EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-EP-2017-025
2017/08/08
CMS-EXO-16-023
Search for third-generation scalar leptoquarks and heavy
right-handed neutrinos in final states with two tau leptons
and two jets in proton-proton collisions at
√
s = 13 TeV
The CMS Collaboration
∗
Abstract
A search is performed for third-generation scalar leptoquarks and heavy right-
handed neutrinos in events containing one electron or muon, one hadronically de-
caying τ lepton, and at least two jets, using a
√
s = 13 TeV pp collision data sample
corresponding to an integrated luminosity of 12.9 fb
−1
collected with the CMS detec-
tor at the LHC in 2016. The number of observed events is found to be in agreement
with the standard model prediction. A limit is set at 95% confidence level on the prod-
uct of the leptoquark pair production cross section and β
2
, where β is the branching
fraction of leptoquark decay to a τ lepton and a bottom quark. Assuming β = 1,
third-generation leptoquarks with masses below 850 GeV are excluded at 95% confi-
dence level. An additional search based on the same event topology involves heavy
right-handed neutrinos, N
R
, and right-handed W bosons, W
R
, arising in a left-right
symmetric extension of the standard model. In this search, W
R
bosons are assumed
to decay to a tau lepton and N
R
followed by the decay of the N
R
to a tau lepton and
an off-shell W
R
boson. Assuming the mass of the right-handed neutrino to be half of
the mass of the right-handed W boson, W
R
boson masses below 2.9 TeV are excluded
at 95% confidence level. These results improve on the limits from previous searches
for third-generation leptoquarks and heavy right-handed neutrinos with τ leptons in
the final state.
Published in the Journal of High Energy Physics as doi:10.1007/JHEP07(2017)121.
c
2017 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license
∗
See Appendix A for the list of collaboration members
arXiv:1703.03995v2 [hep-ex] 5 Aug 2017
1
1 Introduction
A number of extensions of the standard model (SM) have been proposed that predict an en-
hanced production rate for events containing pairs of quarks and pairs of third-generation
leptons. One such theoretical proposal involves the existence of particles called leptoquarks
(LQs), which carry color charge, fractional electric charge, and both lepton and baryon quan-
tum numbers. The LQs arise in many models, including grand unified theories [1], composite-
ness models [2, 3], and superstring theories [4]. If LQs exist, they will decay into a lepton and a
quark. At the CERN LHC, LQ pairs are predominantly produced via gluon-gluon fusion and
quark-antiquark annihilation. Based on the latest experimental constraints reviewed in [5], we
assume that contribution of t-channel production of LQ pairs involving Yukawa coupling of a
LQ, a lepton, and a quark, is small and neglected in this analysis and the main free parameter
is the mass of LQ. However, the branching fraction for the decay of a LQ into a quark and a
charged lepton, β, depends on the details of the model under consideration. In this analysis
we focus on the decay of a pair of third-generation LQs resulting in two τ leptons and two jets
originating from b quark fragmentation.
A similar final state is expected in theories that postulate that the masses of the familiar left-
handed neutrinos arise not from the Higgs field, but from a mechanism that involves the ex-
istence of right-handed neutrinos. One of the appealing features of left-right (L-R) symmetric
extensions [6] of the SM is that these models predict the existence of new heavy charged (W
R
)
and neutral (Z
R
) gauge bosons that could be produced at LHC energies. Heavy neutrinos
(N
e
, N
µ
, N
τ
) naturally arise as the right-handed (RH) partners of the SM neutrinos in these L-R
extensions through the see-saw mechanism [7].
In this paper, we search for these two processes by selecting final states containing two τ lep-
tons and two jets originating from the hadronization of quarks. A search for pair production
of third-generation scalar LQs is pursued by looking for events containing two τ leptons and
two b quarks. We also search for the production of a W
R
boson from quark-antiquark annihi-
lation. A heavy right-handed neutrino is produced from the decay of the W
R
boson following
the decay chain W
R
→ τ + N
τ
, where N
τ
→ τ + W
∗
R
→ τ + qq. In both searches, we focus
on signatures with one of the τ leptons decaying into an electron or a muon, referred to as a
leptonic decay τ
`
in the following, and the other τ lepton decaying hadronically, denoted by τ
h
.
Previous searches for third-generation LQs have been carried out at pp, pp, e
+
e
−
, and ep col-
liders and the most recent results are given in [8, 9] and references therein. The most stringent
lower limit on the mass of scalar third-generation LQs to date, based on the final state with
two τ leptons and two b jets and assuming β = 1, is 740 GeV at 95% confidence level (CL), from
the CMS experiment [10, 11]. Previous searches for heavy neutrinos have been performed at
LEP [12, 13], excluding heavy neutrino masses below approximately 100 GeV. Further searches
at LHC have been performed in the dielectron and dimuon channels and have excluded W
R
bosons with mass up to 3 TeV using data taken at 7 TeV [14] and at 8 TeV [15]. Using 2.1 fb
−1
of data of 13 TeV pp collisions collected in 2015, the CMS experiment searched for heavy neu-
trinos and right-handed charged bosons using events in which both τ leptons decay hadroni-
cally. That analysis excluded W
R
bosons with masses below 2.35 (1.63) TeV at 95% CL, assum-
ing the N
τ
mass is 0.8 (0.2) times the mass of W
R
boson [11]. In the present search, we use a
√
s = 13 TeV pp collision data sample corresponding to an integrated luminosity of 12.9 fb
−1
collected with the CMS detector in 2016.
2 3 Event reconstruction and selection
2 The CMS detector and Monte Carlo event samples
The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diam-
eter, providing a magnetic field of 3.8 T. Within the solenoid volume are a silicon pixel and
strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scin-
tillator hadron calorimeter, each composed of a barrel and two endcap sections. Extensive
forward calorimetry complements the coverage provided by the barrel and endcap detectors.
Muons are detected in gas-ionisation detectors embedded in the steel flux-return yoke outside
the solenoid. A detailed description of the CMS detector, together with a definition of the
coordinate system used and the relevant kinematic variables, can be found in Ref. [16].
The first level of the CMS triggering system, composed of custom hardware processors, uses
information from the calorimeters and the muon detectors to select the most interesting events
in a fixed time interval of less than 4 µs. The high-level trigger processor farm further decreases
the event rate from around 100 kHz to less than 1 kHz.
Background and signal processes are modeled using the following simulated samples. The
PYTHIA v8.205 generator [17] is used to model the signal and diboson (WW, WZ, and ZZ) pro-
cesses. The LQ signal samples are generated with LQ masses ranging from 250 to 1500 GeV in
steps of 50 GeV. The branching fraction of the LQ to a τ lepton and a bottom quark is assumed
to be 100%. The signal samples are normalized to the next-to-next-to-leading order [18, 19].
The W
R
signal samples are generated with W
R
boson masses ranging from 1000 to 4000 GeV in
steps of 500 GeV and the cross sections are computed in Ref. [20]. The MADGRAPH v5.1.5 gen-
erator [21] is used to model W+jets and Z+jets processes. Single top production and tt process
are modelled with the POWHEG 2.0 [22–24] generator. The NNPDF 3.0 [25] Parton Distribu-
tion Functions (PDF) are used, and all simulated samples are interfaced with PYTHIA with the
CUETP8M1 tune [26] to describe parton showering and hadronization. Additional inelastic pp
interactions (pileup) generated by PYTHIA are overlaid on all simulated events, according to the
luminosity profile of the analyzed data. All the generated signal and background samples are
processed with the simulation of the CMS detector based on GEANT4 [27]. Small differences
between data and simulation in trigger, in particle identification and isolation efficiencies, and
in the resolution of the p
T
of jets and missing transverse momentum are corrected by applying
scale factors to simulated events, as detailed below.
3 Event reconstruction and selection
The particle-flow (PF) algorithm [28, 29], which exploits information from all subdetectors, is
used to identify individual particles, such as charged and neutral hadrons, muons, electrons,
and photons. These reconstructed particles are used as input for reconstructing more complex
objects such as τ
h
candidates, jets, and variables like missing transverse momentum.
The reconstructed interaction vertex with the largest value of
∑
i
(p
i
T
)
2
, where p
i
T
is the trans-
verse momentum of the ith track associated with the vertex, is selected as the primary vertex
of the event. This vertex is used as the reference vertex for all the objects reconstructed using
the PF algorithm.
Electrons are reconstructed by matching the energy deposits in the ECAL to tracks recon-
structed in the silicon pixel and strip detectors. The electrons selected in this analysis are
required to have transverse momenta p
T
> 50 GeV and pseudorapidity |η| < 2.1 [30]. The
identification and isolation of electrons are based on a multivariate technique [31] and selected
electrons must satisfy tight electron identification and isolation criteria.
3
Muon reconstruction starts by matching tracks in the silicon tracker with tracks in the outer
muon spectrometer [32]. A global muon track is fitted to the hits from both tracks. Muons
are required to have p
T
> 50 GeV and |η| < 2.1. Quality selection criteria are applied to
the muon tracks to distinguish muons originating from particle collisions with those muons
coming from cosmic rays. In addition, muons are required to pass isolation criteria to separate
prompt muons from those associated with a jet, usually from the semileptonic decays of heavy
quarks.
The hadron-plus-strips algorithm [33, 34] is used to reconstruct τ
h
candidates. It starts from
a jet and searches for candidates produced by the main hadronic decay modes of a τ lepton:
either directly to one charged hadron, or via intermediate ρ(770) and a
1
(1280) mesons to one
charged hadron plus one or two neutral pions, or three charged hadrons. The reconstructed τ
h
is required to have |η| < 2.3 and p
T
> 50 (p
T
> 60) GeV in the LQ (heavy RH neutrino) search.
Hadronic tau lepton decays are identified by a multivariate technique that uses as inputs the
isolation of the τ
h
and variables that are sensitive to its lifetime. A selection criterion is used
that has an efficiency of approximately 65% for identifying hadronically decaying tau leptons
and a probability of less than 1% for misidentifying jets as hadronic tau decays. Additional
criteria are applied to remove electrons and muons reconstructed as τ
h
candidates.
The identified electron or muon and the τ
h
are required to originate from the same vertex and
be spatially separated by ∆R ≡
√
∆φ
2
+ ∆η
2
> 0.5. To suppress background events such
as diboson and Z+jets with bosons decay giving a final state with a pair of leptons, events
containing additional electron or muon candidates with p
T
> 15 GeV, and which pass loose
identification and isolation criteria, are rejected.
Jets are reconstructed using the anti-k
T
algorithm with a distance parameter of R = 0.4 [35, 36]
using PF candidates. The jet energy is corrected for the average contribution from particles
from other proton-proton collisions in the same or neighbouring bunch crossings (pileup) [37].
Additional corrections are applied to better reflect the true total momentum of the particles in
the jet [38]. Selected jets are required to be within |η| < 2.4 and have p
T
> 50 GeV, and to be
separated from the selected electron or muon and the τ
h
by ∆R > 0.5. Further identification
requirements are applied to distinguish genuine jets from those coming from pileup [39].
The transverse momentum imbalance, (
~
p
miss
T
), is calculated as the negative vectorial sum of
transverse momenta of all PF candidates, and corrected by propagating the corrections applied
to identified jets [40]. A correction is applied to account for the effect of additional pileup
interactions. In addition, several filters are employed to veto events with large
~
p
miss
T
caused by
detector effects.
Candidate events were collected using a set of triggers requiring the presence of either an elec-
tron or a muon candidate with p
T
> 45 GeV.
The search for LQs is based on a sample of events containing one light lepton, one τ
h
candidate,
and at least two jets. At least one of the two leading jets is identified as originating from b quark
hadronization (b-tagged) using the combined secondary vertex algorithm [41]. The chosen b
tagging working point corresponds to an identification efficiency of approximately 70% with
about 1% misidentification rate from light quarks. The lepton and τ
h
candidate are required to
have opposite electric charge. There are two possible combinations of two tau candidates, with
two jets, and the combination that minimises the difference in masses between the two resulting
tau candidate-jet systems is chosen. Additionally, the invariant mass of the system formed by
the visible particles of the τ
h
candidate and a jet is required to be greater than 250 GeV.
The search for a W
R
boson decaying into a heavy neutrino uses the same data sample as used