 TeV data from the LHC. The experimental analyses show the clear importance of the

vector boson fusion (VBF) mode in driving the discovery of the hτ

−

decay process. As is

well known, VBF leads to a distinctive topology that can be used to enhance the signal to

background ratio [49]. In contrast, the leading Higgs production mode (gg → h → τ

−

)

suﬀers from formidable SM background [41, 42], and the clean associated W

h/Zh pro-

duction modes (with h → τ

−

, and the vector boson decaying to leptonic ﬁnal states) [50]

suﬀer from very low rates of the signal itself as well as diﬃcult backgrounds.

In this paper, we therefore focus on the vector boson fusion production of the Higgs

boson in association with two forward tagging jets, and explore the prospects of probing

the CP nature of the hτ

−

coupling using the τ

∓

→ π

∓

(−)

decay mode. We perform

a complete Monte Carlo (MC) simulation of the signal and the dominant background

processes, keeping the spin correlation in all steps of the decay chain, and including the

eﬀects of parton shower, hadronization and underlying events. We also carefully study

the impact of the crucial detector resolution uncertainty in determining the momentum

direction of the neutral pions. As we shall show in the subsequent sections, VBF turns out

to be a very promising mode for probing the hτ

−

coupling structure as well.

– 2 –

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