Article
Reference
Measurement of colour flow with the jet pull angle in tt¯ events using the ATLAS detector at √s = 8 TeV
ATLAS Collaboration
ANCU, Lucian Stefan (Collab.), et al.
Abstract
The distribution and orientation of energy inside jets is predicted to be an experimental handle on colour connections between the hard-scatter quarks and gluons initiating the jets. This Letter presents a measurement of the distribution of one such variable, the jet pull angle. The pull angle is measured for jets produced in tt¯ events with one W boson decaying leptonically and the other decaying to jets using 20.3 fb −1 of data recorded with the ATLAS detector at a centre-of-mass energy of s=8 TeV at the LHC. The jet pull angle distribution is corrected for detector resolution and acceptance effects and is compared to various models.
ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Measurement of colour flow with the jet pull angle in tt¯ events using the ATLAS detector at √s = 8 TeV. Physics Letters. B , 2015, vol. 750, p. 475-493
DOI : 10.1016/j.physletb.2015.09.051
Available at:
http://archive-ouverte.unige.ch/unige:75555
Disclaimer: layout of this document may differ from the published version.
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Measurement of colour flow with the jet pull angle in t t ¯ events using the ATLAS detector at √
s = 8 TeV
.ATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received18June2015
Receivedinrevisedform15September 2015
Accepted19September2015 Availableonlinexxxx Editor:W.-D.Schlatter
The distribution and orientation ofenergy inside jets is predicted to be an experimental handle on colour connectionsbetweenthehard-scatterquarksand gluonsinitiatingthejets.ThisLetterpresents ameasurementofthedistributionofonesuchvariable,thejetpullangle.Thepullangleismeasuredfor jetsproducedintt¯eventswithoneW bosondecayingleptonicallyandtheotherdecayingtojetsusing 20.3 fb−1ofdatarecordedwiththeATLASdetectoratacentre-of-massenergyof√s
=8 TeV attheLHC.
Thejetpullangledistributioniscorrectedfordetectorresolutionandacceptanceeffectsandiscompared tovariousmodels.
©2015PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Dueto theconfiningnature ofthe strongforce, directly mea- suring the quantum chromodynamic (QCD) interactions between quarks andgluons is not possible. The strength and directionof the strong force depends on the colour charge of the particles involved.To a good approximation, the radiation patternin QCD canbedescribedthrougha colour-connectionpicture,whichcon- sistsofcolourstringsconnectingquarksandgluonsofonecolour to quarks and gluons of the corresponding anti-colour. An im- portant question is whether there is evidence of these colour connections(colourflow) intheobservable objects:colour-neutral hadronsandthejetsthey form.The studyofenergydistributions insideand betweenjetsin various topologies hasa long history, datingback to the discovery of gluonsin three-jet eventsat PE- TRA[1–4].Colour connections are still a poorly constrained QCD effect,whichmotivatesthededicatedstudypresentedinthisLet- ter.Ifwellunderstood,experimentscanexploitcolourflowtoaid StandardModel(SM) measurements andsearchesforphysics be- yondtheSM(BSM).Asatestthatthecolourflowcanbeextracted from the observable final state, the data are compared to mod- els withsimulated W bosons that are colour-charged or colour- neutral.
One observable predicted to contain information about the colourrepresentationofadijetresonanceliketheW, Z,orHiggs
E-mailaddress:[email protected].
boson, is the jetpullvector [5].The pull vector for a givenjet J withtransversemomentum,pTJ,isdefinedas
vpJ=
i∈J
piT|ri|
pTJ ri. (1)
ThesuminEq.(1)runsoverjetconstituentswithtransversemo- mentum piT and location ri=(yi,φi), defined as the vector differencebetweentheconstituentandthejetaxis(yJ,φJ)inra- pidity (y) –azimuthal angle(φ)space.1 Giventhepull vectorfor jet J1, the angle formed between this pull vector and the vec- tor connecting J1 and another jet J2, (yJ2−yJ1,φJ2−φJ1), is expectedtobesensitive totheunderlyingcolourconnectionsbe- tween thejets. Thisisshowngraphically inFig. 1,andthe angle iscalledthepullangle,denotedθP(J1,J2).Thepull angleissym- metricaroundzerowhenittakesvaluesbetween−π and π and so henceforth θP(J1,J2) refers to the magnitudeof the angle in (y,φ) space with0< θP≤π. Ifthe pull vector is computed using jetsoriginatingfromcolour-connected quarks, θP∼0 since the radiation ispredicted to fall mostly betweenthe two jets. If
1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.
Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedintermsof thepolarangleθasη= −ln tan(θ/2).Separationbetweenobjectsin(η,φ)spaceis definedasR=
(η)2+(φ)2.Therapidityofafour-vectory=0.5logE+p
z E−pz
, whereE istheenergyand pz thecomponentofthemomentumparalleltothe beamaxis.
http://dx.doi.org/10.1016/j.physletb.2015.09.051
0370-2693/©2015PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
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Fig. 1.Diagramillustratingtheconstruction ofthejet pullangleforjet J1 with respectto J2.
thevectoriscomputedusingtwojetswhichdonotoriginatefrom colour-connected quarks, there is no reason to expect θP to be small. Thus θP should be useful for determining colour connec- tions.
One ofthechallenges instudying colourflow isthe selection of a final state with a known colour composition. Colour-singlet W bosonsfromtt¯ eventsprovideanexcellent testinggroundbe- causethesebosonshaveaknowninitial(colourless)stateandsuch eventscanbe selectedwithhighpurity.The firststudyofcolour flowusingW bosonsfromtopquarkdecayswasperformedbythe DØCollaboration[6].IntheDØanalysis,calorimetercellsclustered withinjetswereusedastheconstituentsinEq.(1)andtheresult- ingdistributionwascomparedtoW singlet(nominal)andW octet templates.Theimpactofthecolourflowontheobservableenergy distributionsisverysubtle;theDØresultwasstatisticallylimited andwasnotabletosignificantlydistinguishsingletandoctetradi- ationpatterns.
The analysispresentedin thisLetter isa measurement ofthe jet pull angle in √
s=8 TeV pp collisions at the Large Hadron Collider(LHC) withtheATLAS detector.Instead ofcomparingthe reconstructedjetpullangleindatadirectlytosimulatedtemplates, aswas doneby the DØCollaboration,the jetpull angledistribu- tionisfirstcorrectedfordetectorresolutionandacceptanceeffects.
Thisallowsfordirectcomparisonwithparticle-levelpredictionsfor models ofphysics beyondthe SM aswell assimulations ofnon- perturbativephysicseffectswithvarioustunableparametervalues.
2. Objectandeventselection
The ATLAS detectorindependentlymeasures the inclusiveand charged-particle energy distributions in jets. This allows the jet pull angle to be constructed usingonly the chargedconstituents ofjets,orboththechargedandneutralconstituents.Inthisanaly- sis, both options are usedin order toprovide independent mea- surements with different experimental systematic uncertainties.
Charged-particlemomentaaremeasuredinaseriesoftrackingde- tectors(collectively calledtheinner detector),covering arangeof
|η|<2.5 and immersed ina 2 T magnetic field. Electromagnetic andhadronic calorimeters surroundthe inner detector, with for- ward calorimeters allowing electromagnetic and hadronic energy measurementsupto|η|=4.5.A detaileddescriptionoftheATLAS detectorcanbefoundelsewhere[7].
Theanti-ktalgorithm[8]withradiusparameterR=0.4 isused to reconstructjetsfromclustersof calorimetercells [9]withde-
Table 1
Estimatedcompositionoftheselectedeventsample.Theuncertaintiesare thesuminquadratureofthe statisticaluncertaintiesandeithertheun- certaintiesofthenormalisation method(forthe datadriven W+jet and multi-jetestimates)ortheuncertaintiesofthecross-sectionestimates.
Process Number of events
t¯t 95 400±14 000
W t-channel single top 2730±600
s- andt-channel single top 150±10
W+jets 3710±120
Z+jets 560±270
Dibosons 190±40
Multijets 2500±910
Total SM 105 000±14 000
Data 102 987
posited energy. The clusters are formed using the local cluster weighting (LCW)algorithm[10] andcalibratedto accountforthe detector response as well as to mitigate the contributions from additional ppcollisions per bunch crossing(pileup) [11].The all- particles pull angleis built from the clustersof calorimetercells assignedtoagivenjet.Inordertoimprovetherapidityresolution, jets andclustersare corrected to pointtoward the reconstructed primary vertex2 andthe corrected jet four-vector is used as the axisinEq.(1).
Thecharged-particlespullangleisbuiltfromtracksthatareas- sociated with a given jet [12,13]. Tracks are reconstructed from hits in the inner detector and are required to have |η|<2.5, pT>0.5 GeV, andsatisfy various quality criteria, such asassoci- ation with the primary vertex, in order to suppress tracks from randomhitsandpileuptracks.The charged-particlespull angleis constructed using the four-momentum sumof all the associated tracks(treatedasmassless)toprovidetheaxisinEq.(1).Theall- particlesandcharged-particlespullanglesarelargelyindependent astheyrelyoninformationfromdifferentdetectorsub-systems.
In orderto isolate a pure sample of hadronically decaying W bosons, this analysis targets a tt¯→bbW¯ (→qq¯)W(→ν) final state.
Events areselectedbytriggersrequiringasingle isolatedelec- tron or muon, andthe offline ‘tight’ electron [14] or‘combined’
muon [15] must have pT>25 GeV,and |η|<2.5. Basic quality criteria are imposed, including the existence of atleast one pri- maryvertexassociatedwithatleastfivetrackswithpT>0.4 GeV.
Furthermore, the magnitude EmissT of the missingtransverse mo- mentum [16] is requiredtobe greater than 20 GeV,and EmissT + mT>60 GeV, where mT is the transverse mass of the selected lepton and the EmissT , mT=
2pTEmissT (1−cos(φ−φν)). Events must have≥4 jets with pT>25 GeV. Atleast two ofthesejets mustbetaggedasb-jetsusingthemultivariatediscriminant,MV1 [17],whichusesimpactparameterandsecondaryvertexinforma- tion. The chosen MV1 working point corresponds to an average b-tagging efficiency of 70% for b-jets in simulated t¯t events. At leasttwojetsmustnotbe b-tagged;ofthese, thetwoleading-pT jets with|η|<2.1 arelabelled asthe jetsfrom the hadronically decaying W boson, Ji with pTJ1>pTJ2.The b-taggedjetsandthe jets selected for the pull angle calculation are required to have
|η|<2.1 sothat allconstituentsare withincoverage oftheinner detectorusedfortracking.Thisprocedureselectsasample thatis expected to containapproximately 90% t¯t events.In 45% ofsim- ulated t¯t events both jets selected for the pull anglecalculation contain energyfromthe hadronicallydecaying W boson.Table 1
2 Theprimaryvertexisdefinedasthevertexwiththehighest
p2Tofassociated tracks.
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Fig. 2.Thedetector-level(a)all-particlesand(b)charged-particlespullangle,θP,indataandinsimulation.Theuncertaintybandincludesonlytheexperimentaluncertainties ontheinputstotheeventselectionandthejetpullcalculation.Alargepartoftheuncertaintydisplayedhereaffectstheoverallnormalisation andiscorrelatedbetween theindividualbins.Thiscomponentoftheuncertaintyiscancelledintheunfoldedmeasurementoftheunit-normalised pullangledistribution.
Table 2
MonteCarlosamplesusedinthisanalysis.TheabbreviationsME,PS,PDF,MPI,LOandNLOrespectivelystandformatrixelement,partonshower,partondistributionfunction, multiplepartoninteractions,leadingorderandnext-to-leadingorderinQCD.TunereferstotheusedsetoftunableMCparameters.Thosesamplesmarkedwitha† areused asalternativet¯tsamplestoevaluateuncertaintiesduetothemodellingoftt¯events.Thenominalt¯tgeneratorinthefirstlineisusedtoestimatetheyieldinTable 1.
Process Generator Type Version PDF Tune
tt¯ Powheg[18–20] NLO ME – CT10[21,22] –
+Pythia[23] +PS 6.426.2 CTEQ6L1[24] Perugia2011c[25]
Single top Powheg NLO ME CT10(4f) –
+Pythia +PS 6.426.2 CTEQ6L1 Perugia2011c
W W,W Z,Z Z Sherpa[26] LO multi-leg ME+PS 1.4.1 CT10 Default
W/Z+jets Alpgen[27] LO multi-leg ME 2.1.4 CTEQ6L1 –
+Pythia +PS 6.426.2 CTEQ6L1 Perugia2011c
tt¯† Powheg NLO ME – CT10 –
+Herwig[28] +PS 6.520.2 CT10 AUET2[29]
+Jimmy[30] (MPI) 4.31 – –
tt¯† MC@NLO[31,32] NLO ME 4.06 CT10 –
+Herwig +PS 6.520.2 CT10 AUET2
+Jimmy (MPI) 4.31 – –
showsthepredictedcompositioncomparedtothedatayield.More detailsaboutthevariouscontributionsaregivenbelow.
There are two pull angles calculated per event, one calcu- latedfromtheall-particles pullvectorandonefromthecharged- particlespullvectorofthehighestpT jetassignedtothehadronic W bosondecay.Theall-particlesandcharged-particlespullangles aretheanglesthat thecorresponding pullvectorsmakewiththe direction from J1 to J2. Figs. 2(a) and 2(b) show comparisons between data and simulation for the all-particles and charged- particles pull angles,calculated atdetector level,i.e.fromrecon- structed objects. Both distributions are broadly flat with an en- hancementatsmallangles,consistentwiththeSMprediction.
3. Eventsimulation
MonteCarlo(MC)samplesareproducedinordertodetermine howthe detectorresponse affectsthe pull angleandto estimate some of the non-t¯t contributions in the data.The details of the samplesusedareshowninTable 2.
Aside from the W+jets background, all MC samples are nor- malised to their theoretical cross-sections, calculated to at least next-to-leading order (NLO) precision in QCD [33–38]. For the purpose of comparisonbetween data andthe SM predictionbe- fore unfolding, t¯t events are normalised to a cross-section of 253±15 pb, calculated at next-to-next-to-leading order (NNLO) in QCD including next-to-next-to-leading logarithmic (NNLL) soft gluonterms[39],assumingatop-quarkmassof172.5 GeV.
GeneratedeventsareprocessedwithafullATLAS detectorand trigger simulation [40] based on Geant4 [41] and reconstructed using the same software as the experimental data. The effects of pileup are modelled by adding to the generated hard-scatter eventsmultipleminimum-biaseventssimulatedwithPythia8.160 [42], the A2 set of tuned MC parameters (tune) [43] and the MSTW2008LOPartonDistributionFunction(PDF)set[44].Thedis- tributionofthenumberofinteractionsisthenweightedtoreflect thepileupdistributioninthedata.
Totestthesensitivityofthejetpullangletothesingletnature ofthe W boson, a sample was generated witha colour-octet W boson. The octet W boson is simulatedusing thesame setup as
