Measurement of the mass difference between top quark and antiquark in pp collisions at root s=8 TeV

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https://www.sciencedirect.com/science/article/pii/S0370269317303027

DOI: 10.1016/j.physletb.2017.04.028

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©2017

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

Measurement

of

the

mass

difference

between

top

quark

and

antiquark

in

pp

collisions

at

√

s

=

8 TeV

.TheCMS Collaboration CERN,Switzerland a r t i c l e i n f o a b s t ra c t Articlehistory: Received 29 October 2016

Received in revised form 9 April 2017 Accepted 13 April 2017

Available online 19 April 2017 Editor: M. Doser

Keywords: CMS

Top quark physics

Theinvarianceofthestandardmodel(SM)undertheCPTtransformationpredictsequalityofparticleand antiparticlemasses. Thispredictionistestedbymeasuring themassdifferencebetweenthe topquark and antiquark(mt=mt−mt)thatareproducedinppcollisionsatacenter-of-massenergyof8 TeV,

usingeventswithamuonoranelectronand atleastfourjetsinthefinalstate.Theanalysisisbased ondatacorrespondingtoanintegratedluminosity of19.6 fb−1collectedbytheCMSexperiment atthe LHC,andyieldsavalueofmt= −0.15±0.19(stat)±0.09(syst) GeV,whichisconsistentwiththeSM

expectation.Thisresultissignificantlymoreprecisethanpreviouslyreportedmeasurements.

©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

1. Introduction

Symmetries such as charge conjugation (C), parity or space reflection (P), and time reversal (T) play a fundamental role in thestandardmodel(SM) ofparticlephysics [1–3].Theinvariance of the SM under their combination, the CPT symmetry, predicts equalityof particle andantiparticle masses, and thus far experi-ments have confirmed thisprediction [4].In some extensions of theSM,however,CPT-violatingeffectsarepresent[5–8].Thelarge number oftop quarks produced in proton–proton(pp) collisions at the CERN LHC provides an opportunity to test CPT symme-try in the quark sector for the most massive particle of the SM through a precise measurement of the difference in mass be-tween the top quark (t)and its antiparticle (t), mt≡mt−mt. This quantity has been measured previously in pp collisions at √

s=1.96 TeV by the CDF and D0 experiments, and in pp col-lisions at √s=7 TeV by the ATLAS and CMS experiments. The CDFmeasurement of mt= −3.3±1.4(stat)±1.0(syst) GeV [9] isalmosttwo standarddeviationsawayfromtheSMvalue. How-ever,thesubsequentmeasurements by D0,CMS, CDF,andATLAS, yielding0.8±1.8(stat)±0.5(syst) GeV[10],−0.44±0.46(stat)± 0.27(syst) GeV[11],−1.95±1.11(stat)±0.59(syst) GeV[12],and 0.67±0.61(stat)±0.41(syst) GeV [13], respectively, are all in agreement with CPT symmetry. This article presents a measure-ment performed with the CMS [14] detector at the LHC. It

rep- _{E-mailaddress:[email protected].}

resents the first determination of mt at √s=8 TeV. While the same techniques as for the previous mt measurement by CMS[11] are used,both the statisticalandthesystematic uncer-tainty are significantly reduced. The event selection is optimized fortt productionwhereoneoftheW bosons decayshadronically (t→bW+→bqq,orits charge conjugate) andthe other decays leptonically (t→bW+→b+ν,or its charge conjugate), with  corresponding toeitheranelectronoramuon(includingalso de-cays to τ leptons where the τ decaysleptonically). The data are split into− and+ samplesthat containthree-jet decaysofthe associated top quarks or antiquarks, respectively. For each event category, theideogramlikelihoodmethod[15]isusedtomeasure

mtandmt,fromwhichtheirdifferenceisobtained.

2. TheCMSdetector

The central feature of the CMS apparatus is a supercon-ducting solenoid of 6 m internal diameter, providing a mag-netic field of 3.8 T. The field volume houses a silicon pixel and strip tracker, a crystal electromagnetic calorimeter (ECAL), and a brass/scintillatorhadron calorimeter(HCAL).The innertracker re-constructs charged-particle trajectorieswithin the pseudorapidity range|η|<2.5.Thetrackerprovides animpactparameter resolu-tion of 20–30 μm anda resolution ofthe momentum transverse to thebeamdirection(pT) of1–3% for10 GeV charged particles. Muons are measured for |η|<2.4 using detection planes based onthreetechnologies:drifttubes,cathode-stripchambers,and re-sistiveplatechambers.Matchingoutermuontrajectoriestotracks http://dx.doi.org/10.1016/j.physletb.2017.04.028

0370-2693/©2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3_.

measured inthe silicontrackerprovides a pT resolution of1–6% forthe pT values relevant to this analysis[16]. In the region of |η|<1.74,theHCALcells havewidthsof0.087 inpseudorapidity and0.087 radinazimuth (φ).In the(η,φ) plane, for|η|<1.48, the HCAL cells map onto arrays of 5×5 ECAL crystals to form calorimeter towers that project radially outwards from near the centeroftheCMSdetector.Atlarger valuesof|η|,thesize ofthe towersincreasesandthematchingECALarrayscontainfewer crys-tals.Theenergyresolutionislessthan5% fortheelectronenergies considered in thisanalysis [17,18]. In addition to the barrel and endcapdetectors,CMShasextensiveforwardcalorimetry.A more detaileddescriptionoftheCMSdetector,togetherwithadefinition ofthecoordinatesystemusedandtherelevantkinematicvariables, canbefoundinRef.[14].

3. Dataandsimulation

Thedataused inthisanalysiscorrespond toan integrated lu-minosityof19.6±0.5 fb−1 [19],collectedduringthe2012pp col-lisionrunofthe LHCatacenter-of-massenergyof8 TeV.Events areselected onlineusing atrigger that requiresan isolated elec-tron with pT>27 GeV and |η|<2.5 or an isolated muon with

pT>24 GeV and |η|<2.1. SimulatedMonteCarlo (MC)samples oftt, W, and Z bosonproduction are generated with MadGraph 5.1.3.30 [20] interfacedwith pythia 6.4.26 [21] for parton show-ering. Single top quark events are simulated using the powheg generator [22–25], also interfaced to pythia. For studies of sys-tematiceffects, a sampleof tt events is generated with mc@nlo 3.14[26],combinedwith herwig 6.520[27]forpartonshowering. Allgeneratedeventsare passed througha simulationoftheCMS detector based on Geant4 [28]. The simulation includes the ef-fect of pileup, i.e. additional pp collisions occurring during the same bunch-crossing or immediately preceding or following the primarycrossing.ThetheoreticalcrosssectionsforW bosonandZ bosonproductionare calculated with FEWZ[29] at next-to-next-to-leading-order (NNLO) precision of quantum chromodynamics (QCD).The tt and single top quark production crosssections are fromcalculationsatNNLO[30]andapproximateNNLO[31] preci-sion,respectively.

4. Eventreconstructionandselection

Allevents are reconstructed using the standard CMS particle-flow (PF) techniques [32], where the information from all CMS subdetectorsiscombinedinacoherentmannertoidentifyand re-constructindividual electrons, muons, photons, charged hadrons, and neutral hadrons. Only the charged particles associated with the primary collision vertex are used in the analysis, where the primary vertex is defined as having the largest value of p2 T ofits associated tracks.Reconstructed charged particles fromthe primary collision vertex, with the exception of isolated electrons and muons, and all neutral particles are used for jet clustering. Jetsareformed usingtheanti-kT clustering algorithm[33]witha distance parameter of 0.5. The momentum of a jet, determined from the vectorial sum of the momenta of all particles within a jet, is found from simulation to lie typically within 5–10% of the true jet momentum. Jet energies are corrected for contribu-tionsfromadditionalpileupinteractionsexpectedwithinthearea ofthe jet.Afterwards, simulation-based pT- and η-dependent jet energy scale corrections are applied to all jets both in the data andsimulation [34]. Through thesemeans, a uniform energy re-sponse is achieved at the reconstructed particle level with only weakpileupdependence.Jetsinthedatahaveanadditional resid-ualcorrectionthatisdeterminedbyassumingmomentumbalance

Table 1

Expected and observed yield of events passing the full selection of e++ jets, e−+jets, μ++jets, and μ−+ jets channels. Simulations are used to obtain the expected number of events except for the QCD multijet background, which is de-rived from data, as described in the text. The uncertainties on the event numbers are statistical and reflect the limited number of events in simulation or data for the individual processes.

Sample e++jets e−+jets μ++jets μ−+jets tt 55922±68 55476±68 72020±77 72094±77 W+jets 5448±50 4128±45 7146±59 5174±51 Z/γ∗+jets 835±12 800±11 812±12 816±11 Single top 3107±35 2659±34 3908±40 3412±38 QCD multijet 7922±89 7235±85 7148±85 7173±85 Total 73234±128 70298±123 91034±136 88669±132 Observed 71952 70396 87039 84024

in dijet, photon + jet, and Z + jet events. The jet energy res-olution is measured in the data to be about10% worse than in simulation [34] whichiscorrected by smearingthe jet energyin simulated events by the corresponding amount. The amount of missing transverse momentum (Emiss_T ) is calculated asthe mag-nitudeofthevectorsumofthetransversemomentaofall recon-structed particles [35]. The effect of jet energy scale corrections onthejet momentaispropagatedto Emiss_T .Aparticle-based rela-tive isolation iscomputedforeach leptonandiscorrected onan event-by-eventbasisforcontributionsfrompileupevents[36].The scalar sumofthe transverse momentaof all reconstructed parti-clecandidates,exceptfortheleptonsthemselves,withinaconeof size R=√(η)2+ (φ)2=0.3 (=0.4 formuons)builtaround the lepton direction must be less than 10% of the electron pT andlessthan12%ofthemuon pT.Eventsarerequiredtocontain onlyoneisolatedlightlepton,eitheranelectronwithpT>32 GeV and|η|<2.5 or amuon with pT>25 GeV and|η|<2.1.Events musthaveatleast fourjetswith pT>30 GeV and |η|<2.4.Jets originating from a bottom quark (b jets) are identified with the combined secondary vertex (CSV) algorithm [37,38] which com-bines the information fromreconstructed secondary vertices and fromdisplacedtracks withinthejetstoforma multi-variant dis-criminatoroutput.It istunedsuchthat itsefficiencytotagb jets isabout68% and therateofmistagginglight-flavor,gluon, andc jetsisabout 4% forjetswithin theconsidered pT range,as eval-uated using thenominal tt simulation.Each eventis required to have at least one b-tagged jet. An additional eventselection re-quirementbasedonthe χ2 _{ofakinematicfittothett hypothesis} describedinSection5isalsoappliedintheanalysis. Thenumber ofeventsobservedinthe dataandthe correspondingpredictions fromsimulatedeventsandfromdatacontrolregions(fortheQCD multijetbackground)areshownin Table 1separatelyfore++jets, e− + jets, μ+ + jets,and μ− +jetsevents.ThefractionofQCD multijeteventsisestimatedwithabinnedmaximum-likelihoodfit to the Emiss

T distribution observed in the data, separately in the e+jetsand μ+jetschannels.ThemassshapefortheQCD mul-tijet background is obtainedfrom the data by using a dedicated sample of events where the isolation and identification criteria (for e +jets) ortheisolation criterion alone(for μ + jets)have beeninverted. A differenceoflessthan 6%is foundbetweenthe observed andexpectedtotal yields.Differences betweenthe data andexpectationintheoverallyield donot affectthisanalysis di-rectly, unlike possible differences in the kinematic properties of theeventsorinthe relativefractionsoftheyields ofthevarious processes. Fig. 1showsthedistributionofthetransversemomenta ofthefourleading jetsineach eventforboth the+ +jetsand the− + jetssamples.Theoverallnumberofsimulatedeventsis normalizedtotheeventyieldobservedinthedata,whilekeeping theirrelativefractionsfixedtotheprediction.Ingeneral, thedata

Fig. 1. Comparison of the data to expectation for the transverse momenta of the four leading jets in each event for ++jets events (left) and −+jets events (right). The last bin of each distribution includes all jets with pT>530 GeV. The bin-by-bin ratio of the observed to the simulated spectra one is shown at the bottom of each plot. The

uncertainties are purely statistical. The total simulated event yields are normalized to the observed yields in the data, while keeping the relative fractions of the individual components fixed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

appearto be well modeled by the simulation,except fora small butstatisticallysignificantdeviationinthejettransversemomenta, visibleasaslopeintheratioplots.Thisisrelatedtothemodeling ofthe top quark transverse momentum inthe simulation,which is knownto predict a slightly harder spectrum than observed in data [39–41]. The effect on the top quark mass measurement is approximately100 MeV,asmeasured inanequivalent analysisin the  + jets channel [42]. As it identicallyaffects the + + jets and− + jetssamples, the impacton the mt measurement is negligible.

5. Thekinematicfitandtheideogrammethod

Akinematicfittothett hypothesis[43–45]isemployedinthis analysistoreconstruct themassofthe hadronically decayingtop quarkbyvaryingthemomentaofthetwojetsthatareassignedto theW→qq decay,usinga W bosonmassvalue of80.4 GeV [4] asa constraint, while keepingthe ratioof energyto momentum of each jet fixed. The W+ andW− bosons are assumed to have equal mass in thisprocedure. For each event, the four jets with thelargesttransversemomentum areconsideredin thefit.These fourjetscanbeassociatedwiththefourquarksfromthe hypoth-esizedtt-decay(tt→bbW+W−→bbqqν)in12possibleways. The kinematic fit is performedfor each of these 12 jet-to-quark assignments. However, beforecarrying out thefit,additional cor-rectionsareappliedtothedataandsimulationinordertocorrect jetenergiestothepartonlevel.Thesecorrectionsarederived sep-aratelyforjetsassociatedwithhadronicW boson decaysandfor b jetsinbinsof pT and|η|ofthesejetsbycomparingthe trans-verseenergyofreconstructed jetswiththat ofthecorresponding generatedpartonsinsimulatedtt events.Onlysolutionsforwhich thekinematicfitreturnsa χ2_/n

dof<10 areaccepted,wherendof (=1) isthenumberofdegreesoffreedominthefit.An eventis rejectedifnocombinationofjetspassesthe χ2 _{requirement. For} each combination of jets i, mi is the top quark mass value that yieldsthesmallest χ2

i,andtheuncertainty σi correspondstothe massrangethatiscompatiblewithanincreaseofthe χ2_by1.The valuesofmi, σi,and χi2areusedasinputtotheideogrammethod asdescribedbelow.A comparisonofmiand χi2 betweenthedata and expectation is given in Fig. 2, for the jet combination with thesmallestoverall χ2 _{ineachevent. Agreementisobserved} be-tweenthedataandexpectation. Intheideogrammethod[15],an approximateevent-by-event likelihood model is constructed asa functionofmt.Thislikelihoodcontainsasignalandabackground

term. The signalpartis aweighted sumoverall combinationsof jets,containingtwoterms:acorrectjet-to-quarkassignmentterm andawrongjet-to-quarkassignmentterm.Theshapesofthe back-groundtermandthewrongjet-to-quarkassignmenttermareboth takenfromsimulation,whichrepresentsourbestknowledgeofthe kinematicpropertiesoftheevents.Thecorrectjet-to-quark assign-ment term,ontheother hand,isdefinedbytheconvolution ofa GaussianresolutionfunctionandarelativisticBreit–Wigner distri-bution.TheGaussianfunctionhasawidthequaltotheuncertainty

σi resulting from the kinematic fit for the givencombination of jets. The weights applied in the sum over jet combinations are calculatedforeachcombinationusingthe χ2

i ofthekinematicfit andthecompatibilityoftheb jetassignments,calculatedfromthe known b tagging efficiencyandmistagging rate. The reweighting significantly reduces the contribution of combinations for which the b jet assignments are highly incompatible with the results of the b taggingalgorithm andof combinationsthat badly fulfill the mass constraints. The combined likelihood for the full event sample is calculated asthe product of the individual event like-lihoods for all selectedevents.The fitted topquark massandits statisticaluncertaintyareextractedfromthiscombinedlikelihood. More detailsaboutthe exactimplementationofthe kinematicfit and the ideogram method can be found in Ref. [11]. The event likelihoods arecalculatedundercertainassumptions and simplifi-cations andhencetheresultofthecombinationisfirstcalibrated using pseudo-experiments where the mass shapes are generated from theexpecteddistributions of signal andbackgroundevents. The standard deviation (width) of the pull distribution and the observedbias ontheestimatedtopquark massbeforecalibration are shownasafunctionofthegeneratedmassin Fig. 3.Thepull is defined as pullj= (mj− m)/σj, where mj is the estimated top quarkmassineach pseudo-experiment j, σj the correspond-ing statisticaluncertainty,andmthemeanoftheestimatedtop quark massesoverallpseudo-experiments.Sincethewidthofthe pull distributionisabout1.13,thestatisticaluncertaintyofthe fi-nal massmeasurement needs tobe scaled up by about13%.The biasesarewithin3 GeV formostoftherangeofinterest.The ob-tainedtopquarkmassiscorrectedusingthefittedlinearfunction shown in Fig. 3 (right). The residual bias on the estimated top quarkmassasafunctionofthegeneratedtopquarkmassafter ap-plying thiscalibrationisshownin Fig. 4,separatelyfor+ +jets and − + jets events.These plots demonstrate that an inclusive +jetscalibrationcanbeusedforboththepositiveandnegative channels.

Fig. 2. Comparison between the data and expectation for the fitted top quark mass of the jet-quark assignment with the smallest χ2_{(top) and these smallest χ}2_values

(bottom), for ++jets events (left) and −+jets events (right). The last bin of the top quark mass distributions includes all masses above 980 GeV. The bin-by-bin ratio of the observed spectrum to the simulated one is shown at the bottom of each plot. The uncertainties are purely statistical. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Width of the pull distribution (left) and bias on the estimated top quark mass (right) as a function of the generated top quark mass for +jets events. The dashed blue line represents the ideal outcome.

6. Measurementofmt

Theanalysisisappliedseparatelytoeventswithpositivelyand negativelychargedleptons, foreach ofwhichthetopquark mass is measured using the hadronically decaying top quark. The dif-ference between the two resulting mass measurements is then takenas the final measurement of the mass difference between thetopquarkandantiquark.Intheinclusivee+jetsand μ+jets sample a mass difference of mt = −0.15±0.19(stat) GeV is measured. Inthe individual e + jets and μ + jets channels,

re-spective mass differences of mt= −0.19±0.28(stat) GeV and

mt= −0.13±0.26(stat) GeV areobtained.Theseresultsare com-patible withthehypothesis of CPTconservation.The averagetop quarkmassismeasuredtobemt=172.84±0.10(stat) GeV,which isin agreementwithprevious measurements [42,46,47],even ig-noringsystematicuncertainties.

7. Systematicuncertainties

Manyofthesystematicuncertainties thataffectthe topquark mass measurement have a significantly reduced impact in the

Fig. 4. Residual bias on the estimated top quark mass as a function of the generated top quark mass using ++jets events (left) and − +jets events (right) after the inclusive +jets calibration. The dashed blue line represents the ideal outcome.

Table 2

Summary of systematic uncertainties on mt. For each contribution, the first value

is the observed systematic shift, whereas the second number is the uncertainty of the shift due to the limited number of generated events. In all cases, the larger among the two is considered as the final systematic uncertainty and is indicated in the bold font. The total uncertainty is obtained from the sum in quadrature of the individual terms.

Source Uncertainty inmt(MeV)

Jet energy scale 7±16

Jet energy resolution 7±11

b vs. b jet response 51±1

Signal fraction 27±2

Background charge asymmetry 11.9±0.1 Background composition 28±1

Pileup 9.1±0.3

b tagging efficiency 24±7 b vs. b tagging efficiency 11±7 Method calibration 3±53

Parton distribution functions 9±3

Total 91

massdifferencebecauseoftheircorrelatedeffectontheindividual topquarkandantiquarkmassextractions.Somesystematic uncer-tainties relatedto the modeling ofthe physics processes are not expectedto affectthemt measurement andarenot considered inthisanalysis. Thesearethemodeling ofthehadronization,the underlyingevent,initial- andfinal-stateradiation,thefactorization and renormalization scales, and the matching between matrix-elementandpartonshower calculations. Other effectsconsidered in themeasurement ofmt are included together with additional sources potentially relevant for the mt measurement, such as lepton-chargeidentificationandapossibledifferenceinjetenergy response tob and b quarks. A summaryof theseeffectsis given in Table 2. The effects are evaluated by comparing the nominal simulation to a sample ofsimulated events where the source of thesystematicuncertaintyunderstudyisvariedwithin its uncer-tainty.Since most sources of systematicuncertainty yield only a smallchangeinthemt measurement,statisticaluncertaintieson theobservedchanges areevaluated usingajackknifere-sampling technique[48]andthelargeramongtheestimatedchangeandits statisticaluncertaintyisquotedasthefinalsystematicuncertainty. Thetotalsystematicuncertaintyistakentobe thequadraticsum ofall individual values.The uncertainties presentedhereare sig-nificantlysmaller than thosereported inRef. [11].All systematic uncertainties in the previous result were statistically compatible withzeroandthetotaluncertaintyincludedasizablecomponent fromthelimitedsizeofthe simulateddatasamples.Muchlarger samplesofsimulatedsignalandbackgroundeventshavebeen pro-ducedforthisnewresult, resultinginmoreaccurateestimatesof

the systematic uncertainties and a reduction of the total uncer-tainties.Someuncertainties,suchasthejetenergyscaleandtheb taggingefficiencyalsoprofitfrommoreaccuratecorrections.

Theindividualcontributionstothetotalsystematicuncertainty aredescribedinmoredetailbelow.

Jet energy scale. Since top quarks and antiquarks are produced at the LHC with slightly different rapidity distributions, the

η-dependenceofthe jetenergyscale uncertaintycanlead to aneffectonmt.Toevaluatethiseffecttheenergyofalljets is scaled up/down within their pT- and η-dependent uncer-tainties(rangingbetween1and 5%)[34].Thisresultsinashift inmassdifferenceof7±16 MeV.

Jet energy resolution. Toevaluatethesystematicuncertainty aris-ing from the uncertaintyin the measured jet energy resolu-tion,thejet energyinsimulationissmeared up/downwithin the uncertainty of this jet energy resolution. The jet energy resolution uncertainty is |η|-dependent and ranges between 6 and9% for the jets considered in this analysis. A shift of 7±11 MeV inmt isobserved.

b vs. b jet response. A difference in the fractionof jet energy re-constructed by the detectorbetween b and b jets can intro-duceabiasinthemt measurement.Suchdifferences,caused forexamplebydifferentcrosssectionsforinteractionsof pos-itively and negatively charged kaons in the calorimeter, are expected to be reducedthanks to the PFreconstruction that reliesmostly ontrackingtoreconstructchargedhadrons. The

pT of the reconstructed jets is compared with the original parton pT in two simulatedtt samples: thenominal sample, generatedwith MadGraph withshoweringfrom pythia,anda samplegeneratedwith mc@nlo withshoweringfromherwig_. SimulatedsamplesproducedwiththesetwosetsofMC gener-atorshavebeenobservedtoencompassthedatainvariouskey observables, anddiffersignificantly inseveralaspects, includ-ing therelative productionanddecayratesofdifferentkinds of hadrons in the jets [49]. In both samples the ratio of b to b response asa function ofjet pT is observed to be sta-tistically compatiblewithunity, andan average difference of 0.078±0.040%ismeasured.Whenthedifferenceof0.078%is propagatedtoournominalsample ofsimulatedeventsashift of51±1 MeV isobserved,whichisquotedassystematic un-certainty.

Signal fraction. Achangeinthesignalfraction(ascalculatedfrom Table 1) willbias themeasured top quark mass, sincesignal and background events have different fitted top quark mass distributions. This will alsointroduce a bias in mt because it will influence the + + jetsand − + jetssamples in a differentwaysincethesehaveadifferentsignal fraction. The

signal fractionis changed by a relative ±10%, corresponding totheagreementbetweentheexpectedandobservedtt cross sections inthis channel [50], andthe resulting shiftof 27± 2 MeV istakenasasystematicuncertainty.

Background charge asymmetry. A difference in the estimated charge asymmetry of thebackgrounds leads to different lev-els ofbackgroundandto adifferentbackground composition in the+ + jetsand− + jetschannels,whichcan biasthe mt measurement. Themeasured inclusiveW+/W− produc-tionratioat8 TeVisinagreementwiththeoreticalpredictions within a precisionof2% [51,52],butsince thisratiodepends onthenumberofjets,theuncertaintyisinflatedbyafactorof two,yieldingavariationof4%.WhenthefractionsofW+ and W−eventsarevariedby2%inoppositedirections,thereby af-fecting the relative ratio ofW+ andW− events by 4%, mt changes by 3.72±0.01 MeV.The W + jetsbackground con-tains non-negligible contributions from W+cc and W+bb events, whose relative W+/W− ratio is affected by a larger uncertainty. The relative ratio isvaried by 20%, which corre-sponds to the uncertaintyin themeasured inclusive W+bb production cross section [53], and yields a shift in mt of 9.05±0.02 MeV and5.83±0.02 MeV fortheW+cc andW+bb contributions,respectively.Singletopquarksproducedviathe

t channelalsopossessachargeasymmetry,measuredtobein agreement withtheorypredictionswithin 15%[54].Changing this charge asymmetry by a relative ±15% results in a shift onmt of3.298±0.005 MeV.Thequadraticsumofallthese observedshiftsisquotedasthesystematicuncertainty.

Background composition. Possible residual effects due to the composition ofthe backgroundare evaluated byscaling each background source up anddown, keeping thesignal fraction fixed. A shift in mt is observed when we scale W + jets (1.3±0.3 MeV), Z + jets (1.99±0.03 MeV), t-channel sin-gle top quark production(6.9±0.1 MeV), andtW single top quarkproduction(1.4±0.3 MeV)up/downby30%;andwhen we scale QCD multijet events (26.8±0.3 MeV) up/down by 50%.Thesizeofeachvariationwaschosen tocoverthe mod-eling uncertaintyin predictions of the MC simulation in the phasespaceoftheanalysisor,inthecaseoftheQCDmultijet sample, differences between estimates obtained with differ-ent methods to determine the normalizationfrom data. The systematicuncertaintyisobtainedbysumminginquadrature eachoftheobservedshifts.

Pileup. Pileup collisions are includedin the sample ofsimulated events used inthisanalysis. Events are reweighted to repro-duce the pileup distribution measured in the data. The sys-tematicuncertaintyis estimatedby changingthemeanvalue of the number of interactions by ±6% to account for uncer-tainties in the rate [55] and exact properties of the pileup collisions.Thisresultsinashiftinmt of9.1±0.3 MeV.

b tagging efficiency and b vs. b tagging efficiency. A mismodeling in simulation of the b tagging efficiency can bias the mea-surement by altering the observed b tagging assignments, which areusedin theideogrammethod.Toquantifythe im-pactoftheuncertaintyintheb taggingefficiency,wechange theworkingpointoftheb taggingalgorithm.Working points corresponding to an absolutechangeof±1.2% [37,38] inthe b tagging efficiency produce a shift in mt of 24±7 MeV (“b tagging efficiency”in Table 2). Theuse ofdifferent work-ing points forthe+ + jetsand− + jetssamples,yielding an absolute1.2% differenceinb tagging efficiencybetweenb andb jets,producesashiftof11±7 MeV (“b versusb tagging efficiency”in Table 2).

Misassignment of lepton charge. In this analysis the leptons are only used in the trigger, in the event selection, and in the

splittingofthedatainto++jetsand−+jetssamples,but not in the massreconstruction. A misassignment of the lep-toncharge canaffectthecalibrationandit canalsoleadtoa dilutionofthemeasurement.Formuonsthecharge misassign-mentrateis measuredwithcosmic muons[16] andcollision data [51,52] tobe ofthe order of10−5 to 10−4 inthe con-sidered pT range.Forelectrons thisrateranges between0.1% and0.4% [51,52].This meansthat the systematicuncertainty fromchargemisassignmentisbelow1%ofthemeasuredmt value,whichisnegligibleandisthereforeignored.

Trigger, lepton identification, and lepton isolation. As the trigger is based on an isolated single lepton, and the lepton is not used in the mass reconstruction, no systematic effect is ex-pectedfroman uncertaintyinthetriggerefficiencyoronthe leptonenergyscale.Similarly,theleptonidentificationand iso-lationarealsonotexpectedtoaffectthemeasurement.

Method calibration. Thedifferenceinmassbetweenthe++jets and − + jetssamples in the nominal MadGraph + pythia samplewithmt=172.5 GeV,isfoundtobe3±53 MeV.This result is statistically compatible with zero and confirms our expectationthat thereis no known effect insimulation that would lead to a difference in mass calibration between the twochannels. Thestatisticaluncertaintyisquotedasthe sys-tematicuncertaintyarisingfromthemethodcalibration.Asa further cross-check, eventsare reweighted to simulatea dif-ference in mass between top quarks and antiquarks in the nominalsample, ranging insmall steps from−4 to +4 GeV. A linear relationbetween simulatedandmeasured mass dif-ferenceisobserved,withaslopecompatiblewithunity,anda statisticalprecisionof5%.Ifpropagatedtothefinalresult,this uncertaintyinslopewouldhaveanegligibleimpactonthe fi-naluncertainty.

Parton distribution functions. The choice ofthe parton distribu-tionfunctions(PDFs)canaffectthemt measurementin mul-tipleways.Theydetermine,forexample,thedifferencein pro-duction of W+ and W− events. The simulated samples are generated using the CTEQ 6.6 PDF [56], for which the un-certainties can be described by 22 independent parameters. Varyingeachoftheseparameterswithinthequoted uncertain-tiesandsummingthelargershiftsinquadratureresultsinan uncertaintyinmtof9±3 MeV.

8. Resultsandsummary

DatacollectedbytheCMSexperimentinppcollisionsat√s=

8 TeV and corresponding to an integrated luminosity of 19.6± 0.5 fb−1 have been used to measure the difference in mass be-tweenthetopquarkandantiquark.Themeasuredvalueis

mt= −0.15±0.19 (stat)±0.09 (syst) GeV.

This resultimproves in precision upon previously reported mea-surements[9–13]bymorethanafactoroftwo.Itisinagreement withtheexpectationsfromCPTinvariance,requiringequalparticle andantiparticlemasses.

Acknowledgements

We congratulate our colleagues in the CERN accelerator de-partments for the excellent performance of the LHC and thank thetechnicalandadministrativestaffs atCERNandatother CMS institutes for their contributions to the success of the CMS ef-fort.Inaddition,wegratefullyacknowledgethecomputingcentres and personnel of the Worldwide LHC Computing Grid for deliv-ering so effectivelythe computing infrastructureessential to our analyses. Finally, we acknowledge the enduring support for the

constructionandoperationoftheLHCandtheCMSdetector pro-videdby thefollowingfundingagencies: BMWFWandFWF (Aus-tria);FNRSandFWO(Belgium);CNPq,CAPES,FAPERJ,andFAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST, and NSFC (China); COLCIENCIAS (Colombia); MSESand CSF (Croatia); RPF (Cyprus); SENESCYT(Ecuador);MoER,ERCIUT,andERDF(Estonia);Academy ofFinland,MEC,andHIP(Finland);CEAandCNRS/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); BUAP, CINVESTAV, CONACYT, LNS, SEP, and UASLP-FAI(Mexico);MBIE(New Zealand);PAEC(Pakistan);MSHE andNSC (Poland);FCT (Portugal); JINR (Dubna); MON, RosAtom, RAS,andRFBR(Russia);MESTD (Serbia);SEIDIandCPAN(Spain); Swiss Funding Agencies (Switzerland); MST (Taipei); ThEPCenter, IPST, STAR, and NSTDA (Thailand); TUBITAK and TAEK (Turkey); NASU andSFFR(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 Foun-dation; the Alexander von Humboldt Foundation; the Belgian Federal Science Policy Office; the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technolo-gie (IWT-Belgium); the Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Science and In-dustrial Research, India; the HOMING PLUS programme of the Foundation for Polish Science, cofinanced from European Union, Regional DevelopmentFund, theMobility Plusprogrammeofthe Ministry of Science and Higher Education, the National Science Center (Poland), contractsHarmonia 2014/14/M/ST2/00428, Opus 2013/11/B/ST2/04202, 2014/13/B/ST2/02543 and 2014/15/B/ST2/ 03998, Sonata-bis 2012/07/E/ST2/01406; the Thalis and Aristeia programmes cofinancedby EU-ESF andthe Greek NSRF; the Na-tional Priorities Research Program by Qatar National Research Fund; the Programa Clarín-COFUND del Principado de Asturias; theRachadapisekSompotFundforPostdoctoralFellowship, Chula-longkornUniversityandtheChulalongkornAcademicintoIts 2nd Century Project Advancement Project (Thailand); and the Welch Foundation,contractC-1845.

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TheCMSCollaboration

S. Chatrchyan,V. Khachatryan,A.M. Sirunyan, A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam, T. Bergauer, M. Dragicevic,J. Erö,C. Fabjan1,M. Friedl,R. Frühwirth1, V.M. Ghete, C. Hartl, N. Hörmann, J. Hrubec, M. Jeitler1,W. Kiesenhofer, V. Knünz,M. Krammer1, I. Krätschmer,D. Liko, I. Mikulec,D. Rabady2, B. Rahbaran,H. Rohringer, R. Schöfbeck, J. Strauss, A. Taurok,

W. Treberer-Treberspurg,W. Waltenberger, C.-E. Wulz1

InstitutfürHochenergiephysik,Wien,Austria

V. Mossolov,N. Shumeiko,J. Suarez Gonzalez

NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus

S. Alderweireldt, M. Bansal, S. Bansal,T. Cornelis, E.A. De Wolf,X. Janssen,A. Knutsson, S. Luyckx, L. Mucibello,S. Ochesanu, B. Roland,R. Rougny, H. Van Haevermaet,P. Van Mechelen,N. Van Remortel, A. Van Spilbeeck

UniversiteitAntwerpen,Antwerpen,Belgium

F. Blekman, S. Blyweert,J. D’Hondt, N. Heracleous,A. Kalogeropoulos, J. Keaveney, T.J. Kim, S. Lowette, M. Maes,A. Olbrechts, D. Strom, S. Tavernier,W. Van Doninck,P. Van Mulders, G.P. Van Onsem, I. Villella

VrijeUniversiteitBrussel,Brussel,Belgium

C. Caillol, B. Clerbaux, G. De Lentdecker, L. Favart, A.P.R. Gay, T. Hreus, A. Léonard,P.E. Marage, A. Mohammadi, L. Perniè, T. Reis,T. Seva, L. Thomas, C. Vander Velde, P. Vanlaer, J. Wang

V. Adler,K. Beernaert, L. Benucci, A. Cimmino,S. Costantini, S. Dildick,G. Garcia, B. Klein, J. Lellouch, J. Mccartin, A.A. Ocampo Rios,D. Ryckbosch, M. Sigamani, N. Strobbe,F. Thyssen, M. Tytgat, S. Walsh, E. Yazgan, N. Zaganidis

GhentUniversity,Ghent,Belgium

S. Basegmez, C. Beluffi3,G. Bruno,R. Castello, A. Caudron, L. Ceard, G.G. Da Silveira, C. Delaere, T. du Pree,D. Favart, L. Forthomme,A. Giammanco4,J. Hollar, P. Jez, M. Komm, V. Lemaitre, J. Liao, O. Militaru,C. Nuttens, D. Pagano, A. Pin, K. Piotrzkowski,A. Popov5, L. Quertenmont,M. Selvaggi, M. Vidal Marono, J.M. Vizan Garcia

UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

N. Beliy, T. Caebergs,E. Daubie, G.H. Hammad

UniversitédeMons,Mons,Belgium

G.A. Alves,M. Correa Martins Junior, T. Dos Reis Martins, M.E. Pol, M.H.G. Souza

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

W.L. Aldá Júnior, W. Carvalho, J. Chinellato6,A. Custódio, E.M. Da Costa, D. De Jesus Damiao,

C. De Oliveira Martins, S. Fonseca De Souza, H. Malbouisson, M. Malek,D. Matos Figueiredo, L. Mundim, H. Nogima,W.L. Prado Da Silva, J. Santaolalla, A. Santoro, A. Sznajder,E.J. Tonelli Manganote6,

A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

C.A. Bernardesb, F.A. Diasa,7,T.R. Fernandez Perez Tomeia, E.M. Gregoresb, C. Laganaa, P.G. Mercadanteb,S.F. Novaesa,Sandra S. Padulaa

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

V. Genchev2, P. Iaydjiev2,A. Marinov, S. Piperov, M. Rodozov,G. Sultanov, M. Vutova

InstituteforNuclearResearchandNuclearEnergy,Sofia,Bulgaria

A. Dimitrov, I. Glushkov,R. Hadjiiska, V. Kozhuharov, L. Litov,B. Pavlov, P. Petkov

UniversityofSofia,Sofia,Bulgaria

J.G. Bian,G.M. Chen, H.S. Chen,M. Chen, R. Du,C.H. Jiang, D. Liang, S. Liang, X. Meng,R. Plestina8,J. Tao, X. Wang,Z. Wang

InstituteofHighEnergyPhysics,Beijing,China

C. Asawatangtrakuldee, Y. Ban, Y. Guo, Q. Li, W. Li, S. Liu, Y. Mao,S.J. Qian, D. Wang,L. Zhang, W. Zou

StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

C. Avila,C.A. Carrillo Montoya, 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

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic, M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,K. Kadija, J. Luetic, D. Mekterovic, S. Morovic, L. Sudic

A. Attikis, G. Mavromanolakis,J. Mousa, C. Nicolaou, F. Ptochos,P.A. Razis

UniversityofCyprus,Nicosia,Cyprus

M. Finger,M. Finger Jr.

CharlesUniversity,Prague,CzechRepublic

E. El-khateeb9,T. Elkafrawy9,M.A. Mahmoud10,A. Mohamed11, A. Radi12,9, E. Salama9,12

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

M. Kadastik, M. Müntel,M. Murumaa, M. Raidal, L. Rebane, A. Tiko

NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola,G. Fedi, M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Härkönen,V. Karimäki, R. Kinnunen, M.J. Kortelainen, T. Lampén, K. Lassila-Perini,S. Lehti, T. Lindén, P. Luukka, T. Mäenpää,T. Peltola, E. Tuominen, J. Tuominiemi, E. Tuovinen,L. Wendland

HelsinkiInstituteofPhysics,Helsinki,Finland

T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon,F. Couderc, M. Dejardin, D. Denegri,B. Fabbro, J.L. Faure, F. Ferri, S. Ganjour, A. Givernaud, P. Gras, G. Hamel de Monchenault,P. Jarry,E. Locci, J. Malcles,A. Nayak,J. Rander, A. Rosowsky, M. Titov

IRFU,CEA,UniversitéParis-Saclay,Gif-sur-Yvette,France

S. Baffioni,F. Beaudette, P. Busson, C. Charlot, N. Daci,T. Dahms, M. Dalchenko,L. Dobrzynski, A. Florent, R. Granier de Cassagnac,M. Haguenauer, P. Miné,C. Mironov, I.N. Naranjo,M. Nguyen, C. Ochando, P. Paganini,D. Sabes, R. Salerno,Y. Sirois, C. Veelken, Y. Yilmaz, A. Zabi

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

J.-L. Agram13,J. Andrea, D. Bloch,J.-M. Brom, E.C. Chabert,C. Collard, E. Conte13,F. Drouhin13, J.-C. Fontaine13,D. Gelé, U. Goerlach,C. Goetzmann, P. Juillot, A.-C. Le Bihan, P. Van Hove

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

S. Gadrat

CentredeCalculdel’InstitutNationaldePhysiqueNucleaireetdePhysiquedesParticules,CNRS/IN2P3,Villeurbanne,France

S. Beauceron,N. Beaupere, G. Boudoul,S. Brochet, J. Chasserat, R. Chierici,D. Contardo, P. Depasse, H. El Mamouni,J. Fan, J. Fay, S. Gascon,M. Gouzevitch, B. Ille, T. Kurca, M. Lethuillier,L. Mirabito, S. Perries,J.D. Ruiz Alvarez, L. Sgandurra,V. Sordini, M. Vander Donckt, P. Verdier,S. Viret, H. Xiao

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

Z. Tsamalaidze14

TbilisiStateUniversity,Tbilisi,Georgia

C. Autermann,S. Beranek, M. Bontenackels, B. Calpas,M. Edelhoff, L. Feld, O. Hindrichs, K. Klein, A. Ostapchuk,A. Perieanu, F. Raupach, J. Sammet, S. Schael, D. Sprenger, H. Weber, B. Wittmer, V. Zhukov5

M. Ata, J. Caudron,E. Dietz-Laursonn, D. Duchardt, M. Erdmann,R. Fischer, A. Güth, T. Hebbeker, C. Heidemann,K. Hoepfner, D. Klingebiel,S. Knutzen,P. Kreuzer, M. Merschmeyer,A. Meyer,

M. Olschewski, K. Padeken,P. Papacz, H. Reithler,S.A. Schmitz, L. Sonnenschein, D. Teyssier,S. Thüer, M. Weber

RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany

V. Cherepanov, Y. Erdogan,G. Flügge, H. Geenen, M. Geisler, W. Haj Ahmad, F. Hoehle, B. Kargoll, T. Kress, Y. Kuessel,J. Lingemann2, A. Nowack,I.M. Nugent, L. Perchalla, O. Pooth,A. Stahl

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

I. Asin, N. Bartosik, J. Behr, W. Behrenhoff,U. Behrens, A.J. Bell,M. Bergholz15, A. Bethani,K. Borras, A. Burgmeier,A. Cakir, L. Calligaris,A. Campbell, S. Choudhury, F. Costanza, C. Diez Pardos,S. Dooling, T. Dorland,G. Eckerlin, D. Eckstein, T. Eichhorn, G. Flucke, A. Geiser, A. Grebenyuk,P. Gunnellini, S. Habib,J. Hauk, M. Hempel, D. Horton, H. Jung, M. Kasemann,P. Katsas, C. Kleinwort, M. Krämer, D. Krücker,W. Lange, J. Leonard, K. Lipka,W. Lohmann15,B. Lutz, R. Mankel, I. Marfin,

I.-A. Melzer-Pellmann,A.B. Meyer, G. Mittag, J. Mnich, A. Mussgiller, S. Naumann-Emme, O. Novgorodova, F. Nowak,H. Perrey, A. Petrukhin,D. Pitzl, R. Placakyte, A. Raspereza,

P.M. Ribeiro Cipriano,C. Riedl, E. Ron,M.Ö. Sahin, J. Salfeld-Nebgen,R. Schmidt15, T. Schoerner-Sadenius, M. Schröder, M. Stein, A.D.R. Vargas Trevino, R. Walsh, C. Wissing

DeutschesElektronen-Synchrotron,Hamburg,Germany

M. Aldaya Martin,V. Blobel, H. Enderle, J. Erfle,E. Garutti, K. Goebel, M. Görner, M. Gosselink, J. Haller, R.S. Höing,H. Kirschenmann, R. Klanner, R. Kogler,J. Lange, I. Marchesini,J. Ott, T. Peiffer, N. Pietsch, J. Poehlsen16, D. Rathjens, C. Sander,H. Schettler, P. Schleper, E. Schlieckau, A. Schmidt, M. Seidel, V. Sola, H. Stadie,G. Steinbrück, D. Troendle,E. Usai, L. Vanelderen

UniversityofHamburg,Hamburg,Germany

C. Barth,C. Baus, J. Berger,C. Böser, E. Butz, T. Chwalek, W. De Boer,A. Descroix, A. Dierlamm, M. Feindt,M. Guthoff2, F. Hartmann2,T. Hauth2,H. Held, K.H. Hoffmann, U. Husemann,I. Katkov5, A. Kornmayer2, E. Kuznetsova, P. Lobelle Pardo, D. Martschei, M.U. Mozer, Th. Müller, M. Niegel, A. Nürnberg, O. Oberst,G. Quast, K. Rabbertz, F. Ratnikov,S. Röcker, F.-P. Schilling, G. Schott, H.J. Simonis,F.M. Stober, R. Ulrich, J. Wagner-Kuhr, S. Wayand,T. Weiler, R. Wolf, M. Zeise

InstitutfürExperimentelleKernphysik,Karlsruhe,Germany

G. Anagnostou, G. Daskalakis,T. Geralis,S. Kesisoglou, A. Kyriakis, D. Loukas, A. Markou, C. Markou, E. Ntomari,I. Topsis-Giotis

InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece

A. Agapitos, L. Gouskos,A. Panagiotou, N. Saoulidou, E. Stiliaris

NationalandKapodistrianUniversityofAthens,Athens,Greece

X. Aslanoglou,I. Evangelou, G. Flouris, C. Foudas,P. Kokkas, N. Manthos, I. Papadopoulos, E. Paradas

UniversityofIoánnina,Ioánnina,Greece

G. Bencze,C. Hajdu, P. Hidas, D. Horvath17, F. Sikler,V. Veszpremi, G. Vesztergombi18,A.J. Zsigmond

WignerResearchCentreforPhysics,Budapest,Hungary

N. Beni, S. Czellar, J. Molnar, J. Palinkas, Z. Szillasi

InstituteofNuclearResearchATOMKI,Debrecen,Hungary

J. Karancsi, P. Raics, Z.L. Trocsanyi,B. Ujvari

S.K. Swain19

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

S.B. Beri,V. Bhatnagar, N. Dhingra, R. Gupta, M. Kaur, M.Z. Mehta, M. Mittal, N. Nishu, A. Sharma, J.B. Singh

PanjabUniversity,Chandigarh,India

Ashok Kumar,Arun Kumar, S. Ahuja, A. Bhardwaj,B.C. Choudhary, A. Kumar,S. Malhotra, M. Naimuddin, K. Ranjan,P. Saxena, V. Sharma, R.K. Shivpuri

UniversityofDelhi,Delhi,India

S. Banerjee, S. Bhattacharya, K. Chatterjee,S. Dutta, B. Gomber, Sa. Jain, Sh. Jain,R. Khurana,A. Modak, S. Mukherjee,D. Roy, S. Sarkar, M. Sharan, A.P. Singh

SahaInstituteofNuclearPhysics,Kolkata,India

A. Abdulsalam,D. Dutta, S. Kailas,V. Kumar, A.K. Mohanty2,L.M. Pant, P. Shukla, A. Topkar

BhabhaAtomicResearchCentre,Mumbai,India

H. Arfaei, H. Bakhshiansohi,H. Behnamian, S.M. Etesami20, A. Fahim21, A. Jafari,M. Khakzad, M. Mohammadi Najafabadi,M. Naseri, S. Paktinat Mehdiabadi, B. Safarzadeh22,M. Zeinali

InstituteforResearchinFundamentalSciences(IPM),Tehran,Iran

M. Grunewald

UniversityCollegeDublin,Dublin,Ireland

M. Abbresciaa,b, L. Barbonea,b,C. Calabriaa,b,S.S. Chhibraa,b,A. Colaleoa,D. Creanzaa,c,

N. De Filippisa,c, M. De Palmaa,b, L. Fiorea, G. Iasellia,c, G. Maggia,c, M. Maggia,B. Marangellia,b, S. Mya,c,S. Nuzzoa,b, N. Pacificoa, A. Pompilia,b,G. Pugliesea,c, R. Radognaa,b,G. Selvaggia,b, L. Silvestrisa, G. Singha,b,R. Vendittia,b,P. Verwilligena,G. Zitoa

a_{INFNSezionediBari,Bari,Italy} b_{UniversitàdiBari,Bari,Italy} c_{PolitecnicodiBari,Bari,Italy}

G. Abbiendia,A.C. Benvenutia, D. Bonacorsia,b, S. Braibant-Giacomellia,b,L. Brigliadoria,b, R. Campaninia,b,P. Capiluppia,b,A. Castroa,b, F.R. Cavalloa, G. Codispotia,b, M. Cuffiania,b,

G.M. Dallavallea,F. Fabbria,A. Fanfania,b,D. Fasanellaa,b, P. Giacomellia, C. Grandia, L. Guiduccia,b, S. Marcellinia, G. Masettia,M. Meneghellia,b,A. Montanaria,F.L. Navarriaa,b,F. Odoricia,A. Perrottaa, F. Primaveraa,b, A.M. Rossia,b, T. Rovellia,b, G.P. Sirolia,b,N. Tosia,b, R. Travaglinia,b

a_{INFNSezionediBologna,Bologna,Italy} b_{UniversitàdiBologna,Bologna,Italy}

S. Albergoa,b, G. Cappelloa,M. Chiorbolia,b,S. Costaa,b,F. Giordanoa,b,2,R. Potenzaa,b, A. Tricomia,b, C. Tuvea,b

a_{INFNSezionediCatania,Catania,Italy} b_{UniversitàdiCatania,Catania,Italy}

G. Barbaglia,V. Ciullia,b,C. Civininia, R. D’Alessandroa,b, E. Focardia,b,E. Galloa, S. Gonzia,b, V. Goria,b, P. Lenzia,b, M. Meschinia, S. Paolettia,G. Sguazzonia,A. Tropianoa,b

a_{INFNSezionediFirenze,Firenze,Italy} b_{UniversitàdiFirenze,Firenze,Italy}

L. Benussi,S. Bianco, F. Fabbri, D. Piccolo

P. Fabbricatorea, R. Ferrettia,b,F. Ferroa,M. Lo Veterea,b,R. Musenicha,E. Robuttia, S. Tosia,b

a_{INFNSezionediGenova,Genova,Italy} b_{UniversitàdiGenova,Genova,Italy}

A. Benagliaa, M.E. Dinardoa,b, S. Fiorendia,b,2, S. Gennaia, A. Ghezzia,b,P. Govonia,b, M.T. Lucchinia,b,2, S. Malvezzia, R.A. Manzonia,b,2, A. Martellia,b,2, D. Menascea,L. Moronia,M. Paganonia,b, D. Pedrinia, S. Ragazzia,b, N. Redaellia,T. Tabarelli de Fatisa,b

a_{INFNSezionediMilano-Bicocca,Milano,Italy} b_{UniversitàdiMilano-Bicocca,Milano,Italy}

S. Buontempoa, N. Cavalloa,c, F. Fabozzia,c,A.O.M. Iorioa,b,L. Listaa,S. Meolaa,d,2, M. Merolaa, P. Paoluccia,2

a_{INFNSezionediNapoli,Napoli,Italy} b_{UniversitàdiNapoli‘FedericoII’,Napoli,Italy} c_{UniversitàdellaBasilicata,Potenza,Italy} d_{UniversitàG. Marconi,Roma,Italy}

P. Azzia,N. Bacchettaa,D. Biselloa,b,A. Brancaa,b,R. Carlina,b, P. Checchiaa,T. Dorigoa, M. Galantia,b,2, F. Gasparinia,b_{, U. Gasparini}a,b_{, P. Giubilato}a,b_{,F. Gonella}a_{, A. Gozzelino}a_{, K. Kanishchev}a,c_,

S. Lacapraraa,I. Lazzizzeraa,c, M. Margonia,b, A.T. Meneguzzoa,b,F. Montecassianoa,M. Passaseoa, J. Pazzinia,b, N. Pozzobona,b,P. Ronchesea,b, F. Simonettoa,b,E. Torassaa, M. Tosia,b,S. Vaninia,b, P. Zottoa,b, A. Zucchettaa,b,G. Zumerlea,b

a_{INFNSezionediPadova,Padova,Italy} b_{UniversitàdiPadova,Padova,Italy} c_{UniversitàdiTrento,Trento,Italy}

M. Gabusia,b, S.P. Rattia,b,C. Riccardia,b,P. Vituloa,b

a_{INFNSezionediPavia,Pavia,Italy} b_{UniversitàdiPavia,Pavia,Italy}

M. Biasinia,b, G.M. Bileia,L. Fanòa,b, P. Laricciaa,b, G. Mantovania,b,M. Menichellia, A. Nappia,b,†, F. Romeoa,b, A. Sahaa, A. Santocchiaa,b,A. Spieziaa,b

a_{INFNSezionediPerugia,Perugia,Italy} b_{UniversitàdiPerugia,Perugia,Italy}

K. Androsova,23,P. Azzurria,G. Bagliesia,J. Bernardinia,T. Boccalia,G. Broccoloa,c,R. Castaldia, M.A. Cioccia,23, R. Dell’Orsoa,F. Fioria,c,L. Foàa,c,A. Giassia, M.T. Grippoa,23, A. Kraana, F. Ligabuea,c, T. Lomtadzea, L. Martinia,b, A. Messineoa,b,C.S. Moona,24, F. Pallaa,A. Rizzia,b,A. Savoy-Navarroa,25, A.T. Serbana,P. Spagnoloa,P. Squillaciotia,23,R. Tenchinia, G. Tonellia,b,A. Venturia, P.G. Verdinia, C. Vernieria,c

a_{INFNSezionediPisa,Pisa,Italy} b_{UniversitàdiPisa,Pisa,Italy}

c_{ScuolaNormaleSuperiorediPisa,Pisa,Italy}

L. Baronea,b,F. Cavallaria, D. Del Rea,b,M. Diemoza, M. Grassia,b,C. Jordaa, E. Longoa,b, F. Margarolia,b, P. Meridiania,F. Michelia,b,S. Nourbakhsha,b, G. Organtinia,b, R. Paramattia,S. Rahatloua,b,C. Rovellia, L. Soffia,b,P. Traczyka,b

a_{INFNSezionediRoma,Roma,Italy} b_{UniversitàdiRoma,Roma,Italy}

N. Amapanea,b, R. Arcidiaconoa,c,S. Argiroa,b,M. Arneodoa,c,R. Bellana,b, C. Biinoa, N. Cartigliaa, S. Casassoa,b, M. Costaa,b,P. De Remigisa, A. Deganoa,b,N. Demariaa,C. Mariottia, S. Masellia, E. Migliorea,b,V. Monacoa,b,M. Musicha,M.M. Obertinoa,c,G. Ortonaa,b, L. Pachera,b, N. Pastronea, M. Pelliccionia,2, A. Potenzaa,b, A. Romeroa,b, M. Ruspaa,c,R. Sacchia,b,A. Solanoa,b,A. Staianoa

a_{INFNSezionediTorino,Torino,Italy} b_{UniversitàdiTorino,Torino,Italy}

S. Belfortea,V. Candelisea,b, M. Casarsaa,F. Cossuttia,2,G. Della Riccaa,b, B. Gobboa, C. La Licataa,b, M. Maronea,b, D. Montaninoa,b, A. Penzoa,A. Schizzia,b, T. Umera,b,A. Zanettia

a_{INFNSezionediTrieste,Trieste,Italy} b_{UniversitàdiTrieste,Trieste,Italy}

S. Chang,T.Y. Kim, S.K. Nam

KangwonNationalUniversity,Chunchon,RepublicofKorea

D.H. Kim,G.N. Kim, J.E. Kim, D.J. Kong,S. Lee, Y.D. Oh, H. Park, D.C. Son

KyungpookNationalUniversity,Daegu,RepublicofKorea

J.Y. Kim,Zero J. Kim, S. Song

ChonnamNationalUniversity,InstituteforUniverseandElementaryParticles,Kwangju,RepublicofKorea

S. Choi, D. Gyun,B. Hong,M. Jo, H. Kim, Y. Kim,K.S. Lee, S.K. Park,Y. Roh

KoreaUniversity,Seoul,RepublicofKorea

M. Choi,J.H. Kim, C. Park, I.C. Park, S. Park,G. Ryu

UniversityofSeoul,Seoul,RepublicofKorea

Y. Choi,Y.K. Choi,J. Goh, M.S. Kim, E. Kwon, B. Lee, J. Lee,S. Lee, H. Seo,I. Yu

SungkyunkwanUniversity,Suwon,RepublicofKorea

A. Juodagalvis

VilniusUniversity,Vilnius,Lithuania

H. Castilla-Valdez,E. De La Cruz-Burelo, I. Heredia-de La Cruz26,R. Lopez-Fernandez, J. Martínez-Ortega, A. Sanchez-Hernandez,L.M. Villasenor-Cendejas

CentrodeInvestigacionydeEstudiosAvanzadosdelIPN,MexicoCity,Mexico

S. Carrillo Moreno, F. Vazquez Valencia

UniversidadIberoamericana,MexicoCity,Mexico

H.A. Salazar Ibarguen

BenemeritaUniversidadAutonomadePuebla,Puebla,Mexico

E. Casimiro Linares, A. Morelos Pineda

UniversidadAutónomadeSanLuisPotosí,SanLuisPotosí,Mexico

D. Krofcheck

UniversityofAuckland,Auckland,NewZealand

P.H. Butler,R. Doesburg, S. Reucroft, H. Silverwood

UniversityofCanterbury,Christchurch,NewZealand

M. Ahmad, M.I. Asghar,J. Butt, H.R. Hoorani, W.A. Khan, T. Khurshid,S. Qazi, M.A. Shah, M. Shoaib

NationalCentreforPhysics,Quaid-I-AzamUniversity,Islamabad,Pakistan

H. Bialkowska,M. Bluj, B. Boimska,T. Frueboes, M. Górski, M. Kazana, K. Nawrocki, K. Romanowska-Rybinska,M. Szleper, G. Wrochna,P. Zalewski

G. Brona, K. Bunkowski, M. Cwiok,W. Dominik, K. Doroba, A. Kalinowski, M. Konecki,J. Krolikowski, M. Misiura, W. Wolszczak

InstituteofExperimentalPhysics,FacultyofPhysics,UniversityofWarsaw,Warsaw,Poland

P. Bargassa,C. Beirão Da Cruz E Silva, P. Faccioli, P.G. Ferreira Parracho, M. Gallinaro,F. Nguyen, J. Rodrigues Antunes, J. Seixas2, J. Varela, P. Vischia

LaboratóriodeInstrumentaçãoeFísicaExperimentaldePartículas,Lisboa,Portugal

I. Golutvin, V. Karjavin, V. Konoplyanikov, V. Korenkov,G. Kozlov, A. Lanev,A. Malakhov,V. Matveev, V.V. Mitsyn,P. Moisenz, V. Palichik, V. Perelygin, S. Shmatov, S. Shulha,N. Skatchkov, V. Smirnov, E. Tikhonenko, A. Zarubin

JointInstituteforNuclearResearch,Dubna,Russia

V. Golovtsov, Y. Ivanov,V. Kim27, P. Levchenko, V. Murzin,V. Oreshkin, I. Smirnov, V. Sulimov, L. Uvarov, S. Vavilov, A. Vorobyev,An. Vorobyev

PetersburgNuclearPhysicsInstitute,Gatchina(St.Petersburg),Russia

Yu. Andreev,A. Dermenev, S. Gninenko, N. Golubev, M. Kirsanov,N. Krasnikov, A. Pashenkov, D. Tlisov, A. Toropin

InstituteforNuclearResearch,Moscow,Russia

V. Epshteyn, V. Gavrilov, N. Lychkovskaya,V. Popov, G. Safronov, S. Semenov, A. Spiridonov, V. Stolin, E. Vlasov, A. Zhokin

InstituteforTheoreticalandExperimentalPhysics,Moscow,Russia

V. Andreev,M. Azarkin, I. Dremin, M. Kirakosyan, A. Leonidov, G. Mesyats, S.V. Rusakov,A. Vinogradov

P.N.LebedevPhysicalInstitute,Moscow,Russia

A. Belyaev,E. Boos, V. Bunichev, M. Dubinin7, L. Dudko,A. Ershov, A. Gribushin,V. Klyukhin, N. Korneeva,I. Lokhtin, A. Markina,S. Obraztsov,M. Perfilov, V. Savrin

SkobeltsynInstituteofNuclearPhysics,LomonosovMoscowStateUniversity,Moscow,Russia

I. Azhgirey,I. Bayshev,S. Bitioukov, V. Kachanov, A. Kalinin, D. Konstantinov, V. Krychkine, V. Petrov, R. Ryutin, A. Sobol, L. Tourtchanovitch,S. Troshin, N. Tyurin, A. Uzunian,A. Volkov

StateResearchCenterofRussianFederation,InstituteforHighEnergyPhysics,Protvino,Russia

P. Adzic28, M. Dordevic, M. Ekmedzic,J. Milosevic

UniversityofBelgrade,FacultyofPhysicsandVincaInstituteofNuclearSciences,Belgrade,Serbia

M. Aguilar-Benitez,J. Alcaraz Maestre, C. Battilana,E. Calvo, M. Cerrada,M. Chamizo Llatas2,N. Colino, B. De La Cruz,A. Delgado Peris,D. Domínguez Vázquez, C. Fernandez Bedoya,J.P. Fernández Ramos, A. Ferrando, J. Flix, M.C. Fouz,P. Garcia-Abia, O. Gonzalez Lopez,S. Goy Lopez, J.M. Hernandez, M.I. Josa, G. Merino, E. Navarro De Martino,J. Puerta Pelayo, A. Quintario Olmeda, I. Redondo,L. Romero,

M.S. Soares, C. Willmott

CentrodeInvestigacionesEnergéticasMedioambientalesyTecnológicas(CIEMAT),Madrid,Spain

C. Albajar, J.F. de Trocóniz

UniversidadAutónomadeMadrid,Madrid,Spain

H. Brun, J. Cuevas,J. Fernandez Menendez, S. Folgueras, I. Gonzalez Caballero, L. Lloret Iglesias