EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP/2011-033
2018/10/29
CMS-SUS-10-004
Search for new physics with same-sign isolated dilepton
events with jets and missing transverse energy at the LHC
The CMS Collaboration∗
Abstract
The results of searches for new physics in events with two same-sign isolated leptons,
hadronic jets, and missing transverse energy in the ﬁnal state are presented. The
searches use an integrated luminosity of 35 pb−1 of pp collision data at a centre-of-
mass energy of 7 TeV collected by the CMS experiment at the LHC. The observed
numbers of events agree with the standard model predictions, and no evidence for
new physics is found. To facilitate the interpretation of our data in a broader range
of new physics scenarios, information on our event selection, detector response, and
efﬁciencies is provided.
Submitted to the Journal of High Energy Physics
∗See Appendix A for the list of collaboration members
arXiv:1104.3168v1 [hep-ex] 15 Apr 2011

1
1 Introduction
Events with same-sign isolated lepton pairs from hadron collisions are very rare in the standard
model (SM) but appear very naturally in many new physics scenarios. In particular, they have
been proposed as signatures of supersymmetry (SUSY) [1–3], universal extra dimensions [4],
pair production of T5/3 (a fermionic partner of the top quark) [5], heavy Majorana neutrinos [6],
and same-sign top-pair resonances as predicted in theories with warped extra dimensions [7].
In this paper we describe searches for new physics with same-sign isolated dileptons (ee, eµ,
µµ, eτ, µτ, and ττ), missing transverse energy (Emiss
T ), and hadronic jets. Our choice of signal
regions is driven by two simple observations. First, astrophysical evidence for dark matter [8]
suggests that we concentrate on ﬁnal states with Emiss
T . Second, observable new physics sig-
nals with large cross sections are likely to be produced by strong interactions, and we thus
expect signiﬁcant hadronic activity in conjunction with the two same-sign leptons. Beyond
these simple guiding principles, our searches are as independent of detailed features of new
physics models as possible. The results are based on a data sample corresponding to an inte-
grated luminosity of 35 pb−1 collected in pp collisions at a centre-of-mass energy of 7 TeV by
the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) in 2010.
This paper is organized as follows. The CMS detector is brieﬂy described in Section 2. The re-
construction of leptons, Emiss
T , and jets at CMS is summarized in Section 3. Section 4 describes
our search regions. We perform separate searches based on leptonic and hadronic triggers in
order to cover a wider region in the parameter space of new physics. Electron and muon trig-
gers allow for searches that require less hadronic energy in the event, while hadronic triggers
allow inclusion of lower transverse momentum (pT) electrons and muons, as well as hadronic
τ decays in the ﬁnal state. The dominant backgrounds for all three searches are estimated
from data, as discussed in Section 5. Systematic uncertainties on the predicted number of sig-
nal events and results of these searches are discussed in Sections 6 and 7. We conclude with
a discussion on how to use our results to constrain a wide variety of new physics models in
Section 8.
2 The CMS Detector
A right-handed coordinate system is employed by the CMS experiment, with the origin at the
nominal interaction point, the x-axis pointing to the centre of the LHC, and the y-axis pointing
up (perpendicular to the LHC plane). The polar angle θ is measured from the positive z-axis
and the azimuthal angle φ is measured in the xy plane. The pseudorapidity is deﬁned as η =
− ln [tan ( θ
2 )].
The central feature of the CMS apparatus is a superconducting solenoid, of 6 m internal diam-
eter, 13 m in length, providing an axial ﬁeld of 3.8 T. Within the ﬁeld volume are several par-
ticle detection systems which each feature a cylindrical geometry, covering the full azimuthal
range from 0 ≤ φ ≤ 2π. Silicon pixel and strip tracking detectors provide measurements of
charged particle trajectories and extend to a pseudorapidity of |η| = 2.5. A homogeneous crys-
tal electromagnetic calorimeter (ECAL) and a sampling brass/scintilator hadronic calorimeter
(HCAL) surround the tracking volume and provide energy measurements of electrons, pho-
tons, and hadronic jets up to |η| = 3.0. An iron-quartz ﬁber hadronic calorimeter, which is also
part of the HCAL system, is located in the forward region deﬁned by 3.0 < |η| < 5.0. Muons
are measured in the pseudorapidity range of |η| < 2.4, with detection planes made of three
technologies: drift tubes, cathode strip chambers, and resistive plate chambers. These are in-
strumented outside of the magnet coil within the steel return yoke. The CMS detector is nearly

2 3 Reconstruction of Leptons, Missing Energy, and Jets
hermetic, allowing for energy balance measurements in the plane transverse to the beam direc-
tion. A two-tier trigger system is designed to select the most interesting pp collision events for
use in physics analysis. A detailed description of the CMS detector can be found elsewhere [9].
3 Reconstruction of Leptons, Missing Energy, and Jets
Muon candidates are required to be successfully reconstructed [10] using two algorithms, one
in which tracks in the silicon detector are matched to consistent signals in the calorimeters and
muon system, and another in which a simultaneous global ﬁt is performed to hits in the silicon
tracker and muon system. The track associated with the muon candidate is required to have a
minimum number of hits in the silicon tracker, have a high-quality global ﬁt including a min-
imum number of hits in the muon detectors, and have calorimeter energy deposits consistent
with originating from a minimum ionizing particle.
Electron candidates are reconstructed [11] starting from a cluster of energy deposits in the
ECAL, which is then matched to hits in the silicon tracker. A selection using electron identiﬁca-
tion variables based on shower shape and track-cluster matching is applied to the reconstructed
candidates; the criteria are optimized in the context of the inclusive W → eν measurement [12]
and are designed to maximally reject electron candidates from QCD multijet production while
maintaining approximately 80% efﬁciency for electrons from the decay of W/Z bosons. Elec-
tron candidates within ∆R = √∆φ2 + ∆η2 < 0.1 of a muon are rejected to remove electron
candidates due to muon bremsstrahlung and ﬁnal-state radiation. Electron candidates origi-
nating from photon conversions are suppressed by looking for a partner track and requiring
no missing hits for the track ﬁt in the inner layers of the tracking detectors.
Hadronic τ candidates (τh) are identiﬁed [13] starting with a hadronic jet clustered from the
particles reconstructed using the particle-ﬂow global-event reconstruction algorithm [14]. The
highest-pT charged track within a cone of ∆R < 0.1 around the jet axis is required to have
pT > 5 GeV. A variable size cone of ∆R < 5 GeV/pT is then deﬁned around this track, and the
boosted τ-decay products are expected to be conﬁned within this narrow cone. Only τ candi-
dates with one or three charged hadrons in this cone are selected. The discrimination between
hadronic τ decays and generic QCD jets is based on an ensemble of ﬁve neural networks, each
of which has been trained to identify one of the ﬁve main hadronic τ-decay modes using the
kinematics of the reconstructed charged and neutral pions [15].
All lepton candidates are required to have |η| < 2.4, and be consistent with originating from
the same interaction vertex. Charged leptons from the decay of W/Z bosons, as well as the
new physics we are searching for, are expected to be isolated from other activity in the event.
We calculate a relative measure of this isolation denoted as Rel Iso . This quantity is deﬁned
as the ratio of the scalar sum of transverse track momenta and transverse calorimeter energy
deposits within a cone of ∆R < 0.3 around the lepton candidate direction at the origin, to the
transverse momentum of the candidate. The contribution from the candidate itself is excluded.
In order to suppress the background due to dileptons originating from the same jet, we require
that selected dileptons have a minimum invariant mass of 5 GeV. This helps to keep dileptons
uncorrelated with respect to their Rel Iso observables, which is a feature we exploit in the anal-
ysis. We also remove events with a third lepton of opposite sign and same ﬂavour as one of the
two selected leptons if the invariant mass of the pair is between 76 and 106 GeV. This require-
ment further reduces an already small background contribution from WZ and ZZ production.
Jets and Emiss
T are reconstructed based on the particle-ﬂow technique desribed in [14, 16]. For

3
jet clustering, we use the anti-kT algorithm with the distance parameter R = 0.5 [17]. Jets are
required to pass standard quality requirements [18] to remove those consistent with calorimeter
noise. Jet energies are corrected for residual nonuniformity and nonlinearity of the detector
response derived using collision data [19]. We require jets to have transverse energy above
30 GeV and to be within |η| < 2.5. We deﬁne the HT observable as the scalar sum of the pT of
all such jets with ∆R > 0.4 to the nearest lepton passing all our requirements.
4 Search Regions
The searches discussed in this paper employ two different trigger strategies, electron and muon
triggers in one case, and HT triggers in the other. The leptonic triggers allow for lower HT
requirements, while the HT triggers allow for lower lepton pT, as well as ﬁnal states with
hadronic τ decays. The motivation for covering the widest possible phase space in this search
can be illustrated by an example of a SUSY cascade, shown in Fig. 1, naturally giving rise to
jets, Emiss
T , and same-sign leptons: (gluinos/squarks) → (charged gaugino) → (lightest super-
symmetric particle (LSP) neutralino). The mass difference between the gluino/squarks and the
charged gaugino, typically arbitrary, deﬁnes the amount of hadronic activity one may expect
in the event. The mass difference between the gaugino and a neutralino inﬂuences the lepton
pT spectrum. Depending on the nature of the chargino and neutralino, their mass difference
can be either arbitrary (e.g., wino and bino) or typically small (e.g., higgsinos). Moreover, there
are a number of ways to generate a large production asymmetry between τ and e/µ leptons,
which motivates us to look speciﬁcally for events with a τ.
Figure 1: An example of a process involving the production and decays of SUSY particles,
which gives rise to two same-sign prompt leptons, jets, and missing transverse energy.
In the following we describe the search regions explored by each trigger strategy. As a new-
physics reference point, we use LM0, a point in the constrained Minimal Supersymmetric Stan-
dard Model (CMSSM) [20] deﬁned with the model parameters m0 = 200 GeV, m1/2 = 160 GeV,
tanβ = 10, µ > 0, and A0 = −400 GeV. LM0 is one of the common CMSSM reference points
used in CMS across many analyses. As the abbreviation suggests, LM0 provides squarks and
gluinos with relatively low masses, and thus has a large production cross section. It is beyond
the exclusion reach of the searches performed by LEP and Tevatron, but it has recently been
excluded by searches [21–23] with ATLAS and CMS concurrently with this one. Nonetheless,
we continue to use LM0 as it provides a common model for which to compare our sensitivity
with that of other analyses. Aside from the LM0 point, several SM simulation samples are used
to both validate and complement various background estimation methods that are based on
the data itself. These samples rely on either PYTHIA 6.4 [24] or MADGRAPH [25] for event

4 4 Search Regions
generation and GEANT4 [26] for simulation of the CMS detector. Samples used include tt,
single-t, γ+jets, W+jets, Z+jets, WW, WZ, ZZ, and QCD multijet production. Next-to-leading-
order (NLO) cross sections are used for all samples except for QCD multijet production.
4.1 Searches using Lepton Triggers
We start with a baseline selection inspired by our published tt → `+`− + X (` = e or µ) cross
section measurement [27].
Events are collected using single and dilepton triggers. The detailed implementation of these
triggers evolved throughout the 2010 data-collecting period as the LHC instantaneous lumi-
nosity was increasing. Trigger efﬁciencies are measured from a pure lepton sample collected
using Z → `+`− decays from data. The luminosity-averaged efﬁciency to trigger on events
with two leptons with |η| < 2.4 and pT > 10 GeV, one of which also has pT > 20 GeV, is
very high. For example, the trigger efﬁciency for an LM0 event passing the baseline selection
described below is estimated to be (99 ± 1)%.
One of the electrons and muons must have pT > 20 GeV and the second one must have pT >
10 GeV. Both leptons must be isolated. The isolation requirement is based on the Rel Iso vari-
able introduced earlier. We require Rel Iso < 0.1 for leptons of pT > 20 GeV, and the isolation
sum (i.e., the numerator of the Rel Iso expression) to be less than 2 GeV for pT < 20 GeV.
We require the presence of at least two reconstructed jets, implying HT > 60 GeV. Finally, we
require the missing transverse energy Emiss
T > 30 GeV (ee and µµ) or Emiss
T > 20 GeV (eµ). This
deﬁnes our baseline selection.
Following the guiding principles discussed in the introduction, we deﬁne two search regions.
The ﬁrst has high Emiss
T (Emiss
T > 80 GeV); the second has high HT (HT > 200 GeV). These
Emiss
T and HT values were chosen to obtain an SM background expectation in simulation of 1/3
of an event in either of the two overlapping search regions.(GeV)TH
100 200 300 400 500 600 700 800 900 1000
(GeV)
miss
TE
50
100
150
200
250
300
350
400
data, tight selection
LM0 MC
= 7 TeVsCMS,
-1
= 35 pbintL(GeV)T
H
100 200 300 400 500 600 700 800 900 1000
(GeV)
miss
T
E
50
100
150
200
250
300
350
400
data, loose selection = 7 TeVsCMS,
-1
= 35 pbintL
Figure 2: HT versus Emiss
T scatter plots for baseline region. (Left) Overlay of the three observed
events with the expected signal distribution for LM0. The three observed events all scatter in
the lower left corner of the plot. (Right) Scatter plot of the background in data when only one
of the two leptons is required to be isolated.

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Search for new physics with same-sign isolated dilepton events with jets and missing transverse energy at the LHC

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