JHEP04(2017)018
Published for SISSA by Springer
Received: October 16, 2016
Accepted: March 12, 2017
Published: April 4, 2017
Search for electroweak production of charginos in final
states with two τ leptons in pp collisions at
√
s = 8 TeV
The CMS collaboration
E-mail: cms-publication-committee-chair@cern.ch
Abstract: Results are presented from a search for the electroweak production of super-
symmetric particles in pp collisions in final states with two τ leptons. The data sample
corresponds to an integrated luminosity between 18.1 fb
−1
and 19.6 fb
−1
depending on the
final state of τ lepton decays, at
√
s = 8 TeV, collected by the CMS experiment at the
LHC. The observed event yields in the signal regions are consistent with the expected
standard model backgrounds. The results are interpreted using simplified models describ-
ing the pair production and decays of charginos or τ sleptons. For models describing the
pair production of the lightest chargino, exclusion regions are obtained in the plane of
chargino mass vs. neutralino mass under the following assumptions: the chargino decays
into third-generation sleptons, which are taken to be the lightest sleptons, and the sleptons
masses lie midway between those of the chargino and the neutralino. Chargino masses
below 420 GeV are excluded at a 95% confidence level in the limit of a massless neutralino,
and for neutralino masses up to 100 GeV, chargino masses up to 325 GeV are excluded at
95% confidence level. Constraints are also placed on the cross section for pair production
of τ sleptons as a function of mass, assuming a massless neutralino.
Keywords: Hadron-Hadron scattering (experiments), Supersymmetry
ArXiv ePrint: 1610.04870
Open Access, Copyright CERN,
for the benefit of the CMS Collaboration.
Article funded by SCOAP
3
.
doi:10.1007/JHEP04(2017)018
JHEP04(2017)018
Contents
1 Introduction 1
2 The CMS detector and event reconstruction 3
3 The Monte Carlo samples 4
4 Definition of M
T2
5
5 Event selection for the τ
h
τ
h
channel 6
6 Event selection for the `τ
h
channel 7
7 Backgrounds 7
7.1 The QCD multijet background estimation in the τ
h
τ
h
channel 8
7.2 W+jets background estimation in the τ
h
τ
h
channel 10
7.3 The Drell-Yan background estimation 11
7.4 Misidentified τ
h
in the `τ
h
channels 11
8 Systematic uncertainties 13
9 Results and interpretation 14
10 Summary 16
A Additional information for new model testing 21
The CMS collaboration 30
1 Introduction
Supersymmetry (SUSY) [1–5] is one of the most promising extensions of the standard model
(SM) of elementary particles. Certain classes of SUSY models can lead to the unification of
gauge couplings at high energy, provide a solution to the gauge hierarchy problem without
fine tuning by stabilizing the mass of the Higgs boson against large radiative corrections,
and provide a stable dark matter candidate in models with conservation of R-parity. A
key prediction of SUSY is the existence of new particles with the same gauge quantum
numbers as SM particles but differing by a half-unit in spin (sparticles).
Extensive searches at the LHC have excluded the existence of strongly produced (col-
ored) sparticles in a broad range of scenarios, with lower limits on sparticle masses ranging
up to 1.8 TeV for gluino pair production [6–13]. While the limits do depend on the details
– 1 –
JHEP04(2017)018
˜χ
∓
1
˜ν
τ
˜τ
±
˜χ
±
1
p1
p2
τ
∓
ν
τ
˜χ
0
1
˜χ
0
1
τ
±
ν
τ
1
˜τ
∓
˜τ
±
p1
p2
τ
∓
˜χ
0
1
˜χ
0
1
τ
±
1
Figure 1. Schematic production of τ lepton pairs from chargino (left) or τ slepton (right) pair
production.
of the assumed SUSY particle mass spectrum, constraints on the colorless sparticles are
generally much less stringent. This motivates the electroweak SUSY search described in
this paper.
Searches for charginos (eχ
±
), neutralinos (eχ
0
), and sleptons (
e
`) by the ATLAS and
CMS Collaborations are described in refs. [14–20]. In various SUSY models, the lightest
SUSY partners of the SM leptons are those of the third generation, resulting in enhanced
branching fractions for final states with τ leptons [21]. The previous searches for charginos,
neutralinos, and sleptons by the CMS Collaboration either did not include the possibility
that the scalar τ lepton and its neutral partner (eτ and eν
τ
) are the lightest sleptons [16], or
that the initial charginos and neutralinos are produced in vector-boson fusion processes [18].
An ATLAS search for SUSY in the di-τ channel is reported in ref. [19], excluding chargino
masses up to 345 GeV for a massless neutralino (eχ
0
1
). The ATLAS results on direct eτ
production is improved and updated in ref. [20].
In this paper, a search for the electroweak production of the lightest charginos ( eχ
±
1
) and
scalar τ leptons (eτ ) is reported using events with two opposite-sign τ leptons and a modest
requirement on the magnitude of the missing transverse momentum vector, assuming the
masses of the third-generation sleptons are between those of the chargino and the lightest
neutralino. Two τ leptons can be generated in the decay chain of eχ
±
1
and eτ, as shown in
figure 1. The results of the search are interpreted in the context of SUSY simplified model
spectra (SMS) [22, 23] for both production mechanisms.
The results are based on a data set of proton-proton (pp) collisions at
√
s = 8 TeV
collected with the CMS detector at the LHC during 2012, corresponding to integrated
luminosities of 18.1 and 19.6 fb
−1
in different channels. This search makes use of the
stransverse mass variable (M
T2
) [24, 25], which is the extension of transverse mass (M
T
)
to the case where two massive particles with equal mass are created in pairs and decay
to two invisible and two visible particles. In the case of this search, the visible particles
are both τ leptons. The distribution of M
T2
reflects the scale of the produced particles
and has a longer tail for heavy sparticles compared to lighter SM particles. Hence, SUSY
can manifest itself as an excess of events in the high-side tail of the M
T2
distribution.
Final states are considered where two τ leptons are each reconstructed via hadronic decays
(τ
h
τ
h
), or where only one τ lepton decays hadronically and the other decays leptonically
(`τ
h
, where ` is an electron or muon).
– 2 –
JHEP04(2017)018
The paper is organized as follows. The CMS detector, the event reconstruction, and
the data sets are described in sections 2 and 3. The M
T2
variable is introduced in section 4.
The selection criteria for the τ
h
τ
h
and `τ
h
channels are described in section 5 and 6, respec-
tively. A detailed study of the SM backgrounds is presented in section 7, while section 8 is
devoted to the description of the systematic uncertainties. The results of the search with
its statistical interpretation are presented in section 9. Section 10 presents the summaries.
The efficiencies for the important selection criteria are summarized in appendix A and can
be used to interpret these results within other phenomenological models.
2 The CMS detector and event reconstruction
The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal
diameter that provides a magnetic field of 3.8 T. Within the solenoid volume are a silicon
pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter, and a brass and
scintillator hadron calorimeter, each composed of a barrel and two endcap sections. Muons
are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the
solenoid. Extensive forward calorimetry complements the coverage provided by the barrel
and endcap detectors. A more 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. [26].
To be recorded for further study, events from pp interactions must satisfy criteria
imposed by a two-level trigger system. The first level of the CMS trigger system, composed
of custom hardware processors, uses information from the calorimeters and 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 before data storage [27].
The particle-flow (PF) algorithm [28, 29] reconstructs and identifies each individual
particle with an optimized combination of information from the various elements of the
CMS detector. Jets are reconstructed from the PF candidates with the anti-k
t
clustering
algorithm [30] using a distance parameter of 0.5. We apply corrections dependent on trans-
verse momentum (p
T
) and pseudorapidity (η) to account for residual effects of nonuniform
detector response [31]. A correction to account for multiple pp collisions within the same or
nearby bunch crossings (pileup interactions) is estimated on an event-by-event basis using
the jet area method described in ref. [32], and is applied to the reconstructed jet p
T
. The
combined secondary vertex algorithm [33] is used to identify (“b tag”) jets originating from
b quarks. This algorithm is based on the reconstruction of secondary vertices, together
with track-based lifetime information. In this analysis a working point is chosen such that,
for jets with a p
T
value greater than 60 GeV the efficiency for tagging a jet containing a b
quark is 70% with a light-parton jet misidentification rate of 1.5%, and c quark jet misiden-
tification rate of 20%. Scale factors are applied to the simulated events to reproduce the
tagging efficiencies measured in data, separately for jets originating from b or c quarks,
and from light-flavor partons. Jets with p
T
> 40 GeV and |η| < 5.0 and b-tagged jets with
p
T
> 20 GeV and |η| < 2.4 are considered in this analysis.
– 3 –
JHEP04(2017)018
The PF candidates are used to reconstruct the missing transverse momentum vector
~p
miss
T
, defined as the negative of the vector sum of the transverse momenta of all PF
candidates. For each event, p
miss
T
is defined as the magnitude of ~p
miss
T
.
Hadronically decaying τ leptons are reconstructed using the hadron-plus-strips algo-
rithm [34]. The constituents of the reconstructed jets are used to identify individual τ
lepton decay modes with one charged hadron and up to two neutral pions, or three charged
hadrons. Additional discriminators are used to separate τ
h
from electrons and muons.
Prompt τ leptons are expected to be isolated in the detector. To discriminate them from
quantum chromodynamics (QCD) jets, an isolation variable [35] is defined by the scalar
sum of the transverse momenta of the charged hadrons and photons falling within a cone
around the τ lepton momentum direction after correcting for the effect of pileup. The
“loose”, “medium”, and “tight” working points are defined by requiring the value of the
isolation variable not to exceed 2.0, 1.0, and 0.8 GeV, respectively. A similar measure of
isolation is computed for charged leptons (e or µ), where the isolation variable is divided
by the p
T
of the lepton. This quantity is used to suppress the contribution from leptons
produced in hadron decays in jets.
3 The Monte Carlo samples
The SUSY signal processes and SM samples, which are used to evaluate potential back-
ground contributions, are simulated using CTEQ6L1 [36] parton distribution functions.
To model the parton shower and fragmentation, all generators are interfaced with pythia
6.426 [37]. The SM processes of Z+jets, W+jets, tt, and dibosons are generated using the
MadGraph 5.1 [38] generator. Single top quark and Higgs boson events are generated
with powheg 1.0 [39–42]. In the following, the events from Higgs boson production via
gluon fusion, vector-boson fusion, or in association with a W or Z boson or a tt pair are re-
ferred to as “hX.” Later on, the events containing at least one top quark or one Z boson are
referred to as “tX” and “ZX,” respectively. The masses of the top quark and Higgs boson
are set to be 172.5 GeV [43] and 125 GeV [44], respectively. Since the final state arising from
the pair production of W bosons decaying into τ leptons is very similar to our signal, in
the following figures its contribution is shown as an independent sample labeled as “WW.”
In one of the signal samples, pairs of charginos are produced with pythia 6.426 and
decayed exclusively to the final states that contain two τ leptons, two τ neutrinos, and two
neutralinos, as shown in figure 1 (left). The daughter sparticle in the two-body decay of the
eχ
±
1
can be either a eτ or eν
τ
. In this scenario, no decay modes are considered other than those
shown in figure 1 (left), so for m(eτ) = m(eν
τ
), the two decay chains (via the eτ or eν
τ
) have
50% branching fraction. The masses of the eτ and eν
τ
are set to be equal to the mean value
of the eχ
±
1
and eχ
0
1
masses and consequently are produced on mass shell. If the eτ (eν
τ
) mass is
close to the eχ
0
1
mass, the τ lepton from the eτ (eχ
±
1
) decay will have a low (high) momentum,
resulting in a lower (higher) overall event selection efficiency, producing a weaker (stronger)
limit on the chargino mass. In the case where the eτ (eν
τ
) mass is close to the eχ
±
1
mass, the
situations are opposite. Of the scenarios in which the τ slepton and the τ sneutrino have
the same mass, the scenario with the highest efficiency overall corresponds to the one in
– 4 –