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JHEP10(2017)182

Published for SISSA by Springer

Received: July 11, 2017

Revised: September 8, 2017

Accepted: October 6, 2017

Published: October 26, 2017

Search for new high-mass phenomena in the dilepton

ﬁnal state using 36 fb

−1

of proton-proton collision

data at

√

s = 13 TeV with the ATLAS detector

The ATLAS collaboration

E-mail: atlas.publications@cern.ch

Abstract: A search is conducted for new resonant and non-resonant high-mass phenom-

ena in dielectron and dimuon ﬁnal states. The search uses 36.1 fb

−1

of proton-proton

collision data, collected at

√

s = 13 TeV by the ATLAS experiment at the LHC in 2015

and 2016. No signiﬁcant deviation from the Standard Model prediction is observed. Upper

limits at 95% credibility level are set on the cross-section times branching ratio for reso-

nances decaying into dileptons, which are converted to lower limits on the resonance mass,

up to 4.1 TeV for the E

-motivated Z

. Lower limits on the qq`` contact interaction scale

are set between 2.4 TeV and 40 TeV, depending on the model.

Keywords: Beyond Standard Model, Hadron-Hadron scattering (experiments)

ArXiv ePrint: 1707.02424

Open Access, Copyright CERN,

for the beneﬁt of the ATLAS Collaboration.

Article funded by SCOAP

https://doi.org/10.1007/JHEP10(2017)182

JHEP10(2017)182

dilepton ﬁnal states provides excellent sensitivity to a large variety of phenomena. This

experimental signature beneﬁts from a fully reconstructed ﬁnal state, high signal-selection

eﬃciencies and relatively small, well-understood backgrounds, representing a powerful test

for a wide range of theories beyond the Standard Model (SM).

Models with extended gauge groups often feature additional U(1) symmetries with cor-

responding heavy spin-1 bosons. These bosons, generally referred to as Z

, would manifest

as a narrow resonance through its decay, in the dilepton mass spectrum. Among these mod-

els are those inspired by Grand Uniﬁed Theories, which are motivated by gauge uniﬁcation

or a restoration of the left-right symmetry violated by the weak interaction. Examples

considered in this article include the Z

bosons of the E

-motivated [1, 2] theories as well

as Minimal models [3]. The Sequential Standard Model (SSM) [2] is also considered due to

its inherent simplicity and usefulness as a benchmark model. The SSM manifests a Z

SSM

boson with couplings to fermions equal to those of the SM Z boson.

The most sensitive previous searches for a Z

boson decaying into the dilepton ﬁnal

state were carried out by the ATLAS and CMS collaborations [4, 5]. Using 3.2 fb

−1

pp collision data at

√

s = 13 TeV collected in 2015, ATLAS set a lower exclusion limit at

95% credibility level (CL) on the Z

SSM

pole mass of 3.4 TeV for the combined ee and µµ

channels. Similar limits were set by CMS using the 2015 data sample.

This search is also sensitive to a series of other models that predict the presence of

narrow dilepton resonances. These models include the Randall-Sundrum (RS) model [6]

with a warped extra dimension giving rise to spin-2 graviton excitations, the quantum

black-hole model [7], the Z

∗

model [8], and the minimal walking technicolour model [9]. In

order to facilitate interpretation of the results in the context of these or any other model

predicting a new dilepton resonance, limits are set on the production of a generic Z

-like

excess.

In addition to the search for narrow resonances, results for non-resonant phenomena are

also reported. Such models of these phenomena include an eﬀective four-fermion contact

interaction (CI) between two initial-state quarks and two ﬁnal-state leptons (qq``). Unlike

resonance models, which require suﬃcient energy to produce the new gauge boson, the

presence of a new interaction in the non-resonant regime can be detected at a much lower

energy.

The most stringent constraints from CI searches are also provided by the ATLAS and

CMS collaborations [4, 10], for couplings between quarks and leptons. Using 3.2 fb

−1

of pp

collision data at

√

s = 13 TeV collected in 2015, ATLAS set lower limits on the qq`` CI scale

of Λ = 25 TeV and Λ = 18 TeV at 95% CL for constructive and destructive interference,

respectively, in the case of left-left interactions and assuming a uniform positive prior

probability in 1/Λ

. Similar limits were set by CMS using the 2015 data set. Both the

resonant and non-resonant models considered as the benchmark for this search are further

discussed in section 2.

The presented search utilises the invariant mass spectra of the observed dilepton ﬁnal

states as discriminating variables. The analysis and interpretation of these spectra rely

primarily on simulated samples of signal and background processes. The interpretation

is performed taking into account the expected shape of diﬀerent signals in the dilepton

– 2 –

JHEP10(2017)182

mass distribution. The use of the shape of the full dilepton invariant mass distribution

reduces the uncertainties in the background modelling, thereby increasing the sensitivity

of this search at high masses. This article is structured as follows: section 2 covers the

theoretical motivation of the models considered in this search, followed by a description

of the ATLAS detector in section 3, and a summary in section 4 of the data and Monte

Carlo (MC) samples used. The event selection is motivated and described in section 5, with

details of the background estimation given in section 6, and an overview of the systematic

uncertainty treatment given in section 7. The event yields and main kinematic distributions

are presented in section 8, followed by a description of the statistical analysis in section 9,

and the results in section 10.

2 Theoretical models

2.1 E

-motivated Z

models

In the class of models based on the E

gauge group [1, 2], the uniﬁed symmetry group

can break to the SM in a number of diﬀerent ways. In many of them, E

is ﬁrst broken

to SO(10) × U(1)

, with SO(10) then breaking either to SU(4) × SU(2)

× SU(2)

SU(5) × U(1)

. In the ﬁrst of these two possibilities, a Z

coming from SU(2)

, where

3R stands for the right-handed third component of weak isospin, or a Z

B−L

from the

breaking of SU(4) into SU(3)

× U(1)

B−L

could exist at the TeV scale, where B (L) is the

baryon (lepton) number and (B − L) is the conserved quantum number. Both of these

bosons appear in the Minimal Z

models discussed in the next section. In the SU(5)

case, the presence of U(1)

and U(1)

symmetries implies the existence of associated gauge

bosons Z

and Z

that can mix. When SU(5) is broken down to the SM, one of the U(1)

can remain unbroken down to intermediate energy scales. Therefore, the precise model

is governed by a mixing angle θ

, with the new potentially observable Z

boson deﬁned

by Z

(θ

) = Z

cos θ

+ Z

sin θ

. The value of θ

speciﬁes the Z

boson’s coupling

strength to SM fermions as well as its intrinsic width. In comparison to the benchmark

SSM

, which has a width of approximately 3% of its mass, the E

models predict narrower

signals. The Z

considered here has a width of 0.5% of its mass, and the Z

has a width

of 1.2% of its mass [11, 12]. All other Z

signals in this model, including Z

, Z

, and

, are deﬁned by speciﬁc values of θ

ranging from 0 to π, and have widths between

those of the Z

and Z

2.2 Minimal Z

models

In the Minimal Z

models [3], the phenomenology of Z

boson production and decay is

characterised by three parameters: two eﬀective coupling constants, g

and g

, and the

boson mass. This parameterisation encompasses Z

bosons from many models, including

the Z

belonging to the E

-motivated model of the previous section, the Z

in a left-right

symmetric model [13, 14] and the Z

B−L

of the pure (B − L) model [15]. The minimal

models are therefore particularly interesting for their generality, and because couplings are

being directly constrained by the search. The coupling parameter g

deﬁnes the coupling

of a new Z

boson to the (B − L) current, while the g

parameter represents the coupling

– 3 –

JHEP10(2017)182

B−L

sin θ

cos θ

Min

√

sin θ

Min

0 −

−

√

Table 1. Values for γ

and θ

Min

in the Minimal Z

models corresponding to three speciﬁc Z

bosons: Z

B−L

, Z

and Z

. The SM weak mixing angle is denoted by θ

to the weak hypercharge Y. It is convenient to refer to the ratios ˜g

≡ g

and

˜g

≡ g

, where g

is related to the coupling of the SM Z boson to fermions deﬁned by

= 2M

/v. Here v = 246 GeV is the SM Higgs vacuum expectation value. To simplify

further, the additional parameters γ

and θ

Min

are chosen as independent parameters with

the following deﬁnitions: ˜g

= γ

cos θ

Min

, ˜g

= γ

sin θ

Min

. The γ

parameter measures

the strength of the Z

boson coupling relative to that of the SM Z boson, while θ

Min

determines the mixing between the generators of the (B − L) and weak hypercharge Y

gauge groups. Speciﬁc values of γ

and θ

Min

correspond to Z

bosons in various models, as

is shown in table 1 for the three cases mentioned in this section.

For the Minimal Z

models, the width depends on γ

and θ

Min

, and the Z

interferes

with the SM Z/γ

∗

process. For example, taking the 3R and B − L models investigated in

this search, the width varies from less than 1% up to 12.8% and 39.5% respectively, for

the γ

range considered. The branching fraction to leptons is the same as for the other Z

models considered in this search. Couplings to hypothetical right-handed neutrinos, the

Higgs boson, and to W boson pairs are not considered. Previous limits on the Z

mass

versus γ

were set by the ATLAS experiment. For γ

= 0.2, the range of Z

mass limits at

95% CL corresponding to θ

Min

∈ [0, π] is 1.11 TeV to 2.10 TeV [16].

2.3 Contact interactions

Some models of physics beyond the SM result in non-resonant deviations from the predicted

SM dilepton mass spectrum. Compositeness models motivated by the repeated pattern of

quark and lepton generations predict new interactions involving their constituents. These

interactions may be represented as a contact interaction between initial-state quarks and

ﬁnal-state leptons [17, 18]. Other models producing non-resonant eﬀects are models with

large extra dimensions [19] motivated by the hierarchy problem. This search is sensitive to

non-resonant new physics in these scenarios; however, constraints on these models are not

evaluated in this article.

The following four-fermion CI Lagrangian [17, 18] is used to describe a new interaction

in the process qq → `

−

L =

[η

) (`

) + η

) (`

)

+ η

) (`

) + η

) (`

)] ,

– 4 –

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