Search for a heavy composite Majorana neutrino in events with dilepton signatures from proton-proton collisions at sqrts 13 TeV

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

CERN-EP-2022-181

2023/06/30

CMS-EXO-20-011

Search for a heavy composite Majorana neutrino in events

with dilepton signatures from proton-proton collisions at

√s=13 TeV

The CMS Collaboration*

Abstract

Results are presented of a search for a heavy Majorana neutrino Nℓdecaying into two

same-ﬂavor leptons ℓ(electrons or muons) and a quark-pair jet. A model is consid-

ered in which the Nℓis an excited neutrino in a compositeness scenario. The analysis

is performed using a sample of proton-proton collisions at √s=13 TeV recorded by

the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of

138 fb−1. The data are found to be in agreement with the standard model prediction.

For the process in which the Nℓis produced in association with a lepton, followed by

the decay of the Nℓto a same-ﬂavor lepton and a quark pair, an upper limit at 95%

conﬁdence level on the product of the cross section and branching fraction is obtained

as a function of the Nℓmass mNℓand the compositeness scale Λ. For this model the

data exclude the existence of Ne(Nµ) for mNℓbelow 6.0 (6.1) TeV, at the limit where

mNℓis equal to Λ. For mNℓ≈1 TeV, values of Λless than 20 (23) TeV are excluded.

These results represent a considerable improvement in sensitivity, covering a larger

parameter space than previous searches in pp collisions at 13 TeV.

Published in Physics Letters B as doi:10.1016/j.physletb.2023.137803.

*See Appendix A for the list of collaboration members

arXiv:2210.03082v2 [hep-ex] 29 Jun 2023

1 Introduction

The standard model (SM) of particle physics is an extremely successful theory that has been

extensively veriﬁed against experimental results. Nevertheless, there are several fundamental

aspects of particle phenomenology that are not explained within the SM. One of these is the

appearance of three generations of leptons and quarks, regarded as fundamental fermions in

the SM, and the related question of the mass hierarchy across the generations. A possible

solution to these issues is offered by composite-fermion models [1–10], in which the quarks

and leptons have substructure.

In the composite-fermion scenario, quarks and leptons are assumed to have an internal sub-

structure that would manifest itself at some sufﬁciently high energy scale Λ, the compos-

iteness scale. This scale plays the role of an expansion parameter with which a series of

higher-dimensional operators are constructed in an effective ﬁeld theory (EFT) framework. The

fermions of the SM are considered as bound states of some not-yet-observed fundamental con-

stituents, generically referred to as preons [2]. Two model-independent features [8, 9, 11, 12] are

experimentally relevant: excited states of quarks and leptons with masses lower than or equal

to Λ, and gauge or contact effective interactions (GI or CI) between the ordinary fermions and

these excited states. The gauge interaction involves both fermion and gauge boson ﬁelds, and,

at the lowest order in the EFT expansion, is described by dimension-ﬁve operators. Conversely,

the contact interaction involves only fermion ﬁelds, with corresponding operators of dimension

six.

A particular case of such excited states is a heavy composite Majorana neutrino (Nℓ,ℓ=

e, µ,τ) [13–16], a neutral lepton having a mass above the electroweak energy scale. The in-

troduction of an Nℓis well motivated as an explanation of the baryon asymmetry in the uni-

verse. Indeed, in the framework of baryogenesis via leptogenesis [17, 18], heavy Majorana

fermions are the source of the matter-antimatter asymmetry in CP violating decays in the early

universe, and it has been proposed [19, 20] that Nℓ’s could quantitatively account for the ob-

served asymmetry. Such composite Majorana neutrinos would also lead to observable effects

in neutrinoless double beta decay experiments [14, 16].

As a general phenomenological framework we consider the composite neutrino model given

in Ref. [21], in which the GI and CI enter into both the production and decay of Nℓ’s and are

governed, respectively, by the effective Lagrangians

LGI =g f

√2ΛNσµν(∂µWν)PLℓ+h.c., (1)

LCI =g2

∗η

Λ2¯

q′γµPLq NγµPLℓ+h.c. (2)

Here N,ℓ,W, and qare the Nℓ, charged lepton, W boson, and quark ﬁelds, respectively, PLis

the left-handed chirality projection operator, and gis the SU(2)Lgauge coupling. The effective

coupling for contact interactions, g2

∗, takes the value 4π[21]. The factors fand ηare additional

couplings in the composite model; they are taken here to be unity, a choice that is commonly

adopted in phenomenological studies and experimental analyses of composite-fermion mod-

els. The total amplitude for the production process is given by the coherent sum of the gauge

and contact contributions, as shown in Fig. 1, as well as for the decay modes shown in Fig. 2.

The production cross section via contact interaction is dominant for a wide range of Λvalues,

including the ones to which this search is sensitive.

In this work, we consider a composite neutrino, produced in association with a charged lepton,

that subsequently decays to a charged lepton and a pair of quarks, leading to the experimental

¯q

+

N

¯q

+

N

¯q

+

N

Figure 1: The fermion interaction as a sum of gauge (center) and contact (right) contributions.

+

N=

NN

¯q

++

¯q

Figure 2: Feynman diagrams for the decay of a heavy composite Majorana neutrino to ℓqq′.

signature ℓℓqq′. Because the Nℓis a Majorana lepton at the TeV scale, the expected signal is

characterized by two leptons ℓwith high transverse momentum (pT) that may be of the same

or opposite charge sign, but are of the same ﬂavor. We focus on the cases in which these leptons

are both electrons or both muons, and the quark pair is detected as a wide jet. A shape-based

analysis is performed, searching for evidence of a signal in the distribution of the invariant

mass of the system comprising the two leptons and the quark-pair jet.

The data sample of proton-proton collisions at √s=13 TeV was recorded in 2016–2018 with

the CMS detector at the CERN LHC, and corresponds to an integrated luminosity of 138 fb−1.

A previous search for Nℓwas performed by CMS with a data sample corresponding to 2.3 fb−1

at √s=13 TeV [22], and found agreement between the data and SM expectations. A 95%

conﬁdence level (CL) upper limit on the Majorana neutrino mass mNℓwas placed at about

4.6 TeV for both the electron and muon channels. With the larger statistical power of the current

data sample, the present search explores a wider range of the parameter space (mNℓ,Λ). We

further expand the composite model with recent considerations on the scope of validity of the

effective operators in Eqs. (1) and (2) as derived in Ref. [23]. The unitarity bounds on these

operators are used as guidance to optimize the search and extend the analysis sensitivity to

lower mNℓand higher Λcompared with the previous search. Tabulated results are provided in

the HEPData record for this analysis [24].

More generally, excited states interacting with the SM sector have been extensively searched

for at high-energy collider facilities. The current most stringent bounds come from the recent

LHC experiments. Excited charged leptons (e∗,µ∗) have been searched for in the channel pp →

ℓℓ∗→ℓℓγ[25–30], where they would be produced via CI and then decay via GI, and in the

channel pp →ℓℓ∗→ℓℓqq′[30] where both production and decay proceed through CI.

2 The CMS detector

The central feature of the CMS apparatus is a superconducting solenoid, of 6 m internal di-

ameter, providing a ﬁeld of 3.8 T. Within the ﬁeld volume, there are the inner tracker, the

crystal electromagnetic calorimeter (ECAL), and the brass and scintillator hadron calorimeter

(HCAL). The inner tracker is composed of a pixel detector and a silicon strip tracker, and mea-

sures charged-particle trajectories in the pseudorapidity range |η|<2.5. The ﬁnely segmented

ECAL consists of nearly 76 000 lead-tungstate crystals that provide coverage up to |η|=3.0.

The HCAL consists of a sampling calorimeter, which utilizes alternating layers of brass as an

absorber and plastic scintillator as an active material, covering the range |η|<3, and is ex-

tended to |η|<5 by the forward hadron calorimeters. The muon system covers the region

|η|<2.4 and consists of up to four planes of gas ionization muon detectors installed outside

the solenoid and sandwiched between the layers of the steel ﬂux-return yoke. Events of interest

are selected using a two-tiered trigger system. The ﬁrst level, composed of custom hardware

processors, uses information from the calorimeters and muon detectors to select events at a

rate of around 100 kHz within a ﬁxed latency of about 4 µs [31]. The second level, known as

the high-level trigger, consists of a farm of processors running a version of the full event recon-

struction software optimized for fast processing, and reduces the event rate to around 1 kHz

before data storage [32]. A detailed description of the CMS detector can be found in Ref. [33].

3 Monte Carlo simulation

The signal and the SM backgrounds are simulated using the Monte Carlo (MC) method. The

simulated samples for the signal are generated at leading order (LO) with CALCHEP v3.6 [34],

using the NNPDF 3.0 LO parton distribution functions (PDFs) with the four-ﬂavor scheme [35].

Samples are generated for Λvalues from 4 to 20 TeV, and with mNℓvalues from 0.5 TeV to Λ,

the maximum value consistent with the model.

The background processes simulated are top quark pair production tt, single top quark pro-

duction tW, the Drell–Yan (DY) process, W+jets, diboson production (WW, WZ, ZZ), tt with

vector boson production ttV, and SM production of jets through the strong interaction de-

scribed by quantum chromodynamics (QCD). The tt events are generated at next-to-leading

order (NLO) with POWHEG v2.0 [36–40]. The POWHEG generator is also used to describe tW

production at NLO. The DY, QCD, W+jets, and ttV samples are generated at LO with MAD-

GRAPH5 aMC@NLO v2.2.2 (v2.4.2) [41] for the 2016 (2017–2018) samples. The DY events are

weighted by a pT-dependent Kfactor, a function of the generator-level Z boson momentum

pT(Z). The Kfactor serves both to adjust a mismodeling of the pT(Z)distribution [42, 43] and to

account for higher-order effects in the QCD and EW perturbative expansions. It is a product of

two terms: one obtained as described in Ref. [44] from Drell–Yan NLO samples, produced with

MADGRAPH5 aMC@NLO with the FXFX matching scheme [45], while the other is extracted

from theoretical calculations [42]. The diboson processes are generated with PYTHIA [46] at

LO.

For the simulation of all backgrounds we use the NNPDF 3.0 [35] PDFs for 2016, and NNPDF 3.1

next-to-NLO (NNLO) [47] for 2017–2018 samples. Parton showering and hadronization are de-

scribed by PYTHIA 8.226 (8.230) with the CUETP8M1 [48] (CP5 [49]) tune for 2016 (2017–2018)

samples. Additional collisions in the same or adjacent bunch crossings (pileup) are taken into

account by superimposing simulated minimum bias interactions onto the hard scattering pro-

cess, with a number distribution matching that observed in data. Simulated events are prop-

agated through the GEANT4 [50] based simulation of the CMS detector, tuned for detector-

related differences in each data-taking period. Normalization of the simulated background

samples is performed using the most precise cross section calculations available [39–41, 51–58],

which are generally calculated to NLO or NNLO.

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EUROPEANORGANIZATIONFORNUCLEARRESEARCH(CERN)CERN-EP-2022-1812023/06/30CMS-EXO-20-011SearchforaheavycompositeMajorananeutrinoineventswithdileptonsignaturesfromproton-protoncollisionsat√s=13TeVTheCMSCollaboration*AbstractResultsarepresentedofasearchforaheavyMajorananeutrinoNℓdecayingintotwosame-flavor...

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Search for a heavy composite Majorana neutrino in events with dilepton signatures from proton-proton collisions at sqrts 13 TeV

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