Measurement of the W⁺W⁻ Cross Section in √s=7  TeV <em>pp</em> Collisions with ATLAS

Article

Reference

Measurement of the W

W

Cross Section in √s=7  TeV pp Collisions with ATLAS

ATLAS Collaboration

ABDELALIM ALY, Ahmed Aly (Collab.), et al.

Abstract

This Letter presents a measurement of the W+W− production cross section in √s=7  TeV pp collisions by the ATLAS experiment, using 34  pb−1 of integrated luminosity produced by the Large Hadron Collider at CERN. Selecting events with two isolated leptons, each either an electron or a muon, 8 candidate events are observed with an expected background of 1.7±0.6 events. The measured cross section is 41+20−16(stat)±5(syst)±1(lumi)  pb, which is consistent with the standard model prediction of 44±3  pb calculated at next-to-leading order in QCD.

ATLAS Collaboration, ABDELALIM ALY, Ahmed Aly (Collab.), et al . Measurement of the W

W

Cross Section in √s=7  TeV pp Collisions with ATLAS. Physical Review Letters , 2011, vol.

107, no. 04, p. 041802

DOI : 10.1103/PhysRevLett.107.041802

Available at:

http://archive-ouverte.unige.ch/unige:41126

Disclaimer: layout of this document may differ from the published version.

Measurement of the W

W

Cross Section in ffiffiffi p s

¼ 7 TeV pp Collisions with ATLAS

G. Aadet al.* (ATLAS Collaboration)

(Received 27 April 2011; published 20 July 2011)

This Letter presents a measurement of the W^þW production cross section in ffiffiffi ps

¼7 TeV pp collisions by the ATLAS experiment, using34 pb¹ of integrated luminosity produced by the Large Hadron Collider at CERN. Selecting events with two isolated leptons, each either an electron or a muon, 8 candidate events are observed with an expected background of1:70:6events. The measured cross section is41^þ20₁₆ðstatÞ 5ðsystÞ 1ðlumiÞpb, which is consistent with the standard model prediction of 443 pbcalculated at next-to-leading order in QCD.

DOI:10.1103/PhysRevLett.107.041802 PACS numbers: 14.70.Fm, 12.15.Ji, 13.38.Be, 13.85.Qk

TheW^þW process plays an important role in electro- weak physics. The production rate and kinematic distribu- tions ofW^þWare sensitive to the triple gauge couplings of theW boson [1,2] andW^þW production is an impor- tant background to standard model Higgs boson searches.

For these reasons, the measurement of theW^þWproduc- tion cross section in 7 TeVppcollisions is a milestone in the Large Hadron Collider (LHC) physics program.

W^þWproduction has been previously measured in both e^þe collisions [1] and pp collisions [2], and was also recently measured in pp collisions [3]. In the standard model, the largest production mechanisms ofW^þWpro- ceed via s-channel and t-channel quark annihilation [4], followed by the gluon fusion process [5], which is next-to- next-to-leading order but is enhanced by the large gluon- gluon parton luminosity at the LHC.

CandidateW^þW events are reconstructed in the lep- tonic‘‘decay channel where each‘is either an elec- tron or a muon; included in this selection is W^þW production in which either or both W bosons decay to !‘. This channel provides a significantly better signal to background ratio than the semileptonic or had- ronic channels. Events consistent with pp!W^þWþ X!‘‘þX are selected by requiring two recon- structed oppositely-charged leptons and a large transverse momentum imbalance due to the neutrinos, which escape detection. There are four main backgrounds, all of compa- rable size: (i)Wþjets production with a jet misidentified as a lepton; (ii) Drell-Yan production, which includes Z=!‘‘ where the observed momentum imbalance is due to mismeasurements and Z= !!‘‘þ4; (iii) top production (ttandWt), which also produces two Wbosons, but is not considered signal and is suppressed by

vetoing candidates with jets; (iv) other diboson processes, which includeWZproduction decaying to‘‘‘where one charged lepton is lost,ZZwith oneZdecaying to charged leptons and oneZdecaying to neutrinos, andWwith the photon misidentified as an electron.

The ATLAS detector [6] has a cylindrical geometry [7]

and consists of an inner tracking detector surrounded by a 2 T superconducting solenoid, electromagnetic and had- ronic calorimeters, and a muon spectrometer with a toroidal magnetic field. The inner detector provides precision track- ing for charged particles forjj<2:5. It consists of silicon pixel and strip detectors surrounded by a straw tube tracker that also provides transition radiation measurements for electron identification. The calorimeter system covers the pseudorapidity rangejj<4:9. Forjj<2:5, the electro- magnetic calorimeter is finely segmented and plays an important role in electron identification. The muon spec- trometer has separate trigger and high-precision tracking chambers coveringjj<2:7. The transverse energyET is defined to beEsin, whereEis the energy associated with a calorimeter cell or energy cluster. Similarly, p_T is the momentum component transverse to the beam line.

A three-level trigger system selects events to record for offline analysis. During the data-taking period, the selec- tions for at least one electron or muon were made pro- gressively stricter, culminating in anE_T>15 GeV single electron or p_T>13 GeV single muon requirement. The results presented here use a data sample corresponding to 34 pb¹ collected during 2010, where the subsystems de- scribed were operational.

The signal acceptance is determined from a detailed Monte Carlo simulation. Theqq!W^þWsignal is simu- lated up to next-to-leading order in QCD withMC@NLO[8]

and the gluon fusion process is simulated withGG2WW[9];

the CTEQ6.6 [10] and CTEQ6M [11] parton distribution functions (PDFs), respectively, are used. HERWIG [12] is used to model W leptonic decays, parton showers, and hadronization, and JIMMY [13] is used to simulate the underlying event. The detector response simulation [14]

is based on the GEANT4program [15]. For the table and

*Full author list given at the end of the article.

Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- bution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

figures in this Letter, the standard model expectation for theW^þWsignal is normalized to443 pb, which is the sum of the quark annihilation (97%) and gluon fusion (3%) processes, as calculated byMC@NLOandGG2WW.

The luminosity in a single bunch-crossing was sufficient to produce multiple collisions, observed as multiple verti- ces, in the same recorded event. The vertex with the largest sump²_T of the associated tracks is selected as the primary vertex; this selects vertices with a few highpT tracks over those with many lowerp_T tracks. Additional inclusivepp collisions are also simulated to reproduce the vertex multi- plicity observed in data.

Electrons are reconstructed from a combination of a track found in the inner detector and an electromagnetic calorimeter energy cluster withjj<1:37or1:52<jj<

2:47to avoid the transition region between the barrel and the end-cap electromagnetic calorimeters. Candidate elec- trons must satisfy the ‘‘tight’’ selection [16], which re- quires the following measured quantities to be consistent with those from a promptly produced electron: shower shape, ratio of energy deposited in the hadronic to electromagnetic calorimeters, inner-detector track quality, track-to-shower matching, ratio of calorimeter energy measurement to track momentum, and transition radiation in the straw tube tracker. The electron is required to be isolated such that the sum ofETfor calorimeter energy in a cone of sizeR ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

²þ²

p of 0.3 around the elec- tron is less than 6 GeV, excluding energy associated with the electron cluster. The overall electron reconstruction and identification efficiency is measured from data using W !eand Z!ee candidates. It varies from 78% for the central region (jj<0:8) to 64% in the forward region (2:0<jj<2:47) with a statistical uncertainty of less than 0.4% and a systematic uncertainty of 5% [16] aver- aged over rapidity. The systematic uncertainty is due to background uncertainties in theW andZsamples and the consistency of efficiencies derived from the two samples.

Muon candidates are formed by associating muon spec- trometer (MS) tracks with inner detector (ID) tracks after accounting for energy loss in the calorimeter. A common transverse momentum is determined using a statistical combination of the two tracks and is required to havejj<

2:4. To reject muons from charged or K decays and charged particles from the beam-induced backgrounds, the MS muonp_T must exceed 10 GeV and be consistent with the ID measurement,jp^ID_T p^MS_T j=p^ID_T <0:5. To suppress muons originating from hadronic jets, the sum of thep_T of other tracks with p_T>1 GeV in a cone of R¼0:2 around the muon candidate divided by the muon p_T is required to be less than 0.1. The muon reconstruction and isolation efficiencies are measured in data usingZ! candidates to obtain a combined efficiency of 92 1ðstatÞ 1ðsystÞ%, where the systematic uncertainty is dominated by variations between data-taking periods due to additional collisions in the events [16].

Jets used to discriminate top from W^þW production are reconstructed from calorimeter clusters using the anti-k_T algorithm [17] with a resolution parameter of R¼0:4. Jets within a R <0:3 of an electron are not used because the electrons are in general also reconstructed as jets. The jet energies are calibrated using E_T- and -dependent correction factors [18] based on simulation and validated by test beam and collision data.

In order to suppress the Drell-Yan background, a mo- mentum imbalance of the visible collision products in the plane transverse to the beam axis is required. For this purpose, missing transverse energy is defined as ~E^missT ¼ P ~ET, where ~ET indicate the 2-dimensional transverse vectors for the reconstructed clusters of energy in the calorimeter in the range jj<4:5 and muon momenta.

Since the ~E^missT variable is sensitive to the mismeasurement of an individual lepton or jet, the relative missing trans- verse energy is defined as

E^miss_T;rel¼

E^miss_T sinðÞ if < =2 E^miss_T if =2;

where is the difference in the azimuthal angle between ~E^missT and the nearest lepton or jet. This definition allows events to be removed when ~E^missT points along a lepton direction, which occurs when the lepton momentum is measured lower than the true value or, for events with the two leptons moving in approximately opposite directions, higher than the true value. This generally reduces the contribution from mismeasured leptons giving a higher signal to background ratio than a direct requirement on j~E^missT j. Two important cases are high-mass muonic Drell- Yan events, where the momentum resolution can be com- parable toj~E^missT jinW^þWevents, andZ!, where the real ~E^missT from leptonicdecays is parallel to the momenta of the leptons.

Candidates are selected with two opposite-sign charged leptons withp_T>20 GeV. The leptons are required to be consistent with coming from a primary vertex with at least three associated tracks, which makes the cosmic ray back- ground negligible. For the ee and final states, the resulting sample is dominantly Z!ee and Z! events, while ecandidates are a mix ofZ!,tt, and other backgrounds. In order to suppress Z= !ee and Z= !, ee and events with an invariant mass near theZmass,jm‘‘mZj<10 GeV, orm‘‘<15 GeV are removed and the remainder are required to have E^miss_T;rel>40 GeV. For e events, a less stringent require- mentE^miss_T;rel>20 GeVis made. In order to suppress thett contribution, candidates containing jets withp_T>20 GeV andjj<3are removed. Figure1shows theE^miss_T;rel distri- butions separately for same-flavor (ee and ) and e events with all selections applied except for the E^miss_T;rel requirement; also shown is the number of jets with p_T>20 GeV after final selection except for the jet-veto PRL107,041802 (2011) P H Y S I C A L R E V I E W L E T T E R S 22 JULY 2011

requirement. In all three distributions, the data in the background regions are in agreement with the standard model expectation and the signal is clearly visible.

The acceptance, which converts the observed yield in the kinematically restricted signal region to the inclusive W^þW cross section, is derived from simulation and is corrected with scale factors based on measurements in independent data samples. The scale factors correct for the difference in trigger, lepton reconstruction and identi- fication, and jet-veto efficiencies between data and simu- lation. The efficiency to pass the trigger criteria is close to unity and has small statistical and systematic uncertainties.

For the lepton reconstruction and identification, the scale factors differ from unity by at most a few percent, indicat- ing the accuracy of the simulation, and have systematic uncertainties derived from the efficiency measurements described above. A small smearing is added to the muon p_Tin simulation, so that it replicates theZ!invariant mass distribution in data. The acceptance uncertainty due to the PDF uncertainties is 1.2%.

There are two major sources of systematic uncertainty in the jet-veto efficiency. The first is the modeling of jet production in association withW^þW due to initial state radiation, radiation from the internal line in thet-channel diagram, and additional parton collisions in the samepp collision. The second is the jet-energy scale, which is the correspondence between the true particle jet p_T and the reconstructed jet p_T. To minimize the systematic uncer- tainty due to these two effects, control samples ofZ!‘‘

are used. These are sufficiently large and pure that the jet- veto efficiency can be directly measured and compared to simulation using the same QCD modeling as theW^þW signal. The ratio of the observed to the simulated zero-jet fraction in the Z!‘‘ sample to simulation is used to define a jet-veto scale factor of 0:970:06. The uncer- tainty is due to differences between the jet-veto efficiency inZandW^þWevents which is assessed including effects from the choice of renormalization and fragmentation scales [19].

The overall selection acceptances for signal events are 4:10:1%foree,8:60:1%for , and11:5 0:6%fore. The relative acceptance in event selection are lepton acceptance and identification (18%, 41%, 27%) and the m_ll (85%, 84%, 100%),E^miss_T;rel (41%, 43%, 69%), and jet-veto (64%, 59%, 61%) requirements, where the three percentages indicate the ee,, and echannels, respectively, and each factor is relative to the previous requirement. The contributions from W^þW production where one or both W bosons decays to awhich subse- quently decays to aneorare less than 10% of the final selectedW^þW signal events in all three channels.

With the exception of Wþjets, the backgrounds are derived from simulations, corrected with the same scale factors as applied to the modeling of the signal acceptance.

The backgrounds are scaled to the data sample based on the integrated luminosity and predicted cross sections. The top and WZ processes are simulated with MC@NLO, the ZZ process is simulated with HERWIG, the W is simulated withmadgraphþ^pythia[20,21], and the Drell-Yan process is simulated withALPGEN[22] andPYTHIA[20]. The QCD jet contribution, which is not significant after theE^miss_T;relcut, is modeled withPYTHIAin Fig.1, which includes data below theE^miss_T;relrequirement.

Like the signal acceptance, the background estimates have uncertainties due to the trigger, lepton reconstruction and identification, and jet-veto efficiencies, in addition to the uncertainties on the integrated luminosity and theoreti- cal cross sections. The Drell-Yan and top background estimates have additional uncertainties described below.

Most of the Drell-Yan events are removed by the dilepton invariant mass andE^miss_T;relrequirements, but because of the large cross section some remain as background. The un- certainty on this background due to the simulation ofE^miss_T;rel is assessed using a control sample of Z= !ee and Z= ! events in the Z mass peak region, jm_‘‘

m_Zj<10 GeV, passing a relaxed requirement ofE^miss_T;rel>

30 GeV. Despite theE^miss_T;relrequirement, this sample is still

0 20 40 60 80 100 120 140

10-1 1 10 102

103 ATLAS ∫Ldt = 34 pb^-1

=7TeV

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→e WW+µνµν

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Events / 10 GeV ¹

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ATLAS ∫

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s _Data

Drell-Yan QCD jet Diboson W+jets top

ν µ ν

→e WW

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Events

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ATLAS ∫

=7TeV

s Data

Drell-Yan QCD jet Diboson W+jets top

ν νl

→l Events / 10 GeV WW

[GeV]

miss T, rel

E E^miss_{T, rel} [GeV] Jet Multiplicity

FIG. 1 (color online). E^miss_T;reldistributions for the selectedeeand(left) ande(center) events and the multiplicity distribution for jets withpT>20 GeVandjj<3:0for all three dilepton channels combined (right). The distributions show events with all selection criteria applied except forE^miss_T;relin theE^miss_T;reldistribution and the jet-veto in the jet multiplicity distribution. Simulation is used for the QCD jet andWþjets background contributions in these plots as opposed to the data-driven method used forWþjets in the signal region described in the text. The QCD jet contribution is negligible in the signal region.

dominated byZ!‘‘events in which the observed mo- mentum imbalance is due to a combination of detector resolution, limited detector coverage, and neutrinos from heavy flavor decays. A 64% systematic uncertainty is assigned based on the difference between the observed yield in data and the Monte Carlo prediction, which are statistically consistent.

The top background arises from ttandWt production where the two W bosons decay leptonically. Simulation based onMC@NLOis used to estimate the number of events passing the jet-veto requirement. Similar to the signal acceptance, there are two important systematic uncertain- ties on the jet-veto efficiency: the jet-energy scale and the amount of initial and final state radiation (ISR/FSR). The jet-energy calibration uncertainty [18] corresponds to a 40% change in the top background. The uncertainty due to the ISR/FSR modeling is estimated using theACERMC

[23] generator interfaced toPYTHIA[20], and varying the parameters controlling ISR and FSR in a range consistent with experimental data [24]. The resulting uncertainty of 32% is a combination of the shift in the prediction and the statistical uncertainty on the simulation.

W bosons produced in association with a jet that is misidentified as a lepton contribute to the selected sample.

The rate at which hadronic jets are misidentified as leptons may not be accurately described in the simulation, because these events are due to rare fragmentation processes or interactions with the detector. This background is therefore determined from data using control samples dominated by Wþjets events and subtracting all other components us- ing simulation. The ALPGENþ^HERWIGþ^JIMMY simula- tion of the Wþjets background used in Fig. 1 gives comparable results to this method. The Wþjets data samples are constructed by requiring one electron or muon passing the full selection criteria and a leptonlike jet, which is a reconstructed electron or muon that is selected as likely to be due to a jet. For electrons, the leptonlike jets are electromagnetic clusters matched to tracks in the inner detector that fail the full electron selec- tion. For muons, leptonlike jets are muon candidates that fail at least one of the requirements on isolation, distance from the primary vertex, or ID and MS consistency. These

events are otherwise required to pass the full event selec- tion, treating the leptonlike jet as if it were a fully identified lepton.

The Wþjets background is then estimated by scaling this control sample by a measuredp_T-dependent factorf. The factorfis the ratio of the probability for a jet to satisfy the full lepton identification criteria to the probability to satisfy the leptonlike jet criteria. The factorfis measured in a QCD jet data sample and corrected for the small contribution of true leptons to the sample using simulation.

The systematic uncertainty onfis 36% for both electrons and muons, and is determined from variations of f in different run periods and in data samples containing jets of different energies, which covers differences in the quark-gluon composition between the jets in the QCD jet andWþjets data samples.

The resulting signal and background expectations are shown in TableI. Eight events are observed in the data with a total expected background of1:70:6events. As shown in Fig. 2, the kinematic properties of the observed events are qualitatively consistent with the standard model expec- tation. To estimate the statistical significance of the signal, Poisson-distributed pseudoexperiments are generated, varying the expected background according to its uncer- tainty. The probability to observe 8 or more events in the absence of a signal is 1:210³, which corresponds to a significance of 3.0 standard deviations. The W^þW pro- duction cross section is determined using a maximum likelihood fitting method to combine the three dilepton channels. A cross section of _WþW¼41^þ20₁₆ðstatÞ 5ðsystÞ 1ðlumiÞ pbis measured. The luminosity uncer- tainty for this measurement is 3.4% [25]. The total system- atic uncertainty (11.5%) includes the signal acceptance and efficiency (A=A¼7:4%) and background estimation (N_b=N_b ¼33%) uncertainties. The dominant system- atics uncertainties are due to the jet-veto (7.5%), and the lepton selection and identification (4.3%).

The measured W^þW production cross section is in good agreement with the standard model prediction of 443 pb calculated at next-to-leading order in QCD and the recent measurement by CMS [3]. With the signifi- cantly larger integrated luminosities expected to be TABLE I. Summary of observed events and expected standard model signal and background contributions in the three dilepton channels and their combination. The first uncertainty is statistical, the second systematic.

Final State e^þeE^miss_T;rel ^þE^miss_T;rel eE^miss_T;rel Combined

Observed Events 1 2 5 8

ExpectedW^þW 0:790:020:09 1:610:040:14 4:450:060:44 6:850:070:66 Backgrounds

Drell-Yan 0:000:100:07 0:010:100:07 0:220:060:15 0:230:150:17

WZ,ZZ,W 0:050:010:01 0:100:010:01 0:230:050:02 0:380:040:04 Wþjets 0:080:050:03 0:000:290:10 0:460:120:17 0:540:320:21

Top 0:040:020:02 0:140:060:07 0:350:100:19 0:530:120:28

Total Background 0:170:110:08 0:250:310:15 1:260:170:31 1:680:370:42 PRL107,041802 (2011) P H Y S I C A L R E V I E W L E T T E R S 22 JULY 2011

provided by the LHC, this signal will form the basis of a research program that will include searches for the stan- dard model Higgs boson, anomalous triple gauge cou- plings, and other processes beyond the standard model.

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;

CONICYT, Chile; CAS, MOST and NSFC, China;

COLCIENCIAS, 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, Georgia;

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 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 partners 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 (Taiwan), RAL (UK) and BNL (USA) and in the Tier-2 facilities worldwide.

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(leading lepton) [GeV]

pT

50 100

Events / 10 GeV

0 2

4 Data

WW Bkg

∫Ldt = 34pb^-1

ATLAS s=7TeV

) [GeV]

l-

(l+

pT

0 50 100

Events / 20 GeV

0 5 10

Data WW Bkg

∫Ldt = 34pb^-1

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-)

+l φ(l

∆

0 1 2 3

Events / 0.4

0 2 4 6

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FIG. 2 (color online). Distributions of the leading lepton pT

(left), transverse momentum of the dilepton system (center), and azimuthal angle between the leptons (right) for the sum of the selectedee,andesamples compared to the expecta- tion. The gray band indicates the combined statistical and systematic uncertainty on the sum of the signal and background expectations.

M. Aharrouche,⁸¹S. P. Ahlen,²¹F. Ahles,⁴⁸A. Ahmad,¹⁴⁸M. Ahsan,⁴⁰G. Aielli,^133a,133bT. Akdogan,^18a T. P. A. A˚ kesson,⁷⁹G. Akimoto,¹⁵⁵A. V. Akimov,⁹⁴A. Akiyama,⁶⁷M. S. Alam,¹M. A. Alam,⁷⁶S. Albrand,⁵⁵ M. Aleksa,²⁹I. N. Aleksandrov,⁶⁵F. Alessandria,^89aC. Alexa,^25aG. Alexander,¹⁵³G. Alexandre,⁴⁹T. Alexopoulos,⁹

M. Alhroob,²⁰M. Aliev,¹⁵G. Alimonti,^89aJ. Alison,¹²⁰M. Aliyev,¹⁰P. P. Allport,⁷³S. E. Allwood-Spiers,⁵³ J. Almond,⁸²A. Aloisio,^102a,102bR. Alon,¹⁷¹A. Alonso,⁷⁹M. G. Alviggi,^102a,102bK. Amako,⁶⁶P. Amaral,²⁹ C. Amelung,²²V. V. Ammosov,¹²⁸A. Amorim,^124a,cG. Amoro´s,¹⁶⁷N. Amram,¹⁵³C. Anastopoulos,¹³⁹T. Andeen,³⁴

C. F. Anders,²⁰K. J. Anderson,³⁰A. Andreazza,^89a,89bV. Andrei,^58aM-L. Andrieux,⁵⁵X. S. Anduaga,⁷⁰ A. Angerami,³⁴F. Anghinolfi,²⁹N. Anjos,^124aA. Annovi,⁴⁷A. Antonaki,⁸M. Antonelli,⁴⁷S. Antonelli,^19a,19b A. Antonov,⁹⁶J. Antos,^144bF. Anulli,^132aS. Aoun,⁸³L. Aperio Bella,⁴R. Apolle,¹¹⁸G. Arabidze,⁸⁸I. Aracena,¹⁴³

Y. Arai,⁶⁶A. T. H. Arce,⁴⁴J. P. Archambault,²⁸S. Arfaoui,^29,dJ-F. Arguin,¹⁴E. Arik,^18a,aM. Arik,^18a A. J. Armbruster,⁸⁷O. Arnaez,⁸¹C. Arnault,¹¹⁵A. Artamonov,⁹⁵G. Artoni,^132a,132bD. Arutinov,²⁰S. Asai,¹⁵⁵ R. Asfandiyarov,¹⁷²S. Ask,²⁷B. A˚ sman,^146a,146bL. Asquith,⁵K. Assamagan,²⁴A. Astbury,¹⁶⁹A. Astvatsatourov,⁵²

G. Atoian,¹⁷⁵B. Aubert,⁴B. Auerbach,¹⁷⁵E. Auge,¹¹⁵K. Augsten,¹²⁷M. Aurousseau,^145aN. Austin,⁷³ R. Avramidou,⁹D. Axen,¹⁶⁸C. Ay,⁵⁴G. Azuelos,^93,eY. Azuma,¹⁵⁵M. A. Baak,²⁹G. Baccaglioni,^89a C. Bacci,^134a,134bA. M. Bach,¹⁴H. Bachacou,¹³⁶K. Bachas,²⁹G. Bachy,²⁹M. Backes,⁴⁹M. Backhaus,²⁰

E. Badescu,^25aP. Bagnaia,^132a,132bS. Bahinipati,²Y. Bai,^32aD. C. Bailey,¹⁵⁸T. Bain,¹⁵⁸J. T. Baines,¹²⁹ O. K. Baker,¹⁷⁵M. D. Baker,²⁴S. Baker,⁷⁷F. Baltasar Dos Santos Pedrosa,²⁹E. Banas,³⁸P. Banerjee,⁹³ Sw. Banerjee,¹⁶⁹D. Banfi,²⁹A. Bangert,¹³⁷V. Bansal,¹⁶⁹H. S. Bansil,¹⁷L. Barak,¹⁷¹S. P. Baranov,⁹⁴A. Barashkou,⁶⁵ A. Barbaro Galtieri,¹⁴T. Barber,²⁷E. L. Barberio,⁸⁶D. Barberis,^50a,50bM. Barbero,²⁰D. Y. Bardin,⁶⁵T. Barillari,⁹⁹ M. Barisonzi,¹⁷⁴T. Barklow,¹⁴³N. Barlow,²⁷B. M. Barnett,¹²⁹R. M. Barnett,¹⁴A. Baroncelli,^134aA. J. Barr,¹¹⁸

F. Barreiro,⁸⁰J. Barreiro Guimara˜es da Costa,⁵⁷P. Barrillon,¹¹⁵R. Bartoldus,¹⁴³A. E. Barton,⁷¹D. Bartsch,²⁰ V. Bartsch,¹⁴⁹R. L. Bates,⁵³L. Batkova,^144aJ. R. Batley,²⁷A. Battaglia,¹⁶M. Battistin,²⁹G. Battistoni,^89a F. Bauer,¹³⁶H. S. Bawa,^143,fB. Beare,¹⁵⁸T. Beau,⁷⁸P. H. Beauchemin,¹¹⁸R. Beccherle,^50aP. Bechtle,⁴¹H. P. Beck,¹⁶ M. Beckingham,⁴⁸K. H. Becks,¹⁷⁴A. J. Beddall,^18cA. Beddall,^18cS. Bedikian,¹⁷⁵V. A. Bednyakov,⁶⁵C. P. Bee,⁸³

M. Begel,²⁴S. Behar Harpaz,¹⁵²P. K. Behera,⁶³M. Beimforde,⁹⁹C. Belanger-Champagne,¹⁶⁶P. J. Bell,⁴⁹ W. H. Bell,⁴⁹G. Bella,¹⁵³L. Bellagamba,^19aF. Bellina,²⁹M. Bellomo,^119aA. Belloni,⁵⁷O. Beloborodova,¹⁰⁷ K. Belotskiy,⁹⁶O. Beltramello,²⁹S. Ben Ami,¹⁵²O. Benary,¹⁵³D. Benchekroun,^135aC. Benchouk,⁸³M. Bendel,⁸¹

B. H. Benedict,¹⁶³N. Benekos,¹⁶⁵Y. Benhammou,¹⁵³D. P. Benjamin,⁴⁴M. Benoit,¹¹⁵J. R. Bensinger,²² K. Benslama,¹³⁰S. Bentvelsen,¹⁰⁵D. Berge,²⁹E. Bergeaas Kuutmann,⁴¹N. Berger,⁴F. Berghaus,¹⁶⁹E. Berglund,⁴⁹ J. Beringer,¹⁴K. Bernardet,⁸³P. Bernat,⁷⁷R. Bernhard,⁴⁸C. Bernius,²⁴T. Berry,⁷⁶A. Bertin,^19a,19bF. Bertinelli,²⁹ F. Bertolucci,^122a,122bM. I. Besana,^89a,89bN. Besson,¹³⁶S. Bethke,⁹⁹W. Bhimji,⁴⁵R. M. Bianchi,²⁹M. Bianco,^72a,72b

O. Biebel,⁹⁸S. P. Bieniek,⁷⁷J. Biesiada,¹⁴M. Biglietti,^134a,134bH. Bilokon,⁴⁷M. Bindi,^19a,19bS. Binet,¹¹⁵ A. Bingul,^18cC. Bini,^132a,132bC. Biscarat,¹⁷⁷U. Bitenc,⁴⁸K. M. Black,²¹R. E. Blair,⁵J.-B. Blanchard,¹¹⁵ G. Blanchot,²⁹T. Blazek,^144aC. Blocker,²²J. Blocki,³⁸A. Blondel,⁴⁹W. Blum,⁸¹U. Blumenschein,⁵⁴ G. J. Bobbink,¹⁰⁵V. B. Bobrovnikov,¹⁰⁷S. S. Bocchetta,⁷⁹A. Bocci,⁴⁴C. R. Boddy,¹¹⁸M. Boehler,⁴¹J. Boek,¹⁷⁴

N. Boelaert,³⁵S. Bo¨ser,⁷⁷J. A. Bogaerts,²⁹A. Bogdanchikov,¹⁰⁷A. Bogouch,^90,aC. Bohm,^146aV. Boisvert,⁷⁶ T. Bold,^163,gV. Boldea,^25aN. M. Bolnet,¹³⁶M. Bona,⁷⁵V. G. Bondarenko,⁹⁶M. Boonekamp,¹³⁶G. Boorman,⁷⁶

C. N. Booth,¹³⁹S. Bordoni,⁷⁸C. Borer,¹⁶A. Borisov,¹²⁸G. Borissov,⁷¹I. Borjanovic,^12aS. Borroni,^132a,132b K. Bos,¹⁰⁵D. Boscherini,^19aM. Bosman,¹¹H. Boterenbrood,¹⁰⁵D. Botterill,¹²⁹J. Bouchami,⁹³J. Boudreau,¹²³

E. V. Bouhova-Thacker,⁷¹C. Boulahouache,¹²³C. Bourdarios,¹¹⁵N. Bousson,⁸³A. Boveia,³⁰J. Boyd,²⁹ I. R. Boyko,⁶⁵N. I. Bozhko,¹²⁸I. Bozovic-Jelisavcic,^12bJ. Bracinik,¹⁷A. Braem,²⁹P. Branchini,^134a G. W. Brandenburg,⁵⁷A. Brandt,⁷G. Brandt,¹⁵O. Brandt,⁵⁴U. Bratzler,¹⁵⁶B. Brau,⁸⁴J. E. Brau,¹¹⁴H. M. Braun,¹⁷⁴

B. Brelier,¹⁵⁸J. Bremer,²⁹R. Brenner,¹⁶⁶S. Bressler,¹⁵²D. Breton,¹¹⁵D. Britton,⁵³F. M. Brochu,²⁷I. Brock,²⁰ R. Brock,⁸⁸T. J. Brodbeck,⁷¹E. Brodet,¹⁵³F. Broggi,^89aC. Bromberg,⁸⁸G. Brooijmans,³⁴W. K. Brooks,^31b G. Brown,⁸²H. Brown,⁷E. Brubaker,³⁰P. A. Bruckman de Renstrom,³⁸D. Bruncko,^144bR. Bruneliere,⁴⁸S. Brunet,⁶¹ A. Bruni,^19aG. Bruni,^19aM. Bruschi,^19aT. Buanes,¹³F. Bucci,⁴⁹J. Buchanan,¹¹⁸N. J. Buchanan,²P. Buchholz,¹⁴¹ R. M. Buckingham,¹¹⁸A. G. Buckley,⁴⁵S. I. Buda,^25aI. A. Budagov,⁶⁵B. Budick,¹⁰⁸V. Bu¨scher,⁸¹L. Bugge,¹¹⁷ D. Buira-Clark,¹¹⁸O. Bulekov,⁹⁶M. Bunse,⁴²T. Buran,¹¹⁷H. Burckhart,²⁹S. Burdin,⁷³T. Burgess,¹³S. Burke,¹²⁹

E. Busato,³³P. Bussey,⁵³C. P. Buszello,¹⁶⁶F. Butin,²⁹B. Butler,¹⁴³J. M. Butler,²¹C. M. Buttar,⁵³ J. M. Butterworth,⁷⁷W. Buttinger,²⁷T. Byatt,⁷⁷S. Cabrera Urba´n,¹⁶⁷D. Caforio,^19a,19bO. Cakir,^3aP. Calafiura,¹⁴

G. Calderini,⁷⁸P. Calfayan,⁹⁸R. Calkins,¹⁰⁶L. P. Caloba,^23aR. Caloi,^132a,132bD. Calvet,³³S. Calvet,³³ PRL107,041802 (2011) P H Y S I C A L R E V I E W L E T T E R S 22 JULY 2011

R. Camacho Toro,³³A. Camard,⁷⁸P. Camarri,^133a,133bM. Cambiaghi,^119a,119bD. Cameron,¹¹⁷J. Cammin,²⁰ S. Campana,²⁹M. Campanelli,⁷⁷V. Canale,^102a,102bF. Canelli,³⁰A. Canepa,^159aJ. Cantero,⁸⁰L. Capasso,^102a,102b

M. D. M. Capeans Garrido,²⁹I. Caprini,^25aM. Caprini,^25aD. Capriotti,⁹⁹M. Capua,^36a,36bR. Caputo,¹⁴⁸ C. Caramarcu,^25aR. Cardarelli,^133aT. Carli,²⁹G. Carlino,^102aL. Carminati,^89a,89bB. Caron,^159aS. Caron,⁴⁸

G. D. Carrillo Montoya,¹⁷²A. A. Carter,⁷⁵J. R. Carter,²⁷J. Carvalho,^124a,hD. Casadei,¹⁰⁸M. P. Casado,¹¹ M. Cascella,^122a,122bC. Caso,^50a,50b,aA. M. Castaneda Hernandez,¹⁷²E. Castaneda-Miranda,¹⁷²

V. Castillo Gimenez,¹⁶⁷N. F. Castro,^124aG. Cataldi,^72aF. Cataneo,²⁹A. Catinaccio,²⁹J. R. Catmore,⁷¹A. Cattai,²⁹ G. Cattani,^133a,133bS. Caughron,⁸⁸D. Cauz,^164a,164cP. Cavalleri,⁷⁸D. Cavalli,^89aM. Cavalli-Sforza,¹¹ V. Cavasinni,^122a,122bA. Cazzato,^72a,72bF. Ceradini,^134a,134bA. S. Cerqueira,^23aA. Cerri,²⁹L. Cerrito,⁷⁵F. Cerutti,⁴⁷

S. A. Cetin,^18bF. Cevenini,^102a,102bA. Chafaq,^135aD. Chakraborty,¹⁰⁶K. Chan,²B. Chapleau,⁸⁵J. D. Chapman,²⁷ J. W. Chapman,⁸⁷E. Chareyre,⁷⁸D. G. Charlton,¹⁷V. Chavda,⁸²S. Cheatham,⁷¹S. Chekanov,⁵S. V. Chekulaev,^159a

G. A. Chelkov,⁶⁵M. A. Chelstowska,¹⁰⁴C. Chen,⁶⁴H. Chen,²⁴L. Chen,²S. Chen,^32cT. Chen,^32cX. Chen,¹⁷² S. Cheng,^32aA. Cheplakov,⁶⁵V. F. Chepurnov,⁶⁵R. Cherkaoui El Moursli,^135eV. Chernyatin,²⁴E. Cheu,⁶

S. L. Cheung,¹⁵⁸L. Chevalier,¹³⁶G. Chiefari,^102a,102bL. Chikovani,⁵¹J. T. Childers,^58aA. Chilingarov,⁷¹ G. Chiodini,^72aM. V. Chizhov,⁶⁵G. Choudalakis,³⁰S. Chouridou,¹³⁷I. A. Christidi,⁷⁷A. Christov,⁴⁸ D. Chromek-Burckhart,²⁹M. L. Chu,¹⁵¹J. Chudoba,¹²⁵G. Ciapetti,^132a,132bK. Ciba,³⁷A. K. Ciftci,^3aR. Ciftci,^3a D. Cinca,³³V. Cindro,⁷⁴M. D. Ciobotaru,¹⁶³C. Ciocca,^19a,19bA. Ciocio,¹⁴M. Cirilli,⁸⁷M. Ciubancan,^25aA. Clark,⁴⁹

P. J. Clark,⁴⁵W. Cleland,¹²³J. C. Clemens,⁸³B. Clement,⁵⁵C. Clement,^146a,146bR. W. Clifft,¹²⁹Y. Coadou,⁸³ M. Cobal,^164a,164cA. Coccaro,^50a,50bJ. Cochran,⁶⁴P. Coe,¹¹⁸J. G. Cogan,¹⁴³J. Coggeshall,¹⁶⁵E. Cogneras,¹⁷⁷ C. D. Cojocaru,²⁸J. Colas,⁴A. P. Colijn,¹⁰⁵C. Collard,¹¹⁵N. J. Collins,¹⁷C. Collins-Tooth,⁵³J. Collot,⁵⁵G. Colon,⁸⁴

P. Conde Muin˜o,^124aE. Coniavitis,¹¹⁸M. C. Conidi,¹¹M. Consonni,¹⁰⁴S. Constantinescu,^25aC. Conta,^119a,119b F. Conventi,^102a,iJ. Cook,²⁹M. Cooke,¹⁴B. D. Cooper,⁷⁷A. M. Cooper-Sarkar,¹¹⁸N. J. Cooper-Smith,⁷⁶K. Copic,³⁴

T. Cornelissen,^50a,50bM. Corradi,^19aF. Corriveau,^85,jA. Cortes-Gonzalez,¹⁶⁵G. Cortiana,⁹⁹G. Costa,^89a M. J. Costa,¹⁶⁷D. Costanzo,¹³⁹T. Costin,³⁰D. Coˆte´,²⁹R. Coura Torres,^23aL. Courneyea,¹⁶⁹G. Cowan,⁷⁶ C. Cowden,²⁷B. E. Cox,⁸²K. Cranmer,¹⁰⁸F. Crescioli,^122a,122bM. Cristinziani,²⁰G. Crosetti,^36a,36bR. Crupi,^72a,72b

S. Cre´pe´-Renaudin,⁵⁵C.-M. Cuciuc,^25aC. Cuenca Almenar,¹⁷⁵T. Cuhadar Donszelmann,¹³⁹S. Cuneo,^50a,50b M. Curatolo,⁴⁷C. J. Curtis,¹⁷P. Cwetanski,⁶¹H. Czirr,¹⁴¹Z. Czyczula,¹¹⁷S. D’Auria,⁵³M. D’Onofrio,⁷³ A. D’Orazio,^132a,132bA. Da Rocha Gesualdi Mello,^23aP. V. M. Da Silva,^23aC. Da Via,⁸²W. Dabrowski,³⁷ A. Dahlhoff,⁴⁸T. Dai,⁸⁷C. Dallapiccola,⁸⁴M. Dam,³⁵M. Dameri,^50a,50bD. S. Damiani,¹³⁷H. O. Danielsson,²⁹ D. Dannheim,⁹⁹V. Dao,⁴⁹G. Darbo,^50aG. L. Darlea,^25bC. Daum,¹⁰⁵J. P. Dauvergne,²⁹W. Davey,⁸⁶T. Davidek,¹²⁶

N. Davidson,⁸⁶R. Davidson,⁷¹M. Davies,⁹³A. R. Davison,⁷⁷E. Dawe,¹⁴²I. Dawson,¹³⁹J. W. Dawson,^5,a R. K. Daya,³⁹K. De,⁷R. de Asmundis,^102aS. De Castro,^19a,19bP. E. De Castro Faria Salgado,²⁴S. De Cecco,⁷⁸ J. de Graat,⁹⁸N. De Groot,¹⁰⁴P. de Jong,¹⁰⁵C. De La Taille,¹¹⁵H. De la Torre,⁸⁰B. De Lotto,^164a,164cL. De Mora,⁷¹ L. De Nooij,¹⁰⁵M. De Oliveira Branco,²⁹D. De Pedis,^132aP. de Saintignon,⁵⁵A. De Salvo,^132aU. De Sanctis,^164a,164c

A. De Santo,¹⁴⁹J. B. De Vivie De Regie,¹¹⁵S. Dean,⁷⁷D. V. Dedovich,⁶⁵J. Degenhardt,¹²⁰M. Dehchar,¹¹⁸ M. Deile,⁹⁸C. Del Papa,^164a,164cJ. Del Peso,⁸⁰T. Del Prete,^122a,122bA. Dell’Acqua,²⁹L. Dell’Asta,^89a,89b M. Della Pietra,^102a,iD. della Volpe,^102a,102bM. Delmastro,²⁹P. Delpierre,⁸³N. Delruelle,²⁹P. A. Delsart,⁵⁵ C. Deluca,¹⁴⁸S. Demers,¹⁷⁵M. Demichev,⁶⁵B. Demirkoz,^11,kJ. Deng,¹⁶³S. P. Denisov,¹²⁸D. Derendarz,³⁸ J. E. Derkaoui,^135dF. Derue,⁷⁸P. Dervan,⁷³K. Desch,²⁰E. Devetak,¹⁴⁸P. O. Deviveiros,¹⁵⁸A. Dewhurst,¹²⁹ B. DeWilde,¹⁴⁸S. Dhaliwal,¹⁵⁸R. Dhullipudi,^24,lA. Di Ciaccio,^133a,133bL. Di Ciaccio,⁴A. Di Girolamo,²⁹ B. Di Girolamo,²⁹S. Di Luise,^134a,134bA. Di Mattia,⁸⁸B. Di Micco,²⁹R. Di Nardo,^133a,133bA. Di Simone,^133a,133b R. Di Sipio,^19a,19bM. A. Diaz,^31aF. Diblen,^18cE. B. Diehl,⁸⁷H. Dietl,⁹⁹J. Dietrich,⁴⁸T. A. Dietzsch,^58aS. Diglio,¹¹⁵ K. Dindar Yagci,³⁹J. Dingfelder,²⁰C. Dionisi,^132a,132bP. Dita,^25aS. Dita,^25aF. Dittus,²⁹F. Djama,⁸³R. Djilkibaev,¹⁰⁸

T. Djobava,⁵¹M. A. B. do Vale,^23aA. Do Valle Wemans,^124aT. K. O. Doan,⁴M. Dobbs,⁸⁵R. Dobinson,^29,a D. Dobos,⁴²E. Dobson,²⁹M. Dobson,¹⁶³J. Dodd,³⁴O. B. Dogan,^18a,aC. Doglioni,¹¹⁸T. Doherty,⁵³Y. Doi,^66,a

J. Dolejsi,¹²⁶I. Dolenc,⁷⁴Z. Dolezal,¹²⁶B. A. Dolgoshein,^96,aT. Dohmae,¹⁵⁵M. Donadelli,^23bM. Donega,¹²⁰ J. Donini,⁵⁵J. Dopke,²⁹A. Doria,^102aA. Dos Anjos,¹⁷²M. Dosil,¹¹A. Dotti,^122a,122bM. T. Dova,⁷⁰J. D. Dowell,¹⁷

A. D. Doxiadis,¹⁰⁵A. T. Doyle,⁵³Z. Drasal,¹²⁶J. Drees,¹⁷⁴N. Dressnandt,¹²⁰H. Drevermann,²⁹C. Driouichi,³⁵ M. Dris,⁹J. Dubbert,⁹⁹T. Dubbs,¹³⁷S. Dube,¹⁴E. Duchovni,¹⁷¹G. Duckeck,⁹⁸A. Dudarev,²⁹F. Dudziak,⁶⁴ M. Du¨hrssen,²⁹I. P. Duerdoth,⁸²L. Duflot,¹¹⁵M-A. Dufour,⁸⁵M. Dunford,²⁹H. Duran Yildiz,^3bR. Duxfield,¹³⁹ M. Dwuznik,³⁷F. Dydak,²⁹D. Dzahini,⁵⁵M. Du¨ren,⁵²W. L. Ebenstein,⁴⁴J. Ebke,⁹⁸S. Eckert,⁴⁸S. Eckweiler,⁸¹