JHEP02(2016)062
Published for SISSA by Springer
Received: November 18, 2015
Accepted: January 18, 2016
Published: February 9, 2016
A search for prompt lepton-jets in pp collisions at
√
s = 8 TeV with the ATLAS detector
The ATLAS collaboration
E-mail:
atlas.publications@cern.ch
Abstract: A search is presented for a new, light boson with a mass of about 1 GeV and
decaying promptly to jets of collimated electrons and/or muons (lepton-jets). The analysis
is performed with 20.3 fb
−1
of data collected by the ATLAS detector at the Large Hadron
Collider in proton-proton collisions at a centre-of-mass energy of 8 TeV. Events are required
to contain at least two lepton-jets. This study finds no statistically significant deviation
from predictions of the Standard Model and places 95% confidence-level upp er limits on the
contribution of new phenomena beyond the SM, incuding SUSY-portal and Higgs-portal
models, on the number of events with lepton-jets.
Keywords: Hadron-Hadron scattering
ArXiv ePrint:
1511.05542
Open Access, Copyright CERN,
for the benefit of the ATLAS Collaboration.
Article funded by SCOAP
3
.
doi:10.1007/JHEP02(2016)062
JHEP02(2016)062
Contents
1 Introduction
2
2 The ATLAS detector 2
3 Signal models 3
3.1 SUSY-portal lepton-jet MC simulation 4
3.2 Higgs-portal lepton-jet MC simulation 5
4 Pre-selection of events 6
4.1 Track selection 7
5 Selection of lepton-jets 7
5.1 Lepton-jet definition 7
5.2 Lepton-jet reconstruction 8
5.3 Lepton-jet reconstruction efficiency 9
5.4 Background rejection at the lepton-jet level 10
5.4.1 eLJ variables 11
5.4.2 muLJ variables 12
5.4.3 emuLJ variables 13
5.4.4 LJ variables optimization 13
6 Background estimation at the event level 14
6.1 Low-mass Drell-Yan 14
6.2 Background estimation with the ABCD-likelihood method 15
7 Systematic uncertainties 16
8 Observed events in data and background estimation 19
9 Interpretation and limits 20
10 Conclusions 24
A Expected number of events in MC and 90% CL upper limits on the
expected and observed number of signal events
27
The ATLAS collaboration 35
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JHEP02(2016)062
1 Introduction
In several models of physics beyond the Standard Model (SM) [1–5], the so-called dark mat-
ter (see e.g. ref. [
6] and references therein) is charged under a non-Abelian, dark-sector,
gauge symmetry that is broken at an energy scale O(1 GeV). T he dark-sector ground state
can transition to and from excited states via the emission of a dark gauge boson, referred to
as the dark photon (γ
d
), that couples very weakly to the SM particles via kinetic mixing [
7]
with the SM photon. In these models, the Large Hadron Collider (LHC) could produce
excited dark-sector states via their interactions with particles found in models of super-
symmetry (SUSY) [
1, 3] or with Higgs scalar bosons [4, 5] (here referred to as SUSY-portal
and Higgs-portal models, respectively), which then decay via the emission of dark photons.
If dark photons carry masses of O(1 GeV), then the dark photon produced from the decay
chain of heavier particles such as the SM Higgs boson or SUSY particles would be highly
boosted. Depending on its mass, the dark photon would decay primarily into a collimated
pair of leptons or light hadrons. The leptonic final-state is experimentally easier accessi-
ble, offering a distinct signature that stands out amongst large hadronic backgrounds. A
collimated set of energetic leptons is referred to as a lepton-jet (LJ).
A search is carried out for final-states with two prompt lepton-jets using data accumu-
lated in proton-proton collisions at a centre-of-mass energy
√
s = 8 TeV with the ATLAS
detector [
8]. Many new physics models predict at least two lepton-jets in the final-states
as described in refs. [
3, 4]. The analysis focuses on the presence of lepton-jets and does not
rely on the rest of the event topology. The dark-photon decay width, Γ
ℓ
, and the kinetic
mixing parameter, ǫ, are related through
Γ
ℓ
=
1
3
αǫ
2
m
γ
d
s
1 −
4m
2
ℓ
m
2
γ
d
1 +
2m
2
ℓ
m
2
γ
d
, (1.1)
where α is the fine structure constant and m
γ
d
and m
ℓ
denote the masses of the dark
photons and charged leptons, respectively [
9, 10]. The analysis focuses on dark photons with
prompt-decays, i.e. consistent with zero decay length within the experimental resolution.
Previous searches for prompt lepton-jets, with ATLAS data at
√
s = 7 TeV, resulted in
upper limits on the production of two lepton-jets in a SUSY-portal model [
11] and for a
Higgs-portal model [
12]. The CMS and D0 collaborations also set upper limits on prompt
lepton-jet production [
13–17]. Related searches for non-prompt lepton-jets [18] have been
performed by ATLAS and have set constraints on smaller values of the kinetic mixing
parameter, ǫ ≤ 10
−5
. There are additional constraints on the kinetic mixing parameter
and dark-photon mass, e.g. from beam-dump and fixed target experiments [
9, 19–27], e
+
e
−
collider experiments [
28–33], electron and muon magnetic moment measurements [34, 35]
and astrophysical observations [
36, 37].
2 The ATLAS detector
ATLAS is a multi-purpose detector [
8] consisting of an inner tracking detector (ID), electro-
magnetic and hadronic calorimeters and a muon spectrometer (MS) that employs toroidal
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JHEP02(2016)062
magnets. The ID provides precision tracking of charged particles for pseudorapidity
1
|η| < 2.5 using silicon pixel and silicon microstrip (SCT) detectors and a straw-tube tran-
sition radiation tracker (TRT) that relies on transition radiation to distinguish electrons
from pions in the range |η| < 2.0.
The sensors of the pixel detector have a typical pixel size of 50 ×400 µm and typically
provide three spatial measurements along the track of a charged particle. The innermost
layer with a radial distance to the beamline of about 5 cm is known as the B-layer. The
SCT has sensors with a strip pitch of 80 µm and provides eight measurements for a typical
track. The fine-grained sensors of the semiconductor trackers permit the reconstruction of
the closely aligned tracks of lepton-jet candidates (section
5.1).
The liquid-argon (LAr) electromagnetic (EM) sampling calorimeters cover the range
|η| < 3.2. The calorimeter’s transverse granularity, typically ∆η × ∆φ of 0.025 × 0.025,
and three-fold shower-depth segmentation are used to construct discriminating variables
for evaluating the electromagnetic character of lepton-jet candidates (section
5.4).
A scintillator-tile calorimeter, divided into a barrel and two extended-barrel cylinders,
on each side of the central barrel, provides hadronic calorimetry in the range |η| < 1.7,
while a LAr hadronic end-cap calorimeter provides coverage over 1.5 < |η| < 3.2. The LAr
forward calorimeters provide both, electromagnetic and hadronic energy measurements,
and extend the coverage to |η| ≤ 4.9. The calorimeter system has a minimum depth of
9.7 nuclear interaction lengths at η = 0. The MS is a large tracking system, consisting of
three parts: a magnetic field provided by three toroidal magnets, a set of 1200 chambers
measuring with high spatial precision the tracks of the outgoing muons, a set of triggering
chambers with accurate time-resolution. It covers |η| < 2.7 and provides precision tracking
and triggering for muons.
ATLAS has a three-level trigger system. The Level 1 (L1) trigger is implemented in
hardware, and it uses information from the calorimeters and muon spectrometer to reduce
the event rate to 75–100 kHz. The software-based Level 2 (L2) trigger and the Event Filter
(EF) reduce the event rate to 300–500 Hz of events that are retained for offline analysis.
The L1 trigger generates a list of region-of-interest (RoI) η–φ coordinates. The muon RoIs
have a spatial extent of 0.2 in ∆η and ∆φ in the MS barrel, and 0.1 in the MS endcap.
Electromagnetic calorimeter RoIs have a spatial extent of 0.2 in ∆η and ∆φ. For the
L2 trigger the reconstruction is mostly based on simplified algorithms running on data
localized in the RoI which was reported by L1. At the EF level the trigger system has
access to the full event for processing.
3 Signal models
Two benchmark models are used to interpret the data. In the SUSY-portal model (sec-
tion
3.1), a pair of squarks is produced and the cascade decays of the squarks include dark-
1
ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP)
in the centre of the detector. The z-axis points along the beam pipe. The x-axis points from the IP to
the centre of the LHC ring, an d the y axis points upward. Cylindrical coordinates (r, φ) are used in the
transverse plane, φ being the azimuthal angle around the beam pipe. Pseudorapidity is defined in terms of
the polar angle θ as η = − ln tan(θ/2).
– 3 –
JHEP02(2016)062
g
q
q
q
i
q
i
q
j
q
j
χ
1
o
χ
1
o
χ
d
χ
d
γ
d
γ
d
l
+
l
+
l
-
l
-
g
q
q
q
i
q
i
q
j
q
j
χ
1
o
χ
1
o
χ
d
χ
d
γ
d
γ
d
s
d
s
d
γ
d
γ
d
l
+
l
-
l
+
l
-
l
+
l
-
l
-
l
+
Figure 1. Feynman diagram illustrating the dark-photon production in the 2γ
d
final-state (left),
and 4γ
d
final-state (right).
sector particles and one or more dark photons. In the Higgs-portal model (section
3.2),
the SM Higgs boson decays into a pair of dark fermions, each of which decays into one or
more dark photons in cascades. For both models, the dark photons decay into lepton pairs,
that can be reconstructed as a lepton-jet, or light hadrons, depending on the branching
fractions. Monte Carlo (MC) simulated samples are produced for the two models. All sig-
nal MC events are processed with the Geant4-based ATLAS detector simulation [
38, 39]
and then analysed with the standard ATLAS reconstruction software. The branching ratio
(BR) values for the dark-photon decays to leptons are taken from ref. [4]. In all signal
models used to interpret the results the dark photons are required to decay promptly with
mean life time (cτ ) close to zero. For the Higgs-portal model, long-lived dark photon sam-
ples with cτ = 47 mm are used to extrapolate the signal efficiency of zero cτ dark photons
to non-zero cτ dark photons (section
9).
3.1 SUSY-portal lepton-jet MC simulation
A benchmark SUSY model [
3] is used to simulate SUSY production of dark-sector particles
and dark photons. Simulated samples are produced in several steps. Squark (˜q) pair
events are generated with Madgraph [
40], version 5, in a simplified model with light-
flavour squark pairs with decoupled gluinos [41, 42].
2
Then Bridge [44], interfaced with
Madgraph, simulates squark decays into neutralinos. The neutralinos decay into dark-
sector particles, which decay to SM particles as shown in figure
1. The squarks are set to
decay with a 100% BR into a quark and a neutralino ( ˜χ
0
1
). The neutralinos decay into dark-
sector particles in two ways: ˜χ
0
1
→ γ
d
˜χ
d
or ˜χ
0
1
→ s
d
˜χ
d
, where s
d
is a dark scalar particle
that decays to γ
d
γ
d
and ˜χ
d
is a dark neutralino. In this model, the stable dark-matter
particle is the dark neutralino which is invisible in the detector. For fragmentation and
hadronization Pythia 8[
45, 46] is used, with the CTEQ6L1 1 [47] PDF parton distribution
function (PDF) set, and the AUET2 [48] set of tuned parameters.
As the dark-sector is loosely constrained experimentally, the squark mass, the dark-
photon mass, and all intermediate masses are chosen to correspond to well-motivated nom-
2
This is the same simplified model used in a previous ATLAS search and shown in the third plot of
figure 10 in ref. [
43]. In the analysis context, the fact that gluinos are decoupled implies the 2 → 2
production, such that there are two SUSY particles at the hard scatter producing two lepton-jets per event.
–4–