Search for contact interactions in dilepton events from pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

arXiv:1112.4462v3 [hep-ex] 23 May 2012

at

√

s = 7 TeV with the ATLAS Detector

The ATLAS Collaboration

This Letter presents a search for contact interactions in the dielectron and dimuon channels using data from proton-proton collisions produced by the LHC at √s = 7 TeV and recorded by the ATLAS detector. The data sample, collected in 2011, corresponds to an integrated luminosity of 1.08 and 1.21 fb−1_{in the e}+_e−_{and µ}+_µ−_{channels, respectively. No significant deviations from the}

standard model are observed. Using a Bayesian approach with a prior flat in 1/Λ2

, the following 95% CL lower limits are placed on the energy scale of ℓℓqq contact interactions: Λ− _{> 10.1 TeV}

(Λ+

> 9.4 TeV) in the electron channel and Λ−_{> 8.0 TeV (Λ}+ _{> 7.0 TeV) in the muon channel}

for constructive (destructive) interference in the left-left isoscalar contact interaction model. Limits are also provided for a prior flat in 1/Λ4

.

INTRODUCTION

A wide range of new physics phenomena can produce modifications to the dilepton mass spectra predicted by the standard model (SM) such as quark/lepton compos-iteness, extra dimensions, and new gauge bosons. The predicted form of these deviations is often either a reso-nance or an excess in the number of events in the spectra at high mass. This Letter reports on a search for such an excess in dilepton events produced in proton-proton collisions at the LHC [1]. An interpretation of these data in the context of contact interactions (CI) is presented, including the first limits with the ATLAS detector in the dielectron channel and an update of the search performed using 2010 data in the dimuon channel [2]. A separate pa-per describes the search for new heavy resonances in the dilepton mass spectra performed using the same ATLAS dataset [3].

If quarks and leptons are composite, with at least one common constituent, the interaction of these con-stituents would likely be manifested through an effec-tive four-fermion contact interaction at energies well be-low the compositeness scale. Such a contact interaction could also describe a new interaction with a messenger too heavy for direct observation at the LHC, in analogy with Fermi’s nuclear β decay theory [4].

The Lagrangian for a general contact interaction has the form [5]

L = 2Λg22 [ ηLL ψLγµψLψLγµψL

+ ηRR ψRγµψRψRγµψR

+2ηLRψLγµψLψRγµψR] ,

(1)

where g is a coupling constant chosen to obey g2_{/4π = 1,}

Λ is the contact interaction scale, which in the context of compositeness models is the energy scale below which fermion constituents are bound, and ψL,Rare left-handed

and right-handed fermion fields, respectively. The pa-rameters ηij, where i and j are L or R, define the chiral

structure (left or right) of the new interaction. Specific

models are constructed by setting different combinations of these parameters to assume values of −1, 0 or +1. The addition of this contact interaction term to the SM Lagrangian alters the Drell-Yan (DY) production cross section (q ¯q → Z/γ∗

→ ℓ+_ℓ−_{). The largest deviations,}

either constructive or destructive, are expected at high dilepton invariant mass and are determined by the scale Λ and the sign of the parameter ηij. This analysis

in-terprets the data in the context of the left-left isoscalar model (LLIM), which is commonly used as a benchmark for contact interaction searches [6]. The LLIM is defined by setting ηLL= ±1 and ηRR = ηLR = 0.

With the introduction of a contact interaction, the dif-ferential cross section for the process q ¯q → ℓ+_ℓ−

can be written dσ dmℓℓ = dσDY dmℓℓ − η LL FI(mℓℓ) Λ2 + FC(mℓℓ) Λ4 , (2)

where mℓℓ is the final-state dilepton mass. The

expres-sion above includes a SM DY term, as well as DY-CI in-terference (FI) and pure contact interaction (FC) terms

(see Ref. [7] for the full form of this expression). At the largest Λ values to which this analysis is sensitive, both interference and pure contact interaction terms play a significant role. For example, at dilepton masses greater than 300 GeV and Λ = 9 TeV, the magnitude of the inter-ference term is about 1.5 times that of the pure contact interaction term.

The present analysis focuses on identifying a broad de-viation from the SM dilepton mass spectra, which are expected to be dominated by the DY process. Current experimental bounds on Λ (see below) indicate any devi-ation from a new interaction would appear at masses well above the Z boson peak. Consequently, the search region is restricted to dilepton masses above 150 GeV. The anal-ysis exploits the high pp collision energy of the LHC and the capabilities of the ATLAS detector to identify and reconstruct electrons and muons at high momentum.

Previous searches for contact interactions have been carried out in neutrino scattering [8], as well as at

electron-positron [9–13], electron-proton [14, 15], and hadron colliders [16–24]. In the case of eeqq contact interactions, the best limits in the LLIM for all quark flavors come from e+_e−

experiments with Λ−

> 7.2 TeV and Λ+_{> 12.9 TeV [9] at 95% confidence level (CL) for}

ηLL = −1 and +1, respectively. These limits assume

that contact interactions of electrons with all quark fla-vors are of the same strength. Best limits set in the specific case of first generation quarks are Λ−_{> 9.1 TeV}

and Λ+ _{> 8.6 TeV [13] at 95% CL. In the case of µµqq}

contact interactions, the best limits are Λ− _{> 4.9 TeV}

and Λ+_{> 4.5 TeV from the ATLAS analysis of the 2010}

data [2].

ATLAS DETECTOR AND DATA SAMPLE

ATLAS is a multipurpose particle detector [25]. It con-sists of an inner tracking detector surrounded by a 2 T superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer with a toroidal magnetic field. Charged particle tracks are reconstructed using the inner detector, which comprises a silicon pixel detector, a silicon-strip tracker, and a transition radi-ation tracker, covering the pseudorapidity range |η| < 2.5 [26]. A hermetic calorimeter, which covers |η| < 4.9, surrounds the superconducting solenoid. The liquid-argon electromagnetic calorimeter, which plays an im-portant role in electron identification and measurement, is finely segmented, with readout granularity (η, φ) vary-ing by layer and cells as small as 0.025 × 0.025 ex-tending to |η| < 2.5, to provide excellent energy and position resolution. The electron energy resolution is dominated at high energy by a constant term equal to 1.2% in the barrel (|η| < 1.37) and 1.8% in the endcaps (1.52 < |η| < 2.47). Hadron calorimetry is provided by an iron-scintillator tile calorimeter in the central rapid-ity range |η| < 1.7 and a liquid-argon calorimeter in the rapidity range 1.5 < |η| < 4.9. Another key detector component for this analysis is the muon spectrometer, which is designed to identify muons and measure their momenta with high accuracy. The currently achieved resolution for momenta transverse to the beam line (pT)

of 1 TeV ranges from 15% (central) to 44% (for |η| > 2). The muon system comprises three toroidal magnet sys-tems, a trigger system consisting of resistive plate cham-bers in the barrel and thin-gap chamcham-bers in the endcaps, providing triggering capability up to |η| = 2.4, and a set of precision monitored drift tubes and cathode strip chambers in the region |η| < 2.7.

The data sample for this analysis was collected during LHC operation in the first half of 2011 and corresponds to a total integrated luminosity of 1.08 and 1.21 fb−1_{in the}

e+_e− _{and µ}+_µ− _{channels, respectively. It was collected}

with stable beam conditions and an operational inner de-tector. For the electron (muon) channel, the calorimeter

(muon spectrometer) was also required to be operational. Events were selected by requiring that they pass the sin-gle electron (muon) trigger with a transverse momentum pT threshold of 20 (22) GeV. This analysis follows the

same event selection as the search for new heavy reso-nances. A summary is provided below, a more complete description can be found in Ref. [3].

SIGNAL AND BACKGROUND MODELING

This analysis looks for deviations from the expected SM dilepton spectra. The largest SM contribution comes from DY followed by semileptonic decay of t¯t pairs, electroweak diboson production (W W , W Z, and ZZ), and production of jets in association with a W boson (W +jets). In addition, multi-jet production (QCD) is a significant background in the electron channel. With the exception of QCD, Monte Carlo (MC) simulation was used to model these backgrounds.

DY events were generated with Pythia 6.421 [27] and Mrst2007 LO∗

parton distribution functions (PDFs) [28]. Signal DY+CI samples in the LLIM were generated with the same version of Pythia for the full dilepton differential cross section as shown in Eq. (2). This ensured that the interference term FI was properly

included. All quark flavors contributed to the contact interaction in these signal samples. Diboson processes were produced with Herwig 6.510 [29] using Mrst2007 LO∗_{PDFs. The W +jets background was generated with}

Alpgen[30] and Cteq6l1 [31] PDFs, and the t¯t back-ground with Mc@nlo 3.41 [32] and Cteq6.6 [33] PDFs. For the latter two, Jimmy 4.31 [34] was used to describe multiple parton interactions and Herwig to describe the remaining underlying event and parton showers. Pho-tos[35] was used to handle the final-state photon radia-tion for all MC samples. Furthermore, higher order QCD corrections were implemented via a mass-dependent K-factor defined as the ratio between the next-to-next-to-leading order (NNLO) Z/γ∗

cross section, calculated us-ing Phozpr [36] and Mstw2008 PDFs [37], and the LO cross section. This QCD K-factor was applied to both DY and DY+CI samples. Likewise, DY and DY+CI samples are corrected with a mass-dependent K-factor accounting for higher-order electroweak corrections aris-ing from virtual heavy gauge boson loops that are calcu-lated using Horace [38]. Finally, the generated samples were processed through a full simulation of the ATLAS detector [39] based on the Geant 4 package [40].

For both channels, the QCD multi-jet background is evaluated from data due to poor modeling and low MC statistics. In the electron channel, a reversed elec-tron identification technique is used to select a sample of events in which both electrons fail a subset of the electron identification criteria (see further discussion be-low). This sample is then used to determine the shape

of the QCD background as a function of dielectron in-variant mass. This template shape and the sum of the DY, dibosons, t¯t, and W +jets backgrounds normalized by their cross sections (including higher order correc-tions) are fitted to the observed dielectron mass distri-bution in the range between 70 and 200 GeV to deter-mine the normalization of the QCD contribution. The above QCD background estimate is cross-checked with two other methods described in Ref. [3] in order to de-termine its systematic uncertainty. In particular, these cross-checks set bounds on the potential bias in the QCD mass spectrum introduced by the reversed identification technique. In the muon channel, the QCD background is much smaller and is also evaluated from data. A re-verse isolation method is utilized: a QCD sample is se-lected from data by requiring two non-isolated muons with 0.1 < ΣpT(R < 0.3)/pT(µ) < 1.0, where ΣpT

(R < 0.3) is the sum of the pT of the tracks in a cone

of R = p∆η2_{+ ∆φ}2 _{< 0.3 around the direction of the}

muon. The normalization for this sample is obtained from the ratio of isolated to non-isolated dimuon events in QCD c¯c and b¯b MC, where the isolation requirement is ΣpT(R < 0.3)/pT(µ) < 0.05. Muons from light hadron

decays are not a significant source of background at the high momenta relevant to this analysis.

EVENT SELECTION

Events passing the trigger selection described above are required to have a pair of either electrons or muons with pT greater than 25 GeV to ensure maximal trigger

efficiency. To reject cosmic ray events and beam halo background, events are required to have a reconstructed vertex with at least three charged particle tracks with pT > 0.4 GeV. If several such vertices are found, the

ver-tex with the largest Σp2

T is selected as the primary vertex

of the event, where the sum is over all charged particles associated with the given vertex. Electron candidates are confined in |η| < 2.47, with the detector crack region 1.37 ≤ |η| ≤ 1.52 excluded because of degraded energy reso-lution. Muon candidates are required to be within the inner detector acceptance.

Electron candidates are formed from clusters of cells reconstructed in the electromagnetic calorimeter. Identi-fication criteria on the transverse shower shape, the lon-gitudinal leakage into the hadronic calorimeter, and the association to an inner detector track are applied to the cluster to satisfy the medium electron definition [41]. The electron energy is obtained from the calorimeter mea-surements and its direction from the associated inner detector track. A hit in the first layer of the pixel de-tector is required (if an active pixel layer is traversed) to suppress background from photon conversions. Further QCD jet background suppression is achieved by demand-ing the highest pT electron in the event to be isolated.

To that effect, the sum of the calorimeter transverse mo-menta around the electron direction ΣpT(R < 0.2) must

be less than 7 GeV. The core of the electron energy depo-sition is excluded and the sum is corrected for transverse shower leakage and pile-up from additional pp collisions. In addition, the two electron candidates are not required to have opposite charge because of possible charge mis-identification either due to bremsstrahlung or to the lim-ited momentum resolution of the inner detector at very high pT. If the event contains more than two selected

electrons, the two electrons with the highest pT are

cho-sen. For these selection criteria, the overall event accep-tance for signal events has a small dependence on the dielectron mass above 500 GeV and a value of approxi-mately 65% at 1 TeV.

Muon candidates are required to be of opposite charge and are reconstructed independently in both the inner detector and the muon spectrometer. The momentum is taken from a combined fit to the measurements from both subsystems. To obtain optimal momentum resolu-tion and accurate modeling by the simularesolu-tion, the muon candidates are required to satisfy the following require-ments in the muon spectrometer: have at least three hits in each of the inner, middle, and outer detectors, and at least one hit in the non-bending xy plane. To sup-press background from cosmic rays, requirements are im-posed on the muon impact parameter and primary vertex (PV): transverse impact parameter |d0| < 0.2 mm, z

co-ordinate with respect to the PV |z0 − zPV| < 1 mm,

and z position of the PV |zPV| < 200 mm. Muons

are also required to be isolated to reduce background from jets: ΣpT(R < 0.3)/pT(µ) < 0.05, as explained

in the previous section. If more than one opposite-sign muon pair is found in an event, the pair with the largest pT(µ+) + pT(µ−) is chosen. The overall event

accep-tance for signal events has only a weak dependence on the dimuon mass with a value of approximately 40% at 1 TeV. Stringent requirements on the presence of hits in all three layers of the muon spectrometer and the limited three-layer geometrical coverage are the primary reason for the lower acceptance relative to the electron channel. Extensive comparisons between data and MC simula-tion were performed at the level of single-lepton distri-butions to confirm that the simulation reproduces the selected data well, especially at high momentum.

Figure 1 displays the dielectron and dimuon mass spec-tra for all selected events with invariant mass greater than 70 GeV. The expected event yields for the differ-ent processes are obtained by first normalizing each MC process by its cross section (including higher order cor-rections) and then normalizing the total MC event yield plus the data-derived QCD background to the data in the Z peak region (dilepton mass between 70 and 110 GeV). Good agreement is observed between the data and the SM prediction over the whole dilepton mass range. A quantitative comparison is provided in Tables I and II. A

[GeV] ee m 80 100 200 300 400 1000 2000 Events -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6 10 ATLAS -1 L dt = 1.08 fb

∫

ee: = 7 TeV s Data 2011 ee → DY DiBoson t t W+jets QCD = 3 TeV -Λ = 3 TeV + Λ = 5 TeV -Λ = 5 TeV + Λ = 7 TeV -Λ = 7 TeV + Λ = 12 TeV -Λ = 12 TeV + Λ [GeV] µ µ m 80 100 200 300 400 1000 2000 Events -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6 10 ATLAS -1 L dt = 1.21 fb

∫

: µ µ = 7 TeV s Data 2011 µ µ → DY Diboson t t W+jets QCD = 3 TeV -Λ = 3 TeV + Λ = 5 TeV -Λ = 5 TeV + Λ = 7 TeV -Λ = 7 TeV + Λ = 12 TeV -Λ = 12 TeV + Λ

FIG. 1. Dielectron (top) and dimuon (bottom) invariant mass distributions for data (points) and Monte Carlo simulation (histograms). The open histograms correspond to the distri-butions expected in the presence of contact interactions with different values of Λ for both constructive (solid histograms) and destructive (dashed histograms) interference.

slight excess of events observed at high dimuon mass is consistent with a statistical fluctuation. The most signif-icant deviation in the number of dimuon events occurs for events with mass greater than 800 GeV. In this region, the Poisson probability for observing 5 or more events where 2.1 are expected is 6.2%. The muon tracks in the five data events were inspected in detail and no problem was found.

SYSTEMATIC UNCERTAINTIES

Since the MC event yields are normalized to the num-ber of events observed in the Z peak region, only mass-dependent systematic uncertainties need to be consid-ered, except for a 5% overall uncertainty in the knowl-edge of the Z/γ∗ _{cross section in the normalization}

re-gion. This overall uncertainty is required since the cross section change due to the new physics is defined with

re-spect to the SM cross section. The dominant uncertain-ties are of theoretical origin but experimental sources are also considered.

Theoretical uncertainties in the predicted event yields arise from the limited knowledge of PDFs, αS, QCD and

electroweak K-factors, and Z/γ∗

cross section. The finite available MC statistics are also taken into account. The uncertainty due to the PDF and αSis estimated using the

Mstw2008 PDF eigenvector set and additional PDFs corresponding to variations in αS. The resulting effect

is about 4% at the Z pole growing with increasing mass to 10% at 1.5 TeV. Uncertainties due to QCD and elec-troweak K-factors are estimated to grow from 0.3% and 0.4% at the Z pole to 3% and 4.5% at 1.5 TeV, respec-tively, for both electron and muon channels. Estimates for the QCD K-factor are obtained by varying the renor-malization and factorization scales independently by fac-tors of two, then adding the impact of those variations linearly. The uncertainty in the electroweak K-factor is evaluated from the effect of neglecting real boson emis-sion, varying the electroweak scheme definitions as im-plemented in Pythia and Horace, as well as of the ef-fect of higher order electroweak and O(ααS) corrections.

For the electron channel, the QCD background estimate is subject to an uncertainty derived from a comparison with different background estimate methods (see discus-sion above). For the muon channel, the QCD background uncertainty is negligible.

Experimental uncertainties originate from the en-ergy/momentum resolution, as well as the trigger, re-construction and identification efficiencies. In the elec-tron channel, the uncertainty in the constant term, which dominates the energy resolution at high energy, has a negligible impact on the analysis. Knowledge of the en-ergy scale also has a negligible effect. The electron re-construction and identification uncertainty results in a 1.5% effect, which is estimated by studying the impact of the isolation requirement on the dielectron mass distri-bution. In the muon channel, the momentum resolution is dominated by the quality of the muon spectrometer alignment. The uncertainty in the alignment is evalu-ated directly from dedicevalu-ated toroid field-off runs and re-dundant momentum measurements in overlapping small and large chambers. These experimental uncertainties are found to have minimal impact on the dimuon mass distribution. Finally, a systematic error growing from 0.3% at the Z pole to 4.5% at 1.5 TeV is assigned to the muon reconstruction efficiency to account conserva-tively for its small pT dependence due to occasional large

energy loss from bremsstrahlung.

STATISTICAL ANALYSIS

The data analysis proceeds with a Bayesian method to compare the observed event yields with the expected

TABLE I. Expected and observed numbers of events in the electron channel. The total expected yield is normalized to the data in the Z peak control region between 70 and 110 GeV. The errors quoted originate from both systematic uncertainties and limited MC statistics, except the error on the total expected yield in the normalization region which is given by the square root of the number of observed events.

mee[GeV] 70-110 110-130 130-150 150-170 170-200 200-240 240-300 DY 258482 ± 410 3185 ± 110 1183 ± 46 608 ± 28 473 ± 24 312 ± 18 196 ± 9 t¯t 218 ± 36 87 ± 7 64 ± 5 51 ± 2 51 ± 2 37 ± 2 30 ± 1 Dibosons 368 ± 19 31 ± 2 24 ± 2 15 ± 1 16 ± 1 14 ± 1 8 ± 1 W+jets 150 ± 100 57 ± 11 40 ± 9 27 ± 6 26 ± 6 20 ± 5 14 ± 4 QCD 332 ± 60 80 ± 42 50 ± 20 32 ± 7 29 ± 7 19 ± 15 12 ± 9 Total 259550 ± 510 3440 ± 120 1361 ± 50 733 ± 30 595 ± 25 401 ± 24 260 ± 13 Data 259550 3419 1362 758 578 405 256 mee[GeV] 300-400 400-550 550-800 800-1200 1200-1800 1800-3000 DY 105.0 ± 5.0 41.0 ± 2.2 12.8 ± 0.8 2.5 ± 0.2 0.29 ± 0.05 <0.05 t¯t 14.9 ± 0.8 4.5 ± 0.2 1.0 ± 0.1 0.10 ± 0.02 <0.05 <0.05 Dibosons 7.5 ± 1.1 2.1 ± 0.4 1.0 ± 0.3 0.3 ± 0.1 <0.05 <0.05 W+jets 9.0 ± 3.2 3.5 ± 1.6 1.0 ± 0.7 0.2 ± 0.3 <0.05 <0.05 QCD 5.5 ± 4.4 1.5 ± 1.2 0.3 ± 0.2 <0.05 <0.05 <0.05 Total 141.9 ± 8.0 52.6 ± 3.0 16.1 ± 1.1 3.0 ± 0.4 0.33 ± 0.05 <0.05 Data 147 48 17 3 0 0

TABLE II. Expected and observed numbers of events in the dimuon channel. The total expected yield is normalized to the data in the Z peak control region between 70 and 110 GeV. The errors quoted originate from both systematic uncertainties and limited MC statistics, except the error on the total expected yield in the normalization region which is given by the square root of the number of observed events.

mµµ[GeV] 70-110 110-130 130-150 150-170 170-200 200-240 240-300 DY 236405 ± 320 3133 ± 90 1076 ± 36 548 ± 22 417 ± 18 249 ± 13 153 ± 7 t¯t 193 ± 21 70 ± 9 51 ± 7 34 ± 4 38 ± 4 30 ± 3 21 ± 2 Diboson 307 ± 16 25 ± 2 19 ± 2 13 ± 2 12 ± 1 10 ± 1 8 ± 1 W+jets 1 ± 1 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 QCD 1 ± 1 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 Total 236908 ± 490 3229 ± 90 1147 ± 37 595 ± 22 467 ± 19 290 ± 13 182 ± 8 Data 236908 3211 1132 621 443 279 195 mµµ[GeV] 300-400 400-550 550-800 800-1200 1200-1800 1800-3000 DY 80.8 ± 3.9 31.0 ± 1.7 9.2 ± 0.6 1.8 ± 0.2 0.22 ± 0.04 <0.05 t¯t 11.7 ± 1.2 3.5 ± 0.3 0.7 ± 0.1 0.06 ± 0.02 <0.05 <0.05 Diboson 6.7 ± 1.1 1.0 ± 0.4 0.7 ± 0.3 <0.05 <0.05 <0.05 W+jets <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 QCD <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 Total 99.3 ± 4.2 35.5 ± 1.8 10.6 ± 0.7 1.9 ± 0.2 0.22 ± 0.04 <0.05 Data 83 39 12 5 0 0

yields for a range of different contact interaction model parameters. Specifically, the number µ of expected events in each of the mass bins defined in Tables I and II is

µ = nDY+CI(θ, ¯ν) + nnon−DY bg(¯ν) , (3)

where nDY+CI(θ, ¯ν) is the number of events predicted by

the Pythia DY+CI MC for a particular choice of con-tact interaction model parameter θ, nnon−DY bg(¯ν) is the

number of non-DY background events, and ¯ν represents the set of Gaussian nuisance parameters that account for systematic uncertainties in these numbers as discussed

above. The parameter θ corresponds to a choice of energy scale Λ and interference parameter ηLL. The complete

set of µ values used in this analysis is shown in Tables III and IV for the electron and muon channels, respectively. For each mass bin, a second order polynomial is used to model the dependence of µ on 1/Λ2_.

The likelihood of observing a set of ¯n events in N in-variant mass bins is given by a product of Poisson prob-abilities for each mass bin k:

L(¯n | θ, ¯ν) = N Y k=1 µnk k e−µk nk! . (4)

TABLE III. Expected numbers of events in the signal region of the analysis for various contact interaction scales with con-structive (Λ−_{) and destructive (Λ}+_{) interference in the electron channel. The errors quoted originate from both systematic}

uncertainties and limited MC statistics.

mee[GeV] 150-170 170-200 200-240 240-300 300-400 Λ−_{= 3 TeV 785 ± 29 649 ± 26 467 ± 22 383 ± 19 343 ± 12} Λ−_{= 4 TeV 781 ± 28 647 ± 26 437 ± 21 326 ± 17 223 ± 7} Λ−_{= 5 TeV 734 ± 27 612 ± 24 405 ± 19 298 ± 16 181 ± 6} Λ−_{= 7 TeV 691 ± 26 638 ± 25 406 ± 19 259 ± 15 163 ± 5} Λ−_{= 12 TeV 721 ± 26 604 ± 24 336 ± 17 234 ± 14 149 ± 5} Λ+ = 3 TeV 770 ± 28 642 ± 24 424 ± 20 331 ± 17 269 ± 9 Λ+ = 4 TeV 745 ± 27 591 ± 23 385 ± 19 277 ± 16 166 ± 5 Λ+_{= 5 TeV 702 ± 25 607 ± 23 350 ± 17 258 ± 15 151 ± 5} Λ+ = 7 TeV 672 ± 25 600 ± 23 399 ± 19 251 ± 14 142 ± 5 Λ+ = 12 TeV 749 ± 27 593 ± 23 403 ± 19 274 ± 15 137.9 ± 4.4 mee[GeV] 400-550 550-800 800-1200 1200-1800 1800-3000 Λ−_{= 3 TeV 286 ± 11 269 ± 12 207 ± 11 112 ± 8 30.3 ± 3.4} Λ−_{= 4 TeV 132 ± 5 109.1 ± 4.9 80.5 ± 4.1 29.8 ± 2.3 10.9 ± 1.3} Λ−_{= 5 TeV 82.7 ± 3.6 57.3 ± 2.6 35.5 ± 1.9 14.8 ± 1.1 3.9 ± 0.5} Λ−_{= 7 TeV 68.3 ± 2.9 29.0 ± 1.4 11.2 ± 0.7 4.0 ± 0.4 1.08 ± 0.17} Λ−_{= 12 TeV 57.3 ± 2.6 18.8 ± 1.1 5.0 ± 0.4 1.00 ± 0.13 0.15 ± 0.05} Λ+ = 3 TeV 215 ± 8 239 ± 11 185 ± 10 107 ± 7 28.4 ± 3.2 Λ+_{= 4 TeV 100.9 ± 3.8 78.1 ± 3.6 60.7 ± 3.2 31.5 ± 2.2 8.7 ± 1.0} Λ+ = 5 TeV 64.4 ± 2.7 36.2 ± 1.8 25.4 ± 1.3 12.7 ± 0.9 3.5 ± 0.4 Λ+ = 7 TeV 56.3 ± 2.5 20.3 ± 1.1 7.7 ± 0.5 3.58 ± 0.28 0.83 ± 0.12 Λ+_{= 12 TeV 52.4 ± 2.4 14.4 ± 0.9 3.29 ± 0.28 0.46 ± 0.08 0.075 ± 0.026}

According to Bayes’ theorem, the posterior probability for the parameter θ given ¯n observed events is

P(θ | ¯n) = 1

ZLM(¯n | θ)P (θ), (5) where Z is a normalization constant and the marginal-ized likelihood LM corresponds to the likelihood after

all nuisance parameters have been integrated out. This integration is performed assuming that the nuisance pa-rameters are correlated across all mass bins and that they affect both signal and background expectations, except for the electroweak K-factor that only affects the DY and DY+CI components. The prior probability P (θ) is cho-sen to be flat in 1/Λ2_{, motivated by the form of Eq. (2).}

The 95% CL limit is then obtained by finding the value Λlim satisfyingR₀θlimP(θ | ¯n) dθ = 0.95, where θ = 1/Λ2.

The above calculations have been performed with the Bayesian Analysis Toolkit (BAT) [42].

RESULTS

To test the consistency between the data and the standard model, a likelihood ratio test was performed by producing a set of 1000 SM-like pseudoexperiments and comparing the likelihood ratio between the sig-nal+background and pure background hypotheses ob-tained in the data to the results of the pseudoexperi-ments. The signal+background likelihood is evaluated at

the Λ value that maximizes it. The derived p-value, cor-responding to the probability of observing a fluctuation in the pseudoexperiments that is at least as signal-like as that seen in the data (i.e. with a maximum likelihood ratio greater or equal to that obtained in the data), is es-timated to be 39% (79%) in the electron channel and 21% (5%) in the muon channel for constructive (destructive) interference. These values indicate that there is no sig-nificant evidence for contact interactions in the analyzed data and thus limits are set on the contact interaction scale Λ.

Using the Bayesian method described above, the ex-pected 95% CL lower limit values on the energy scale Λ are found to be 9.6±1.0 TeV for constructive interference (ηLL= −1) and 9.3±1.0 TeV for destructive interference

(ηLL = +1) in the electron channel. The corresponding

expected limits in the muon channel are 8.9±0.9 TeV and 8.6±0.9 TeV. The quoted uncertainties correspond to the 68% range of limits surrounding the median value of all limits obtained with a set of 1000 pseudoexperiments. Stronger limits are expected in the electron channel than in the muon channel due to the significantly larger ac-ceptance for the dielectron selection.

The observed limits (at 95% CL) are Λ− _{> 10.1 TeV}

(Λ+_{> 9.4 TeV) in the electron channel and Λ}−_{> 8.0 TeV}

(Λ+ _{> 7.0 TeV) in the muon channel for constructive}

(destructive) interference. These limits are summarized in Table V.

If instead of choosing the prior to be flat in 1/Λ2_{, it}

TABLE IV. Expected numbers of events in the signal region of the analysis for various contact interaction scales with con-structive (Λ−_{) and destructive (Λ}+_{) interference in the muon channel. The errors quoted originate from both systematic}

uncertainties and limited MC statistics.

mµµ[GeV] 150-170 170-200 200-240 240-300 300-400 Λ−_{= 3 TeV 638 ± 28 547 ± 26 371 ± 22 285 ± 19 263 ± 13} Λ−_{= 4 TeV 618 ± 27 513 ± 24 287 ± 18 228 ± 15 163 ± 7} Λ−_{= 5 TeV 572 ± 26 478 ± 23 357 ± 20 206 ± 14 131 ± 6} Λ−_{= 7 TeV 571 ± 25 496 ± 23 294 ± 18 187 ± 14 113 ± 5} Λ−_{= 12 TeV 606 ± 26 441 ± 22 252 ± 16 191 ± 14 100.0 ± 3.8} Λ+ = 3 TeV 602 ± 26 417 ± 21 332 ± 19 205 ± 15 186 ± 10 Λ+ = 4 TeV 575 ± 25 456 ± 22 286 ± 17 182 ± 13 112 ± 5 Λ+_{= 5 TeV 554 ± 25 483 ± 23 289 ± 17 167 ± 12 102.0 ± 4.0} Λ+ = 7 TeV 557 ± 24 435 ± 21 292 ± 18 196 ± 14 102.0 ± 4.4 Λ+ = 12 TeV 576 ± 25 421 ± 21 256 ± 16 186 ± 13 100.0 ± 3.9 mµµ[GeV] 400-550 550-800 800-1200 1200-1800 1800-3000 Λ−_{= 3 TeV 192 ± 11 185 ± 12 151 ± 12 60 ± 9 18 ± 7} Λ−_{= 4 TeV 97 ± 5 73 ± 5 50.3 ± 4.0 17.9 ± 2.9 6.8 ± 2.4} Λ−_{= 5 TeV 62.8 ± 3.3 37.5 ± 2.2 21.8 ± 1.8 8.7 ± 1.3 2.8 ± 1.0} Λ−_{= 7 TeV 45.9 ± 2.4 19.5 ± 1.2 7.9 ± 0.7 3.0 ± 0.5 0.59 ± 0.38} Λ−_{= 12 TeV 38.0 ± 2.0 14.0 ± 0.9 2.90 ± 0.26 0.91 ± 0.24 0.11 ± 0.14} Λ+ = 3 TeV 152 ± 9 153 ± 11 131 ± 11 63 ± 8 19 ± 6 Λ+_{= 4 TeV 68.2 ± 3.7 54.7 ± 3.7 42.8 ± 3.4 22.2 ± 2.7 7.4 ± 2.1} Λ+ = 5 TeV 51.0 ± 2.5 24.9 ± 1.6 15.7 ± 1.3 8.0 ± 1.0 2.9 ± 0.8 Λ+ = 7 TeV 33.9 ± 1.8 13.3 ± 0.8 5.04 ± 0.43 1.85 ± 0.32 0.63 ± 0.30 Λ+_{= 12 TeV 34.0 ± 1.8 10.1 ± 0.6 1.73 ± 0.18 0.25 ± 0.12 0.07 ± 0.10}

pure CI term in Eq. (2), the observed limit in the elec-tron channel becomes weaker by 0.9 TeV (0.8 TeV) for constructive (destructive) interference. The correspond-ing shift to lower values is 0.4 TeV (0.3 TeV) in the muon channel, see Table V.

The quoted limits have been obtained with electroweak corrections applied to both DY and DY+CI samples for consistency, although part of the electroweak corrections cannot be computed reliably due to the unknown new physics represented by the contact interaction. However, this particular choice does not affect the observed limits significantly and results in slightly more conservative lim-its. The limits obtained without electroweak corrections improve by less than 0.1 TeV.

Finally, a limit is set on the combination of the elec-tron and muon channels, assuming lepton universality, by computing a combined posterior probability for the two channels. The following sources of systematic un-certainty are treated as fully correlated between the two channels: PDF and αS, QCD and electroweak K-factors,

and Z/γ∗

cross section for normalization. The resulting combined limits are Λ−

> 10.2 TeV and Λ+ _{> 8.8 TeV}

for the 1/Λ2_{prior. Table V summarizes all limits for the}

two priors considered in this study and shows that the combined limit is weaker than the electron channel limit in the case of destructive interference. This is due to the slight excess of events observed in the muon channel for masses greater than 800 GeV, an excess that gives an increase in the likelihood that is stronger for destructive interference than it is for constructive interference.

TABLE V. Expected and observed 95% CL lower limits on the contact interaction energy scale Λ for the electron and muon channels, as well as for the combination of those channels. Separate results are provided for the different choices of flat priors: 1/Λ2 _{and 1/Λ}4_.

Channel Prior Expected limit (TeV) Observed limit (TeV)

Constr. Destr. Constr. Destr.

e+ e− _1/Λ2 _{9.6 9.3 10.1 9.4} 1/Λ4 8.9 8.6 9.2 8.6 µ+_µ− _1/Λ2 _{8.9 8.6 8.0 7.0} 1/Λ4 _{8.3 7.9 7.6 6.7} Comb. 1/Λ2 10.4 10.1 10.2 8.8 1/Λ4 _{9.6 9.4 9.4 8.4} CONCLUSIONS

A search for contact interactions in e+_e−

and µ+_µ−

events produced in proton-proton collisions at √s = 7 TeV has been performed. The analysis uses early 2011 run data amounting to 1.08 (1.21) fb−1_{of pp collisions in}

the electron (muon) channel collected with the ATLAS detector. The dilepton mass distributions do not display significant deviations from the standard model. Using a Bayesian approach with a 1/Λ2 _{prior, as was done in}

most previous searches at hadron colliders, the follow-ing 95% CL limits are set on the energy scale of contact interactions: Λ− _{> 10.1 TeV (Λ}+ _{> 9.4 TeV) in the}

electron channel and Λ−

> 8.0 TeV (Λ+ _{> 7.0 TeV) in}

the muon channel for constructive (destructive) interfer-ence in the left-left isoscalar compositeness model. Some-what weaker limits are obtained with a prior flat in 1/Λ4_.

These limits are the most stringent to date on µµqq con-tact interactions and exceed the best existing limits set by a single experiment on eeqq contact interactions for light-quark flavors.

ACKNOWLEDGEMENTS

We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CON-ICYT, Chile; CAS, MOST and NSFC, China; COL-CIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; ARTEMIS, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNAS, Geor-gia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; RCN, Norway; MNiSW, Poland; GRICES and FCT, Portugal; MERYS (MECTS), Romania; MES of Rus-sia and ROSATOM, RusRus-sian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Can-tons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the Royal Society and Lever-hulme 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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M. Ciubancan25a_{, A. Clark}49_{, P.J. Clark}45_{, W. Cleland}122_{, J.C. Clemens}82_{, B. Clement}55_{, C. Clement}145a,145b_,

R.W. Clifft128_{, Y. Coadou}82_{, M. Cobal}163a,163c_{, A. Coccaro}171_{, J. Cochran}63_{, P. Coe}117_{, J.G. Cogan}142_,

J. Coggeshall164, E. Cogneras176, J. Colas4, A.P. Colijn104, N.J. Collins17, C. Collins-Tooth53, J. Collot55, G. Colon83_{, P. Conde Mui˜no}123a_{, E. Coniavitis}117_{, M.C. Conidi}11_{, M. Consonni}103_{, V. Consorti}48_,

S. Constantinescu25a_{, C. Conta}118a,118b_{, F. Conventi}101a,i_{, J. Cook}29_{, M. Cooke}14_{, B.D. Cooper}76_,

A.M. Cooper-Sarkar117_{, K. Copic}14_{, T. Cornelissen}173_{, M. Corradi}19a_{, F. Corriveau}84,j_{, A. Cortes-Gonzalez}164_,

G. Cortiana98_{, G. Costa}88a_{, M.J. Costa}166_{, D. Costanzo}138_{, T. Costin}30_{, D. Cˆot´e}29_{, R. Coura Torres}23a_,

L. Courneyea168_{, G. Cowan}75_{, C. Cowden}27_{, B.E. Cox}81_{, K. Cranmer}107_{, F. Crescioli}121a,121b_{, M. Cristinziani}20_,

G. Crosetti36a,36b_{, R. Crupi}71a,71b_{, S. Cr´ep´e-Renaudin}55_{, C.-M. Cuciuc}25a_{, C. Cuenca Almenar}174_,

T. Cuhadar Donszelmann138_{, M. Curatolo}47_{, C.J. Curtis}17_{, C. Cuthbert}149_{, P. Cwetanski}60_{, H. Czirr}140_,

P. Czodrowski43_{, Z. Czyczula}174_{, S. D’Auria}53_{, M. D’Onofrio}72_{, A. D’Orazio}131a,131b_{, P.V.M. Da Silva}23a_,

C. Da Via81_{, W. Dabrowski}37_{, T. Dai}86_{, C. Dallapiccola}83_{, M. Dam}35_{, M. Dameri}50a,50b_{, D.S. Damiani}136_,

H.O. Danielsson29_{, D. Dannheim}98_{, V. Dao}49_{, G. Darbo}50a_{, G.L. Darlea}25b_{, C. Daum}104_{, W. Davey}20_,

T. Davidek125_{, N. Davidson}85_{, R. Davidson}70_{, E. Davies}117,c_{, M. Davies}92_{, A.R. Davison}76_{, Y. Davygora}58a_,

E. Dawe141_{, I. Dawson}138_{, J.W. Dawson}5,∗_{, R.K. Daya-Ishmukhametova}22_{, K. De}7_{, R. de Asmundis}101a_,

S. De Castro19a,19b_{, P.E. De Castro Faria Salgado}24_{, S. De Cecco}77_{, J. de Graat}97_{, N. De Groot}103_{, P. de Jong}104_,

C. De La Taille114, H. De la Torre79, B. De Lotto163a,163c, L. de Mora70, L. De Nooij104, D. De Pedis131a,

A. De Salvo131a, U. De Sanctis163a,163c, A. De Santo148, J.B. De Vivie De Regie114, S. Dean76, W.J. Dearnaley70, R. Debbe24, C. Debenedetti45, D.V. Dedovich64, J. Degenhardt119, M. Dehchar117, C. Del Papa163a,163c,

J. Del Peso79_{, T. Del Prete}121a,121b_{, T. Delemontex}55_{, M. Deliyergiyev}73_{, A. Dell’Acqua}29_{, L. Dell’Asta}21_,

M. Della Pietra101a,i_{, D. della Volpe}101a,101b_{, M. Delmastro}4_{, N. Delruelle}29_{, P.A. Delsart}55_{, C. Deluca}147_,

S. Demers174_{, M. Demichev}64_{, B. Demirkoz}11,k_{, J. Deng}162_{, S.P. Denisov}127_{, D. Derendarz}38_{, J.E. Derkaoui}134d_,

F. Derue77_{, P. Dervan}72_{, K. Desch}20_{, E. Devetak}147_{, P.O. Deviveiros}104_{, A. Dewhurst}128_{, B. DeWilde}147_,

S. Dhaliwal157_{, R. Dhullipudi}24 ,l_{, A. Di Ciaccio}132a,132b_{, L. Di Ciaccio}4_{, A. Di Girolamo}29_{, B. Di Girolamo}29_,

S. Di Luise133a,133b_{, A. Di Mattia}171_{, B. Di Micco}29_{, R. Di Nardo}47_{, A. Di Simone}132a,132b_{, R. Di Sipio}19a,19b_,

M.A. Diaz31a_{, F. Diblen}18c_{, E.B. Diehl}86_{, J. Dietrich}41_{, T.A. Dietzsch}58a_{, S. Diglio}85_{, K. Dindar Yagci}39_,

J. Dingfelder20_{, C. Dionisi}131a,131b_{, P. Dita}25a_{, S. Dita}25a_{, F. Dittus}29_{, F. Djama}82_{, T. Djobava}51b_,

M.A.B. do Vale23c_{, A. Do Valle Wemans}123a_{, T.K.O. Doan}4_{, M. Dobbs}84_{, R. Dobinson}29,∗_{, D. Dobos}29_,

E. Dobson29,m_{, J. Dodd}34_{, C. Doglioni}49_{, T. Doherty}53_{, Y. Doi}65,∗_{, J. Dolejsi}125_{, I. Dolenc}73_{, Z. Dolezal}125_,

B.A. Dolgoshein95,∗_{, T. Dohmae}154_{, M. Donadelli}23d_{, M. Donega}119_{, J. Donini}33_{, J. Dopke}29_{, A. Doria}101a_,

A. Dos Anjos171_{, M. Dosil}11_{, A. Dotti}121a,121b_{, M.T. Dova}69_{, J.D. Dowell}17_{, A.D. Doxiadis}104_{, A.T. Doyle}53_,

Z. Drasal125_{, J. Drees}173_{, N. Dressnandt}119_{, H. Drevermann}29_{, C. Driouichi}35_{, M. Dris}9_{, J. Dubbert}98_{, S. Dube}14_,

E. Duchovni170_{, G. Duckeck}97_{, A. Dudarev}29_{, F. Dudziak}63_{, M. D¨uhrssen}29_{, I.P. Duerdoth}81_{, L. Duflot}114_,

M-A. Dufour84_{, L. Duguid}75_{, M. Dunford}29_{, H. Duran Yildiz}3a_{, R. Duxfield}138_{, M. Dwuznik}37_{, F. Dydak}29_,

M. D¨uren52_{, W.L. Ebenstein}44_{, J. Ebke}97_{, S. Eckweiler}80_{, K. Edmonds}80_{, C.A. Edwards}75_{, N.C. Edwards}53_,

W. Ehrenfeld41_{, T. Ehrich}98_{, T. Eifert}142_{, G. Eigen}13_{, K. Einsweiler}14_{, E. Eisenhandler}74_{, T. Ekelof}165_,

M. El Kacimi134c_{, M. Ellert}165_{, S. Elles}4_{, F. Ellinghaus}80_{, K. Ellis}74_{, N. Ellis}29_{, J. Elmsheuser}97_{, M. Elsing}29_,

D. Eriksson145a_{, J. Ernst}1_{, M. Ernst}24_{, J. Ernwein}135_{, D. Errede}164_{, S. Errede}164_{, E. Ertel}80_{, M. Escalier}114_,

C. Escobar122_{, X. Espinal Curull}11_{, B. Esposito}47_{, F. Etienne}82_{, A.I. Etienvre}135_{, E. Etzion}152_{, D. Evangelakou}54_,

H. Evans60_{, L. Fabbri}19a,19b_{, C. Fabre}29_{, R.M. Fakhrutdinov}127_{, S. Falciano}131a_{, Y. Fang}171_{, M. Fanti}88a,88b_,

A. Farbin7_{, A. Farilla}133a_{, J. Farley}147_{, T. Farooque}157_{, S.M. Farrington}117_{, P. Farthouat}29_{, P. Fassnacht}29_,

D. Fassouliotis8_{, B. Fatholahzadeh}157_{, A. Favareto}88a,88b_{, L. Fayard}114_{, S. Fazio}36a,36b_{, R. Febbraro}33_,

P. Federic143a_{, O.L. Fedin}120_{, W. Fedorko}87_{, M. Fehling-Kaschek}48_{, L. Feligioni}82_{, D. Fellmann}5_{, C. Feng}32d_,

E.J. Feng30_{, A.B. Fenyuk}127_{, J. Ferencei}143b_{, J. Ferland}92_{, W. Fernando}108_{, S. Ferrag}53_{, J. Ferrando}53_{, V. Ferrara}41_,

A. Ferrari165_{, P. Ferrari}104_{, R. Ferrari}118a_{, A. Ferrer}166_{, M.L. Ferrer}47_{, D. Ferrere}49_{, C. Ferretti}86_,

A. Ferretto Parodi50a,50b_{, M. Fiascaris}30_{, F. Fiedler}80_{, A. Filipˇciˇc}73_{, A. Filippas}9_{, F. Filthaut}103_,

M. Fincke-Keeler168_{, M.C.N. Fiolhais}123a,h_{, L. Fiorini}166_{, A. Firan}39_{, G. Fischer}41_{, P. Fischer}20_{, M.J. Fisher}108_,

M. Flechl48_{, I. Fleck}140_{, J. Fleckner}80_{, P. Fleischmann}172_{, S. Fleischmann}173_{, T. Flick}173_{, L.R. Flores Castillo}171_,

M.J. Flowerdew98_{, M. Fokitis}9_{, T. Fonseca Martin}16_{, D.A. Forbush}137_{, A. Formica}135_{, A. Forti}81_{, D. Fortin}158a_,

J.M. Foster81_{, D. Fournier}114_{, A. Foussat}29_{, A.J. Fowler}44_{, K. Fowler}136_{, H. Fox}70_{, P. Francavilla}11_,

S. Franchino118a,118b_{, D. Francis}29_{, T. Frank}170_{, M. Franklin}57_{, S. Franz}29_{, M. Fraternali}118a,118b_{, S. Fratina}119_,

S.T. French27_{, F. Friedrich}43_{, R. Froeschl}29_{, D. Froidevaux}29_{, J.A. Frost}27_{, C. Fukunaga}155_,

E. Fullana Torregrosa29_{, J. Fuster}166_{, C. Gabaldon}29_{, O. Gabizon}170_{, T. Gadfort}24_{, S. Gadomski}49_,

G. Gagliardi50a,50b_{, P. Gagnon}60_{, C. Galea}97_{, E.J. Gallas}117_{, V. Gallo}16_{, B.J. Gallop}128_{, P. Gallus}124_{, K.K. Gan}108_,

Y.S. Gao142,e_{, V.A. Gapienko}127_{, A. Gaponenko}14_{, F. Garberson}174_{, M. Garcia-Sciveres}14_{, C. Garc´ıa}166_{, J.E. Garc´ıa}

Navarro166_{, R.W. Gardner}30_{, N. Garelli}29_{, H. Garitaonandia}104_{, V. Garonne}29_{, J. Garvey}17_{, C. Gatti}47_,

G. Gaudio118a_{, O. Gaumer}49_{, B. Gaur}140_{, L. Gauthier}135_{, I.L. Gavrilenko}93_{, C. Gay}167_{, G. Gaycken}20_,

J-C. Gayde29, E.N. Gazis9, P. Ge32d, C.N.P. Gee128, D.A.A. Geerts104, Ch. Geich-Gimbel20, K. Gellerstedt145a,145b, C. Gemme50a_{, A. Gemmell}53_{, M.H. Genest}55_{, S. Gentile}131a,131b_{, M. George}54_{, S. George}75_{, P. Gerlach}173_,

A. Gershon152_{, C. Geweniger}58a_{, H. Ghazlane}134b_{, N. Ghodbane}33_{, B. Giacobbe}19a_{, S. Giagu}131a,131b_,

V. Giakoumopoulou8_{, V. Giangiobbe}11_{, F. Gianotti}29_{, B. Gibbard}24_{, A. Gibson}157_{, S.M. Gibson}29_{, L.M. Gilbert}117_,

V. Gilewsky90_{, D. Gillberg}28_{, A.R. Gillman}128_{, D.M. Gingrich}2,d_{, J. Ginzburg}152_{, N. Giokaris}8_{, M.P. Giordani}163c_,

R. Giordano101a,101b_{, F.M. Giorgi}15_{, P. Giovannini}98_{, P.F. Giraud}135_{, D. Giugni}88a_{, M. Giunta}92_{, P. Giusti}19a_,

B.K. Gjelsten116_{, L.K. Gladilin}96_{, C. Glasman}79_{, J. Glatzer}48_{, A. Glazov}41_{, K.W. Glitza}173_{, G.L. Glonti}64_,

J.R. Goddard74_{, J. Godfrey}141_{, J. Godlewski}29_{, M. Goebel}41_{, T. G¨opfert}43_{, C. Goeringer}80_{, C. G¨ossling}42_,

T. G¨ottfert98_{, S. Goldfarb}86_{, T. Golling}174_{, S.N. Golovnia}127_{, A. Gomes}123a,b_{, L.S. Gomez Fajardo}41_{, R. Gon¸calo}75_,

J. Goncalves Pinto Firmino Da Costa41_{, L. Gonella}20_{, A. Gonidec}29_{, S. Gonzalez}171_{, S. Gonz´alez de la Hoz}166_,

G. Gonzalez Parra11_{, M.L. Gonzalez Silva}26_{, S. Gonzalez-Sevilla}49_{, J.J. Goodson}147_{, L. Goossens}29_,

P.A. Gorbounov94_{, H.A. Gordon}24_{, I. Gorelov}102_{, G. Gorfine}173_{, B. Gorini}29_{, E. Gorini}71a,71b_{, A. Goriˇsek}73_,

E. Gornicki38_{, S.A. Gorokhov}127_{, V.N. Goryachev}127_{, B. Gosdzik}41_{, M. Gosselink}104_{, M.I. Gostkin}64_,

I. Gough Eschrich162_{, M. Gouighri}134a_{, D. Goujdami}134c_{, M.P. Goulette}49_{, A.G. Goussiou}137_{, C. Goy}4_,

S. Gozpinar22, I. Grabowska-Bold37, P. Grafstr¨om29, K-J. Grahn41, F. Grancagnolo71a, S. Grancagnolo15, V. Grassi147, V. Gratchev120, N. Grau34, H.M. Gray29, J.A. Gray147, E. Graziani133a, O.G. Grebenyuk120, T. Greenshaw72, Z.D. Greenwood24,l, K. Gregersen35, I.M. Gregor41, P. Grenier142, J. Griffiths137,

N. Grigalashvili64_{, A.A. Grillo}136_{, S. Grinstein}11_{, Y.V. Grishkevich}96_{, J.-F. Grivaz}114_{, M. Groh}98_{, E. Gross}170_,

J. Grosse-Knetter54_{, J. Groth-Jensen}170_{, K. Grybel}140_{, V.J. Guarino}5_{, D. Guest}174_{, C. Guicheney}33_,

A. Guida71a,71b_{, S. Guindon}54_{, H. Guler}84,n_{, J. Gunther}124_{, B. Guo}157_{, J. Guo}34_{, A. Gupta}30_{, Y. Gusakov}64_,

V.N. Gushchin127_{, A. Gutierrez}92_{, P. Gutierrez}110_{, N. Guttman}152_{, O. Gutzwiller}171_{, C. Guyot}135_{, C. Gwenlan}117_,

C.B. Gwilliam72_{, A. Haas}142_{, S. Haas}29_{, C. Haber}14_{, H.K. Hadavand}39_{, D.R. Hadley}17_{, P. Haefner}98_{, F. Hahn}29_,

S. Haider29_{, Z. Hajduk}38_{, H. Hakobyan}175_{, D. Hall}117_{, J. Haller}54_{, K. Hamacher}173_{, P. Hamal}112_{, M. Hamer}54_,

A. Hamilton144b,o_{, S. Hamilton}160_{, H. Han}32a_{, L. Han}32b_{, K. Hanagaki}115_{, K. Hanawa}159_{, M. Hance}14_{, C. Handel}80_,

P. Hanke58a_{, J.R. Hansen}35_{, J.B. Hansen}35_{, J.D. Hansen}35_{, P.H. Hansen}35_{, P. Hansson}142_{, K. Hara}159_{, G.A. Hare}136_,

T. Harenberg173_{, S. Harkusha}89_{, D. Harper}86_{, R.D. Harrington}45_{, O.M. Harris}137_{, K. Harrison}17_{, J. Hartert}48_,

F. Hartjes104_{, T. Haruyama}65_{, A. Harvey}56_{, S. Hasegawa}100_{, Y. Hasegawa}139_{, S. Hassani}135_{, M. Hatch}29_{, D. Hauff}98_,

S. Haug16_{, M. Hauschild}29_{, R. Hauser}87_{, M. Havranek}20_{, B.M. Hawes}117_{, C.M. Hawkes}17_{, R.J. Hawkings}29_,

A.D. Hawkins78_{, D. Hawkins}162_{, T. Hayakawa}66_{, T. Hayashi}159_{, D. Hayden}75_{, H.S. Hayward}72_{, S.J. Haywood}128_,

E. Hazen21_{, M. He}32d_{, S.J. Head}17_{, V. Hedberg}78_{, L. Heelan}7_{, S. Heim}87_{, B. Heinemann}14_{, S. Heisterkamp}35_,

L. Helary4_{, C. Heller}97_{, M. Heller}29_{, S. Hellman}145a,145b_{, D. Hellmich}20_{, C. Helsens}11_{, R.C.W. Henderson}70_,

M. Henke58a_{, A. Henrichs}54_{, A.M. Henriques Correia}29_{, S. Henrot-Versille}114_{, F. Henry-Couannier}82_{, C. Hensel}54_,

T. Henß173_{, C.M. Hernandez}7_{, Y. Hern´andez Jim´enez}166_{, R. Herrberg}15_{, A.D. Hershenhorn}151_{, G. Herten}48_,

R. Hertenberger97_{, L. Hervas}29_{, N.P. Hessey}104_{, E. Hig´on-Rodriguez}166_{, D. Hill}5,∗_{, J.C. Hill}27_{, N. Hill}5_,

K.H. Hiller41_{, S. Hillert}20_{, S.J. Hillier}17_{, I. Hinchliffe}14_{, E. Hines}119_{, M. Hirose}115_{, F. Hirsch}42_{, D. Hirschbuehl}173_,