Inclusive and differential measurements of the t(t)over-bar charge asymmetry in pp collisions at root s=8 TeV

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DOI: 10.1016/j.physletb.2016.03.060

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©2016

by Elsevier. All rights reserved.

DIRETORIA DE TRATAMENTO DA INFORMAÇÃO

Cidade Universitária Zeferino Vaz Barão Geraldo

CEP 13083-970 – Campinas SP

Fone: (19) 3521-6493

http://www.repositorio.unicamp.br

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Inclusive

and

differential

measurements

of

the

t¯

t charge

asymmetry

in

pp

collisions

at

√

s

=

8 TeV

.

CMS

Collaboration



CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received11July2015

Receivedinrevisedform23February2016 Accepted21March2016

Availableonline29March2016 Editor: M.Doser Keywords: CMS Physics Topquark Chargeasymmetry

Thet¯t chargeasymmetryismeasuredinproton–protoncollisionsatacentre-of-massenergyof8 TeV. The data, collected with the CMS experiment atthe LHC, correspondto an integrated luminosity of 19.7 fb−1_{.Selectedeventscontainanelectronoramuonand fourormorejets,whereatleastonejet}

isidentifiedasoriginating fromb-quarkhadronization.Theinclusivechargeasymmetryisfoundtobe 0.0010±0.0068 (stat)±0.0037 (syst).Inaddition,differentialchargeasymmetriesasafunctionofrapidity, transversemomentum,andinvariantmassofthet¯t systemarestudied.ForthefirsttimeattheLHC,the measurementsarealsoperformedinareducedfiducialphasespaceoftopquarkpairproduction,with an integratedresult of−0.0035±0.0072 (stat)±0.0031 (syst).Allmeasurementsare consistentwithin twostandarddeviationswithzeroasymmetryaswellaswiththepredictionsofthestandardmodel.

©2016CERNforthebenefitoftheCMSCollaboration.PublishedbyElsevierB.V.Thisisanopenaccess articleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Thetopquarkoffersanexcellent opportunitytosearch for de-viationsfromthestandardmodel(SM),asitslargemassmakesit uniqueamongallquarks.Apossiblehintfornewphysicsinthetop quarksectoristhediscrepancybetweenthemeasuredt

¯

t forward– backward asymmetry and the SM expectations, reported by the CDF [1,2] and D0 [3–5] Collaborations at the Tevatron. Although this discrepancy has become smaller as the measurements and SM calculations[6,7] havebeenrefined, it hasgenerateda num-beroftheoreticalexplanationsinvokingcontributionsfromphysics beyond the SM (BSM). These have in turn led to models based onaxigluons orZ bosonsasmediatorsin thet

¯

t production pro-cess.An overviewofthetheoretical explanationscan befound in Ref.[8]andreferencestherein.

Athadroncolliderstopquarkpairsareproducedpredominantly intheprocessesofgluon–gluonfusionandquark–antiquark anni-hilation.Atleadingorder(LO),thet

¯

t productionissymmetricwith respecttotheexchange ofthetopquarkandantiquark.Athigher orders, QCD radiative corrections to the qq

¯

→

tt process

¯

induce an asymmetry inthe differential distributions of top quarks and antiquarks.Theinterferencebetweeninitial- andfinal-state radia-tion(ISRandFSR) processes,aswell asthe interferencebetween the Born and box diagrams, generate a correlation between the

 _{E-mailaddress:cms-publication-committee-chair@cern.ch.}

direction of the top quark momentum and that of the incoming quark [9]. Similarly, the direction of the top antiquark momen-tumisrelatedtothat oftheincomingantiquark.Theseprocesses induce a forward–backward asymmetry

(

AFB

)

atthe Tevatron pp

¯

collider. Thecharge-symmetric pp collisions atthe CERNLHC re-sultinadifferenteffect.AttheLHC,thelargeraveragemomentum fraction of the valence quarks leads to an excess of top quarks produced in the forward andbackward directions, while the top antiquarksareproducedmorecentrally.Thismakesthedifference inthe absolutevaluesoftherapidities1 _{ofthetopquark and} an-tiquark,



|

y

|

= |

yt|

− |

y_¯t|, a suitable observable to measure the t

¯

t chargeasymmetry attheLHC experiments.Usingthe sensitive variable,thechargeasymmetrycanbedefinedas

AC

=

N+

−

N−

N+

+

N−

,

(1)

where N+ andN− representthenumberof eventswithpositive and negative valuesof



|

y

|

, respectively. Theoretical predictions forthisobservableareoforder1% intheSM[10,11],butits sensi-tivitytonewphysicsmakesmeasurementsoftheeffectinteresting even when theprecision isnot highenough to establishthe ex-istence of the SM charge asymmetry. Both the CMS and ATLAS

1 _{Therapidityisdefinedasy= (1/2)ln[(E+p}

z)/(E−pz)],whereE denotesthe

particleenergyandpzitsmomentumcomponentalongthecounterclockwisebeam

direction. http://dx.doi.org/10.1016/j.physletb.2016.03.060

0370-2693/©2016CERNforthebenefitoftheCMSCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

Collaborationshavepublished resultsbased onthedatacollected ata centre-of-mass energy

√

s

=

7 TeV, which are in agreement withtheSMpredictions[12–15].

Toshed lightonthe possibleexistenceandthenatureofnew physicscontributions,itiscrucialtomeasurenotonlytheinclusive asymmetrybutalsoACasafunctionofvariablesmagnifyingthet

¯

t chargeasymmetry.ForthispurposeEq.(1)ismodifiedtoconsider onlyeventsinaspecificbinofthegivenvariable.

Inthisletter,we presentan inclusivemeasurement andthree differentialmeasurements of the t

¯

t charge asymmetry.The three differentialvariables,which are eachsensitive toa different con-tributiontothe chargeasymmetry,includethe t

¯

t system rapidity

|

yt¯t|,itstransversemomentum ptTt¯,anditsinvariantmassmt¯t.The measurements use the data collected with the CMS experiment in2012corresponding toan integratedluminosity of19.7 fb−1at

√

s

=

8 TeV.

Thevariable

|

y_t¯t| issensitive to the ratioofthe contributions from the qq and

¯

gg initial states to t

¯

t production. The charge-symmetric gluon fusion process is dominant in the central re-gion,whilet

¯

t productionthroughqq annihilation

¯

mostlyproduces eventswiththe tt pair

¯

at largerrapidities, whichimplies an en-hancementofthechargeasymmetrywithincreasing

|

yt¯t|[10].

Theratioofthepositiveandnegativecontributionstothe over-allasymmetrydependsonpt_T¯t.IntheSMtheinterferencebetween the Born and the box diagramsleads to a positive contribution, while the interference between ISR and FSR results in a nega-tivecontribution.Thepresenceofadditionalhardradiationimplies, onaverage,ahighertransverse momentum(pT)ofthet

¯

t system. Consequently,ineventswithlargevaluesofpt_T¯t,thenegative con-tributionfromtheISR–FSRinterferenceisenhanced[10].

The charge asymmetry is expected to depend on m_t¯t since the contribution of the qq initial

¯

state process is enhanced for larger values of this variable. It is also sensitive to BSM contri-butions;newheavy particles could be exchanged betweeninitial quarks and antiquarks and contribute to the t

¯

t production (see, e.g. Ref. [16] and references therein). The amplitudes associated withthese new contributions would interfere with those of the SMprocesses,anddepending onthemodeltheycouldleadto an increasingt

¯

t chargeasymmetrywithincreasingm_t¯t.

Becauseonlyapartofthet

¯

t phasespaceisexperimentally ac-cessible, measurements of the charge asymmetry that are to be comparedtotheoretical predictionsnecessarilyincludean extrap-olationto a more well-defined phase space. To this end a fidu-cial phase space is defined that emulates the restrictions of the measurablephasespacewhileallowingforthecalculationof the-oretical predictions. This minimizes the need for extrapolation, which can be subject to unpredictable uncertainties ifthere are significant BSM contributions. An additional extrapolation to the fullphasespaceoftopquark pairproductionisprovidedaswell, whichallowsforaneasiercomparisontotheresultsofother mea-surementsandtheoreticalcalculations.

2. CMSdetector

The central feature of the CMS apparatus is a superconduct-ingsolenoidof 6 minternal diameter,providing amagnetic field of3.8 T.Withinthesolenoidvolumeare asilicon pixelandstrip tracker,aleadtungstatecrystalelectromagneticcalorimeter(ECAL), and a brass and scintillator hadron calorimeter, each composed ofa barreland two endcapsections.The inner tracker measures trajectories of charged particles within the pseudorapidity range

|

η

|

<

2

.

5,whilethecalorimetersprovidecoverageupto

|

η

|

=

3

.

0. The pseudorapidity is definedas

η

= −

ln

(

tan

θ/

2

)

, withthe po-larangle

θ

beingmeasuredrelativetothecounterclockwisebeam direction. The ECAL hasan energyresolution of 3% or better for

therangeofelectronenergiesrelevantforthisanalysis. Extensive forward calorimetry complements the coverage provided by the barrelandendcapdetectors.Muonsaremeasuredinthe pseudora-pidity range

|

η

|

<

2

.

4 usinggas-ionizationdetectorsembedded in thesteelflux-returnyokeoutsidethesolenoid.Matchingmuonsto tracksmeasuredinthesilicontrackerresultsinarelative pT reso-lutionformuonswith20

<

pT

<

100 GeV of1.3–2.0%inthebarrel andbetter than6% inthe endcaps.The pT resolution inthe bar-relisbetterthan10%formuonswithpT upto1 TeV[17].A more detaileddescriptionoftheCMSdetector,togetherwithadefinition ofthecoordinatesystemusedandtherelevantkinematicvariables, canbefoundinRef.[18].

3. Simulatedsamples

For several steps of the measurement, samples of simulated events are used to model both the signal process and the back-ground contributions arising from the production of single top quarks and vector bosons in association with jets (W

+

jets and Z

+

jets).AnadditionalbackgroundcontributionfromQCDmultijet events ismodelled usinga template derived from data; see Sec-tion6.Topquarkpairsareproducedwiththenext-to-leadingorder (NLO)generator powheg,version1.0[19–22],usingtheCT10[23] partondistributionfunctions(PDF).Theelectroweakproductionof singletopquarks,inthet-channelandinassociationwithaW bo-son(tW-channel),issimulatedusing powheg andtheCTEQ6MPDF set [24].The productionofelectroweak vectorbosons in associa-tionwithjetsissimulatedusing MadGraph,version5.1.3.30[25], andtheCTEQ6L1[24]PDFset.

For the simulation of t

¯

t and single top quark events the top quark mass is set to 172.5 GeV. For all samples, pythia, version 6.426[26],isusedforthedescriptionofthepartonshoweringand hadronization. The simulations include additional proton–proton interactions in the same bunch crossing (in-time pileup) and in earlier/later bunch crossings (out-of-time pileup) with the same frequencyofoccurrenceasobservedinthedata.

Differential cross section measurements [27] have shownthat the pT spectrum of the top quarks in t

¯

t events is significantly softer than the one generated by the used simulation programs. To correct for this effect, the simulated t

¯

t sample is reweighted accordingtoscalefactorsderivedfromthesemeasurements.

4. Eventselection

Theanalysisusest

¯

t eventsinwhichoneoftheW bosonsfrom a topquark decay subsequentlydecaysinto anelectron ormuon and the corresponding neutrino, and the other W boson decays into a pair of quarks. We therefore select events containing one electron ormuonandfour ormorejets, atleastone ofwhich is identifiedasoriginatingfromthehadronizationofabottomquark. To be considered forthe offline analysis, the eventsmust passa single-electronorasingle-muontriggerwith pT thresholds of27 and24 GeVfortheelectronandmuon,respectively.

The particle-flow(PF)algorithm [28,29]is usedto reconstruct electrons,muons,andjetsintheevent.Thealgorithmreconstructs andidentifieseachindividualparticlewithanoptimized combina-tionofinformationfromthevariouselementsoftheCMSdetector. ThereconstructedPFcandidatesaredividedintofiveclasses: elec-trons,muons,photons,chargedhadrons,andneutralhadrons.

The primary vertex of the event [30] is identified as the re-constructed vertex with the highest sum of squared transverse momenta ofthe associated chargedparticles. For an eventto be accepted, theprimary vertexmust satisfy criteriaon its location withinthedetectorvolume,aswellasonthequalityofits recon-struction.

Electroncandidatesare requiredtohavea transverse momen-tum larger than 30 GeV and be within

|

η

|

<

2

.

5, excluding the transitionregionbetweentheECALbarrelandendcapsof1

.

44

<

|

η

sc|

<

1

.

57 sincethereconstruction ofan electron objectin this region is not optimal, where

η

sc is the pseudorapidity of the electroncandidatesupercluster [31].Furthermore,electron candi-dates are selected basedon the value ofa multivariate discrimi-nant,whichcombinesdifferentvariablesrelatedtocalorimetryand trackingparameters,butalso pT and

η

oftheelectron candidate. The electron definition also encompasses a conversion rejection method aimed at identifying electrons from photon conversions. Detailedinformationon theelectronreconstruction canbe found inRef.[31].

Muonsare required to have

|

η

|

<

2

.

1 and pT

>

26 GeV, with further requirements on the quality of the muon reconstruction andthecompatibilitywiththeprimaryvertexoftheevent.The

η

requirementreflects thecoverage ofthe single-muontrigger. De-tailsonthemuonreconstructioncanbefoundinRef.[17].

Additionally, electron and muon candidates must be isolated. Theisolationisquantifiedbythevariable I

rel,definedasthesum ofreconstructedtransversemomentaofnearbyPFobjectsdivided bytheleptontransversemomentum(p_T),correctedforpileup ef-fects[31]usingtheeffectivearea(in

η

–

φ

space)oftheleptonand theenergydensityintheevent.Electronsandmuonsarerequired to have I

rel

<

0

.

1 and

<

0

.

12, respectively, usingisolation cones withradiiof0

.

3 and0

.

4 in

η

–

φ

space.

Events with additional electrons and muons are vetoed. The lepton veto is based on a looser definition of the lepton iden-tification. In this definition, electrons must have pT

>

20 GeV,

|

η

|

<

2

.

5 and I_rel

<

0

.

15, while passing a loose criterion on the valueofthemultivariatediscriminant.Muonsarerequiredtohave pT

>

10 GeV,

|

η

|

<

2

.

5,andIrel

<

0

.

2.

JetsareclusteredfromPFparticleswiththeanti-kT [32] algo-rithmwithadistanceparameterof0.5.Chargedhadronsidentified asoriginatingfrompileup verticesare removed before clustering intojets.Furthercorrections[33]tothejetenergyareapplied, de-pendingonjet

η

andpT,thejetareain

η

–

φ

space,andthemedian pTdensityoftheevent.Theselectedjetsmustliewithin

|

η

|

<

2

.

5 and are required to have pT

>

30 GeV. The jet pT resolution in dataisapproximately10%worse comparedtosimulations. To ac-count for this, the transverse momenta of jets in the simulated samplesare smearedaccordingly.Finally,jetsfromthe hadroniza-tionofb quarksareidentifiedusingthemediumworkingpointof thecombinedsecondaryvertexalgorithm[34].Thebtag identifi-cationefficiencyofthisalgorithmforbjetswithpT

>

30 GeV and

|

η

|

<

2

.

4 varies between60and 70%,while the misidentification rateforjetsarisingfromlightquarksorgluonsisabout1%[35].

With the applied event selection we find a total of 171 121 eventswithan electroninthefinal state,hereafter referred toas the electron

+

jets channel, and192 123events inthe muon

+

jets channel.

5. Definitionofafiducialphasespace

Because of the offline event selection, only a subset of the events collected by the CMSdetector is used in the analysis. To allowforacomparisonofthemeasurements withthetheoretical calculations,anextrapolationtoawell-definedphasespaceneeds to be performed. The extrapolation relieson a correct modelling oftheratioofthenumberofeventsinthemeasuredphasespace tothatintheextrapolatedone;sucha ratio,however,maybe af-fectedbynewphysics.Thesimpleapproach,whichisextrapolation tothefullphasespace oft

¯

t production,entailsa largedependence onthemodelassumptions.

As an alternative,afiducialphasespace is definedusing gener-ator-levelselectioncriteriathatmimicthereconstruction-level cri-teriaappliedduringthenominalselection.Theratioofthenumber of fiducial events to the number ofreconstruction-level selected events,determinedfromsimulation,isthenappliedtothedatato estimatethedistributionofanobservableinthefiducialregion.

Because ofthe physicalandtopologicalsimilarityof eventsin the selectedandfiducialphase spaces,newphysics contributions areexpectedtoaffectbothinapproximatelythesameway,leaving the ratiounchanged. Thusthisextrapolationto thefiducialphase space is nearly model-independent. It should be noted that the desired model-independence is achieved by using a purely mul-tiplicativecorrection; a priorsubtractionofnon-fiducial t

¯

t events intheselectedphasespacewouldrequirealargerrelianceonthe model assumptions, as there wouldbe no cancellationof uncer-tainties.

Jetsofgeneratedparticlesinsimulatedeventsareusedto em-ulatetheselectionstepsactingonreconstructedjets.Hadron-level particlesareclusteredintojetsusingtheanti-kT algorithmwitha distanceparameter of0.5, asusedforthereconstructed jets. The clustering includes charged leptons and neutrinos, except those created in the leptonic decay of W bosons originating from top quarks. It should be notedthat theselection criteria forcharged leptons areappliedonlytoleptonsoriginatingfromtopquark de-cays.

Using these objects the following selection requirements are applied.Theeventneedstocontainexactlyoneelectron(ormuon) with pT

>

30

(

26

)

GeV and

|

η

|

<

2

.

5

(

2

.

1

)

. Any event that con-tains an additional electron (or muon) with pT

>

20

(

10

)

GeV and

|

η

|

<

2

.

5 is rejected. At least four generator-level jets with pT

>

30 GeV,

|

η

|

<

2

.

5 are required. The eventis rejectedif the axes of anysuch jets havean angular separationof



R

<

0

.

4 to the lepton, where



R

=



(

η

)

2

_{+ (φ)}

2 _{is calculatedusing the} differencesintheazimuthalangles

φ

andpseudorapidities

η

.This criterion serves as an emulation of the lepton isolation criteria, which use similar radii and are hard to implementfor theoreti-calcalculations.

Thefiducialregioncontainsabout10%oftheeventsofthefull phasespace.Roughly50%oftheeventsinthefiducialregionpass the selection outlinedin Section 4,withthelargest inefficiencies caused bythe lepton selectionandtrigger requirements.In com-parison, only 1.5% ofthe events outside the fiducial region fulfil the eventselectioncriteria, making up about20% ofthe selected events.

6. Estimationofbackgroundcontributions

For the estimation of the background contributions we make use of the discriminating power of the transverse mass of the W boson,mW

T,andofM3,theinvariantmassofthecombinationof three jetsthat corresponds tothe largestvectoriallysummed pT. Thisestimationisnecessaryforthesubtractionofthebackground contributionsofthemeasureddata,asdescribedinSection7.The mW

T variableis calculated fromthe transversemomentum of the charged lepton



p_T andthemissing transverse momentumvector



pmiss_T . The latter is defined as the pT imbalance of the recon-structedPFobjects,takingintoaccountthepropagationofjet en-ergycorrectionstothisobservable.Itsmagnitudeisreferred toas Emiss_T .Neglectingtheleptonmasses,mW_T isdefinedas

mW_T

=



2

(

Emiss_T p

T

− 

pmissT

· 

pT

).

(2)

Thebackgroundestimationismadewithabinnedmaximum like-lihood fitfor eachlepton channel. Asimultaneous fitin mW_T and M3 is performed in two disjointdata samples, corresponding to

Fig. 1. Comparisonofthecombinedlepton+jetsdatawithsimulatedcontributionsforthedistributionsinmW

T andM3.Thesimulatedsignalandbackgroundcontributions arenormalizedtotheresultsofthefitsinTable 1.Thelastbinineachhistogramincludestheoverflowvalues.Additionally,theratioofthedatatothesumofthesimulated contributionsisshown,withthestatisticaluncertaintiesofthesimulatedcontributions(includingtheuncertaintiesinthefit)indicatedbythebluehatchedregion.(For interpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

Table 1

Numberofeventsfor backgroundand t¯t contributions fromfits todata, along withtheirstatisticaluncertainties.Thecorrelationsbetweentheindividualvalues havebeentakenintoaccountforthedeterminationoftheuncertaintyonthe to-talbackgroundyield.TheuncertaintiesquotedforthesingletopquarkandZ+jets backgroundsaredrivenbytheconstraintsusedasinputsforthelikelihoodfit.

Process Electron+jets Muon+jets

Single top quark (t+tW) 7016±1328 7302±1663

W+jets 22 508±1460 20 522±1606 Z+jets 2345±510 2046±415 QCD multijet 6136±1201 4199±588 Total background 38 005±1491 34 096±1495 t¯t 133 130±1521 158 058±1538 Observed data 171 121 192 123 mW

T

<

50 GeV and

>

50 GeV.ThemWT distribution isfittedinthe low-mW_T sample, which isrich in QCD multijetevents andyields agooddiscrimination betweentheQCDmultijetprocessand pro-cesses containing a genuine W boson. The distribution of M3 is notasdependentonthechoiceofeventsample;itisfittedinthe complementary high-mW_T sample to avoidusing the sameevents forbothfits.

Forthet

¯

t,W

+

jets,Z

+

jets,andsingletopquarkprocesses, sim-ulatedsamplesareused tomodelthe shapesofthemW

T and M3 distributions. The contribution from multijet background events isestimatedfromdatacontrol samplescontaining nonisolatedor poorlyidentified leptons.Rate constraintscorresponding to Gaus-sianuncertainties of20% areintroduced intothe likelihood func-tion for the Z

+

jets andsingle top quark processes according to therespectiveNLOcrosssections,whiletheratesoftheother pro-cessesarefree parametersofthefit.Thewidthoftheconstraints ismotivatedbytheuncertaintiesofmeasurementsandtheoretical calculationsofthecorrespondingcrosssections[36–40].Adetailed descriptionofthefittingprocedurecanbefoundinRef.[41].

Table 1summarizestheresultsofthefits.Fig. 1showsthetwo variablesusedfortheestimationofthebackground,withthe indi-vidualsimulatedcontributions normalizedto theresultsfromthe fit.

7.Measurementofthet

¯

t chargeasymmetry

Themeasurementofthe t

¯

t charge asymmetry isbasedonthe reconstructedfour-momenta ofthe t and

¯

t quarks ineach event.

We reconstruct the leptonically decaying W boson from



p_T and



pmiss_T and associate the measured jets in the eventwith quarks in the t

¯

t decay chain.The association is done usinga likelihood criterion based on the b tagging discriminator valuesof the jets and the corresponding reconstructed masses of the top quarks and W bosons.The reconstruction procedure is described in de-tailin Ref.[41].

The reconstructed top quark andantiquark four-momenta are usedtoobtainthe inclusiveanddifferentialdistributionsof



|

y

|

, andthe charge asymmetry is calculatedfrom thenumber of en-trieswith



|

y

|

>

0 and

<

0. Incaseofthe differential measure-ments,theasymmetriesarecalculatedseparatelyforthedifferent bins in the kinematic variable Vi, where Vi is either

|

yt¯t|, ptT¯t, or m_t¯t.

Toallowforacomparisonoftheresultingasymmetryandthe predictions from theory, the reconstructed distributions of



|

y

|

and the three kinematic variables are corrected for background contributions,resolution,andselectionefficiency.

Inthefirstcorrectionstep,thedistributionsofbackground pro-cesses,asusedinSection6,arenormalizedtotheestimatedrates (see Table 1) and subtracted from the data, assuming Gaussian uncertaintiesinthebackgroundratesaswell asinstatistical fluc-tuationsinthebackgroundtemplates.Thecorrelationsamongthe individualbackgroundratesaretakenintoaccount.

The resulting background-subtracted distributions are trans-latedfromthereconstructionleveltopartonlevelwithinthephase space of the selected events. Afterwards, acceptance corrections areapplied,correctingeithertothefiducialphasespacedescribed inSection 5or tothe fullphase space. Apartfromthislast step, the measurements forboth phase spaces are identical. After the correctionshavebeenapplied,theresultingdistributionsare inde-pendentofthedetectorandanalysisspecifications.

The above corrections are obtained by applying an unfolding proceduretothedata[42] throughageneralizedmatrixinversion method.Inthismethod,theresolutionandselectioneffectsare de-scribedbyaresponsematrixR thattranslatesthetruespectrum



x intothemeasuredspectrum w



=

R



x.Asreconstructionand selec-tion effects factorize, the response matrix R can be seen as the product of a migration matrix, describing reconstruction effects, andadiagonalmatrixcontainingtheselectionefficiencies, describ-ingacceptanceeffects.Boththemigrationmatrixandtheselection efficienciesare determinedfromsimulatedt

¯

t events.Asthe com-ponentscorresponding totheelectron

+

jetsandmuon

+

jets

chan-Table 2

Thebinrangesfortheindividualbinsofthedifferentialmeasurements.Two differ-entchoicesofbinningareusedforthedistributionofmt¯t.

Bin |yt¯t| pTt¯t(GeV) mt¯t(GeV) mt¯t(GeV)

1 0–0.34 0–41 0–430 0–420 2 0.34–0.75 41–92 430–530 420–500 3 0.75–∞ 92–∞ 530–∞ 500–600 4 600–750 5 750–900 6 900–∞

nels are found to be very similar, they are combined to yield a methodthatcanbeappliedtothesummeddataofbothchannels. Inthiscombinationthe individualcomponentsare scaled accord-ing to the scale factors obtained via the background estimation. The unfolding procedure usedin the inclusivemeasurement, de-scribedindetailinRef.[41],isalsousedforthetwo-dimensional distributionsofthedifferentialmeasurements.

Thisanalysisuses12binsfortheunfolded



|

y

|

distributionin theinclusivemeasurementand8binsforthesamedistributionin the differential measurements. The unfolded Vi distributions use

3 bins,with one additional measurement beingperformedusing 6binsinm_t¯t.Theadditionalmeasurementprovidesfinelygrained resultsintheregionofhighmtt¯.The rangesforthebinsinthese distributionsaregiveninTable 2.Itshouldbenotedthatthe out-ermostbinsof



|

y

|

extendtoinfinity.

Inthecorrespondingreconstructedspectrathenumbersofbins along both axesare doubled, asis recommendedforthe applied unfoldingprocedure [42].The choiceofthebinedges for



|

y

|

is differentineach binof Vi,resultingin differentamounts of

ver-ticaloverlap betweenhorizontallyneighbouring bins inthe two-dimensionaldistributions(forillustrationseethebinninginFig. 2, bottomright).

Tolimitthemagnificationofstatisticaluncertaintiesduetothe unfolding procedure, a regularization is applied that suppresses solutions with large fluctuationsbetweenneighbouring bins. The strength of the regularization is determined by minimizing the statisticalcorrelationsbetweenbinsintheunfoldedspectrum. Dif-ferentstrengthsareusedfortheregularizationalongthesensitive variable within each binof the kinematic variable. Similarly, the regularization along the kinematicvariable isadjusted separately foreachbinofthekinematicvariable.

Separatemigrationmatricesareusedfortheinclusive measure-ment andforeachofthe differentialmeasurements.Fig. 2shows the migration matricesfortheinclusive measurementand, asan example, for the differential measurement in m_t¯t. For the inclu-sivemeasurementthemigrationmatrixdescribesthemigrationof selectedeventsfromtruevaluesof



|

y

|

tothereconstructed val-ues. Forthe migration matricesof the differentialmeasurements not onlythemigrationbetweenbinsof



|

y

|

hastobetakeninto account,butalsothemigrationbetweenbinsofVi.Fora

measure-ment in3unfoldedbinsof Vi thesemigrationmatricesfeaturea

grid of 6

×

3 bins in Vi, witheach of these bins representing a

16

×

8 migrationmatrix describingthemigration between differ-ent



|

y

|

values.

The values of



|

y

|

and Vi also affect the probability for an

event to fulfil the eventselection criteria. The selection efficien-ciesrelativetothefullphasespacefortheinclusivemeasurement andforthedifferentialmeasurementinm_tt¯ aredepictedinFig. 2. The selection efficiency ofthe fiducial phase spaceis definedby the ratioofall selected eventsto theeventspresentin the fidu-cial phasespace. Itshould be notedthatthe selectedeventsalso include eventsthat do not passthe criteriaof thefiducial phase space;their influenceisimplicitlycorrected forintheacceptance correction because of theway theselection efficiency isdefined.

Fig. 2. Migrationmatrixbetweengenerated(|y|gen)andreconstructed(|y|rec)rapiditydifferences(topleft)andselectionefficiencywithrespecttothefullphasespace asafunctionof|y|gen(topright)oftheinclusivemeasurement.Migrationmatrix(bottomleft)andselectionefficiency(bottomright)forthemeasurementdifferential in mt¯t.

Thus thiscorrection ismultiplicative innature, whichis justified by the inherent similarityof theseevents to the events that are intendedtobemeasured.

One limiting factor for the precision of the analysis is the presence of sizeable statistical fluctuations in the response ma-trices as they are obtained from simulated events. To mitigate thiseffect, one can exploit an approximate symmetry of the re-sponsematrixunderchargeconjugation.Foreventsresultingfrom a charge-symmetric initial state like gluon–gluon fusion it can beassumedthatreconstructioneffectsalsohaveapredominantly charge-symmetricbehaviour.Fromthisreasoning,thesymmetryis enforced for this analysisby averaging those bins of the gluon– gluoncontributiontotheresponsematrixthatcorrespondtoeach otherunderchargeconjugation.

The correctness of the unfolding procedure has been verified withpseudo-experiments,eachofwhichprovidesarandomly gen-eratedsampledistributionfromthetemplatesusedintheanalysis.

8. Estimationofsystematicuncertainties

The measured charge asymmetry AC is affected by several sourcesofsystematicuncertainty. Effectsalteringthe directionof thereconstructedtopquark momentacanchangethevalueofthe reconstructedcharge asymmetry.Systematicuncertaintieswithan impactonthedifferentialselectionefficiency,aswellasvariations inthe ratesandmodelling ofbackground contributions,can also biastheresult.Toevaluate eachsourceofsystematicuncertainty, anewbackgroundestimationisperformedandthemeasurement is repeated on data using modified simulated samples. The dif-ferences in unfolded asymmetries are then used to construct a systematicasymmetrycovariancematrixinalooseanalogyto sta-tisticalcovariancematrices.Foranuncertaintydescribedbya sin-glesystematicshiftacovarianceof

cov

(

x

,

y

)

= (

x

−

xnom

) (

y

−

ynom

)

(3) isused,withx and y referringtobinsoftheasymmetry distribu-tionresultingfromthesystematicshiftandxnom and ynom being theresultsofthenominalmeasurement.Foruncertaintiesthatare determinedusingexactlytwo variations(indexedby1and2)the absolutevaluesofthemaximal shiftsobservedineach resultbin,



xmax and



ymax, are determined separately; the covariance is thendefinedas

cov

(

x

,

y

)

= 

xmax



ymaxsign



(

x1

−

x2

) (

y1

−

y2

)



.

(4)

Thisprocedurecorrespondstoasymmetrizationofthelargest ob-servedshiftsandthusconstitutesamoreconservativeuncertainty estimatethananapproachbasedonadirectanalogywith statisti-calcovariancedefinitions.Thecovariancematricesofallsystematic uncertainties areadded up toyield aresultant matrix wherethe diagonalelementsarethevariances.

Inthefollowing,a summaryofthestudiedsources of system-aticuncertaintyisgiven.

Thecorrections to thejet energy scale andjet energy resolu-tionarevariedwithin their

η

- and pT-dependentuncertaintiesto estimatetheireffectsonthemeasurement.Theeffectofvariations inthefrequencyofoccurrenceofpileupeventsisdetermined us-ing reweighted simulatedsamples. Differencesbetween dataand simulationsintheb taggingefficiencyandtheleptonselection ef-ficiencyare determined asscale factorsthat depend on pT, or

η

andpT,respectively.Effectsduetouncertaintiesonthesescale fac-tors arestudied by varying them asa functionof

η

within their uncertainties and, in the case of the lepton selection efficiency, alsoasafunctionoftheleptoncharge.Theeffectofleptoncharge misidentificationisverysmallandisneglected.

Toestimatetheinfluenceofapossiblemismodellingofthe sim-ulatedW

+

jets background, themeasurement isrepeatedusinga W

+

jets templatedetermined froma sidebandregionindata, de-finedbyaninversionoftherequirementofab-taggedselectedjet. Thetemplateisreweightedtoaccountforthedifferencesbetween the signal and sidebandregions, which are determined from the simulation.

The uncertainty in the multijet background modelling in the electron

+

jetschannelisdeterminedbyreplacingthenominal tem-plate, which isestimated usingtwo sidebandregions defined ei-therbyinvertedisolationorbyinvertedidentificationcriteria,with templates derived from only one of the sideband regions each. Meanwhile,inthemuon

+

jetschannel,onlythetemplatefromthe isolation-invertedsamplecanbeused,soaconservativeestimation oftheuncertaintyinthisbackgroundcontributionisperformedby taking themaximum deviation out of three scenarios where the multijettemplateisreplacedwiththet

¯

t signaltemplate,withthe simulated W

+

jets template, or with a template obtained by in-verting thesignofthe sensitivevariable inthemultijet template itself.

Incontrasttotheothersystematiceffects,theuncertaintydue totheunfoldingmethodisestimatedbyunfoldingsimulated sam-ples instead of data. The simulated t

¯

t events are reweighted to reproduce theobserved asymmetries inthe differential measure-mentsbasedondata,andtheresultingreconstruction-levelspectra are unfolded. The deviations between the unfolded asymmetries andthereweightedtrueasymmetriesaretakentobeameasureof themodeldependenceoftheunfoldingprocedureintheobserved pointinphasespace.Theactualuncertaintyofeachmeasurement isestimatedasthesquare rootoftheaveragesquareddeviations producedbytheunfoldinginthethreereweightingscenarios cor-respondingtothethreekinematicvariables.

To estimate the uncertainty resulting from possible mismod-elling of the t

¯

t signal, samples of simulated t

¯

t events produced with MadGraph arecomparedtosamplesproducedwith powheg, bothinterfacedto pythia forthemodellingofthepartonshower. Ina similarwaytheimpactofa possiblemismodellingofparton shower andhadronization isstudied by using herwig [43,44],as opposedto pythia,forthesimulationofthesignal,withthe hard-scattering matrix element being simulated by either powheg or mc@nlo_{[45].Asameasureoftheuncertaintyrelatedtothe} per-formed reweighting asa function ofthe top quark pT,described inSection3,themeasurementisrepeatedusingsampleswithout reweighting. Finally, the impact ofvariations in the renormaliza-tionandfactorizationscales(

μ

Rand

μ

F)inthesimulatedt

¯

t events isdetermined usingdedicatedsamplesgeneratedatscales varied upanddownbyfactorsof2.

The systematicuncertainty on the measured asymmetry from the choice of PDFs for the colliding protons is estimated using the LHAPDF [46] package and the uncertainty in the CT10 [23], MSTW2008[47],andNNPDF2.1[48]PDFsets.

Thecontributions ofthedifferentsourcesofsystematic uncer-taintiestothetotaluncertaintyoftheinclusivemeasurementsare summarized in Table 3. The table also showsthe ranges of sys-tematic uncertainties in the 3-binned differential measurements to illustrate the magnitudes of the individual contributions. Be-cause the measurements in the two phase spaces differ only by the acceptance corrections, the uncertainties can be seen to be-havesimilarlyforthetwocases.

9. Results

Table 4 givesthevaluesofthe measuredinclusiveasymmetry atthedifferentstagesoftheanalysis,whiletheunfolded



|

y

|

dis-tributionsforthefiducialandfullphasespacesareshowninFig. 3.

Table 3

UncertaintiesfortheinclusivemeasurementofACandrangesofuncertaintiesforthedifferentialmeasurements usingthreebinsforthekinematicvariable.Numbersaregivenformeasurementsinthefiducialphasespace(fid. PS)andinthefullphasespace(fullPS).

Uncertaintysource Inclusive ACuncertainty Differential ACuncertainty

fid. PS full PS fid. PS full PS

Jet energy scale 0.0020 0.0018 0.0009–0.0066 0.0008–0.0063

Jet energy resolution 0.0003 0.0003 0.0005–0.0020 0.0005–0.0020

Pileup 0.0006 0.0006 0.0002–0.0027 0.0003–0.0027

b tagging 0.0009 0.0008 0.0002–0.0033 0.0002–0.0032

Lepton selection efficiency 0.0009 0.0009 0.0005–0.0016 0.0005–0.0017

W+jets background 0.0005 0.0007 0.0003–0.0030 0.0005–0.0025

QCD multijet background 0.0010 0.0009 0.0008–0.0030 0.0011–0.0028

Unfolding 0.0012 0.0022 0.0004–0.0023 0.0011–0.0033

Generator 0.0002 0.0005 0.0008–0.0058 0.0007–0.0043

Hadronization 0.0010 0.0011 0.0007–0.0046 0.0008–0.0040

Top quark pTreweighting 0.0000 0.0002 0.0000–0.0014 0.0001–0.0015

μRandμFscales 0.0002 0.0007 0.0008–0.0057 0.0009–0.0064

PDF 0.0002 0.0003 0.0004–0.0014 0.0004–0.0012

Total syst. uncertainty 0.0031 0.0037 0.0043–0.0120 0.0041–0.0115

Statistical uncertainty 0.0072 0.0068 0.0078–0.0181 0.0078–0.0172

Total uncertainty 0.0078 0.0077 0.0094–0.0217 0.0094–0.0207

Table 4

Themeasuredinclusiveasymmetryatthedifferentstagesoftheanalysisandthecorrespondingtheoretical pre-dictionsfromtheSM.

Asymmetry ( AC)

Reconstructed 0.0036±0.0017 (stat)

Background-subtracted 0.0008±0.0023 (stat)

Corrected for migration effects −0.0042±0.0072 (stat)

Fiducial phase space −0.0035±0.0072 (stat)±0.0031 (syst)

Theoretical prediction [Bernreuther, Si][11,49] 0.0101±0.0010

Full phase space 0.0010±0.0068 (stat)±0.0037 (syst)

Theoretical prediction [Kühn, Rodrigo][10] 0.0102±0.0005 Theoretical prediction [Bernreuther, Si][11,49] 0.0111±0.0004

Fig. 3. Unfoldedinclusive|y|distributioninthefiducialphasespace(left)andinthefullphasespace(right).Theuncertaintiesonthedatapointsrepresentthestatistical uncertaintiesduetothelimitedamountsofdataandsimulatedevents.ThemeasuredvaluesarecomparedtoNLOpredictionsfortheSMbasedoncalculationsbyKühnand Rodrigo(K&R)[10]andBernreutherandSi(B&S)[11,49].

Thelattertwo distributionsare shownin theformofnormalized differentialcrosssectionsasafunctionof



|

y

|

.Allunfolded quan-tities correspond to the parton level. The statistical uncertainty ofall quotedresults encompassesthesubdominanteffects ofthe limitednumberofsimulatedeventsusedforthemeasurement.It shouldbenotedthattheacceptancecorrectionsforthetwophase spaces differ as a function of



|

y

|

; as a result, statistical fluc-tuations in the data can have different effects on the measured asymmetries.

Theuncertaintyinthe theoreticalpredictionby Kühnand Ro-drigo[10] is estimatedby varying thetop quark mass,the PDFs,

and the

μ

R and

μ

F scales, with the scale uncertainties being the dominant effect. The uncertainty in the theoretical predic-tion by Bernreuther and Si [11,49] contains only the effects of variations of the

μ

R and

μ

F scales. The t

¯

t chargeasymmetry for the fiducialphase spaceis computedwiththe t

¯

t productionand semileptonic/non-leptonic t

¯

t decay matrix elements at NLO. The top quark decaymatrix elements atNLO containadditional scale dependencies. This results in a larger scale uncertainty as com-paredto thecharge asymmetry forthe full phasespace. Another recentCMSanalysisoftheinclusivechargeasymmetry inthefull phasespace[50],whichusesaslightlymoremodel-dependent

ap-Fig. 4. Correctedasymmetryasafunctionof|ytt¯|(upperleft),ptT¯t(upperright),andmt¯t(lowerleftandlowerright).Thelatterisshownintwodifferentbinnings.Allresults correspondtothefiducialphasespace.ThemeasuredvaluesarecomparedtoanNLOpredictionfortheSMbasedoncalculationsbyBernreutherandSi(B&S)[11,49].The innerbarsindicatethestatisticaluncertainties,whiletheouterbarsrepresentthestatisticalandsystematicuncertaintiesaddedinquadrature.

proach to achieve lower uncertainties, and a recently published ATLASmeasurement ofinclusiveanddifferentialcharge asymme-tries[51] both yieldresultsthat are comparableto theones pre-sentedhere.

The results of the differential measurements in the fiducial phase space are shown in Fig. 4, and the extrapolation to the full phase spacein Fig. 5. The measured valuesare compared to predictionsfrom SM calculations[10,11,49] aswell as to predic-tions froman effectivefield theory [52,53]. The latteris capable ofreproducing the CDF results [2]by introducing an anomalous effectiveaxial-vector couplingtothe gluon attheone-loop level. Thegluon–quarkvertexis treatedintheapproximation ofan ef-fective field theorywith ascale fornew physics contributionsof order1.5–2.0 TeV.Predictionsfortheasymmetry asafunctionof pt_T¯t arenotavailableforthistheoryandforoneoftheSM calcula-tions.Becauseoftheimportance oftheregionofhighm_t¯t forthe detectionof newphysics, we provide an additional, more finely-graineddifferentialmeasurementofthe asymmetry asa function ofthisobservable.

Both of the inclusive results yield values that are slightly smallerthantheSMpredictions,withthelargerdeviationbeingin thefiducialphase spaceandcorrespondingto 1.7standard devia-tions.Thedifferentialmeasurementsshowagoodagreementwith theSM predictions.Forthebenchmark modelinvolvingan effec-tive axial-vector couplingofthe gluon, themeasurement athigh mt¯t excludesnewphysics scales below1

.

5 TeV at the95% confi-dencelevel.

10. Summary

Inclusiveanddifferentialmeasurementsofthecharge asymme-try in t

¯

t production at the LHC are presented.The data sample,

collected inproton–protoncollisionsat

√

s

=

8 TeV withtheCMS detector, corresponds to an integrated luminosity of 19.7 fb−1_. Events with top quark pairs decaying into the electron

+

jets and muon

+

jetschannelsareselectedandafullt

¯

t eventreconstruction is performed to determine the four-momenta of the top quarks andantiquarks. The observeddistributions are thencorrected for acceptanceandreconstructioneffects.ForthefirsttimeattheLHC, acceptancecorrectionstothet

¯

t chargeasymmetry areperformed notonlytothefullphasespacebutalsotoafiducialphasespace. Within two standard deviations, all measured values are consis-tent withthe predictions of the standard model and no hint of new physics contributions isobserved. The charge asymmetry in thehigh-mass regionisabouttwo standarddeviationsbelowthe predictions froman effective field theory withthe scale fornew physicsat1.5 TeV.

Acknowledgements

WethankWernerBernreuther,Zong-GuoSi,JohannKühn, Ger-manRodrigo,EmidioGabrielli,AntonioRacioppi,andMarttiRaidal forkindlyprovidingtheoreticalpredictionsforthedifferential dis-tributionsthataremeasuredinthisletter.

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technicalandadministrative staffsatCERN andatother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentresand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythe computinginfrastructureessential to ouranalyses. Finally, we acknowledge the enduring support for the construc-tionandoperation oftheLHC andtheCMSdetectorprovidedby thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS

Fig. 5. Correctedasymmetryasafunctionof|ytt¯|(upperleft),ptT¯t(upperright),andmt¯t(lowerleftandlowerright).Thelatterisshownintwodifferentbinnings.Allresults correspondtothefullphasespace.ThemeasuredvaluesarecomparedtoNLOpredictionsfortheSMbasedoncalculationsbyKühnandRodrigo(K&R)[10]andBernreuther andSi(B&S)[11,49],aswellastothepredictionsofamodelfeaturinganeffectiveaxial-vectorcouplingofthegluon(EAG)[52,53].Theinnerbarsindicatethestatistical uncertainties,whiletheouterbarsrepresentthestatisticalandsystematicuncertaintiesaddedinquadrature.

and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES(Bulgaria);CERN;CAS,MoST,andNSFC(China); COLCIENCIAS (Colombia);MSESandCSF(Croatia);RPF(Cyprus);MoER,ERCIUT andERDF(Estonia);Academy ofFinland,MEC,andHIP(Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); MSIP and NRF (Republic of Korea); LAS (Lithuania); MOE and UM(Malaysia); CINVESTAV, CONACYT,SEP,andUASLP-FAI(Mexico);MBIE(NewZealand);PAEC (Pakistan);MSHEandNSC(Poland);FCT(Portugal);JINR(Dubna); MON,RosAtom,RASandRFBR(Russia);MESTD(Serbia);SEIDIand CPAN(Spain);SwissFundingAgencies(Switzerland);MST(Taipei); ThEPCenter,IPST,STARandNSTDA(Thailand);TUBITAKandTAEK (Turkey);NASUandSFFR(Ukraine); STFC(United Kingdom);DOE andNSF(USA).

Individuals have received support from the Marie-Curie pro-gramme and the European Research Council and EPLANET (Eu-ropean Union); the Leventis Foundation; the A.P. Sloan Founda-tion; the Alexander von Humboldt Foundation; the Belgian Fed-eral Science Policy Office; the Fonds pour la Formation à la Recherchedansl’Industrieetdansl’Agriculture (FRIA-Belgium);the AgentschapvoorInnovatiedoorWetenschapenTechnologie (IWT-Belgium);the Ministry ofEducation,Youth andSports (MEYS) of theCzechRepublic;theCouncilofScienceandIndustrialResearch, India;theHOMINGPLUSprogrammeoftheFoundation forPolish Science, cofinanced from European Union, Regional Development Fund;the CompagniadiSan Paolo(Torino); theConsorzio per la Fisica (Trieste);MIUR project20108T4XTM(Italy); the Thalisand Aristeia programmes cofinanced by EU-ESF andthe Greek NSRF; the National Priorities Research Program by Qatar National Re-search Fund;the RachadapisekSompot FundforPostdoctoral

Fel-lowship,ChulalongkornUniversity(Thailand);andtheWelch Foun-dation.

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CMSCollaboration

V. Khachatryan,

A.M. Sirunyan,

A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam,

E. Asilar,

T. Bergauer,

J. Brandstetter,

E. Brondolin,

M. Dragicevic,

J. Erö,

M. Flechl,

M. Friedl,

R. Frühwirth

1

,

V.M. Ghete,

C. Hartl,

N. Hörmann,

J. Hrubec,

M. Jeitler

1

,

V. Knünz,

A. König,

M. Krammer

1

,

I. Krätschmer,

D. Liko,

T. Matsushita,

I. Mikulec,

D. Rabady

2

,

B. Rahbaran,

H. Rohringer,

J. Schieck

1

,

R. Schöfbeck,

J. Strauss,

W. Treberer-Treberspurg,

W. Waltenberger,

C.-E. Wulz

1

InstitutfürHochenergiephysikderOeAW,Wien,Austria

V. Mossolov,

N. Shumeiko,

J. Suarez Gonzalez

NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus

S. Alderweireldt,

T. Cornelis,

E.A. De Wolf,

X. Janssen,

A. Knutsson,

J. Lauwers,

S. Luyckx,

S. Ochesanu,

R. Rougny,

M. Van De Klundert,

H. Van Haevermaet,

P. Van Mechelen,

N. Van Remortel,

A. Van Spilbeeck

UniversiteitAntwerpen,Antwerpen,Belgium

S. Abu Zeid,

F. Blekman,

J. D’Hondt,

N. Daci,

I. De Bruyn,

K. Deroover,

N. Heracleous,

J. Keaveney,

S. Lowette,

L. Moreels,

A. Olbrechts,

Q. Python,

D. Strom,

S. Tavernier,

W. Van Doninck,

P. Van Mulders,

G.P. Van Onsem,

I. Van Parijs

VrijeUniversiteitBrussel,Brussel,Belgium

P. Barria,

C. Caillol,

B. Clerbaux,

G. De Lentdecker,

H. Delannoy,

G. Fasanella,

L. Favart,

A.P.R. Gay,

A. Grebenyuk,

T. Lenzi,

A. Léonard,

T. Maerschalk,

A. Marinov,

L. Perniè,

A. Randle-conde,

T. Reis,

T. Seva,

C. Vander Velde,

P. Vanlaer,

R. Yonamine,

F. Zenoni,

F. Zhang

3

UniversitéLibredeBruxelles,Bruxelles,Belgium

K. Beernaert,

L. Benucci,

A. Cimmino,

S. Crucy,

D. Dobur,

A. Fagot,

G. Garcia,

M. Gul,

J. Mccartin,

A.A. Ocampo Rios,

D. Poyraz,

D. Ryckbosch,

S. Salva,

M. Sigamani,

N. Strobbe,

M. Tytgat,

W. Van Driessche,

E. Yazgan,

N. Zaganidis

GhentUniversity,Ghent,Belgium

S. Basegmez,

C. Beluffi

4

,

O. Bondu,

S. Brochet,

G. Bruno,

R. Castello,

A. Caudron,

L. Ceard,

G.G. Da Silveira,

C. Delaere,

D. Favart,

L. Forthomme,

A. Giammanco

5

,

J. Hollar,

A. Jafari,

P. Jez,

M. Komm,

V. Lemaitre,

A. Mertens,

C. Nuttens,

L. Perrini,

A. Pin,

K. Piotrzkowski,

A. Popov

6

,

L. Quertenmont,

M. Selvaggi,

M. Vidal Marono

UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

N. Beliy,

G.H. Hammad

UniversitédeMons,Mons,Belgium

W.L. Aldá Júnior,

G.A. Alves,

L. Brito,

M. Correa Martins Junior,

M. Hamer,

C. Hensel,

C. Mora Herrera,

A. Moraes,

M.E. Pol,

P. Rebello Teles

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

E. Belchior Batista Das Chagas,

W. Carvalho,

J. Chinellato

7

,

A. Custódio,

E.M. Da Costa,

D. De Jesus Damiao,

C. De Oliveira Martins,

S. Fonseca De Souza,

L.M. Huertas Guativa,

H. Malbouisson,

D. Matos Figueiredo,

L. Mundim,

H. Nogima,

W.L. Prado Da Silva,

A. Santoro,

A. Sznajder,

E.J. Tonelli Manganote

7

,

A. Vilela Pereira

S. Ahuja

a

,

C.A. Bernardes

b

,

A. De Souza Santos

b

,

S. Dogra

a

,

T.R. Fernandez Perez Tomei

a

,

E.M. Gregores

b

,

P.G. Mercadante

b

,

C.S. Moon

a

,

8

,

S.F. Novaes

a

,

Sandra S. Padula

a

,

D. Romero Abad,

J.C. Ruiz Vargas

a_{UniversidadeEstadualPaulista,SãoPaulo,Brazil} b_{UniversidadeFederaldoABC,SãoPaulo,Brazil}

A. Aleksandrov,

V. Genchev

†

,

R. Hadjiiska,

P. Iaydjiev,

S. Piperov,

M. Rodozov,

S. Stoykova,

G. Sultanov,

M. Vutova

InstituteforNuclearResearchandNuclearEnergy,Sofia,Bulgaria

A. Dimitrov,

I. Glushkov,

L. Litov,

B. Pavlov,

P. Petkov

UniversityofSofia,Sofia,Bulgaria

M. Ahmad,

J.G. Bian,

G.M. Chen,

H.S. Chen,

M. Chen,

T. Cheng,

R. Du,

C.H. Jiang,

R. Plestina

9

,

F. Romeo,

S.M. Shaheen,

J. Tao,

C. Wang,

Z. Wang,

H. Zhang

InstituteofHighEnergyPhysics,Beijing,China

C. Asawatangtrakuldee,

Y. Ban,

Q. Li,

S. Liu,

Y. Mao,

S.J. Qian,

D. Wang,

Z. Xu,

W. Zou

StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

C. Avila,

A. Cabrera,

L.F. Chaparro Sierra,

C. Florez,

J.P. Gomez,

B. Gomez Moreno,

J.C. Sanabria

UniversidaddeLosAndes,Bogota,Colombia

N. Godinovic,

D. Lelas,

D. Polic,

I. Puljak,

P.M. Ribeiro Cipriano

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic,

M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,

K. Kadija,

J. Luetic,

S. Micanovic,

L. Sudic

InstituteRudjerBoskovic,Zagreb,Croatia

A. Attikis,

G. Mavromanolakis,

J. Mousa,

C. Nicolaou,

F. Ptochos,

P.A. Razis,

H. Rykaczewski

UniversityofCyprus,Nicosia,Cyprus

M. Bodlak,

M. Finger

10

,

M. Finger Jr.

10

CharlesUniversity,Prague,CzechRepublic

A.A. Abdelalim

11

,

A. Awad

12

,

13

,

A. Mahrous

14

,

A. Radi

13

,

12

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

B. Calpas,

M. Kadastik,

M. Murumaa,

M. Raidal,

A. Tiko,

C. Veelken

NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola,

J. Pekkanen,

M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Härkönen,

V. Karimäki,

R. Kinnunen,

T. Lampén,

K. Lassila-Perini,

S. Lehti,

T. Lindén,

P. Luukka,

T. Mäenpää,

T. Peltola,

E. Tuominen,

J. Tuominiemi,

E. Tuovinen,

L. Wendland

J. Talvitie,

T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon,

F. Couderc,

M. Dejardin,

D. Denegri,

B. Fabbro,

J.L. Faure,

C. Favaro,

F. Ferri,

S. Ganjour,

A. Givernaud,

P. Gras,

G. Hamel de Monchenault,

P. Jarry,

E. Locci,

M. Machet,

J. Malcles,

J. Rander,

A. Rosowsky,

M. Titov,

A. Zghiche

DSM/IRFU,CEA/Saclay,Gif-sur-Yvette,France

I. Antropov,

S. Baffioni,

F. Beaudette,

P. Busson,

L. Cadamuro,

E. Chapon,

C. Charlot,

T. Dahms,

O. Davignon,

N. Filipovic,

A. Florent,

R. Granier de Cassagnac,

S. Lisniak,

L. Mastrolorenzo,

P. Miné,

I.N. Naranjo,

M. Nguyen,

C. Ochando,

G. Ortona,

P. Paganini,

S. Regnard,

R. Salerno,

J.B. Sauvan,

Y. Sirois,

T. Strebler,

Y. Yilmaz,

A. Zabi

LaboratoireLeprince-Ringuet,EcolePolytechnique,IN2P3–CNRS,Palaiseau,France

J.-L. Agram

15

,

J. Andrea,

A. Aubin,

D. Bloch,

J.-M. Brom,

M. Buttignol,

E.C. Chabert,

N. Chanon,

C. Collard,

E. Conte

15

,

X. Coubez,

J.-C. Fontaine

15

,

D. Gelé,

U. Goerlach,

C. Goetzmann,

A.-C. Le Bihan,

J.A. Merlin

2

,

K. Skovpen,

P. Van Hove

InstitutPluridisciplinaireHubertCurien,UniversitédeStrasbourg,UniversitédeHauteAlsaceMulhouse,CNRS/IN2P3,Strasbourg,France

S. Gadrat

CentredeCalculdel’InstitutNationaldePhysiqueNucleaireetdePhysiquedesParticules,CNRS/IN2P3,Villeurbanne,France

S. Beauceron,

C. Bernet,

G. Boudoul,

E. Bouvier,

C.A. Carrillo Montoya,

J. Chasserat,

R. Chierici,

D. Contardo,

B. Courbon,

P. Depasse,

H. El Mamouni,

J. Fan,

J. Fay,

S. Gascon,

M. Gouzevitch,

B. Ille,

F. Lagarde,

I.B. Laktineh,

M. Lethuillier,

L. Mirabito,

A.L. Pequegnot,

S. Perries,

J.D. Ruiz Alvarez,

D. Sabes,

L. Sgandurra,

V. Sordini,

M. Vander Donckt,

P. Verdier,

S. Viret,

H. Xiao

UniversitédeLyon,UniversitéClaudeBernardLyon1,CNRS–IN2P3,InstitutdePhysiqueNucléairedeLyon,Villeurbanne,France

T. Toriashvili

16

GeorgianTechnicalUniversity,Tbilisi,Georgia

I. Bagaturia

17

TbilisiStateUniversity,Tbilisi,Georgia

C. Autermann,

S. Beranek,

M. Edelhoff,

L. Feld,

A. Heister,

M.K. Kiesel,

K. Klein,

M. Lipinski,

A. Ostapchuk,

M. Preuten,

F. Raupach,

S. Schael,

J.F. Schulte,

T. Verlage,

H. Weber,

B. Wittmer,

V. Zhukov

6

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

M. Ata,

M. Brodski,

E. Dietz-Laursonn,

D. Duchardt,

M. Endres,

M. Erdmann,

S. Erdweg,

T. Esch,

R. Fischer,

A. Güth,

T. Hebbeker,

C. Heidemann,

K. Hoepfner,

D. Klingebiel,

S. Knutzen,

P. Kreuzer,

M. Merschmeyer,

A. Meyer,

P. Millet,

M. Olschewski,

K. Padeken,

P. Papacz,

T. Pook,

M. Radziej,

H. Reithler,

M. Rieger,

F. Scheuch,

L. Sonnenschein,

D. Teyssier,

S. Thüer

RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany

V. Cherepanov,

Y. Erdogan,

G. Flügge,

H. Geenen,

M. Geisler,

F. Hoehle,

B. Kargoll,

T. Kress,

Y. Kuessel,

A. Künsken,

J. Lingemann

2

,

A. Nehrkorn,

A. Nowack,

I.M. Nugent,

C. Pistone,

O. Pooth,

A. Stahl

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

M. Aldaya Martin,

I. Asin,

N. Bartosik,

O. Behnke,

U. Behrens,

A.J. Bell,

K. Borras,

A. Burgmeier,

A. Cakir,

L. Calligaris,

A. Campbell,

S. Choudhury,

F. Costanza,

C. Diez Pardos,

G. Dolinska,

S. Dooling,

T. Dorland,

G. Eckerlin,

D. Eckstein,

T. Eichhorn,

G. Flucke,

E. Gallo,

J. Garay Garcia,

A. Geiser,

A. Gizhko,