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我们研究了LHC敏感性,以在Z'模型的环境中探测长寿命的重中微子N。 当通过Z'对产生N对时,我们着重研究N的位移顶点特征,这些特征在LHC实验的内部跟踪器内部衰减为轻子和射流。 我们用强子跟踪或射流结合一个或两个位移顶点的当前长寿命粒子搜索策略来探索大型强子对撞机的作用范围。 我们关注两个动机良好的模型,即最小U(1)B-L情景及其U(1)X扩展。 我们发现搜索至少一个位移的顶点可以覆盖参数空间的很大一部分,轻中微子混合的低至| V lN |。 2≈10 -17,并且l = e,μ可通过GeV尺度重中微子质量访问。
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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
0
decays at the LHC
Cheng-Wei Chiang,
a
Giovanna Cottin,
a,b
Arindam Das
c
and Sanjoy Mandal
d,e
a
Department of Physics, National Taiwan University,
Taipei 10617, Taiwan
b
Instituto de F´ısica, Pontificia Universidad Cat´olica de Chile,
Avenida Vicu˜na Mackenna 4860, Santiago, Chile
c
Department of Physics, Osaka University,
Toyonaka, Osaka 560-0043, Japan
d
Institute of Physics, Sachivalaya Marg,
Bhubaneswar 751005, India
e
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
0
models. We focus on displaced vertex signatures of N when pair produced
via a Z
0
, 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)
X
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
lN
|
2
≈ 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,
c
The Authors.
Article funded by SCOAP
3
.
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
0
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
i
, enabling the seesaw mechanism of light neu-
trino mass generation. The N
i
can be produced from a SM-like Higgs associated with the
B −L breaking, or pair produced at colliders via a Z
0
. 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
0
decaying to lepton pairs by the CMS collab-
oration placed a bound on the Z
0
mass to be m
Z
0
> 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
0
just below 5 TeV. For a broad review on Z
0
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
0
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
0
, 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)
X
extension of the SM [48], as it has been shown in refs. [15, 49]
that an enhancement in the Z
0
production is possible, providing increased sensitivity to
more complex scenarios in the search for displaced heavy neutrinos when they come from
a Z
0
. The lifetime of N as a function of the lightest neutrino mass under the general
U(1)
X
-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
0
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)
c
× SU(2)
L
× U(1)
Y
×
U(1)
X
, where U(1)
X
is realized as a linear combination of the SM U(1)
Y
and the U(1)
B−L
symmetries [51], known as the non-exotic U(1)
X
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
i
(with i = 1, 2, 3) [52, 53]. A new scalar
field Φ is also introduced to break the U(1)
X
symmetry by attaining a VEV. The particle
content is given in table 1.
– 2 –
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