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JHEP12(2019)070

Published for SISSA by Springer

Received: August 30, 2019

Revised: October 26, 2019

Accepted: November 26, 2019

Published: December 9, 2019

Displaced heavy neutrinos from Z

decays at the LHC

Cheng-Wei Chiang,

Giovanna Cottin,

a,b

Arindam Das

and Sanjoy Mandal

d,e

Department of Physics, National Taiwan University,

Taipei 10617, Taiwan

Instituto de F´ısica, Pontiﬁcia Universidad Cat´olica de Chile,

Avenida Vicu˜na Mackenna 4860, Santiago, Chile

Department of Physics, Osaka University,

Toyonaka, Osaka 560-0043, Japan

Institute of Physics, Sachivalaya Marg,

Bhubaneswar 751005, India

Homi Bhabha National Institute, BARC Training School Complex,

Anushakti Nagar, Mumbai 400094, India

E-mail: chengwei@phys.ntu.edu.tw, gfcottin@uc.cl,

arindam.das@hetmail.phys.sci.osaka-u.ac.jp, smandal@iopb.res.in

Abstract: We study the LHC sensitivity to probe a long-lived heavy neutrino N in the

context of Z

models. We focus on displaced vertex signatures of N when pair produced

via a Z

, decaying to leptons and jets inside the inner trackers of the LHC experiments.

We explore the LHC reach with current long-lived particle search strategies for either one

or two displaced vertices in association with hadronic tracks or jets. We focus on two

well-motivated models, namely, the minimal U(1)

B−L

scenario and its U(1)

extension.

We ﬁnd that searches for at least one displaced vertex can cover a signiﬁcant portion of the

parameter space, with light-heavy neutrino mixings as low as |V

≈ 10

−17

, and l = e, µ

accessible across GeV scale heavy neutrino masses.

Keywords: Beyond Standard Model, Neutrino Physics

ArXiv ePrint: 1908.09838

Open Access,

 The Authors.

Article funded by SCOAP

https://doi.org/10.1007/JHEP12(2019)070

JHEP12(2019)070

Contents

1 Introduction 1

2 The model 2

3 LHC sensitivity with displaced vertex searches 6

3.1 Reach with ATLAS 1DV ID 7

3.2 Reach with CMS 2DV + jets 9

4 Conclusions 12

A Useful formulas 15

1 Introduction

The observed light mass of neutrinos in the Standard Model (SM) begs for new physics

explanations. The eﬀects of such new physics are being actively looked for at the Large

Hadron Collider (LHC) and several other experimental facilities worldwide. Such light

neutrino masses can be explained by employing the so-called seesaw mechanism [1–3] that

in its simplest form introduces new, heavy right-handed neutrinos, which can mix with

the light neutrinos in the SM. For small enough values of the mixing, the heavy neutrinos

can be long-lived, leading to macroscopic decays in the LHC experiments. Such decays

can be reconstructed as displaced vertices (DVs) inside the inner trackers of the LHC

detectors. Growing attention to these signatures has taken place over recent years (for a

recent state-of-art review of long-lived particle searches at the LHC, see ref. [4]), as null

results at the LHC may point to the possibilities that the new physics has more complex

decay patterns, and that its eﬀects may have been overlooked or misidentiﬁed by standard

searches. New physics may become evident in these spectacular displaced signatures, as

the SM can hardly mimic them. Their very small backgrounds will continue to make them

attractive and their current study is of top importance for the high luminosity run of the

LHC and future experimental facilities [5–7].

Heavy neutrinos are predicted in several models of new physics beyond the SM. Of

particular interest are the B − L extensions of the SM, which have an extended gauge

symmetry U(1)

B−L

[8] and an associated new heavy Z

vector boson. The U(1)

B−L

sym-

metry can be broken spontaneously by the addition of a new SM-singlet Higgs ﬁeld that

attains a vacuum expectation value (VEV), and the theory includes three-generations of

right-handed (or sterile) heavy neutrinos N

, enabling the seesaw mechanism of light neu-

trino mass generation. The N

can be produced from a SM-like Higgs associated with the

B −L breaking, or pair produced at colliders via a Z

. It can further decay with a displaced

vertex (DV) depending on its couplings and mass.

– 1 –

JHEP12(2019)070

Dedicated experimental searches for a Z

decaying to lepton pairs by the CMS collab-

oration placed a bound on the Z

mass to be m

> 4.5 TeV [9] (assuming a SM-like gauge

coupling). Recently the ATLAS collaboration analyzed the full Run 2 dataset [10], exclud-

ing a Z

just below 5 TeV. For a broad review on Z

models and early LHC strategies, see

ref. [11] and references therein (see also refs. [12–17] for other collider studies in the B −L

case). ATLAS has now implemented a search that targets at displaced heavy neutrinos as

benchmark, in the scenario where only one extra right-handed neutrino (produced in W

boson decays) is added to the SM [18]. Current and proposed displaced strategies in several

other heavy neutrino models are an attractive focus of research in recent years [19–42].

LHC constraints for the minimal U(1)

B−L

model were addressed through a global ﬁt in

ref. [43] for several choices of model parameters, but signatures involving displaced heavy

neutrinos were not considered. Recent works on displaced neutrinos in U(1)

B−L

models

have focused on displaced signatures coming from Higgs bosons due to a higher production

cross section [44, 45]. For this reason, production via a Z

has had less attention. Early

displaced strategies for a simpliﬁed model were recast in ref. [35], with focus on a benchmark

scenario with relatively unboosted N. Recently, the authors in ref. [46] estimated the reach

of future lifetime frontier experiments (like FASER [6, 47] and MATHUSLA [5]) on a rather

light N and Z

, of O(GeV) masses. In this work, we focus on higher masses and the LHC

capabilities running at high luminosity, by reinterpreting ongoing DV searches at ATLAS

and CMS. We also investigate prospects in a more general scenario than the B −L model,

the so-called non-exotic U(1)

extension of the SM [48], as it has been shown in refs. [15, 49]

that an enhancement in the Z

production is possible, providing increased sensitivity to

more complex scenarios in the search for displaced heavy neutrinos when they come from

a Z

. The lifetime of N as a function of the lightest neutrino mass under the general

U(1)

-extended models have been studied in refs. [17, 50].

The paper is organized as follows. We summarize the model under study in the

section 2. We also extract the exclusion region in the parameter space of new gauge

coupling and Z

mass using the Drell-Yan processes measured by both ATLAS and CMS.

In section 3, we discuss the ATLAS and CMS displaced searches, reinterpret their results,

and identify discovery prospects at the high luminosity LHC. We summarize and conclude

in section 4.

2 The model

We consider an extension of the SM to have the gauge group SU(3)

× SU(2)

× U(1)

U(1)

, where U(1)

is realized as a linear combination of the SM U(1)

and the U(1)

B−L

symmetries [51], known as the non-exotic U(1)

extension of the SM [48]. The model is

free from all the gauge and mixed gauge-gravity anomalies due to the presence of three

generations of right-hand neutrinos (RHNs) N

(with i = 1, 2, 3) [52, 53]. A new scalar

ﬁeld Φ is also introduced to break the U(1)

symmetry by attaining a VEV. The particle

content is given in table 1.

– 2 –

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