Article
Reference
Measurement of W boson angular distributions in events with high transverse momentum jets at √s = 8 TeV using the ATLAS detector
ATLAS Collaboration
ANCU, Lucian Stefan (Collab.), et al.
Abstract
The W boson angular distribution in events with high transverse momentum jets is measured using data collected by the ATLAS experiment from proton–proton collisions at a centre-of-mass energy s=8 TeV at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb−1 . The focus is on the contributions to W+jets processes from real W emission, which is achieved by studying events where a muon is observed close to a high transverse momentum jet. At small angular separations, these contributions are expected to be large. Various theoretical models of this process are compared to the data in terms of the absolute cross-section and the angular distributions of the muon from the leptonic W decay.
ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Measurement of W boson angular distributions in events with high transverse momentum jets at √s = 8 TeV using the ATLAS detector. Physics Letters. B , 2017, vol. 765, p. 132-153
DOI : 10.1016/j.physletb.2016.12.005
Available at:
http://archive-ouverte.unige.ch/unige:90873
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Contents lists available atScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Measurement of W boson angular distributions in events with high transverse momentum jets at √
s = 8 TeV using the ATLAS detector
.TheATLAS Collaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received23September2016
Receivedinrevisedform30November2016 Accepted2December2016
Availableonline6December2016 Editor: W.-D.Schlatter
TheWbosonangulardistributionineventswithhightransversemomentumjetsismeasuredusingdata collectedbytheATLASexperimentfromproton–protoncollisionsatacentre-of-massenergy√
s=8 TeV atthe LargeHadronCollider,correspondingto anintegratedluminosity of20.3 fb−1.The focusison the contributionsto W+jets processesfromreal W emission, whichis achievedbystudying events whereamuonisobservedclosetoahightransversemomentumjet.Atsmallangularseparations,these contributionsare expectedtobelarge.Varioustheoreticalmodelsofthisprocessarecomparedtothe dataintermsoftheabsolutecross-sectionandtheangulardistributionsofthemuonfromtheleptonic Wdecay.
©2016PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Precision measurements of Standard Model processes at the LargeHadronCollider(LHC)arecrucialforprobingthefundamen- talstructure ofthe strongandelectroweakinteractions. Thedata sample corresponding to an integrated luminosity of 20.3 fb−1 collected by the ATLAS experimentfromproton–proton (pp) col- lisions ata centre-of-mass energy√
s=8 TeV at theLHC allows detailedstudyofperturbative quantum chromodynamics (pertur- bativeQCD,pQCD)andrealandvirtual electroweak(EW)correc- tionsthatimpactmeasurementsofW+jets production.
Athighenergies, realemissionofweak bosons indijetevents can contribute significantly to inclusive W +jets measurements [1–5].In leading-order(LO) calculationsof W+1-jet production, theW bosonisbalancedbytherecoilhadronicjet,oftenreferred toasback-to-backproduction.Atnext-to-leadingorder(NLO),QCD and EW corrections to W+1-jet processes appear, both as real and virtual contributions. In the case of real W boson emission from an initial- or final-state quark, thesecontributions scale as O
αln2pT,j/mW
,where α isthegauge couplingoftheunified EW theory, pT,j isthe transverse momentumof thejet andmW is the W boson mass, andhave a collinear enhancement in the distribution of the angular distance between the W boson and the closest jet. The collinear enhancement arises from collinear andinfrared divergences whichwould be presentin the limit of
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vanishing W boson mass, but which are regulated by its finite mass.Theprocedurestocorrectlyaccountforcollinearpartonradi- ation,suchasmasslessgluonemission,arewellknownandledto theintroductionof(Sudakov)partonshoweringoftheinitial- and final-state partons inMonte Carlo generators forQCD as well as quantum electrodynamics(QED)contributions.Ananalogouspro- cedure is available for the emission of real W bosons [6]. The effectofrealW bosonemissioncanbeprobedbyisolatingevents forwhichthecancellationbetweenrealandvirtual correctionsis incomplete, forexample by studying the region ofsmall angular separationbetweena jetandtheW boson.Thisregionalsocon- tains LOcontributionsfrom W+2-jets, aswell ascorrectionsto thatprocess,whichmustbeincludedforaccuratepredictions.
Duetothiscomplexmixtureof W+1-jet andW+2-jetpro- cesses,andtherelevantQCDandEWcorrectionstoboth,compar- isons of measurements to predictions using multiple approaches for estimating those corrections are crucial. Comparisons of the measured angular spectra of the muon from the W boson with fixed-order predictions at NLO and next-to-next-to-leading-order (NNLO)andwithprogramswithelectroweakpartonshowershelp inunderstandingtheaccuracyofthesepredictions.
The measurements presented here focus on events that con- tain amuon andajet withtransverse momentum pT>500 GeV.
Inthiskinematicregime,contributionstoW+jets processesfrom realWbosonemissionareenhancedintheregionofsmallangular separation betweenthe W boson decayproducts andtheclosest jet.Theangularseparationisdefinedasthedistancebetween the http://dx.doi.org/10.1016/j.physletb.2016.12.005
0370-2693/©2016PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
muon and the closest jet, R(μ,jet)=
(φ)2+(η)2,1 here- afterreferred toasR.Measurementsofthisangularseparation thusprovideprecision testsofpQCD andelectroweakpredictions forthe rate andpattern of real W boson emission. Real W bo- sonemission,alsotermedcollinearW production,isthedominant processforeventswithR<2.4,andthusR<2.4 isreferredto asthecollinearregion.Thesignificanceofthishigher-ordercontri- butionatsmallR isshowninRef.[5].ForeventswithR>2.4, theW bosonisbalancedbyahadronicrecoilthatmayconsistof oneormorejets.
Thesemeasurements of the R distribution probe a newre- gionofphasespacethathasnotbeenexplicitlystudiedindetail.
MeasurementsofW+jets productionbyboththeATLASandCMS experimentsoftenremove portions ofthe collinearregion by re- quiringthat the lepton (e or μ) is separatedfromany jet by an angulardistanceofR>0.5[7,8].Byrelaxingthisrequirementto R>0.2 and focusing on the distribution ofangular separation betweenthemuonandtheclosestjetineventswithatleastone veryhigh pT jet (pT>500 GeV),itispossible toexplicitlytarget realW emissionwiththismeasurement.
Collinear W production may constitute an important back- ground in searches for beyond the Standard Model physics that involve Lorentz-boosted top quarks [9], either in rare topologies or at highenergies. If the W decay products are collinear with one of the jets, the structure of that jet can begin to resemble thatofthethree-prongedstructureofaboostedtop quark.While the ratefor collinear W production issuppressed relative to di- jetproductionwithnoW emission,hadronicW decayscancause a large increase in the measured jet mass.The resultis that W emission fromquarks at very high pT can yield single jets with definitesubstructurethat resemble theboostedtop-quarksignals beingsearchedfor.
2. TheATLASdetector
TheATLASdetector[10,11]providesnearlyfullsolidanglecov- eragearoundtheppcollisionpointattheLHC.
The inner detector(ID) comprises a silicon pixeltracker clos- esttothebeamline,amicrostripsilicontracker,andastraw-tube transition-radiationtrackeratradii upto108 cm.Athinsolenoid surrounding the tracker provides a 2 T axial magnetic field en- ablingthemeasurementofcharged-particlemomenta.Theoverall IDacceptancespans thefull azimuthalrangein φ,andtherange
|η|<2.5 forparticlesoriginatingnearthenominalLHCinteraction region[12].
Theelectromagnetic (EM)andhadroniccalorimeters are com- posedofmultiplesubdetectorsspanning |η|<4.9.TheEM barrel calorimeterusesaliquid-argon(LAr)activemedium,togetherwith leadabsorbers, andcovers |η|<1.45.Intheregion|η|<1.7,the hadroniccalorimeteris constructedfromsteelabsorberandscin- tillatortilesandisseparatedintobarrel(|η|<1.0) andextended- barrel (0.8<|η|<1.7) sections. The endcap (1.375<|η|<3.2) andforward (3.1<|η|<4.9) regions are instrumented withLAr calorimetersforEMaswellashadronicenergymeasurements.
Amuon spectrometer with threelarge air-core toroid magnet systemssurroundsthecalorimeters.Themuonspectrometermea- suresthe momentum ofmuons from their tracks, which are re- constructedwiththreelayersofhigh-precisiontrackingchambers.
1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.
Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthez-axis.Thepseudorapidityisdefinedintermsofthe polarangleθasη= −ln tan(θ/2).
These chambers provide coverage in the range |η|<2.7, while dedicatedfastchambersallowtriggeringintheregion|η|<2.4.
Athree-level triggersystemis usedtorecord events foranal- ysis. The different parts of the trigger system are referred to as the Level-1 trigger, the Level-2 trigger, andthe Event Filter[13].
The Level-1 trigger isimplemented in hardware anduses a sub- set of detector informationto reduce the event rateto a design value of at most75 kHz. The Level-1 trigger is followed by two software-based triggers, the Level-2 trigger and the Event Filter, whichtogetherreducetheeventratetoafewhundredHz.
3. Dataandsimulatedsamples
The measurementpresented hereisbased onthe entire2012 pp datasetata centre-of-mass energyof √
s=8 TeV. Eventsare required tomeet baseline quality criteria during stableLHC run- ningperiods.Thesedataqualitycriteriaprimarilyrejectdatawith significantcontaminationfromdetectornoiseorissuesintheread- out [14] basedupon individual assessmentsforeach subdetector.
The resulting dataset corresponds to an integrated luminosity of 20.3 fb−1. The absolute luminosity scale is derived from beam- separationscansperformedinNovember2012.Theuncertaintyin theintegratedluminosityis±1.9%[15].
Simulated eventsfrom Monte Carlo(MC) generators are used forcalculatingthe signal efficiencyandestimatingbackground in thesignalregion.TheeventsaresimulatedusingaGEANT4-based [16] full detectorsimulation [17].In additionto thehard scatter, each event isoverlaid witha number ofadditional pp collisions (pile-up) extracted from the distribution of the average number of pp interactions per bunch crossing μ observed indata. These additionalppcollisionsaregeneratedwithPYTHIA v8.160[18]us- ingthe ATLASA2setoftunedparameters (A2tune)[19] andthe MSTW2008LOpartondistributionfunction(PDF)set[20].
Events containing W+jets are generated with ALPGEN 2.14 [21], which implements MLM matching [22] of the matrix ele- mentcalculationwithpartonshowering.TheW bosonisproduced aspartofthematrixelement calculations,allowing simulationof bothcollinearandback-to-backW+jets production.Inthelatter, the W boson isbalancedby thehadronicrecoilsystem. Thema- trixelements provided by ALPGENare configured to allowup to fivepartonsinthefinal stateinadditiontotheW boson,includ- ing heavy-flavour production aswell. The generator is interfaced withPYTHIA v6.427 [23]forpartonshoweringandfragmentation.
The CTEQ6L1PDF set[24] isused.A K-factorisapplied tothese samples to correct the normalisation to a NNLO pQCD inclusive cross-sectioncalculatedwithFEWZ[25]andtheMSTW2008NNLO PDFset.Asample ofeventsisalsogeneratedwithPYTHIA v8.210 andusingtheCT10NLOPDFset[26]inwhich W bosonradiation canbeproducedviaaweakpartonshower.
DijeteventsaregeneratedwithPYTHIA v8.165.Top-quark pair production is simulated with POWHEG-r2129 [27–30] interfaced withPYTHIA v6.426withtheP2011C[31] tune forpartonshow- ering and fragmentation. Diboson production is simulated with MC@NLO v4.07[32].Additionalsamplesofdibosonproductionare generated using SHERPA v1.43 [33] and these are used to esti- matetheoretical uncertainties inthediboson backgroundestima- tion.TheabovesamplesareallgeneratedusingtheCT10NLOPDF set. Events containing Z+jets aregeneratedwith ALPGENusing the same configuration asthe W+jets simulationabove. Single top-quark productionis a negligible background forthis analysis andisnotincluded.
All samplesare normalisedto their calculatedinclusive cross- sections.However,fortheW+jets,dijets,t¯tand Z+jets samples, thereisanadditionalcorrectionappliedtothenormalisation,de- rivedfromthecomparisonofdataandMonteCarlosimulationsin
thesignalregionandcontrolregions.Theprocess ofderiving this correctionisexplainedindetailinSection4.
4. Objectandeventselections 4.1. Baselineeventselection
The topologyof collinear W production involvestwo back-to- backhigh-pT jets, one ofwhichemitsa nearby W boson.Events arerequiredtocontainatleastonejetwithpT>500 GeV,asthis is found to be sufficient to probe the kinematic region of inter- est.The probability ofa collinear W emission fromsuch a jetis estimatedbyPYTHIA v8.210tobe0.15%.Overhalfoftheproduc- tionof W+jets inthephase spaceprobed inthismeasurement isinthecollinear region.Arequirementforasecond high-pT jet isnot applied. Althoughboth jetsinitiallyrecoil fromeach other andhavesimilar pT,thejetthatemitsthecollinear W bosoncan loseasignificantamountofenergytothemuonandneutrino,nei- therofwhicharereconstructedaspartofthejetenergy.Requiring asecond high-pT jet wouldimpose an implicitmaximumonthe energycarriedbytheW bosonanditsdecayproducts.
The analysis focuses on the leptonic decays of W bosons to muonsin order to ensurea high reconstruction purity, andthus events are required to have exactly one muon. Events that con- tain an electron are rejected, which reduces the background by removingmixed-flavourdileptonic(electronplusmuon)t¯t decays.
ControlregionsareusedtoestablishthenormalisationofMCsim- ulationsofseveralbackgroundprocesses.Theseregionsaredefined by inverting various selection criteriaused in the final measure- ment.
Torejectnon-collision background[34],eventsarerequiredto contain at least one primary vertex consistent with the beam- interactionregion,reconstructedfromatleasttwotrackseachwith ptrackT >400 MeV.Theprimaryhard-scattervertexisdefinedasthe vertexwiththe highest
(ptrackT )2.Toreject rareeventscontam- inatedby spurious signalsin thedetector, all anti-kt [35,36] jets withradius parameter R=0.4 andpjetT >20 GeV (seebelow)are requiredtosatisfytheloosestjet-qualityrequirementsdiscussedin Ref.[34].Thesecriteriaare designedtoreject non-collision back- ground and significant transient noise in the calorimeters while maintaining an efficiency for good-quality events greater than 99.8% with ashigh a rejection of contaminated events aspossi- ble.Inparticular,thisselectionisveryefficientinrejecting events thatcontainfakejetsduetocalorimeternoise.
4.2. Triggerselection
Eventsusedinthisanalysisareselectedbyrequiringthatthey passatleast oneoftwo single-muontriggers [37].The firsttrig- gerrequiresan isolated muon with pT>24 GeV andthe second triggerrequiresamuonwithpT>36 GeV withnoisolation crite- riaapplied. Thetrack-based isolation usedinthe triggerrequires thatthescalarsumofthe pT ofalltrackswithinaconeofradius R=0.2 aroundthemuonislessthan12%ofthemuonpT. 4.3. Objectreconstruction
Muons are reconstructed by combining tracks in the ID with tracks inthe muon spectrometer[38].They are requiredto have pT>25 GeV and|η|<2.4.Toreducecontaminationfromsemilep- tonicb-decays,in-flightpionandkaondecaysandcosmicmuons, their longitudinal impact parameter withrespect to the primary vertexz0 mustsatisfy|z0|sinθ <0.5 mm andtheirtransverseim- pactparameterwithrespecttotheprimaryvertexd0mustsatisfy
|d0|/σ(d0)<3.Theselectedofflinereconstructedmuonmustalso matchtheonlinemuonthatpassedthetrigger.
Jetsarebuiltusingtheanti-kt algorithm witharadius param- eterof R=0.4 fromlocallycalibratedthree-dimensionaltopolog- ical energy clusters[39]. The resulting jetsare required to have pT>100 GeV and|η|<2.1.
Thenumberofb-taggedjetsforagiveneventiscalculatedus- ing the MV1tagger[40] onjetsbuilt usingtheanti-kt algorithm with R=0.4.The jetsconsideredforb-tagginghave pT>25 GeV and are reconstructed within |η|<2.1. The MV1 tagger is con- figured to have a b-tagging efficiency of 70% in semileptonic t¯t events.
Electronsare reconstructedfroma combinationofa calorime- ter energy cluster and a matched ID track [41,42]. They must meet a set of identification criteria (the so-called medium crite- riaofRef. [41]). Theyare alsorequiredtohave pT>20 GeV and
|η|<2.47,excludingthetransitionregionbetweenthebarreland theendcapcalorimeters(1.37<|η|<1.52).Toreducethecontam- inationfromsemileptonicb-decaysandmisidentification,thesame impactparameterrequirementsusedformuonsare appliedalong withanisolationrequirement.Thisisolationistrack-basedandre- quiresthatthescalarsumofthepTofalltracksinaconeofradius R=0.2 aroundtheelectronbelessthan15%oftheelectron pT. 4.4. Measurementselection
Toselectthe W+jets signal,eventsarerequiredtocontainat least one jetwith pT>500 GeV,exactly one muon, nob-tagged jets, a primary vertexand no electrons. Any additional jetswith pT>100 GeV are included in the analysis. The leading jet, de- fined as the jet with the highest pT, is not necessarily the one closest to the muon. The R distance is always measured with respecttotheclosest jet.Themuonisrequiredtobeisolatedus- ing both track-based andcalorimeter-basedisolation criteria. The trackisolationrequiresthat thescalarsumofthe pT ofall tracks in acone ofradiusR=0.2 aroundthe muonbe lessthan 10%
ofthemuon pT.The calorimeterisolation requiresthatthescalar sumofthe pTinallcalorimetercellsinaconeofradiusR=0.2 aroundthemuonbelessthan40%ofthemuonpT.Applyingthese isolation criteria significantly reduces the background from dijet events, where muons mostly originate from heavy-flavouror in- flightdecaysandarenon-isolated.Theb-tagvetoalsoreducesthe backgroundfromtt,¯ whichgeneratestwob-quarksintheirdecay, byover80%,whileonly10%oftheW+jets signalisrejected.Re- quirements on missingtransverse momentum were not found to improvethesignalselectionorbackgroundrejection.Theefficiency of theisolation requirement was studied bothin simulatedsam- ples andinsituusingdataeventscontaining high-pT topquarks, and the results from the two studies were in agreement. How- ever,intheextremelycollinearregion,wherethedistancebetween the muonandtheclosest jetisR<0.2,the limitedsize ofthe eventsampledidnotallowthesameconclusion.Asaresult,events where R<0.2 arealsoexcluded. Thiscausesapproximately 2%
oftheW+jets signaltoberejected.
4.5. Controlregiondefinitionsandbackgroundestimation
For the final state with atleast one high-pT jet anda single muon, thedominantbackgroundprocesses thatcontribute to the signalregionaredijets,t¯tandZ+jets.Inaddition,thereisasmall background contribution from diboson production. These are all modelledusingthesimulatedsamplesdescribedinSection3.
Foreachofthethreemainbackgroundprocesses,acontrolre- gionutilisinganeventselectiondifferentfromthesignalregionis definedsuchthatmostoftheeventsinthiscontrolregionarefrom
