Table Of ContentMeasurement of the b-jet cross-section with associated vector boson
production with the ATLAS experiment at the LHC
HeatherM.GrayaonbehalfoftheATLASCollaboration
CERN
2 Abstract. Ameasurementofthecross-sectionforvectorbosonproductioninassociationwithjetscontaining
1 b-hadronsispresentedusing35pb−1 ofdatafromtheLHCcollectedbytheATLASexperimentin2010.Such
0 processes are not only important testsof pQCD but also large, irreducible backgrounds tosearches such as a
2 low massHiggsboson decaying topairsofb-quarks whentheHiggsisproduced inassociationwithavector
n boson.TheoreticalpredictionsoftheV+bproductionratehavelargeuncertaintiesandpreviousmeasurements
a havereporteddiscrepancies. Cross-sectionsmeasuredusingintheelectronandmuonchannelswillbeshown.
J Comparisonswillbemadetorecenttheoreticalpredictionsatthenext-to-leadingorderinα .
S
4
2
Vectorbosonproductioninassociationwithjets(V+jets) taining additional jets are vetoed to control backgrounds.
] isoftenusedasatestinggroundforperturbativeQCDcal- TheZ+b-jetcross-sectionismeasuredatthejetlevel,such
x
e culations. Despite substantial progress in the understand- thateventswithtwojetsenterthedistributionstwice.The
- ingandmodellingofinclusivejetproductioninvectorbo- W+b-jetcross-section,ismeasuredattheeventlevel,with
p
son events,less study hasbeenmade of heavy-flavourjet eachevententeringeachdistributiononce.
e
production.V+jetsisalsoanimportantbackgroundtomany
h
TheATLASdetectorconsistsofaninnertrackingsys-
[ searchesattheLHC.TheseincludesearchesfortheHiggs
boson produced in association with a vector boson and tem(|η|<2.5)surroundedbya2Tsuper-conductingsole-
1 noid,electromagneticandhadroniccalorimeters(|η|<4.9),
with the Higgs decaying to a pair of b-quarks [1] or su-
v and a muon spectrometer(|η| < 2.7) [7]. Eventsare col-
persymmetric models with b-quarks in the final state[2].
6 lectedusingsingleelectronormuonhigh p triggersand
Significantprogresshasbeenmaderecentlyinincreasing T
7
requiredtocontainatleastonereconstructedprimaryver-
the accuracy of theoretical calculations [3,4]. Challenges
9
tex.Leptonsarerequiredtohave p > 20GeVwithelec-
4 to theoretical calculations stem from the non-necessarily T
tronswithin|η|< 2.47andmuonswithin|η|< 2.4.Events
. negligibleb-quarkmassandtheinterplaybetweencontri-
1 containingaZbosonareselectedbyrequiringtwoisolated
butionsfromb-quarkproductionininitialandfinalstates.
0
WediscussresultsofameasurementoftheWandZ+b- leptons of the same flavour but with opposite charge and
2
withtheinvariantmassoftheleptonstobeconsistentwith
1 jet cross-sections made by the ATLAS experiment using
: 35 pb−1 of LHC data collected in 2010 [5,6]. Both mea- theZbosonmass:76<mll <106GeV.Eventscontaining
v surements are challenging, but in different respects: the a W boson are selected by requiring exactly one isolated
Xi Z+b-jet is statistically limited by its small cross-section, lepton with missing transverse energy (ETmiss > 25 GeV)
r while the W+b-jet process has a larger cross-section, but and transverse mass, mW = 2pℓpν(1−cos(φell−φν)),
a also largerbackgroundsresultingina lowsignaltoback- T q T T
consistentwiththatofaWboson(mW >40GeV).
groundratio. T
TheV+b-jetcross-sectionsare measuredin a fiducial Jetsarereconstructedusingtheanti-k algorithmwith
T
phase space, bycorrectingthe resultsback to the particle a radius parameter R = 0.4 and required to have p >
T
levelwhileaccountingforalldetectoreffects.Thecutsap- 25 GeV. Jets with anangularseparation∆R < 0.5froma
pliedtodefinethefiducialcross-section,closelymatching selectedleptonareremoved.Jetsarerequiredtohave|η|<
theexperimentalacceptance,arelistedinTable1.Thelep- 2.1toensuregoodb-taggingperformancewiththefulljet
tonmomentumincludestheenergyfromphotonsradiated containedwithin the InnerDetector acceptance.Jets con-
within a cone of radius 0.1. The neutrino pT is used as tainingb-quarksareidentifiedusingthedecaylengthsig-
thetransversemissingenergy.Particleleveljetsarerecon- nificancebetweentheprimaryandasecondaryvertex[8].
structedusingtheanti-kT algorithmusingallstable parti- Jets with a significance greater than 5.85 are tagged as
cles(τ>10ps).Ab-jetisdefinedasajetwithab-hadron b-jets. In a simulated tt¯sample such a cut has a 50% b-
with pT > 5GeV withanangularseparationfromthejet jetidentificationefficiencyandamis-tagrateof10%and
∆R<0.3. 0.5%forcandlightjetsrespectively[8].Theb-taggingef-
The Z+b-jetcross-section is measured inclusively for ficiencywasmeasuredindatausingab-jetenrichedsam-
eventscontainingatleastoneb-jet.DuetothelargerW+b- pleobtainedbyselectingjetscontainingmuons,whichare
jet cross-section a differential measurement is made for producedpredominantlyfromb-hadrondecay.Thedistri-
eventscontainingeitheroneortwob-jets,buteventscon- bution of the muon transverse momentum relative to the
jetaxis,prel,isthenusedtodiscriminatebetweenb,c-and
T
a e-mail:[email protected] lightjets,beforeandafterb-tagging.
EPJWebofConferences
Table1.Definitionofthephasespaceforthefiducialcrosssec-
VV
tioRneqfourirtehmeeWnt+b-jetandZ+b-jWetm+ebasurements[5Z,6+].b Events / 5 GeEvents / 5 Ge110022 ATLAS ∫ L dt = 36 pb-1 DtZWZt((a(→→t→aµτ µτ2+)νµ0 )+-1 )+ 0j+e j( tejsetsst s= 7 TeV)
1100 Diboson
Leptonp pℓ >20GeV Single top
T T
Leptonη |ηℓ|<2.5
11
Dileptonmass - 76−106GeV
Neutrinop pν >25GeV -
T T
Wtransversemass mT >40GeV - 1100--11
Jet p pj >25GeV
T T
Jetrapidity |yj|<2.1 6600 8800 110000 112200 114400 116600 118800 220000 222200 224400
mmµµ++µµ-- [[GGeeVV]]
Jetmultiplicity n≤2 -
b-jetmultiplicity n =1orn =2 n ≥1
b b b Fig.1.Di-leptonmassdistributionforeventswithatleastoneb-
Jet-leptonseparation ∆R(ℓ,jet)>0.5 - taggedjetwith p >25GeVand|y| <2.1forthemuonchannel
T
intheZ+b-jetanalysis[5].Thecontributionestimatedfromthe
simulatedMCsamplesofthesignalandvariousbackgroundsis
Table2.Numberofsignalandbackgroundeventsin35pb−1 of
shown. The multi-jet background, estimated with a data-driven
data [5,6]. The W+b numbers show the ALPGEN predictions
method,isnotshown.
normalised to the inclusive NNLO W cross-section, while the
Z+jetyieldhasbeennormalisedtothefitresult.TheQCDback-
groundinbothcaseshasbeenestimatedfromdata.Thetopback- eV120
G ATLAS Data 2010, s= 7 TeV
gdraotau-nddrivinenthmeeWth+odb,-jwethailnealfyosrisZw+basiteswtiamsaetesdtimuasitnedg afropmartisaimlly- nts/ 8 100 Electron + 1 or 2 jets WW++bc
e W+light
ulation. Thesmaller singletopand dibosons backgrounds were Ev 80 multi-jet
estimatedfromsimulation.Uncertaintiesarenotindicated. tt
60 Single top
Other EW
Z+≥1b-jet W+1b-jet W+2-jet
∫
40 L dt = 35 pb-1
V+b 64 43 45
V+c 60 192 81 20
V+l 0.0 68 37
Top 6 19.1 77 0 50 100 150
Missing E [GeV]
SingleTop 0.3 31 41 T
QCD 1.0 18 15.8
Fig.2.MissingETdistributionintheW+b-jetelectronchannel
Diboson 0.5 5.8 4.6
in the combined 1- and 2- jet bin after applying the b-tagging
requirement [6]. All backgrounds, but QCD multi-jet, are nor-
malisedtotheirMCprediction.
Theestimatednumberofsignalandbackgroundevents
in 35 pb−1 of data is shown in Table. 2. The dilepton in-
variant mass in the muon channel for the Z+b-jet analy- enceinefficiencybetweenpromptandnon-promptmuons
sisisshowninFig.1.ThedominantbackgroundisZ+jet topassthestandardselectioncriteria.Intheelectronchan-
eventswith a lightor c-jet has beenmistagged as a b-jet. neltheQCDbackgroundisestimatedfromafittothemiss-
Otherbackgrounds,includingtopandsingle-top,aresmall ingenergydistributionwithatemplateobtainedbyrevers-
andthereforeestimatedfromsimulation.Thesmallback- ingcertainelectronidentificationcriteria(seeFig.2).The
groundfrommulti-jetproduction,referredtoasQCD,was 50% uncertainty on the QCD backgroundestimate trans-
estimatedfromdata,byfittinganexponentialdistribution latesintoa7%uncertaintyontheW+b-jetcross-section.
in the dilepton mass in a QCD enriched sample obtained Amaximumlikelihoodfittothesecondaryvertexmass
witharelaxedleptonselectioncriteria. distributionwasusedtodiscriminatebetweenb−,c−and
In the W+b-jet measurement, particularly in the 2-jet light-jetsandtoextracttheflavourfractiononastatistical
selection,thecontributionfromnonW+jetbackgroundsis basis.Theothernon-V+jetsbackgroundswereallowedto
largewithaS/B0.1-0.2.ThereforeboththeQCDmultijet floatwithinuncertainties.Thetemplateshapesforeachjet
andthetopbackgroundwereestimatedfromdata.Thetop flavourwereestimatedfromsimulationandthesystematic
background was estimated in a control region with more uncertaintiesontheshapewerederivedbycomparingdata
than 4 jets and extrapolated into the signal region using andMCinmulti-jetsamplesenrichedineitherlightorb-
Monte Carlo (MC) simulation. The uncertainty on the b- andc-jets.ExamplesofthefitsareshowninFig.3and4.
taggingefficiencycancelsinthisratiothereforeobtaining For the Z+b-jet measurement, the fit is performedfor all
an estimate largely independent of the b-tagging uncer- b-jetsintheelectronandmuonchannelscombined,while
tainty. The veto on additionaljets was used to reducethe fortheW+b-jetmeasurementitisperformedseparatelyby
topbackground. leptonflavourandjetmultiplicity.
Heavy-flavour jet production is the dominant process Figure3showsthesecondaryvertexmassdistribution
contributingtotheQCDmulti-jetbackground.AstheQCD for W+1-jet events. The distribution is shown both with
backgroundislargeandhasalargeuncertaintyaverytight theW+jetcontributionnormalisedtotheNNLOWcross-
requirementonleptonisolationwasintroduced.Inthemuon section (left) and after the W+jet contribution has been
channel the QCD background largely results from non- scaled by the fit results(right).The secondaryvertexdis-
promptmuonsandwasextractedbyexploitingthe differ- tributionfortheZ+b-jetanalysisisshownwiththeflavour
2011HadronColliderPhysicsSymposium
Events/0.5 GeV12468000000 AETleLcAtroSn + 1 Jets ∫ L dt DWWWQtSO=t iaCtn+++3htgDbcla5eigl r e2 phE 0TtbW1o-10p, s= 7 TeV Events / 0.5 GeV 123456789000000000 AETleLcAtroSn + 1 jet ∫ L dtDWWWmtSO t=iatun+++h tl3gbclateiigl5- re2j he E0ptttoW1bp0-1, s= 7 TeV ≥ν→ 1 b-jet) [pb] + lW 112050 EEMNA(A(PbbllLLYLuee--jjPPOToeeccGGttnHtt rr 5 oofoIEECrFAnnnoNNhN lmyCaa S++ n nhfM rd.JJao EIInmMMM .a uMMMnoYYdEn )PCSh)an. Data 20∫1L0d,t =s A=3T75L TpAebSV-1
0 2 Secondary V4ertex Mass [GeV]6 00 1 2 Sec3ondary V4ertex Ma5ss [GeV]6 σ(
5
Fig.3.Thesecondary vertexmassdistributionforb-taggedjets
inW+b-jeteventscontaininganelectronandasinglejet[6].The
distribution is shown with the b, c and light contributions nor- 0
1 jet 2 jet 1+2 jet
malisedtotheinclusiveNNLOW+jetcross-section(left)andto
thefitresults(right).
Fig.5.Comparisonofthemeasuredandpredictedfiducialcross-
sections of b-jet production in association with a W boson [6].
VV Thecross-sectionisshowninexclusivebinsfor1,2and1+2jets
ee
Jets / 0.5 GJets / 0.5 G45450000 dZZZbaa+++tcbclaikg gh2rt0o1u0n d(ss=7TeV) ataonnddNstLoeOptahproarsteeedlfiyrcotfimoornAlseLpotbPotGnaiEflnNeadvaounusdrisnP.gyTtthhheieam.5eflaasvuoreumrnenutmibsecromscphaermede
∫
L dt = 36 pb-1
Table3.Measuredandpredictedfiducialcross-sectionofinclu-
3300
siveb-jetproductioninassociationwithaZ-boson[5]
2200 ATLAS Cross-section[pb]
Sherpa 3.29±0.04(statonly)
1100
ALPGEN 2.23±0.01(statonly)
MCFM 3.88±0.58
00
00 11 22 33 44 55 66 77 88 99 1100 ATLAS 3.55+0.82(stat.)±0.12(syst)
SSVV00 mmaassss [[GGeeVV]] −0.74
Fig.4.Thesecondary vertexmassdistributionforb-taggedjets
quarksandcontributionwithamasslessbquarkintheini-
in selected Z+b events. The b, c and light templates have been
tialstate treatedusingaschemebasedonbquarksPDFs.
normalised to the fit results. The small contribution from other
TheW+b-jetcross-sectionissystematicallyfoundtohave
backgroundsisalsoindicated[5].
a small excess over theoretical predictions, both in the 1
and2-jetexclusivebinsandthe1+2-jetexclusivebin.How-
evertheexcessissmallandthemeasurementisconsistent
contributionsnormalisedtothefitresults.Thereisnovisi-
blecontributionfromZ+lightjetsbecausethebestfitvalue atthe1.5σlevel.
wasconsistentwithzerowithinuncertainties. In conclusion,ATLASmeasurementsof vectorboson
in association with b-jets using the first 35 pb−1 of data
ThefiducialV+jetcross-sectionswereobtainedbyun-
havebeenpresented.Despitelargeuncertainties,theZ+b-
foldingthemeasuredb-fractionusingMCsimulationsam-
jet cross-section is found to be consistent with NLO pre-
ples. Alpgen was used for the W+b-jetmeasurementand
dictions,whileasmallexcessatthe1.5σlevelisobserved
SherpafortheZ+b-jetmeasurement.Themeasuredcross-
intheW+b-jetmeasurement.
sectionshaveuncertaintiesfromthebackgrounds,template
shapeandtheunfoldingfactors.
Uncertaintiesontheb-taggingefficiencyresultinasys-
References
tematicuncertaintyof10%(12%)fortheZ(W)+b-jetmea-
surement. The unfolding factor is sensitive to the accu-
1. ATLAS Collaboration, ATLAS-CONF-2011-103,
racy of the Monte Carlo modelling such as the b-jet p
T https://cdsweb.cern.ch/record/1369826/
spectrum and uncertainty on the opening angle between
2. ATLAS Collaboration, submitted to PRL,
the pairs of b-quarks.The b-jet p spectrum is important
T arXiv:1112.3832
because the b-tagging efficiency depends on p and the
T 3. Campbell,J.M.etal,Phys.Rev.D.79(2009)034023.
openingangledistributiondeterminesthe probabilitythat
4. Caola,F.etal,arXiv:1107.3714,(2011)
the pair of b-quarks will be reconstructed as a single jet.
5. ATLAS Collaboration, Phys. Lett. B 706 (2012) 295-
Uncertaintiesonthejetenergyscaleresultsina4%(7%)
313,arXiv:1109.1403
uncertaintyintheZ(W)+b-jetmeasurement.
6. ATLAS Collaboration, Phys. Lett. B 707 (2012) 418-
TheresultsfortheZ+b-jetmeasurementaresummaris-
437,arXiv:1109:1470
ed inTable 3.The Z+b-jetcross-sectionagreeswellwith
7. ATLASCollaboration,JINST01(2008),S08003
theNLOMCFMpredictionof3.88±0.58pb[9,10].
8. ATLAS Collaboration, ATLAS-CONF-2011-089
TheresultsfortheW+b-jetmeasurementaresummaris-
(2011),https://cdsweb.cern.ch/record/1356198
edinFig.5.ThemeasurementsarecomparedtoNLOpre-
9. Campbell, J.M. et al, Nucl.Phys.Proc.Suppl. 205-206
dictions obtained using the 5 flavour number scheme [3]
(2010)10-15,arXiv:1007.3492
after includinga non-perturbativecorrectionto the cross-
10. Campbell, John M. et al. Phys.Rev. D69 (2004)
sectionof0.93±0.07[6]. Thecorrectionincludesa con-
074021,hep-ph/0312024
tribution from a bb¯ pair in the final state with massive b-
