We present a search for a kinematic edge in the invariant mass distribution
of two opposite-sign same-flavor leptons, in final states with jets and missing
transverse energy. The analysis makes use of $19.4$ fb$^{-1}$ proton-proton
collision data at $\sqrt{s} = 8$ TeV. The data have been recorded with the CMS
experiment. Complementary methods have been used for the background estimation,
which when combined achieve a total uncertainty of $5\%$ ($10\%$) for leptons
in the central (forward) rapidity of the detector. We do not observe a
statistically significant signal and the results are consistent with the
background-only hypothesis.Comment: ICNFP2014 conference proceedings, presented in August 2014, prepared
  for submission in EP

Theofilatos, Konstantinos

English

arXiv

We present a search for a kinematic edge in the invariant mass distribution of two opposite-sign same-flavor leptons, in final states with jets and missing transverse energy. The analysis makes use of 19.4 fb−1 proton-proton collision data at √s = 8 TeV. The data have been recorded with the CMS experiment. Complementary methods have been used for the background estimation, which when combined achieve a total uncertainty of 5% (10%) for leptons in the central (forward) rapidity of the detector. We do not observe a statistically significant signal and the results are consistent with the background-only hypothesis

Konstantinos Theofilatos

EDP Sciences OAI-PMH repository (1.2.0)

Search for a dileptonic edge with CMS

Crossref

Search for a dileptonic edge with CMS
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1Institute for Particle Physics, ETH Zürich, Rämistrasse 101, 8092 Zürich, Switzerland
Abstract. We present a search for a kinematic edge in the invariant mass distribution of
two opposite-sign same-flavor leptons, in final states with jets and missing transverse en-
ergy. The analysis makes use of 19.4 fb−1 proton-proton collision data at
√
s = 8 TeV. The
data have been recorded with the CMS experiment. Complementary methods have been
used for the background estimation, which when combined achieve a total uncertainty of
5% (10%) for leptons in the central (forward) rapidity of the detector. We do not observe
a statistically significant signal and the results are consistent with the background-only
hypothesis.
A search for new physics in final states with an opposite-sign same-flavor lepton pair (dilepton),
jets and missing transverse energy (MET) is presented here. We search for evidence of production
of a new heavy neutral particle, which is produced in the cascade of a strongly interacting heavier
resonance. This report has been compiled from results that have been published here [1]. We use
proton-proton collision data at
√
s = 8 TeV. The total accumulated luminosity is 19.4 fb−1. The data
have been recorded with the CMS experiment [2].
The invariant mass distribution of the dilepton system can exhibit a rise that increases with dilepton
mass followed by a sharp cut-off, resembling the shape of a triangle with an edge, if the two leptons
originate from the decay of a heavy neutral particle. In Supersymmetry (SUSY), such a signal is
exemplified by the decay of the heavy neutralino (χ̃2
0
), which is produced within the cascade of a
squark (q̃) – the SUSY partner of a quark. An example cascade is depicted in Fig. 1. The cascade
can also start from a gluino, the SUSY partner of the gluon, if that is heavier than the squark resulting
to an additional energetic jet in the final state. In either cases, the location of the dileptonic mass
endpoint is given by Eq. 1.
mmax = mχ̃02
√√⎛⎜⎜⎜⎜⎜⎜⎝1 −
⎛⎜⎜⎜⎜⎜⎝ m̃mχ̃0
2
⎞⎟⎟⎟⎟⎟⎠
2
⎞⎟⎟⎟⎟⎟⎟⎠
⎛⎜⎜⎜⎜⎝1 −
(
mχ̃0
1
m̃
)2⎞⎟⎟⎟⎟⎠ (1)
If the mass difference between the two neutralinos is small, or the mass of the slepton (̃) is large,
the χ̃2
0
decay can proceed through a virtual Z0∗ boson exchange, resulting to a direct 3-body decay.
The endpoint in that case is given by the mass difference of the two neutralinos, mmax

= mχ̃0
2
− mχ̃0
1
and the invariant mass distribution of the dilepton system is not anymore an exact triangle and it has
more rounded shape. If there is sufficient energy to produce a Z0 boson, i.e. when mχ̃0
2
− mχ̃0
1
∼ mZ
this will be preferred against a possible 3-body decay and the dilepton mass distribution will exhibit
ae-mail: konstantinos.theofilatos@cern.ch
 
 
DOI: 10.1051/
C© Owned by the authors, published by EDP Sciences, 2015
/
0 0
201epjconf
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0 059
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5
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 (2015)
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Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20159504066
q ± ∓
q̃ χ̃0
2 ̃
∓ χ̃0
1
invisible
Figure 1. An example SUSY process that will lead to an excess of events in the invariant mass distribution of
the two leptons m. The invariant mass distribution of the two leptons will exhibit a threshold effect reaching a
maximum endpoint. The location of the endpoint solely depends on the masses of the particles involved and can
be used to constrain different physics models.
an excess of events at mZ ∼ 91 GeV. Furthermore, similar cascades involving higher mass neutralino
eigenstates mχ̃0
x=2,3,4
decaying to lighter neutralinos can occur producing similar topologies.
Measuring the possible different endpoints and their σ × BR is a way to constrain candidate new
physics models with data [3]. In addition, since the above discussion was only based on kinematic
argumentation, any massive heavy neutral particle (SUSY or not) that decays through the aforemen-
tioned cascades may give rise to similar phenomenology.
In SUSY, the lightest neutralino χ̃0
1
is massive, neutral, weakly interacting and is most often as-
sumed to be stable. This assumption is not a necessity, but is experimentally well motivated and
can explain the evidence of dark matter in the universe. The stability of dark matter particles in the
universe implies the conservation of a new quantum number and thus the dark matter particles are
expected to be always pair produced in a collider, if there is sufficient ŝ available. The stability of
χ̃0
1
can be enforced in SUSY by the R-parity conservation [6]. Being a carbon copy of the SM, we
expect that SUSY particles will be predominantly produced via the strong interaction pp → q̃q̃, g̃q̃
and g̃g̃. In each signal event, two independent cascades will be formed, originating from the decays
of the squarks and gluinos that will decay producing jets and gauge bosons until the lightest SUSY
particle is produced χ̃0
1
. We therefore expect at least two χ̃0
1
in the final state. A stable χ̃0
1
will escape
from detection like a heavy neutrino, producing missing transverse energy in the detector.
Both CMS and ATLAS Collaborations have extensively studied the dilepton signature as a search
strategy before the LHC start-up [4][5]. Previous results of a search for kinematic edge have been
published by CMS at
√
s = 7 TeV [7]. In addition, a search for a Z0 boson accompanied by jets and
MET has been also published by CMS at
√
s = 7 TeV [8]. This update, uses
√
s = 8 TeV data and
unifies the experimental methods deployed earlier in the corresponding CMS 7 TeV papers [7][8].
We select events with two leptons having pT > 20 GeV and |η| < 2.4, with pT denoting the
transverse momentum and η the pseudorapidity. We also count the number of jets produced in the
event having pT > 40 GeV and |η| < 3.0. In the jet counting, we exclude jets that are within ΔR =√
δφ2 + δη2 < 0.4 away from the leptons. We require that the two leptons are well separated (ΔR >
0.3). The signal region is defined as (Njets = 2 ∪ MET > 150 GeV) ∩ (Njets ≥ 3 ∪ MET > 100 GeV),
and the phase space is further subdivided to “central” region (Fig. 2) where both leptons have |η| < 1.4
and “forward” region when at least one lepton lays in the forward region 1.6 < |η| < 2.4 (Fig. 3).
A priori these two regions of the phase space, have different S/B. We expect that the leptons that
will originate from the decay of heavy resonance, will be more centrally distributed in rapidity with
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at
a/
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d
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2
Figure 2. Comparison between the observed and estimated SM background dilepton mass distributions in the
central region, where the SM backgrounds are evaluated from control samples in data.
respect to the standard model (SM) background processes that mimic the same topology. The main
SM background is top pair production,
tt̄ → W+W−bb̄ → +−bb̄νν̄ (2)
which results in equal yields of same-flavor (SF = ee and μμ) and opposite-flavor (OF = eμ) events.
This is due to the lepton universality in the decays of the two W bosons, which decay with equal
probability to all lepton flavors. We use a control sample of OF data that satisfies all signal selection
criteria, but the flavor composition which is SF for the signal and OF for the control sample. The OF
data are corrected for triggering and lepton reconstruction efficiency effects such as to match the SF
selection efficiency. The measured multiplicative correction factor R(SF/OF) = NSF/NOF is consistent
with unit within uncertainties and is very robust against changes in the muon over electron efficiency
ratio (r = εμ/εe). This can be understood by considering that R(SF/OF) = 0.5 × (r + r−1) × RT with
RT a constant factor related to the triggering efficiencies of ee, μμ and eμ paths when they reach their
plateau. It is easy to verify that 0.5 × (r + r−1) stays close to 1.0 even when r varies significantly,
due to cancellation efficiency effects when combining electron and muons channels to form the SF
sample. We measure the efficiency correction factors and their uncertainty, by two complementary
methods, which are detailed in [1]. The results of the two methods are consistent and the two methods
are finally combined. The final uncertainty of R(SF/OF) is 4% (7%) for the central (forward) region.
Another important source of background comes from Drel–Yan (DY) events, which could exhibit
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50 100 150 200 250 300
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at
a/
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gn
d
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1
2
Figure 3. Comparison between the observed and estimated SM background dilepton mass distributions in the
forward region, where the SM backgrounds are evaluated from control samples in data.
large apparent missing energy due to detector resolution and reconstruction effects. This background
is not flavor symmetric and therefore the OF control sample is not alone sufficient to cope with it. We
deploy two independent methods to estimate the DY background, the MET templates and the Jet-Z-
Balance (JZB) method. The former uses photon+MET data reweighed to match the DY kinematics
while the latter exploits the sign symmetry of the JZB variable, which is an estimate of the momentum
imbalance in DY events [8]. The total uncertainty on the background is 5% (10%) for the central
(forward) region. The region of phase space with moderate MET and hadronic activity HT1 that is
probed by this analysis, is not easily accessible to inclusive searches for jets+MET that use MET
and HT triggers. The achieved level of precision allows to search for soft signals that would have
been missed in more inclusive “all-hadronic” final states, which typically suffer from much higher
SM background uncertainties.
A counting experiment is performed for 20 < m < 70 GeV while the search for kinematic
edge is performed in 20 < m < 300 GeV by fitting the signal and background shapes in data
(Fig. 4). The background shape normalization is performed with a Gaussian constraint taking into
account the measured value of R(SF/OF) and its uncertainty. For the signal, we use a triangular shape
assumption, which is the nominal shape expected from kinematics for two sequential two-body decays
as e.g. in SUSY when the decay occurs via an intermediate slepton. The local significance of the
counting experiment is 2.6σ (0.3σ) in the central (forward) signal region. The best fitted value for the
1HT is defined as the scalar sum of pT of all jets
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-2
-1
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Figure 4. Fit results for the signal-plus-background hypothesis in comparison to the measured dilepton mass
distributions, in the central region, projected on the SF event sample. The combined fit shape is shown as a blue,
solid line. The individual fit components are indicated by dashed lines. The flavor-symmetric background is
denoted as FS and is displayed with a black dashed line. The Drell–Yan contribution is denoted as DY and is
displayed with a red dashed line. The extracted signal component is denoted as Signal and is displayed with a
green dashed line.
Table 1. Results of the counting experiment.
Central Forward
Observed data events (Ndata) 860 163
Estimated background (Nb) 730 ± 40 157 ± 16
Ns = Ndata − Nb 130+48−49 6+20−21
Significance [σ] 2.6 0.3
location of the kinematic edge, assuming a triangular signal shape, is at 78.7 ± 1.4 GeV (Fig. 4), with
significance of 2.4σ. The fit is simultaneous to both central and forward signal regions. The reduced
significance of the fit can be understood by the mixing with equal weight of the central and forward
regions. The comparison of data versus simulation of the background and three different benchmark
models is shown in Fig. 5. These reference signal models are based on sbottom pair production with
masses as indicated in the legend. The decay mode of each sbottom quark is b̃ → χ̃0
2
b → χ̃0
1
Z0∗b
with Z0∗ boson being off-shell and decaying to leptons, quark–antiquark and invisible with their SM
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)ττDY+jets (
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tMadgraph t
single-top
Other SM
Scaling Uncert.
JEC & Pileup Uncert.
 = 150 GeV
2
0
χ
∼ = 225 GeV m
b
~m
 = 275 GeV
2
0
χ
∼ = 350 GeV m
b
~m
 = 150 GeV
2
0
χ
∼ = 400 GeV m
b
~m
μμee+
Central Signal Region
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at
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/ M
C
0
0.5
1
1.5
2 Data / Background Only
Data / Background + Signal
Data / Background + Signal
Data / Background + Signal
Figure 5. Data compared with SM simulation in the central signal region. Example signal scenarios based on
the pair production of bottom-squarks is also overlaid.
branching fractions. As a result, about 13% of the signal events contain at least one SF dilepton in
the final state. The mass difference mχ̃0
2
− mχ̃0
1
has been fixed to 70 GeV, producing the characteristic
endpoint expected for a 3-body decay.
The results of the counting experiment are summarized in Tab. 1. In the forward signal region,
which is experimentally more challenging, we record as much data as expected from the SM back-
ground. In the central signal region, although the yield of signal candidates Ns = 130
+48
−49 is sizable, it
is not sufficiently large to reject the background-only hypothesis. The possibility that the observed ex-
cess is solely a background fluctuation of statistical nature, can not be excluded. This analysis probes
a phase space that is not easily accessible to inclusive searches for jets+MET and it does that with
foremost precision. We will have to wait for its repetition with 13 TeV LHC data in order to wipe out
the possibility that this is a first hint of new physics.
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sign same-flavor leptons, jets, and missing transverse energy in pp collisions at
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s = 8 TeV, CMS-
PAS-SUS-12-019, 2014
[2] CMS Collaboration, “The CMS experiment at the CERN LHC”, JINST 3 S08004, 2008
EPJ Web of Conferences
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[3] I. Hinchliffe et al., “Precision SUSY measurements at CERN LHC”, Phys. Rev. D 55 5520-5540
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ICNFP 2014
04066-p.7


We present a search for a kinematic edge in the invariant mass distribution of two opposite-sign same-flavor leptons, in final states with jets and missing transverse energy. The analysis makes use of $19.4$ fb$^{-1}$ proton-proton collision data at $\sqrt{s} = 8$ TeV. The data have been recorded with the CMS experiment. Complementary methods have been used for the background estimation, which when combined achieve a total uncertainty of $5\%$ ($10\%$) for leptons in the central (forward) rapidity of the detector. We do not observe a statistically significant signal and the results are consistent with the background-only hypothesis.We present a search for a kinematic edge in the invariant mass distribution of two opposite-sign same-flavor leptons, in final states with jets and missing transverse energy. The analysis makes use of $19.4$ fb$^{-1}$ proton-proton collision data at $\sqrt{s} = 8$ TeV. The data have been recorded with the CMS experiment. Complementary methods have been used for the background estimation, which when combined achieve a total uncertainty of $5\%$ ($10\%$) for leptons in the central (forward) rapidity of the detector. We do not observe a statistically significant signal and the results are consistent with the background-only hypothesis.We present a search for a kinematic edge in the invariant mass distribution of two opposite-sign same-flavor leptons, in final states with jets and missing transverse energy. The analysis makes use of 19.4 fb^-1 proton-proton collision data at sqrts = 8 TeV. The data have been recorded with the CMS experiment. Complementary methods have been used for the background estimation, which when combined achieve a total uncertainty of 5% (10%) for leptons in the central (forward) rapidity of the detector. We do not observe a statistically significant signal and the results are consistent with the background-only hypothesis

Search for a dileptonic edge with CMS

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