A search for prompt lepton-jets in pp collisions at root s=7 TeV with the ATLAS detector

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)

CERN-PH-EP-2012-319

Submitted to: Phys. Lett. B

A search for prompt lepton-jets in pp collisions at

√

s

= 7 TeV

with the ATLAS detector

The ATLAS Collaboration

Abstract

We present a search for a light (mass < 2 GeV) boson predicted by Hidden Valley supersymmetric

models that decays into a final state consisting of collimated muons or electrons, denoted “lepton-jets”.

The analysis uses 5 fb

−1

of

√

s

= 7 TeV proton–proton collision data recorded by the ATLAS detector

at the Large Hadron Collider to search for the following signatures: single lepton-jets with at least four

muons; pairs of lepton-jets, each with two or more muons; and pairs of lepton-jets with two or more

electrons. This study finds no statistically significant deviation from the Standard Model prediction and

places 95% confidence-level exclusion limits on the production cross section times branching ratio of

light bosons for several parameter sets of a Hidden Valley model.

A search for prompt lepton-jets in pp collisions at

√

s

= 7 TeV with the ATLAS

detector

ATLAS Collaboration

Abstract

We present a search for a light (mass < 2 GeV) boson predicted by Hidden Valley supersymmetric models that decays into a final state consisting of collimated muons or electrons, denoted “lepton-jets”. The analysis uses 5 fb−1 _of

√

s = 7 TeV proton–proton collision data recorded by the ATLAS detector at the Large Hadron Collider to search for the following signatures: single lepton-jets with at least four muons; pairs of lepton-jets, each with two or more muons; and pairs of lepton-jets with two or more electrons. This study finds no statistically significant deviation from the Standard Model prediction and places 95% confidence-level exclusion limits on the production cross section times branching ratio of light bosons for several parameter sets of a Hidden Valley model.

1. Introduction

A light boson at the GeV scale, in a model where a Hidden Valley sector is weakly coupled to the Standard Model (SM) sector [1–3], has been proposed to explain several recently observed anomalies in cosmic-ray and dark matter direct-detection experiments. These obser-vations include an unexpected excess of cosmic elec-trons and/or positrons [4–7] and signals from certain dark matter direct-detection experiments [8–10]. The proposed boson could be created at particle accelera-tors and produce distinctive final states of tightly colli-mated “lepton-jets” consisting of close by electrons or muons [11–15]. Such lepton-jet decays are also a gener-ically interesting signature that may be produced by rare decays of, for instance, Z or Higgs bosons [16]. Upper limits on lepton-jet production have already been set by previous analyses of collider data [17, 18].

In Hidden Valley models, the universe consists of SM and supersymmetric (SUSY) particles, together with an additional spectrum of dark matter particles charged un-der a hidden gauge group (called the dark sector). Cer-tain particles called messengers are charged under both the dark sector and the SM and SUSY gauge symme-tries, permitting decay chains through the normal and dark sectors. For example, the lightest supersymmetric particle, which cannot decay to SM particles due to R-parity conservation, can decay into less-massive dark-sector states ending with the lightest particle in the dark sector, a dark photon denoted γD. This dark photon can

decay into light SM fermions by kinetic mixing [19]

of the dark gauge sector and SM gauge symmetries. These models aim to explain the excess of cosmic-ray positrons, in the absence of any observed proton ex-cess, with a dark boson γD that has a mass below the

proton–antiproton kinematic threshold of ∼2 GeV. Such low-mass dark photons can decay to electrons, muons, and pions, whereas decays to protons are kinematically forbidden. Due to the boost of the γD, the light SM

de-cay products are highly collimated, providing a striking signature for new physics.

The data is interpreted in a model where a pair of squarks is produced and each of the squarks cascade de-cays into dark-sector particles, including one or more dark-photons. The dark-photons decay into pairs of lep-tons, forming lepton-jets. Additionally, dark-sector par-ticles may radiate multiple dark photons, increasing the lepton multiplicities and number of the lepton-jets [16]. The amount of radiation is determined by the dark sec-tor gauge coupling parameter αd. Setting αd = 0.0

re-sults in a simple lepton-jet with two hard leptons. Larger values of αd may produce lepton-jets with four, six,

eight, or more prompt leptons from the decay of over-lapping dark photons, albeit with reduced boost. The transverse momentum (pT) of the leptons increases with

dark photon mass, but decreases with αd. This paper

considers values of αd of 0.0, 0.1, and 0.3, and dark

photon masses (mγD) of 150, 300, and 500 MeV. For

mγD = 150 MeV, the dark photon is below the muon–

antimuon threshold and can only decay to electrons. With mγD ≥ 300 MeV, the dark photon decays to

20% of the decays produce pion pairs. These nine signal operating points cover a wide range of phase space from low-multiplicity lepton-jets containing leptons of only one flavour, to high-multiplicity lepton-jets containing a mix of electrons and muons.

The data samples used in this analysis were col-lected with the ATLAS detector during the 2011 run of the Large Hadron Collider at centre-of-mass energy √

s= 7 TeV and correspond to 4.5 fb−1of integrated lu-minosity for the muon analyses and 4.8 fb−1for the elec-tron analysis [20, 21], after their respective data quality requirements have been applied. This paper considers lepton-jets in three signatures: single muon-jets with four or more muons, pairs of muon-jets each with two or more muons, and pairs of electron-jets each with two or more electrons. The selection is designed to enhance the signal relative to the SM backgrounds, the largest of which is multi-jet production. In multi-jet production the background arises from either real leptons from the decay of SM particles, from hadrons that are misidenti-fied as leptons, or in the case of electrons, from photon conversions. All other SM background sources are ex-pected to be negligible after the final selection cuts are applied. The multi-jet background is reduced through a variety of selection cuts, and the remaining background is estimated with two different data-driven techniques.

No requirements are made on the remaining activity in the event beyond the one or two lepton-jets in order to avoid introducing a strong model dependence in the analysis. For example, no cuts are made on the presence of other particles or jets, the event track multiplicity, or the presence of missing transverse energy.

2. The ATLAS detector

ATLAS is a general purpose detector [22] consist-ing of an inner trackconsist-ing detector (ID) embedded in a 2 T solenoid, electromagnetic and hadronic calorime-ters and a muon spectrometer (MS) employing toroidal magnets. The ID provides precision tracking of charged particles for |η| < 2.5 using silicon pixel and microstrip detectors and a straw-tube transition radiation tracker (TRT) that relies on transition radiation to distinguish electrons from pions in the range |η| < 2.0. Liquid-argon (LAr) electromagnetic sampling calorimeters, with excellent energy and position resolution, cover the range |η| < 3.2 with a typical granularity of∆η × ∆φ of 0.025 × 0.025. A scintillator-tile calorimeter, which is divided into a large barrel and two smaller extended-barrel cylinders, one on each side of the central bar-rel, provides hadronic calorimetry in the range |η| < 1.7. In the end-caps (|η| > 1.5), LAr is also used

for the hadronic calorimeters, matching the outer |η| limit of end-cap electromagnetic calorimeters. The LAr forward calorimeters provide both electromagnetic and hadronic energy measurements, and extend the cover-age to |η|= 4.9. The calorimeter system has a minimum depth of 9.7 interaction lengths at η= 0. The MS covers |η| < 2.7 and provides triggering and precision tracking for muons.1

A three-level trigger system is used to select events. The Level 1 (L1) trigger is implemented in hardware and uses information from the calorimeters and muon sub-detectors to reduce the event rate to a design value of at most 75 kHz. This is followed by two software-based trigger levels, Level 2 (L2) and Event Filter (EF), which together reduce the event rate to 300 Hz on av-erage. The L1 trigger generates a list of Regions of In-terest (RoI) η–φ coordinates with associated thresholds. The muon RoI have a spatial extent of 0.2 in∆η and ∆φ in the MS barrel, and 0.1 in the MS endcap. The electro-magnetic calorimeter RoI have a spatial extent of 0.2 in ∆η and ∆φ. At L2, most reconstruction uses simplified algorithms running on data localized to an RoI which was reported by L1. At the EF level, the trigger system has access to the full event for processing.

3. Event reconstruction and selection

The analysis used only data from stable running peri-ods, and required events to have a primary collision ver-tex containing at least three tracks with pT > 400 MeV

in order to remove cosmic rays.

3.1. Electron-jet channel

Events containing electron-jets were selected using single-electron triggers with an online pT threshold of

20 or 22 GeV, the latter being used after there was a sub-stantial increase in the instantaneous luminosity during 2011. To ensure proper modelling of the trigger accep-tance, events were required to contain at least one recon-structed electron with pT > 35 GeV, above which the

trigger efficiency is constant. The reconstructed electron was required to match an electron reconstructed above the pTthreshold in the trigger system with a separation

in R (∆R ≡ p(∆φ)2+ (∆η)2_{) less than 0.2.}

1_{ATLAS uses a right-handed coordinate system with its origin at}

the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle θ as η= − ln tan(θ/2).

The electron-jet candidates were built from elec-tromagnetic clusters with minimum transverse energy ET > 10 GeV inside the calorimeter fiducial region

(|η| < 2.47, excluding the barrel/end-cap transition re-gion 1.37 < |η| < 1.52 where there is substantial dead material that is difficult to model accurately). At least two tracks from the primary vertex (transverse impact parameter |d0| < 1 mm) having pT > 10 GeV were

re-quired to have∆R < 0.1 of the cluster position in the second sampling layer of the calorimeter. Additional requirements were made on the number of hits along the track in the silicon pixel and silicon microstrip de-tectors to suppress backgrounds from photon conver-sions. The analysis required two lepton-jet candidates in each event, with one cluster matching the electron re-constructed in the trigger system. The invariant mass of the two highest-pTtracks associated with each

electron-jet had to be less than 2 GeV.

The background for the electron-jets analysis comes primarily from multi-jet events, and to a lesser extent from photon+jet events. Five variables were used to re-duce the remaining background for electron-jet candi-dates. The electron cluster energy concentration shown in Fig. 1(a), Rη2, must exceed 0.92. Rη2 is defined as the ratio of total energy in 3 × 7 cells to the total en-ergy in 7 × 7 cells in η–φ in the second sampling layer of the electromagnetic calorimeter. The electron cluster lateral shower width in the calorimeter, wη2, shown in

Fig. 1(b), must be less than 0.0115, where

wη2= s P iEi×ηi2 P iEi − P iEi×ηi P iEi !2 . (1) Here Eiand ηirepresent the energy and pseudorapidity

of the ith _{cell in a 3 × 5 η − φ window in the second}

sampling layer of the electromagnetic calorimeter. The ratio of the number of high-threshold hits [22], indica-tive of transition radiation, to the total number of hits from the TRT associated with each track, fHT, was

re-quired to be greater than 0.05 to remove pions. The fHT

distribution per track is shown in Fig. 1(c). The sharp peak at zero arises from tracks matched to an electron candidate outside of the TRT acceptance. A scaled iso-lation variable is defined as the transverse energy within 0.1 < ∆R < 0.4 around the cluster divided by cluster ET; events were required to have scaled isolation below

30% as shown in Fig. 1(d). Finally, a requirement that the fraction of the lepton-jet energy found in the elec-tromagnetic calorimeter, fEMmust be larger than 0.98,

was used to reject activity from hadrons, as shown in Fig. 1(e).

3.2. Muon-jet channels

Single muon-jet events were selected from events sat-isfying a trigger with a single muon having more than 18 GeV in pT. Candidates for double muon-jets were

taken with either a single-muon trigger with a pT

thresh-old of 18 GeV or a three-muon trigger with a pT

thresh-old of 6 GeV. The muon triggers were complemen-tary, as the three-muon trigger has reduced efficiency for high-pT muons from a single lepton-jet which may

be too close together to produce more than one RoI. Muon candidates must have been reconstructed in both the ID and the MS and have |η| < 2.5. Addi-tional requirements were made on the number of asso-ciated hits in the silicon pixel and microstrip detectors, as well as on the number of track segments in the MS. The muons were required to come from the primary ver-tex by imposing a |d0| < 1 mm cut on the tracks. The

muon-jets were reconstructed in an iterative procedure using all candidate muons, by seeding the jet candi-date with the highest-pT muon, and adding all muons

within ∆R = 0.1. Additional jets were formed using the remaining muons, again seeding the muon-jet with the remaining highest-pTmuon. For the double

muon-jet analysis, two muons with pT > 11 GeV were

re-quired per jet with the additional requirement that the leading muon pT be greater than 23 GeV for the

sin-gle muon trigger events. For the sinsin-gle muon-jet analy-sis, four muons were required per jet with pT > 19, 16,

14 GeV, respectively, for the three highest-pT muons,

and pT > 4 GeV for all additional muons.

Within a muon-jet, the two muons closest in pTwere

required to have an invariant mass less than 2 GeV. A scaled isolation variable was formed by summing the ET of all calorimeter cells within∆R = 0.3 of any of

the muon-jet’s component muons while excluding cells found within∆R = 0.05 of the muons, and dividing by the muon-jet pT. The scaled isolation was required to be

less than 0.3 (0.15) per muon-jet for the double (single) muon-jet analyses, to suppress muons from hadronic jets.

As noted earlier, a signature of the dark matter sig-nal is a muon-jet composed of two or more muon tracks confined to a narrow cone. One source of collimated muons arises from the decay of low-mass states, since the opening angle is in inverse proportion to the Lorentz boost. The background from boosted low-mass states with an invariant mass less than 3.5 GeV is displayed in Fig. 2 showing the opening angle ∆R vs invariant mass for all dimuon pairs. This plot was produced using the same muon selection used for the muon-jet analy-sis, excluding the∆R requirement. The invariant mass

Cluster Energy Concentration 0.4 0.5 0.6 0.7 0.8 0.9 1 Events / 0.01 0 2 4 6 8 10 12 14 16 18 20 3 10 × Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data ATLAS -1 L dt = 4.8 fb

∫

= 7 TeV s (a)

Electron Cluster Lateral Shower Width 8 10 12 14 16 18 20 22 24 -3 10 × Events / 0.0005 0 2 4 6 8 10 12 14 16 18 20 22 3 10 × Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data ATLAS -1 L dt = 4.8 fb

∫

= 7 TeV s (b)

Fraction of High Threshold TRT Hits 0 0.1 0.2 0.3 0.4 0.5 0.6 Events / 0.02 0 0.05 0.1 0.15 0.2 0.25 6 10 × Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data ATLAS -1 L dt = 4.8 fb

∫

= 7 TeV s (c) Calorimeter Isolation 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Events / 0.02 0 2 4 6 8 10 12 3 10 × Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data ATLAS -1 L dt = 4.8 fb

∫

= 7 TeV s (d)

Fraction of Energy in the EM Calorimeter 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 Events / 0.01 2 10 3 10 4 10 5 10 Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data Z MC + Jets MC γ Multi-jet MC Signal MC Data ATLAS -1 L dt = 4.8 fb

∫

= 7 TeV s (e)

Figure 1: Distributions of the five discriminating variables used after selection of events which have passed the trigger and contain two or more electron-jet candidates, shown separately for the multi-jet, Z → ee and γ+ jets backgrounds as well as for the signal sample with αd = 0.0 and mγD = 150 MeV. The signal MC normalization is arbitrary. The

dashed black vertical line shows the cut values at (a) cluster energy concentration Rη2 ≥ 0.92, (b) electron cluster lateral shower width wη2≤ 0.0115, (c) fraction of high threshold TRT hits fHT≥ 0.05, (d) calorimeter isolation ≤ 0.3,

and (e) fraction of energy in the EM calorimeter fEM ≥ 0.98. The hadronic jet and γ+ jets distributions are shown

here from Pythia MC for illustrative purposes.

of muon pairs falls off smoothly, interrupted by easily observable narrow peaks produced by low-mass reso-nances such as φ (∼1 GeV), and ω and ρ (∼0.7 GeV). For smaller opening angles,∆R . 0.03, the low-mass resonances barely stand out from the rest of the back-ground. It was not practical to exclude the ω/ρ and φ peak regions from the analysis. However, the J/ψ was removed for the electron-jet and muon-jet search by a 2 GeV mass cut. A second smoothly falling distribution is also visible in this figure from events with an additional three or more muons, one of which has a high enough pT to fire the trigger, producing an additional

combina-torial background.

4. Signal and background estimation

Both MC and data-driven methods were used for background and efficiency estimations. Various SM processes can mimic the signal due to misreconstructed objects, such as jets misidentified as electrons, or chance overlap of leptons. We have considered MC hadronic multi-jet events, γ+ jets events, W → `ν+ jets, Z →`+`−_{+ jets, t¯t and diboson (WW, WZ, ZZ) events}

at √s = 7 TeV. Pythia6 [23] was used for all sam-ples except t¯t, WW, WZ, ZZ for which MC@NLO [24] was used. The contribution from WZ and ZZ back-grounds, when one of the bosons is off-shell, was mod-eled with Sherpa [25]. Of all the backgrounds

consid-Figure 2: The opening angle∆R vs invariant mass for all muon pairs in the 4.5 fb−1_{data sample. The}

over-laid red points show the profile of the∆R distribution for each dimuon mass bin. The position of each point is the mean value of the vertical slice and its width is the RMS. The secondary distribution running along the top of the distribution arises from events with more than two muons, where a third muon triggers at a higher pT,

allowing for combinations of dimuon pairs with a larger opening angle.

ered, only the hadronic multi-jet and γ + jets events contribute significantly to the final background expec-tation. In addition, signal MC simulation was generated using MadGraph [26] with the CTEQ6L1 set of parton distribution functions [27], and a custom-made Mathe-matica [28] package to model the dark-sector cascade decay described in Refs. [11, 16], followed by Pythia6 for hadronization. All MC samples include the effect of multiple pp interactions per bunch crossing and are assigned an event weight such that the distribution of the number of pp interactions matches that in data. The mean momentum of the dark photons depends strongly on αd and therefore the acceptance of the lepton-jets

also depends on this parameter. At αd = 0.0 the mean

momentum of the dark photon is 73, 76, and 82 GeV for mγD = 150, 300, and 500 MeV, respectively, with

no cuts applied. For αd = 0.1, the mean dark photon

momentum decreases to 30.4, 35.9, and 41.6 GeV. At αd= 0.3 the mean values are 21.1, 25.7, and 30.9 GeV.

All MC events were processed with the Geant4 based ATLAS detector simulation [29, 30] and then analyzed

with the standard ATLAS reconstruction software. Due to the very small acceptance for hadronic jets passing our signal criteria, O(10−3_{) to O(10}−4_{) for jets}

with 50 < pT < 400 GeV, there were too few MC

events to accurately estimate background yields. The background MC samples were used to help establish the event selection criteria, based on characteristics of the background. All the samples were required to satisfy the trigger conditions, with efficiencies ranging from 40% to 75% for the lepton-jet models considered.

4.1. Background estimation with the ABCD-likelihood method

In the lepton pT and dilepton invariant mass ranges

relevant to this study, the level of the background is best estimated using a data-driven method, rather than by MC simulation where the number of events is low and the backgrounds may be poorly modeled. This pa-per uses an ABCD-likelihood method to determine the lepton-jet backgrounds which was cross-checked with a tag-and-probe fake-rate estimate. The traditional im-plementation of the ABCD method consists of using two uncorrelated or loosely correlated variables from the event selection to define four regions labeled A, B, C and D, as illustrated in Fig. 3. The background in the signal region is estimated by taking the ratio of events in the adjacent regions. This method breaks down in the presence of significant signal contamination in the side-band regions, or when there are too few events. The ABCD-likelihood method addresses both of these issues. A likelihood function, formed from the product of Poisson probability functions describing the signal and background expectations, is fit to all four of the re-gions simultaneously.

The likelihood takes the form:

L(nA, nB, nC, nD|µ, θµ)= Y i=A,B,C,D e−µiµni i ni! (2)

where nA, nB, nC, and nDare the numbers of events

observed in each of the four regions, and µA, µB, µC,

and µDare linear combinations of signal (µ) and

multi-jet background (µU) expectations. In region A, the ex-pected number of events µA = µU+ µ. In region B,

µB = µUτB+ µb, where τB is the ratio of background

events expected in region B to that in region A and b gives the signal contamination in region B. Similarly, the expected number of events in region C is expressed as µC= µUτC+µc. In region D, µD= µUτBτC+µd, such

that the multi-jet background contribution is determined using the product of the ratios. The signal contamina-tion coefficients are taken from MC simulation for each

signal sample, while µU_{and the τ}

ivalues are allowed to

float in a simultaneous fit to the four data regions. For the electron-jet analysis, the ABCD-likelihood method used boundaries at Rη2 = 0.92 and fEM = 0.98

on the second highest-ETlepton-jet to define these four

regions. In photon+jet events, the photon will typi-cally deposit more energy in the EM calorimeter than the hadronic jet. Using the subleading cluster to esti-mate the background thus accounts for both the pho-ton+jet and multi-jet backgrounds. The double (single) muon-jet analysis used the scaled isolation variable and the pTcut on the fourth (third) muon in the event,

asso-ciated with a muon-jet. The two-dimensional distribu-tions with the A, B, C and D regions are shown in Fig. 3. In the absence of signal, the numbers of events predicted in region A for the single muon-jet channel, the double muon-jet channel, and the double electron-jet channel are 3.0 ± 1.0, 0.5 ± 0.3, and 15.2 ± 2.7, respectively. The quoted errors are statistical only.

4.2. Background estimation using jet probabilities Both the electron-jet and muon-jet analyses used a tag-and-probe method to cross-check the amount of background in the signal region using back-to-back hadronic jet pairs with pT > 30 GeV and |∆φ| > 2 but

with different selection criteria.

For the electron-jet analysis, the tag was chosen by matching a jet with fEM < 0.9 to a trigger jet. Using

the highest-ETelectromagnetic cluster within∆R = 0.4

of the probe jet as a seed for the electron-jet, the fake rate was extracted from the probe jets that satisfied the electron-jet criteria, as well as the probability for such electron-jets to pass the electron trigger. Jet triggers with different pTthresholds were used to determine the

rates over the full range of probe-jet pT values. These

probabilities were then used to calculate event weights for the inclusive multi-jet MC sample to estimate the number of events which would pass the electron trigger and electron-jet selection requirements. This method predicted 14.55+0.23_−0.04background events after all analy-sis cuts were applied to the data. The quoted error is statistical only.

The double muon-jet analysis used two criteria to se-lect either light-quark or b-quark jets by requiring that either the tag jets contain no muons and no b-tag, or the tag jets have a b-tag. The probe jets were then used to determine the probability that a hadronic jet could sat-isfy the muon selection criteria and the probability that it could satisfy the muon-jet selection criteria, as a func-tion of the probe-jet pT. The ratio of these two

proba-bilities was used in events containing three muons (of which at least two formed a muon-jet and the third was

embedded in a hadronic jet) to estimate the background from multi-jet production, accounting for the flavour of the hadronic jet. This method predicted 2.2 ± 0.9 events from multi-jet production. The quoted error is statistical only.

The fake rates for muon-jets and electron-jets were found to be consistent with those obtained from the ABCD-likelihood method, which were discussed in Section 4.1 and are summarized in Table 1. This cross-check thus validates the background estimates.

5. Results and interpretation

Table 1 shows the number of events passing all anal-ysis cuts compared to the background expectation from the ABCD-likelihood method. A slight excess is ob-served in both the single and the double muon-jet sig-nal regions corresponding to p-values (the probabil-ity the background process would produce at least this many events) of 0.06 and 0.04, respectively. The ac-ceptance times trigger, reconstruction, and selection ef-ficiency for the various signal points are listed in Ta-ble 2. It ranges from about 0.4% to 10% depending on the model parameter αd, the mass of the dark-photon,

and the analysis channel. The estimate of the back-ground from the ABCD-likelihood method has a large statistical error, which reduces the expected sensitiv-ity of the analysis. The systematic uncertainty on the ABCD-likelihood method due to correlation between the variables, 3% (4%) for the single (double) muon-jets channel, is small by comparison.

Table 3 lists the systematic uncertainties on the sig-nal yields. The possible mismodelling of track recon-struction at very small opening angles (“Offline ∆R Ef-ficiency” in Table 3) introduces a ∼10% systematic er-ror on the signal acceptance. The size of the systematic uncertainty on the acceptance was estimated by measur-ing the trackmeasur-ing efficiency using a tag-and-probe method with J/ψ data and MC. For∆R > 0.05 the data and MC agree to within ∼4%. However, a systematic variation of ∼10% is observed in the efficiency for the smaller ∆R region, which is probably due to a slightly softer pT distribution in the MC than in the data. Systematic

errors are also assigned to the determination of the lu-minosity, the modelling of the trigger acceptance, the modelling of the lepton reconstruction efficiency, and the modelling of each of the analysis cuts.

The 95% confidence-level upper limits on the number of expected events from new phenomena producing col-limated pairs of prompt leptons were calculated using the CLs method [31] with a log-likelihood ratio (LLR) test statistic. Ensembles of pseudo-experiments were

EM Fraction 0.7 0.75 0.8 0.85 0.9 0.95 1 2 η R 0.7 0.75 0.8 0.85 0.9 0.95 1 0 0.5 1 1.5 2 2.5 3 3.5 4 ATLAS Data -1 L dt = 4.8 fb

∫

_{s= 7 TeV} A B C D (a)

Lepton-Jet Scaled Isolation 0 0.5 1 1.5 2 2.5 [GeV] T 4th Muon p 0 5 10 15 20 25 30 35 40 45 50 0 0.5 1 1.5 2 2.5 3 ATLAS Data -1 L dt = 4.5 fb

∫

_{s= 7 TeV} A B C D (b) Lepton-Jet Isolation 0 0.5 1 1.5 2 2.5 [GeV] T Third Muon p 0 5 10 15 20 25 30 35 40 45 50 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 ATLAS Data -1 L dt = 4.5 fb

∫

_{s= 7 TeV} A B C D (c) EM Fraction 0.7 0.75 0.8 0.85 0.9 0.95 1 2 η R 0.7 0.75 0.8 0.85 0.9 0.95 1 0.1 1 10 ATLAS Signal MC A B C D (d)

Lepton-Jet Scaled Isolation 0 0.5 1 1.5 2 2.5 [GeV]_T 4th Muon p 0 5 10 15 20 25 30 35 40 45 50 0 1 2 3 4 5 6 7 ATLAS Signal MC A B C D (e) Lepton-Jet Isolation 0 0.5 1 1.5 2 2.5 [GeV]_T Third Muon p 0 5 10 15 20 25 30 35 40 45 50 0 2 4 6 8 10 12 14 ATLAS Signal MC A B C D (f)

Figure 3: Variables used for ABCD-likelihood method from data (top) and MC signal (bottom) using αd= 0.1 and

mγD = 300 MeV for the electron channel and αd = 0.0 and mγD = 300 MeV for the two muon channels. The dashed

black lines show the cuts used to define the four regions. Shown are (left) Electron-jet: Rη2 vs EM fraction; (centre)

double muon-jet: scaled isolation vs pTof the fourth muon; (right) single muon-jet: scaled isolation vs pTof the third

muon.

Electron LJ 1 Muon LJ 2 Muon LJ

Data 15 7 3

All background 15.2±2.7 3.0 ± 1.0 0.5 ± 0.3

Table 1: The number of events in the signal region (A) observed in data and expected from background sources estimated with the ABCD-likelihood method. The quoted error is statistical only.

generated for the signal-only hypothesis and the sig-nal+background hypothesis, varying the LLR accord-ing to the statistical and systematic uncertainties. The upper limits were determined by performing a scan of p-values corresponding to LLR values larger than the one observed in data. For broad applicability, the limits are expressed in terms of the signal cross section times

branching ratio to the final state under consideration, us-ing the expected signal acceptance for each of the pair-ings of dark photon masses and dark sector gauge cou-pling parameter values in Table 4.

The observed limits in the electron-jets channel are in good agreement with the expected limits, and are the first inclusive study of prompt electron-jets at the LHC. The limits in the muon-jet channels are slightly higher than expected as a result of the slight excesses, but are within 2σ of the SM expectation for both chan-nels. The limits on the production of prompt lepton-jets in the muon-jet channel improve upon previous results by an order of magnitude.

Electron LJ [%] 1 Muon LJ [%] 2 Muon LJ [%]

Luminosity 3.9 3.9 3.9

Trigger Efficiency 1.5 2.0 3.6

Offline ∆R Efficiency 13.0 10.7 10.7

Lepton Momentum Scale 0.6 1.0 1.0

Isolation 5.2 < 0.1 < 0.1

Rη2and wη2Efficiency 8.0 NA NA

fHTEfficiency 1.0 NA NA

fEMEfficiency 3.0 NA NA

Muon Momentum Resolution NA < 1.0 < 1.0

Table 3: Contributions to the systematic uncertainty on the signal yields for the three different lepton-jet (LJ) channels given as percentages. A “NA” means this source does not apply.

Signal Parameters Electron LJ 1 Muon LJ 2 Muon LJ αd mγD[MeV] A× [%] A× [%] A× [%]

0.0 150 3.01 ± 0.30 0.0 300 2.7 ± 0.5 4.3 ± 0.9 9.2 ± 0.9 0.0 500 1.8 ± 0.5 1.7 ± 1.3 8.5 ± 1.1 0.10 150 2.69 ± 0.23 0.10 300 1.04 ± 0.19 3.7 ± 0.5 7.10 ± 0.39 0.10 500 1.17 ± 0.23 5.0 ± 0.8 8.1 ± 0.6 0.30 150 2.49 ± 0.22 0.30 300 0.80 ± 0.13 2.16 ± 0.29 7.47 ± 0.42 0.30 500 0.37 ± 0.10 3.16 ± 0.46 6.23 ± 0.43

Table 2: The acceptance times trigger, reconstruction, and selection efficiency (A×) expected in the signal re-gion (A) from various signal hypotheses for the three different lepton-jet (LJ) channels. Note that the mγD =

150 MeV dark photon cannot decay to muons.

6. Conclusions

A search for collimated pairs of muons or electrons, lepton-jets, has been performed using nearly 5 fb−1

of pp collisions at √s = 7 TeV recorded with the ATLAS detector at the LHC. Such final states have been proposed as a possible explanation of recently ob-served anomalies in cosmic-ray and dark matter direct-detection experiments. No significant excess of data compared to the SM expectation was observed in any of the three channels, and 95% confidence-level upper limits have been computed on the cross section times branching ratio for several parameter values of a Hid-den Valley model. The limits range from 0.017 to 1.2 pb.

7. Acknowledgements

We thank CERN for the very successful operation of the LHC, as well as the support staff from our

institu-Signal Parameters Electron LJ 1 Muon LJ 2 Muon LJ αd mγD[MeV] Obs (Exp) pb Obs (Exp) pb Obs (Exp) pb

0.0 150 0.082 (0.082) - -0.0 300 0.11 (0.11) 0.060 (0.035) 0.017 (0.011) 0.0 500 0.20 (0.21) 0.15 (0.090) 0.019 (0.012) 0.10 150 0.096 (0.10) - -0.10 300 0.37 (0.37) 0.064 (0.036) 0.018 (0.011) 0.10 500 0.39 (0.39) 0.053 (0.035) 0.018 (0.011) 0.30 150 0.11 (0.11) - -0.30 300 0.40 (0.40) 0.099 (0.055) 0.020 (0.012) 0.30 500 1.2 (1.2) 0.066 (0.043) 0.022 (0.015)

Table 4: Observed (Expected) upper limits for cross sec-tion times branching ratio (σ× BR) to the final state un-der consiun-deration, in units of pb for the three different lepton-jet (LJ) channels.

tions without whom ATLAS could not be operated effi-ciently.

We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWF and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT and NSRF, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; BRF and RCN, Norway; MNiSW, Poland; GRICES and FCT, Portugal; MERYS (MECTS), Romania; MES of Russia and ROSATOM, Russian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Cantons of Bern and Geneva, Switzerland; NSC,

Taiwan; TAEK, Turkey; STFC, the Royal Society and Leverhulme Trust, United Kingdom; DOE and NSF, United States of America.

The crucial computing support from all WLCG part-ners is acknowledged gratefully, in particular from CERN and the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Tai-wan), RAL (UK) and BNL (USA) and in the Tier-2 fa-cilities worldwide.

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The ATLAS Collaboration

G. Aad48_{, T. Abajyan}21_{, B. Abbott}111_{, J. Abdallah}12_,

S. Abdel Khalek115_{, A.A. Abdelalim}49_{, O. Abdinov}11_,

R. Aben105_{, B. Abi}112_{, M. Abolins}88_,

O.S. AbouZeid158_{, H. Abramowicz}153_{, H. Abreu}136_,

B.S. Acharya164a,164b,a_{, L. Adamczyk}38_{, D.L. Adams}25_,

T.N. Addy56_{, J. Adelman}176_{, S. Adomeit}98_,

P. Adragna75, T. Adye129, S. Aefsky23, J.A. Aguilar-Saavedra124b,b, M. Agustoni17, S.P. Ahlen22, F. Ahles48, A. Ahmad148, M. Ahsan41, G. Aielli133a,133b, T.P.A. Åkesson79, G. Akimoto155, A.V. Akimov94, M.A. Alam76, J. Albert169, S. Albrand55, M. Aleksa30, I.N. Aleksandrov64, F. Alessandria89a_{, C. Alexa}26a_{, G. Alexander}153_,

G. Alexandre49_{, T. Alexopoulos}10_{, M. Alhroob}164a,164c_,

M. Aliev16_{, G. Alimonti}89a_{, J. Alison}120_,

B.M.M. Allbrooke18_{, L.J. Allison}71_{, P.P. Allport}73_,

S.E. Allwood-Spiers53_{, J. Almond}82_,

A. Aloisio102a,102b_{, R. Alon}172_{, A. Alonso}79_,

F. Alonso70_{, A. Altheimer}35_{, B. Alvarez Gonzalez}88_,

M.G. Alviggi102a,102b_{, K. Amako}65_{, C. Amelung}23_,

V.V. Ammosov128,∗_{, S.P. Amor Dos Santos}124a_,

A. Amorim124a,c, S. Amoroso48, N. Amram153, C. Anastopoulos30, L.S. Ancu17, N. Andari115, T. Andeen35, C.F. Anders58b, G. Anders58a, K.J. Anderson31, A. Andreazza89a,89b, V. Andrei58a, M-L. Andrieux55, X.S. Anduaga70, S. Angelidakis9, P. Anger44, A. Angerami35, F. Anghinolfi30, A. Anisenkov107_{, N. Anjos}124a_{, A. Annovi}47_,

A. Antonaki9_{, M. Antonelli}47_{, A. Antonov}96_,

J. Antos144b_{, F. Anulli}132a_{, M. Aoki}101_{, S. Aoun}83_,

L. Aperio Bella5_{, R. Apolle}118,d_{, G. Arabidze}88_,

I. Aracena143_{, Y. Arai}65_{, A.T.H. Arce}45_{, S. Arfaoui}148_,

J-F. Arguin93_{, S. Argyropoulos}42_{, E. Arik}19a,∗_,

M. Arik19a_{, A.J. Armbruster}87_{, O. Arnaez}81_{, V. Arnal}80_,

A. Artamonov95_{, G. Artoni}132a,132b_{, D. Arutinov}21_,

S. Asai155, S. Ask28, B. Åsman146a,146b, L. Asquith6, K. Assamagan25,e, A. Astbury169, M. Atkinson165, B. Aubert5, E. Auge115, K. Augsten126,

M. Aurousseau145a, G. Avolio30, D. Axen168, G. Azuelos93, f, Y. Azuma155, M.A. Baak30, G. Baccaglioni89a, C. Bacci134a,134b, A.M. Bach15, H. Bachacou136_{, K. Bachas}154_{, M. Backes}49_,

M. Backhaus21_{, J. Backus Mayes}143_{, E. Badescu}26a_,

P. Bagnaia132a,132b_{, Y. Bai}33a_{, D.C. Bailey}158_{, T. Bain}35_,

J.T. Baines129_{, O.K. Baker}176_{, S. Baker}77_{, P. Balek}127_,

E. Banas39_{, P. Banerjee}93_{, Sw. Banerjee}173_{, D. Banfi}30_,

A. Bangert150_{, V. Bansal}169_{, H.S. Bansil}18_{, L. Barak}172_,

S.P. Baranov94_{, T. Barber}48_{, E.L. Barberio}86_,

D. Barberis50a,50b_{, M. Barbero}21_{, D.Y. Bardin}64_,

T. Barillari99_{, M. Barisonzi}175_{, T. Barklow}143_,

N. Barlow28_{, B.M. Barnett}129_{, R.M. Barnett}15_,

A. Baroncelli134a_{, G. Barone}49_{, A.J. Barr}118_,

F. Barreiro80_{, J. Barreiro Guimar˜aes da Costa}57_,

R. Bartoldus143_{, A.E. Barton}71_{, V. Bartsch}149_,

A. Basye165_{, R.L. Bates}53_{, L. Batkova}144a_,

J.R. Batley28_{, A. Battaglia}17_{, M. Battistin}30_,

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P.H. Beauchemin161_{, R. Beccherle}50a_{, P. Bechtle}21_,

H.P. Beck17, K. Becker175, S. Becker98,

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O. Beltramello30_{, O. Benary}153_{, D. Benchekroun}135a_,

K. Bendtz146a,146b_{, N. Benekos}165_{, Y. Benhammou}153_,

E. Benhar Noccioli49_{, J.A. Benitez Garcia}159b_,

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E. Berglund105_{, J. Beringer}15_{, P. Bernat}77_,

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D. Boscherini20a_{, M. Bosman}12_{, H. Boterenbrood}105_,

J. Bouchami93_{, J. Boudreau}123_,

E.V. Bouhova-Thacker71_{, D. Boumediene}34_,

C. Bourdarios115_{, N. Bousson}83_{, A. Boveia}31_,

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J. Bracinik18_{, P. Branchini}134a_{, A. Brandt}8_,

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B. Brelier158_{, J. Bremer}30_{, K. Brendlinger}120_,

R. Brenner166_{, S. Bressler}172_{, T.M. Bristow}145b_,

D. Britton53_{, F.M. Brochu}28_{, I. Brock}21_{, R. Brock}88_,

F. Broggi89a_{, C. Bromberg}88_{, J. Bronner}99_,

G. Brooijmans35_{, T. Brooks}76_{, W.K. Brooks}32b_,

G. Brown82_{, P.A. Bruckman de Renstrom}39_,

D. Bruncko144b_{, R. Bruneliere}48_{, S. Brunet}60_,

A. Bruni20a_{, G. Bruni}20a_{, M. Bruschi}20a_,

L. Bryngemark79, T. Buanes14, Q. Buat55, F. Bucci49, J. Buchanan118, P. Buchholz141, R.M. Buckingham118, A.G. Buckley46, S.I. Buda26a, I.A. Budagov64,

B. Budick108, V. B¨uscher81, L. Bugge117, O. Bulekov96, A.C. Bundock73, M. Bunse43, T. Buran117,

H. Burckhart30, S. Burdin73, T. Burgess14, S. Burke129, E. Busato34_{, P. Bussey}53_{, C.P. Buszello}166_{, B. Butler}143_,

J.M. Butler22_{, C.M. Buttar}53_{, J.M. Butterworth}77_,

W. Buttinger28_{, M. Byszewski}30_{, S. Cabrera Urb´an}167_,

D. Caforio20a,20b_{, O. Cakir}4a_{, P. Calafiura}15_,

G. Calderini78_{, P. Calfayan}98_{, R. Calkins}106_,

L.P. Caloba24a_{, R. Caloi}132a,132b_{, D. Calvet}34_,

S. Calvet34_{, R. Camacho Toro}34_{, P. Camarri}133a,133b_,

D. Cameron117_{, L.M. Caminada}15_,

R. Caminal Armadans12_{, S. Campana}30_,

M. Campanelli77, V. Canale102a,102b, F. Canelli31, A. Canepa159a, J. Cantero80, R. Cantrill76, M.D.M. Capeans Garrido30, I. Caprini26a, M. Caprini26a, D. Capriotti99, M. Capua37a,37b, R. Caputo81, R. Cardarelli133a, T. Carli30, G. Carlino102a, L. Carminati89a,89b, S. Caron104, E. Carquin32b_{, G.D. Carrillo-Montoya}145b_,

A.A. Carter75_{, J.R. Carter}28_{, J. Carvalho}124a,i_,

D. Casadei108_{, M.P. Casado}12_{, M. Cascella}122a,122b_,

C. Caso50a,50b,∗_{, A.M. Castaneda Hernandez}173, j_,

E. Castaneda-Miranda173_{, V. Castillo Gimenez}167_,

N.F. Castro124a_{, G. Cataldi}72a_{, P. Catastini}57_,

A. Catinaccio30_{, J.R. Catmore}30_{, A. Cattai}30_,

G. Cattani133a,133b_{, S. Caughron}88_{, V. Cavaliere}165_,

P. Cavalleri78, D. Cavalli89a, M. Cavalli-Sforza12, V. Cavasinni122a,122b, F. Ceradini134a,134b,

A.S. Cerqueira24b, A. Cerri15, L. Cerrito75, F. Cerutti15, S.A. Cetin19b, A. Chafaq135a, D. Chakraborty106, I. Chalupkova127, K. Chan3, P. Chang165,

B. Chapleau85, J.D. Chapman28, J.W. Chapman87, D.G. Charlton18_{, V. Chavda}82_{, C.A. Chavez Barajas}30_,

S. Cheatham85_{, S. Chekanov}6_{, S.V. Chekulaev}159a_,

G.A. Chelkov64_{, M.A. Chelstowska}104_{, C. Chen}63_,

H. Chen25_{, S. Chen}33c_{, X. Chen}173_{, Y. Chen}35_,

Y. Cheng31_{, A. Cheplakov}64_,

R. Cherkaoui El Moursli135e_{, V. Chernyatin}25_,

E. Cheu7_{, S.L. Cheung}158_{, L. Chevalier}136_,

G. Chiefari102a,102b_{, L. Chikovani}51a,∗_{, J.T. Childers}30_,

A. Chilingarov71_{, G. Chiodini}72a_{, A.S. Chisholm}18_,

R.T. Chislett77_{, A. Chitan}26a_{, M.V. Chizhov}64_,

G. Choudalakis31_{, S. Chouridou}137_{, I.A. Christidi}77_,

A. Christov48_{, D. Chromek-Burckhart}30_{, M.L. Chu}151_,

J. Chudoba125_{, G. Ciapetti}132a,132b_{, A.K. Ciftci}4a_,

R. Ciftci4a_{, D. Cinca}34_{, V. Cindro}74_{, A. Ciocio}15_,

M. Cirilli87_{, P. Cirkovic}13b_{, Z.H. Citron}172_,

M. Citterio89a_{, M. Ciubancan}26a_{, A. Clark}49_,

P.J. Clark46_{, R.N. Clarke}15_{, W. Cleland}123_,

J.C. Clemens83, B. Clement55, C. Clement146a,146b, Y. Coadou83, M. Cobal164a,164c, A. Coccaro138, J. Cochran63, L. Coffey23, J.G. Cogan143,

J. Coggeshall165, J. Colas5, S. Cole106, A.P. Colijn105, N.J. Collins18, C. Collins-Tooth53, J. Collot55, T. Colombo119a,119b, G. Colon84, G. Compostella99, P. Conde Mui˜no124a_{, E. Coniavitis}166_{, M.C. Conidi}12_,

S.M. Consonni89a,89b_{, V. Consorti}48_,

S. Constantinescu26a_{, C. Conta}119a,119b_{, G. Conti}57_,

F. Conventi102a,k_{, M. Cooke}15_{, B.D. Cooper}77_,

A.M. Cooper-Sarkar118_{, K. Copic}15_{, T. Cornelissen}175_,

M. Corradi20a_{, F. Corriveau}85,l_{, A. Cortes-Gonzalez}165_,

G. Cortiana99_{, G. Costa}89a_{, M.J. Costa}167_,

D. Costanzo139_{, D. Cˆot´e}30_{, L. Courneyea}169_,

G. Cowan76_{, B.E. Cox}82_{, K. Cranmer}108_{, F. Crescioli}78_,

M. Cristinziani21, G. Crosetti37a,37b, S. Cr´ep´e-Renaudin55, C.-M. Cuciuc26a,

C. Cuenca Almenar176, T. Cuhadar Donszelmann139, J. Cummings176, M. Curatolo47, C.J. Curtis18, C. Cuthbert150, P. Cwetanski60, H. Czirr141, P. Czodrowski44, Z. Czyczula176, S. D’Auria53, M. D’Onofrio73_{, A. D’Orazio}132a,132b_,

M.J. Da Cunha Sargedas De Sousa124a_{, C. Da Via}82_,

W. Dabrowski38_{, A. Dafinca}118_{, T. Dai}87_{, F. Dallaire}93_,

C. Dallapiccola84_{, M. Dam}36_{, M. Dameri}50a,50b_,

D.S. Damiani137_{, H.O. Danielsson}30_{, V. Dao}104_,

G. Darbo50a_{, G.L. Darlea}26b_{, J.A. Dassoulas}42_,

W. Davey21_{, T. Davidek}127_{, N. Davidson}86_,

R. Davidson71_{, E. Davies}118,d_{, M. Davies}93_,

O. Davignon78, A.R. Davison77, Y. Davygora58a, E. Dawe142, I. Dawson139,

R.K. Daya-Ishmukhametova23, K. De8,

R. de Asmundis102a, S. De Castro20a,20b, S. De Cecco78, J. de Graat98, N. De Groot104, P. de Jong105,

C. De La Taille115, H. De la Torre80, F. De Lorenzi63, L. De Nooij105_{, D. De Pedis}132a_{, A. De Salvo}132a_,

U. De Sanctis164a,164c_{, A. De Santo}149_,

J.B. De Vivie De Regie115_{, G. De Zorzi}132a,132b_,

W.J. Dearnaley71_{, R. Debbe}25_{, C. Debenedetti}46_,

B. Dechenaux55_{, D.V. Dedovich}64_{, J. Degenhardt}120_,

J. Del Peso80_{, T. Del Prete}122a,122b_{, T. Delemontex}55_,

M. Deliyergiyev74_{, A. Dell’Acqua}30_{, L. Dell’Asta}22_,

M. Della Pietra102a,k_{, D. della Volpe}102a,102b_,

S. Demers176_{, M. Demichev}64_{, B. Demirkoz}12,m_,

S.P. Denisov128_{, D. Derendarz}39_{, J.E. Derkaoui}135d_,

F. Derue78_{, P. Dervan}73_{, K. Desch}21_{, E. Devetak}148_,

P.O. Deviveiros105_{, A. Dewhurst}129_{, B. DeWilde}148_,

S. Dhaliwal158_{, R. Dhullipudi}25,n_,

A. Di Ciaccio133a,133b_{, L. Di Ciaccio}5_,

C. Di Donato102a,102b_{, A. Di Girolamo}30_,

B. Di Girolamo30_{, S. Di Luise}134a,134b_{, A. Di Mattia}152_,

B. Di Micco30, R. Di Nardo47, A. Di Simone133a,133b, R. Di Sipio20a,20b, M.A. Diaz32a, E.B. Diehl87, J. Dietrich42, T.A. Dietzsch58a, S. Diglio86, K. Dindar Yagci40, J. Dingfelder21, F. Dinut26a, C. Dionisi132a,132b, P. Dita26a, S. Dita26a, F. Dittus30, F. Djama83, T. Djobava51b, M.A.B. do Vale24c,

A. Do Valle Wemans124a,o_{, T.K.O. Doan}5_{, M. Dobbs}85_,

D. Dobos30_{, E. Dobson}30,p_{, J. Dodd}35_{, C. Doglioni}49_,

T. Doherty53_{, Y. Doi}65,∗_{, J. Dolejsi}127_{, Z. Dolezal}127_,

B.A. Dolgoshein96,∗_{, T. Dohmae}155_{, M. Donadelli}24d_,

J. Donini34_{, J. Dopke}30_{, A. Doria}102a_{, A. Dos Anjos}173_,

A. Dotti122a,122b_{, M.T. Dova}70_{, A.D. Doxiadis}105_,

A.T. Doyle53_{, N. Dressnandt}120_{, M. Dris}10_,

J. Dubbert99_{, S. Dube}15_{, E. Dubreuil}34_{, E. Duchovni}172_,

G. Duckeck98_{, D. Duda}175_{, A. Dudarev}30_{, F. Dudziak}63_,

M. D¨uhrssen30, I.P. Duerdoth82, L. Duflot115, M-A. Dufour85, L. Duguid76, M. Dunford58a, H. Duran Yildiz4a, R. Duxfield139, M. Dwuznik38, M. D¨uren52, W.L. Ebenstein45, J. Ebke98,

S. Eckweiler81, W. Edson2, C.A. Edwards76, N.C. Edwards53, W. Ehrenfeld21, T. Eifert143, G. Eigen14_{, K. Einsweiler}15_{, E. Eisenhandler}75_,

T. Ekelof166_{, M. El Kacimi}135c_{, M. Ellert}166_{, S. Elles}5_,

F. Ellinghaus81_{, K. Ellis}75_{, N. Ellis}30_{, J. Elmsheuser}98_,

M. Elsing30_{, D. Emeliyanov}129_{, R. Engelmann}148_,

A. Engl98_{, B. Epp}61_{, J. Erdmann}176_{, A. Ereditato}17_,

D. Eriksson146a_{, J. Ernst}2_{, M. Ernst}25_{, J. Ernwein}136_,

D. Errede165_{, S. Errede}165_{, E. Ertel}81_{, M. Escalier}115_,

H. Esch43_{, C. Escobar}123_{, X. Espinal Curull}12_,

B. Esposito47, F. Etienne83, A.I. Etienvre136, E. Etzion153, D. Evangelakou54, H. Evans60, L. Fabbri20a,20b, C. Fabre30, R.M. Fakhrutdinov128, S. Falciano132a, Y. Fang33a, M. Fanti89a,89b, A. Farbin8, A. Farilla134a, J. Farley148, T. Farooque158, S. Farrell163, S.M. Farrington170, P. Farthouat30, F. Fassi167,

P. Fassnacht30_{, D. Fassouliotis}9_{, B. Fatholahzadeh}158_,

A. Favareto89a,89b_{, L. Fayard}115_{, P. Federic}144a_,

O.L. Fedin121_{, W. Fedorko}168_{, M. Fehling-Kaschek}48_,

L. Feligioni83_{, C. Feng}33d_{, E.J. Feng}6_{, A.B. Fenyuk}128_,

J. Ferencei144b_{, W. Fernando}6_{, S. Ferrag}53_,

J. Ferrando53_{, V. Ferrara}42_{, A. Ferrari}166_{, P. Ferrari}105_,

R. Ferrari119a_{, D.E. Ferreira de Lima}53_{, A. Ferrer}167_,

D. Ferrere49_{, C. Ferretti}87_{, A. Ferretto Parodi}50a,50b_,

M. Fiascaris31_{, F. Fiedler}81_{, A. Filipˇciˇc}74_{, F. Filthaut}104_,

M. Fincke-Keeler169_{, M.C.N. Fiolhais}124a,i_,

L. Fiorini167_{, A. Firan}40_{, G. Fischer}42_{, M.J. Fisher}109_,

E.A. Fitzgerald23_{, M. Flechl}48_{, I. Fleck}141_,

J. Fleckner81_{, P. Fleischmann}174_{, S. Fleischmann}175_,

G. Fletcher75_{, T. Flick}175_{, A. Floderus}79_,

L.R. Flores Castillo173_{, A.C. Florez Bustos}159b_,

M.J. Flowerdew99_{, T. Fonseca Martin}17_{, A. Formica}136_,

A. Forti82_{, D. Fortin}159a_{, D. Fournier}115_{, A.J. Fowler}45_,

H. Fox71, P. Francavilla12, M. Franchini20a,20b, S. Franchino119a,119b, D. Francis30, T. Frank172, M. Franklin57, S. Franz30, M. Fraternali119a,119b, S. Fratina120, S.T. French28, C. Friedrich42, F. Friedrich44, D. Froidevaux30, J.A. Frost28, C. Fukunaga156, E. Fullana Torregrosa127, B.G. Fulsom143_{, J. Fuster}167_{, C. Gabaldon}30_,

O. Gabizon172_{, T. Gadfort}25_{, S. Gadomski}49_,

G. Gagliardi50a,50b_{, P. Gagnon}60_{, C. Galea}98_,

B. Galhardo124a_{, E.J. Gallas}118_{, V. Gallo}17_,

B.J. Gallop129_{, P. Gallus}126_{, K.K. Gan}109_{, Y.S. Gao}143,g_,

A. Gaponenko15_{, F. Garberson}176_,

M. Garcia-Sciveres15_{, C. Garc´ıa}167_,

J.E. Garc´ıa Navarro167_{, R.W. Gardner}31_{, N. Garelli}143_,

V. Garonne30_{, C. Gatti}47_{, G. Gaudio}119a_{, B. Gaur}141_,

L. Gauthier136, P. Gauzzi132a,132b, I.L. Gavrilenko94, C. Gay168, G. Gaycken21, E.N. Gazis10, P. Ge33d, Z. Gecse168, C.N.P. Gee129, D.A.A. Geerts105, Ch. Geich-Gimbel21, K. Gellerstedt146a,146b, C. Gemme50a, A. Gemmell53, M.H. Genest55, S. Gentile132a,132b, M. George54, S. George76, D. Gerbaudo12_{, P. Gerlach}175_{, A. Gershon}153_,

C. Geweniger58a_{, H. Ghazlane}135b_{, N. Ghodbane}34_,

B. Giacobbe20a_{, S. Giagu}132a,132b_{, V. Giangiobbe}12_,

F. Gianotti30_{, B. Gibbard}25_{, A. Gibson}158_,

S.M. Gibson30_{, M. Gilchriese}15_{, D. Gillberg}30_,

A.R. Gillman129_{, D.M. Gingrich}3, f_{, J. Ginzburg}153_,

N. Giokaris9_{, M.P. Giordani}164c_{, R. Giordano}102a,102b_,

F.M. Giorgi16_{, P. Giovannini}99_{, P.F. Giraud}136_,

D. Giugni89a, M. Giunta93, B.K. Gjelsten117, L.K. Gladilin97, C. Glasman80, J. Glatzer21, A. Glazov42, G.L. Glonti64, J.R. Goddard75, J. Godfrey142, J. Godlewski30, M. Goebel42, T. G¨opfert44, C. Goeringer81, C. G¨ossling43, S. Goldfarb87, T. Golling176, D. Golubkov128, A. Gomes124a,c_{, L.S. Gomez Fajardo}42_{, R. Gonc¸alo}76_,

J. Goncalves Pinto Firmino Da Costa42_{, L. Gonella}21_,

S. Gonz´alez de la Hoz167_{, G. Gonzalez Parra}12_,

M.L. Gonzalez Silva27_{, S. Gonzalez-Sevilla}49_,

J.J. Goodson148_{, L. Goossens}30_{, P.A. Gorbounov}95_,

H.A. Gordon25_{, I. Gorelov}103_{, G. Gorfine}175_,

B. Gorini30_{, E. Gorini}72a,72b_{, A. Goriˇsek}74_,

E. Gornicki39_{, A.T. Goshaw}6_{, M. Gosselink}105_,

D. Goujdami135c_{, M.P. Goulette}49_{, A.G. Goussiou}138_,

C. Goy5_{, S. Gozpinar}23_{, I. Grabowska-Bold}38_,

P. Grafstr¨om20a,20b_{, K-J. Grahn}42_{, E. Gramstad}117_,

F. Grancagnolo72a_{, S. Grancagnolo}16_{, V. Grassi}148_,

V. Gratchev121_{, H.M. Gray}30_{, J.A. Gray}148_,

E. Graziani134a_{, O.G. Grebenyuk}121_{, T. Greenshaw}73_,

Z.D. Greenwood25,n_{, K. Gregersen}36_{, I.M. Gregor}42_,

P. Grenier143_{, J. Griffiths}8_{, N. Grigalashvili}64_,

A.A. Grillo137, K. Grimm71, S. Grinstein12, Ph. Gris34, Y.V. Grishkevich97, J.-F. Grivaz115, A. Grohsjean42, E. Gross172, J. Grosse-Knetter54, J. Groth-Jensen172, K. Grybel141, D. Guest176, C. Guicheney34,

E. Guido50a,50b, T. Guillemin115, S. Guindon54, U. Gul53, J. Gunther125, B. Guo158, J. Guo35, P. Gutierrez111_{, N. Guttman}153_{, O. Gutzwiller}173_,

C. Guyot136_{, C. Gwenlan}118_{, C.B. Gwilliam}73_,

A. Haas108_{, S. Haas}30_{, C. Haber}15_{, H.K. Hadavand}8_,

D.R. Hadley18_{, P. Haefner}21_{, F. Hahn}30_{, Z. Hajduk}39_,

H. Hakobyan177_{, D. Hall}118_{, G. Halladjian}62_,

K. Hamacher175_{, P. Hamal}113_{, K. Hamano}86_,

M. Hamer54_{, A. Hamilton}145b,q_{, S. Hamilton}161_,

L. Han33b_{, K. Hanagaki}116_{, K. Hanawa}160_{, M. Hance}15_,

C. Handel81_{, P. Hanke}58a_{, J.R. Hansen}36_{, J.B. Hansen}36_,

J.D. Hansen36, P.H. Hansen36, P. Hansson143, K. Hara160, T. Harenberg175, S. Harkusha90, D. Harper87, R.D. Harrington46, O.M. Harris138, J. Hartert48, F. Hartjes105, T. Haruyama65, A. Harvey56, S. Hasegawa101, Y. Hasegawa140, S. Hassani136, S. Haug17, M. Hauschild30, R. Hauser88,

M. Havranek21_{, C.M. Hawkes}18_{, R.J. Hawkings}30_,

A.D. Hawkins79_{, T. Hayakawa}66_{, T. Hayashi}160_,

D. Hayden76_{, C.P. Hays}118_{, H.S. Hayward}73_,

S.J. Haywood129_{, S.J. Head}18_{, V. Hedberg}79_,

L. Heelan8_{, S. Heim}120_{, B. Heinemann}15_,

S. Heisterkamp36_{, L. Helary}22_{, C. Heller}98_,

M. Heller30_{, S. Hellman}146a,146b_{, D. Hellmich}21_,

C. Helsens12_{, R.C.W. Henderson}71_{, M. Henke}58a_,

A. Henrichs176, A.M. Henriques Correia30,

S. Henrot-Versille115, C. Hensel54, C.M. Hernandez8, Y. Hern´andez Jim´enez167, R. Herrberg16, G. Herten48, R. Hertenberger98, L. Hervas30, G.G. Hesketh77, N.P. Hessey105, R. Hickling75, E. Hig´on-Rodriguez167, J.C. Hill28, K.H. Hiller42, S. Hillert21, S.J. Hillier18, I. Hinchliffe15_{, E. Hines}120_{, M. Hirose}116_{, F. Hirsch}43_,

D. Hirschbuehl175_{, J. Hobbs}148_{, N. Hod}153_,

M.C. Hodgkinson139_{, P. Hodgson}139_{, A. Hoecker}30_,

M.R. Hoeferkamp103_{, J. Hoffman}40_{, D. Hoffmann}83_,

M. Hohlfeld81_{, M. Holder}141_{, S.O. Holmgren}146a_,

T. Holy126_{, J.L. Holzbauer}88_{, T.M. Hong}120_,

L. Hooft van Huysduynen108_{, S. Horner}48_,

J-Y. Hostachy55_{, S. Hou}151_{, A. Hoummada}135a_,

J. Howard118_{, J. Howarth}82_{, I. Hristova}16_{, J. Hrivnac}115_,

T. Hryn’ova5_{, P.J. Hsu}81_{, S.-C. Hsu}138_{, D. Hu}35_,

Z. Hubacek30_{, F. Hubaut}83_{, F. Huegging}21_,

A. Huettmann42_{, T.B. Huffman}118_{, E.W. Hughes}35_,

G. Hughes71_{, M. Huhtinen}30_{, M. Hurwitz}15_,

N. Huseynov64,r_{, J. Huston}88_{, J. Huth}57_{, G. Iacobucci}49_,

G. Iakovidis10_{, M. Ibbotson}82_{, I. Ibragimov}141_,

L. Iconomidou-Fayard115_{, J. Idarraga}115_{, P. Iengo}102a_,

O. Igonkina105_{, Y. Ikegami}65_{, M. Ikeno}65_{, D. Iliadis}154_,

N. Ilic158, T. Ince99, P. Ioannou9, M. Iodice134a, K. Iordanidou9, V. Ippolito132a,132b, A. Irles Quiles167, C. Isaksson166, M. Ishino67, M. Ishitsuka157,

R. Ishmukhametov109, C. Issever118, S. Istin19a, A.V. Ivashin128, W. Iwanski39, H. Iwasaki65,

J.M. Izen41, V. Izzo102a, B. Jackson120, J.N. Jackson73, P. Jackson1_{, M.R. Jaekel}30_{, V. Jain}2_{, K. Jakobs}48_,

S. Jakobsen36_{, T. Jakoubek}125_{, J. Jakubek}126_,

D.O. Jamin151_{, D.K. Jana}111_{, E. Jansen}77_{, H. Jansen}30_,

J. Janssen21_{, A. Jantsch}99_{, M. Janus}48_{, R.C. Jared}173_,

G. Jarlskog79_{, L. Jeanty}57_{, I. Jen-La Plante}31_,

G.-Y. Jeng150_{, D. Jennens}86_{, P. Jenni}30_,

A.E. Loevschall-Jensen36_{, P. Jeˇz}36_{, S. J´ez´equel}5_,

M.K. Jha20a_{, H. Ji}173_{, W. Ji}81_{, J. Jia}148_{, Y. Jiang}33b_,

M. Jimenez Belenguer42_{, S. Jin}33a_{, O. Jinnouchi}157_,

M.D. Joergensen36, D. Joffe40, M. Johansen146a,146b, K.E. Johansson146a, P. Johansson139, S. Johnert42, K.A. Johns7, K. Jon-And146a,146b, G. Jones170, R.W.L. Jones71, T.J. Jones73, C. Joram30, P.M. Jorge124a, K.D. Joshi82, J. Jovicevic147, T. Jovin13b, X. Ju173, C.A. Jung43, R.M. Jungst30, V. Juranek125_{, P. Jussel}61_{, A. Juste Rozas}12_,

S. Kabana17_{, M. Kaci}167_{, A. Kaczmarska}39_,

P. Kadlecik36_{, M. Kado}115_{, H. Kagan}109_{, M. Kagan}57_,

E. Kajomovitz152_{, S. Kalinin}175_{, L.V. Kalinovskaya}64_,

S. Kama40_{, N. Kanaya}155_{, M. Kaneda}30_{, S. Kaneti}28_,

T. Kanno157_{, V.A. Kantserov}96_{, J. Kanzaki}65_,

B. Kaplan108_{, A. Kapliy}31_{, D. Kar}53_{, M. Karagounis}21_,

K. Karakostas10_{, M. Karnevskiy}58b_{, V. Kartvelishvili}71_,

A.N. Karyukhin128, L. Kashif173, G. Kasieczka58b, R.D. Kass109, A. Kastanas14, M. Kataoka5,

Y. Kataoka155, J. Katzy42, V. Kaushik7, K. Kawagoe69, T. Kawamoto155, G. Kawamura81, S. Kazama155, V.F. Kazanin107, M.Y. Kazarinov64, R. Keeler169, P.T. Keener120, R. Kehoe40, M. Keil54,

G.D. Kekelidze64_{, J.S. Keller}138_{, M. Kenyon}53_,

H. Keoshkerian5_{, O. Kepka}125_{, N. Kerschen}30_,

B.P. Kerˇsevan74_{, S. Kersten}175_{, K. Kessoku}155_,

J. Keung158_{, F. Khalil-zada}11_{, H. Khandanyan}146a,146b_,

A. Khanov112_{, D. Kharchenko}64_{, A. Khodinov}96_,

A. Khomich58a_{, T.J. Khoo}28_{, G. Khoriauli}21_,

A. Khoroshilov175_{, V. Khovanskiy}95_{, E. Khramov}64_,

J. Khubua51b_{, H. Kim}146a,146b_{, S.H. Kim}160_,