Article
Reference
Measurement of the top quark mass in the tt¯→dilepton channel from
√s = 8 TeV ATLAS data
ATLAS Collaboration
ANCU, Lucian Stefan (Collab.), et al.
Abstract
The top quark mass is measured in the tt¯→dilepton channel ( lepton=e,μ ) using ATLAS data recorded in the year 2012 at the LHC. The data were taken at a proton–proton centre-of-mass energy of s=8 TeV and correspond to an integrated luminosity of about 20.2 fb−1 . Exploiting the template method, and using the distribution of invariant masses of lepton– b-jet pairs, the top quark mass is measured to be mtop=172.99±0.41 (stat)±0.74 (syst) GeV , with a total uncertainty of 0.84 GeV . Finally, a combination with previous ATLAS mtop measurements from s=7 TeV data in the tt¯→dilepton and tt¯→lepton+jets channels results in mtop=172.84±0.34 (stat)±0.61 (syst) GeV , with a total uncertainty of 0.70 GeV .
ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Measurement of the top quark mass in the tt¯→dilepton channel from √s = 8 TeV ATLAS data. Physics Letters. B , 2016, vol.
761, p. 350-371
DOI : 10.1016/j.physletb.2016.08.042
Available at:
http://archive-ouverte.unige.ch/unige:86546
Disclaimer: layout of this document may differ from the published version.
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Measurement of the top quark mass in the t ¯ t → dilepton channel from
√ s = 8 TeV ATLAS data
.TheATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received8June2016
Receivedinrevisedform21July2016 Accepted8August2016
Availableonlinexxxx Editor:W.-D.Schlatter
Thetopquarkmassismeasuredinthett¯→dilepton channel(lepton=e,μ)usingATLASdatarecorded intheyear2012attheLHC.Thedataweretakenataproton–protoncentre-of-massenergyof√s
=8 TeV and correspond toan integratedluminosity ofabout 20.2 fb−1.Exploitingthe template method,and usingthedistributionofinvariant massesoflepton–b-jet pairs,thetopquark massismeasuredtobe mtop=172.99±0.41 (stat)±0.74 (syst) GeV,withatotaluncertaintyof0.84 GeV.Finally,a combination withpreviousATLASmtop measurementsfrom√
s=7 TeV datainthett¯→dilepton andt¯t→lepton+ jets channelsresultsinmtop=172.84±0.34 (stat)±0.61 (syst) GeV,withatotaluncertaintyof0.70 GeV.
©2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Themassofthetopquark (mtop)isanimportantparameterof theStandardModel (SM)ofparticlephysics.Precisemeasurements ofmtop provide crucialinformation forglobal fits ofelectroweak parameters[1–3]whichhelpassesstheinternalconsistencyofthe SM and to probe its extensions. In addition, the value of mtop affectsthe stability oftheSM Higgs potential, whichhascosmo- logicalimplications [4–6].Manymeasurementsofmtop havebeen performedby the Tevatron and LHC Collaborations. Combining a selection of those, the first Tevatron+LHC mtop result is mtop= 173.34±0.27 (stat)±0.71 (syst) GeV,withatotaluncertaintyof 0.76 GeV [7].Meanwhile, a numberof newresultshave become available[8–13],some ofwhichare moreprecise thanthe above combination. The latest ATLAS results in the t¯t→lepton+jets and t¯t→dilepton decay channels, both with electrons (e) and muons(μ)inthefinalstate[14],aremtop=172.33±0.75 (stat)± 1.02 (syst) GeV andmtop=173.79±0.54 (stat)±1.30 (syst) GeV, respectively.
ThisLetterpresentsanewmeasurementofmtopobtainedinthe t¯t→dilepton decay channel using2012 datataken ata proton–
proton(pp) centre-of-mass energy of√
s=8 TeV, withan inte- gratedluminosityofabout20.2 fb−1.Theanalysisexploitsthede- cayt¯t→W+W−bb¯→+−νν¯bb,¯ whichisrealisedwhenbothW bosonsdecayintoachargedleptonanditscorrespondingneutrino.
Inthe analysis, thet¯t decaychannels ee,eμ and μμ (including
τ→e,μ) are combined andreferred toas thedilepton channel.
Single-top-quarkevents withthe samelepton final statesare in-
E-mailaddress:[email protected].
cluded in the signal. Given the larger data sample compared to Ref.[14],theeventselectionwasoptimisedtoachievethesmallest total uncertainty. The measurement is based on the implemen- tation of the template method described in Ref. [14], which is calibrated using signal Monte Carlo (MC) samples. Consequently, thetopquarkmassmeasuredinthiswaycorrespondstothemass definitionusedintheMCprogram.
2. ATLASdetector
TheATLASexperiment[15]attheLHCisamulti-purposeparti- cledetectorwithaforward–backwardsymmetriccylindricalgeom- etryandanear4π coverageinsolidangle.1Itconsistsofaninner tracking detectorsurrounded by a thin superconducting solenoid providing a2 T axial magnetic field, electromagneticandhadron calorimeters,andamuon spectrometer.The innertrackingdetec- torcoversthepseudorapidityrange|η|<2.5.Itconsistsofsilicon pixel,siliconmicrostrip,andtransitionradiationtrackingdetectors.
Lead/liquid-argon(LAr)samplingcalorimetersprovideelectromag- netic(EM)energymeasurementswithhighgranularity.A hadronic (steel/scintillator-tile) calorimetercovers thecentral pseudorapid- ityrange(|η|<1.7).Theend-capandforwardregions areinstru- mented with LArcalorimeters forEM andhadronic energymea-
1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.
Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeing theazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedintermsof thepolarangleθ asη= −ln tan(θ/2).Angulardistanceismeasured inunitsof R≡
(η)2+(φ)2. http://dx.doi.org/10.1016/j.physletb.2016.08.042
0370-2693/©2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
JID:PLB AID:32230 /SCO Doctopic: Experiments [m5Gv1.3; v1.185; Prn:25/08/2016; 16:08] P.2 (1-22)
2 The ATLAS Collaboration / Physics Letters B•••(••••)•••–•••
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surementsupto |η|=4.9.The muonspectrometersurroundsthe calorimetersandisbasedonthreelargeair–coretoroidsupercon- ductingmagnetswitheightcoilseach.Itsbendingpowerisinthe rangefrom2.0to7.5 T m. Itincludesasystemofprecision track- ingchambersandfastdetectorsfortriggering.A three-leveltrigger system is used to select events. The first-level trigger is imple- mentedinhardwareandusesasubsetofthedetectorinformation toreduce the acceptedeventratetoatmost75 kHz. Thisisfol- lowed by two software-based trigger levels that together reduce the accepted rateto 400 Hz on average depending on the data- takingconditionsduring2012.
3. DataandMCsamples
Thisanalysisisbasedon ppcollisiondatarecordedin2012at
√s=8 TeV.Theintegrateddataluminosityamountsto 20.2 fb−1 withan uncertaintyof 1.9% determined withthe procedures de- scribedinRef.[16].
The modellingoft¯t and single-top-quarksignal eventsandof mostbackgroundprocessesreliesonMCsimulations.Forthesim- ulationofsignaleventsthePowheg-Boxprogram[17–19]isused.
The simulation of the top quark pair [20] and single-top-quark productionintheW t-channel[21] usesmatrixelementsatnext- to-leadingorder (NLO)inthestrongcouplingconstant αS,withthe NLO CT10[22] partondistributionfunction (PDF)andtheparam- eterhdamp= ∞.Thehdampparametersetstheresummationscale, whichcontrolsthetransitionfromthematrixelementtothepar- ton shower (PS) simulation. Given the event selection described belowrequiresleptonic decayproducts oftwo W bosons, single- top-quarkeventsinthe s-channelandt-channelare foundnotto contributetothesample.
ThePythia(v6.425)program [23]withtheP2011C[24] setof tunedparameters(tune)andthecorrespondingCTEQ6L1PDF[25]
are employed to provide the parton shower, hadronisation and underlying-eventmodelling. The uncertaintiesdueto QCDinitial- and final-state radiation (ISR/FSR) modelling are estimated with samples generated with the Powheg-Box program interfaced to the Pythia program for which the parameters of the generation arevariedtospantherangescompatiblewiththeresultsofmea- surementsoft¯t productioninassociationwithjets[26–28].
Formtop hypothesis testing, thett¯ andsingle-top-quark event samplesaregeneratedforfivevaluesofmtopintherange167.5 to 177.5 GeV instepsof2.5 GeV.Foreachmtopvalue, theMCsam- ples are normalised accordingto the best available cross-section calculations, which for mtop =172.5 GeV are σt¯t =253+−1315pb [29–34]fortt¯productionand σW t=22.4±1.5 pb[35]forsingle- top-quarkproduction inthe W t-channel. The PDF + αS-induced uncertainties in these cross-sections are calculated using the PDF4LHCprescription[36]withtheMSTW200868% CL NNLOPDF [37,38], CT10 NNLO PDF [22,39] andNNPDF2.3 5f FFN PDF [40], and are added in quadrature with the uncertainties due to the choicesofthefactorisationandrenormalisationscales.
The simulation of W± or Z boson production in association withjetsisperformedwiththeAlpgen(v2.13)program[41]inter- facedtothePythia6programusingtheCTEQ6L1PDFandthecor- respondingAUET2tune[42].Dibosonproductionprocesses(W W, W Z and Z Z) aresimulatedusingthe Alpgenprogram interfaced totheHerwig(v6.520)program[43]withtheAUET2tuneandto theJimmy (v4.31)program[44].Allsamplesare simulatedtaking intoaccount the effectsofmultiple soft pp interactions (pile-up) registered in the 2012 data. These interactions are modelled by overlaying simulated hits fromevents withexactly one inelastic (signal)collisionperbunchcrossingwithhitsfromminimum-bias events that are produced with the Pythia (v8.160) program [45]
usingtheA2Mtune[46]andtheMSTW2008 LO PDF.Forthisanal- ysis,theobservedvaluesofthepile-up-related quantitiesμ,the meannumberofinteractionsperbunchcrossing,andnvtx,theav- eragenumberofverticesperevent,areμ=20.7 andnvtx=9.2.
Finally,the samplesundergoa simulationoftheATLAS detec- tor[47]basedonGeant4[48],andarethenprocessedthroughthe same reconstruction software as the data. A number of samples usedtoassesssystematicuncertaintiesareproducedwithafaster version ofthesimulationwhich,inadditiontothefullsimulation of thetracking, usessmearing functionsandinterpolates particle behaviourandcalorimeterresponse,basedonresolutionfunctions measured infull-simulationstudies,toapproximatetheresultsof thefullsimulation.
4. Dataselectionandeventreconstruction
Triggers basedon isolatedsingle electrons ormuonswithen- ergy or momentum thresholds of24 GeV are used.The detector objects resulting fromthe top quark pair decayare electronand muon candidates,jetsandmissingtransversemomentum (EmissT ).
Inthe following,the termlepton isusedforchargedleptons (ex- cluding τ leptons)exclusively.
Electron candidates [49] are required to have a transverse energy of ET>25 GeV, a pseudorapidity of the corresponding EM cluster of |ηcluster|<2.47, with the transition region 1.37<
|ηcluster|<1.52 between the barrel and the end-cap calorimeter excluded. The muon candidates [50] are required to have trans- verse momentum pT>25 GeV and |η|<2.5.Toreduce thecon- tamination by leptons from heavy-flavour decays inside jets or fromphotonconversions, referredtoasnon-prompt (NP)leptons, strictisolationcriteriaareappliedtotheamountofactivityinthe vicinityoftheleptoncandidate[49,50].
Jetsarebuiltfromtopologicalclustersofcalorimetercells[51]
with the anti-kt jet clustering algorithm [52] using a radius pa- rameter of R=0.4. Jets are reconstructed using the local clus- terweighting (LCW) andglobalsequentialcalibration (GSC)algo- rithms[53–55]andrequiredtosatisfy pT>25 GeV and|η|<2.5.
MuonsreconstructedwithinaR=0.4 conearoundtheaxisofa jet withpT>25 GeV arenotconsideredascharged-leptoncandi- dates.Inaddition,jetswithinaR=0.2 conearoundanelectron candidate are removed and finally electrons within a R=0.4 cone around anyofthe remaining jetsare discarded. The identi- fication of jetscontaining b-hadrons, b-tagging,is usedforevent reconstruction and background suppression. In the following, ir- respective of their origin, jetstagged by the b-tagging algorithm are referred to as b-tagged jets, whereas those not tagged are referred to as untagged jets. Similarly, whether they are tagged or not, jets originating from bottom quarks are referred to as b-jetsandthosefrom(u,d,c,s)-quarksorgluonsaslightjets.The working point of the neural-network-based MV1 b-tagging algo- rithm [56] corresponds toan averageb-tagging efficiency of70%
forb-jetsinsimulatedtt¯eventsandrejectionfactorsof5forjets containing a c-hadron and 137 for jets containing only lighter- flavour hadrons. Tomatchtheb-tagging performanceinthedata, pT- and η-dependent scale factors [56], obtained from dijetand t¯t→dilepton events, are applied to MC jets depending on their true flavour.Thereconstruction ofthe EmissT is basedonthe vec- torsumofenergydepositsinthecalorimeters,projectedontothe transverse plane.Muonsare includedinthe EmissT usingtheir re- constructedmomentuminthetrackingdetectors[57].
The contribution of events wrongly reconstructed as tt¯→ dilepton events due to the presence of objects misidentified as leptons (fake leptons),is estimatedfromdata[58].Thetechnique employedusesfake-leptonandreal-leptonefficienciesthatdepend on ηand pT,measured ina background-enhancedcontrol region
