Measurement of the production cross section for $Z/γ^*$ in association with jets in pp collisions at $\sqrt{s}$ = 7 TeV with the ATLAS Detector

0.09 MB·English

by  Evelin Meoni

Measurement of the production cross section for Z/γ in association with
∗
jets in pp collisions at √s = 7 TeV with the ATLAS Detector
EvelinMeoni1,a onbehalfoftheATLASCollaboration
1Institut de F´ısica d’Altes Energies (IFAE). Edifici Cn, Universitat Auto`noma de Barcelona (UAB), E-08193 Bellaterra
(Barcelona),Spain.
2
1 Abstract. WepresentresultsontheproductionofjetsofparticlesinassociationwithaZ/γ boson,inproton-
∗
0 protoncollisionsat √s=7TeVwiththeATLASdetectorattheLHC.Theanalysisincludesthefull2010dataset,
2 collectedwithalowrateofmultipleproton-protoncollisionsintheaccelerator,correspondingtoanintegrated
luminosityof36pb 1.InclusivejetcrosssectionsinZ/γ events,withZ/γ decayingintoelectronormuonpairs,
n − ∗ ∗
a aremeasuredforjetswithtransversemomentum pT >30GeVandjetrapidity|y|<4.4.Themeasurementsare
J comparedtonext-to-leading-orderperturbativeQCDcalculations,andtopredictionsfromdifferentMonteCarlo
3 generatorsimplementingleading-ordermatrixelementssupplementedbypartonshowers.
2

] 1 Introduction theeventsareselectedtohavetwooppositelychargedre-
x
constructedelectronswithtransverseenergyE >20GeV,
e T
- Themeasurementofthecrosssectionsfortheproduction pseudorapidityin the range ηe < 2.47 (where the transi-
p of hadronicjets in association with a Z/γ∗ bosonat LHC tionregionbetweencalorime|ter|sections1.37< ηe <1.52
e providesastringenttestofperturbativequantumchromo- isexcluded).Inthemuonchannel,theeventsar|es|elected
h
dynamics(pQCD). Moreover,since these processes form tohavetwooppositelychargedreconstructedmuonswith
[
importantbackgroundsfor searches of new physics,their transversemomentump >20GeV,pseudorapidityinthe
T
1 detailed measurementis a first step in the discovery pro- range η <2.4.Inbothcases,adileptoninvariantmassin
µ
v gramatLHC. theran|ge|66GeV<m <116GeVisrequired.Jetsarere-
ll
5 Thiscontributionpresentsareviewofthemeasurementsof constructedusingtheanti-ktalgorithm[3]withadistance
74 jet productionin eventswith a Z/γ∗ boson using 36 pb−1 parameter R=0.4. The inputs to the anti-kt jet algorithm
ofdatacollectedbytheATLASexperiment[1]in2010at are clusters of calorimeter cells seeded by cells with en-
4
. √s=7TeV.Duringthisperiod,theluminosityofthema- ergy that is significantly above the measured noise. The
1 chinehasgrownroughlyexponentiallywithrunningtime measuredjet transversemomentump is correctedto the
0 upto2.1 1032cm−2s−1.TheATLASdetectorperformed particlelevelscaleusinganaveragecTorrection,computed
2 ×
wellthroughoutthe2010runanditsresponsewasquickly as a function of jet transverse momentum and pseudora-
1
: understood. pidity,andextractedfromMCsimulation.Inthisanalysis,
v Themeasurements,describedindetailin[2],arecompared jetsareselectedwithcorrectedp >30GeVand y <4.4
T
Xi withtheavailablenext-to-leading-order(NLO)pQCDcal- separatedfromeachofthetwoleptonsbya∆R>0|.5|.
culations, providinga validation of the theoryin the new ThebackgroundcontaminationisestimatedusingMCsim-
r
a kinematicregime,andwiththeMonteCarlo(MC)predic- ulatedsamples,exceptfortheQCD multijetcontribution,
tionsthatincludeleading-order(LO)matrixelementssup- where a data-driven method is employed for each chan-
plemented by parton showers. The latter are affected by nel.Atemplatefitisusedintheelectronchannel,whereas
largescaleuncertaintiesandneedtobetunedandvalidated the di-muonmass versusthe muonisolation plane is em-
usingdata. ployed in the muon case. In the electron channel, the to-
talbackgroundincreasesfrom5%to17%astheinclusive
jetmultiplicity(N )increasesandisdominatedbymulti-
jet
2 Event Selection and Background jetprocesses,followedbytt¯anddibosonprocesses.Inthe
estimation muonchannel,thebackgroundincreasesfrom2%to10%
asN increases,dominatedbytt¯anddibosonprocesses.
jet
Events are required to have a reconstructed primary ver-
tex with at least 3 tracks associated to it. Events are se-
lectedwithaZ/γ bosondecayingintoapairofelectrons 3 Cross Section Measurement
∗
(e+e )ormuons(µ+µ ).Inbothcasesasingleleptontrig-
− − Thejetmeasurementsarecorrectedfordetectoreffectsback
ger,electronormuon,isemployed.Intheelectronchannel,
to the particle level using a bin-by-bin correction proce-
a e-mail:[email protected] dure,based on ALPGEN [4] MC simulatedsamples, that
EPJWebofConferences
acccranoaodtdrniriotuerancislb.ts0suoA.t1fitaooacprnrcasojoreutfutirncnosdltmeestlhelfetceohvtreileoeltp,pnhthteoheoneffietffiodlceniicprseietneorcncatnyidcoykinaaitn.noedefdmtrwheaestiiotchZlsui/ntγiino∗acnlicudoedeffnneeettichfiotesf- +≥→-) [pb] N)+e ejet 11110000234 ∫apjTne LttA i >-dTk t3t L =0jeA 3tGsS6,e RVpb, =-|1y 0je.t|4 <, 4.4 ZDASPB/LhlYaaγPetT*car(GHk p→2HEIaA0 aNe1 t(+ 0ne+ o- ()Hr m+sE a=jRel iWt7zs eTIGde Vto) data)
TinhtehetofitadluccrioalssresgeicotnioannsdatrheeminecalsuusrievdejaestfduinffcetrieonntsiaolfcrNojsest γσ*((Z/ 1
sections are measured as functions of jet p and y. The
T | | 10-1
ldoeifaffdpeiTrnegnantjiedatl|iycnr|oZos/fsγt∗sheeecvtelieonantdssiwnagritehjeaatlts(olheimagshetaeossnuterpeaTdn)daastnwfduonsjececttiosonninds ata / NLO 0111....82461 1Dthaetoar e2t0ic1a0l u/ nBclaec2rtkaHinatties 3 4
thefinalstate,respectively.Forthelatter,thecrosssection D 0.6
ismeasuredalsoasafunctionoftheinvariantmassandthe MC 11..46 1DNNatLaO 2 0u1n0c e/ rAtaLiPn2tGieEsN 3 4
angularseparationofthetwoleadingjets. a / 1.21
Themeasureddifferentialcrosssectionsaredefinedasfunc- Dat 00..68
tionofagivenξas: MC 11..46 1Data 2010 / She2rpa 3 4
a / 1.21
ddσξ = L1∆1ξ(NData−Nbackg)×U(ξ) (1) Dat 00..68 ≥1 ≥2 ≥3 ≥4
N
jet
where,foreachbininξ,N andN denotethenum-
Data backg
berofentries(eventsorjets)observedindataandtheback-
Fig.1.MeasuredcrosssectionforZ/γ ( e+e )+jetsproduction
ground prediction, respectively, and ∆ξ is the bin width, ∗ → −
asafunctionof theinclusivejetmultiplicity.Inthisandinfig-
U(ξ) is the correctionfactor, and L is the total integrated
ures2-4theerrorbarsindicatethestatisticaluncertaintyandthe
luminosity.
dashedareasthestatisticalandsystematicuncertaintiesaddedin
quadrature.ThemeasurementsarecomparedtoNLOpQCDpre-
A detailed estimation of the systematic uncertainties dictions fromBlackHat and toMC predictionsfromALPGEN,
that affect the measurement has been carried out. In par- SherpaandPYTHIA.
ticulartheimpactofuncertaintiesrelatedtothejetenergy
scale,thejetenergyresolution,thebackgroundestimation,
theleptonsandtheunfoldinghavebeenevaluated.Anad- toatleastfourjetsinthefinalstatefortheelectronchan-
ditional3.4%uncertaintyonthetotalintegratedluminosity nel.
is also taken into account.The main systematic source is The measured ratio of cross sections for N and N -1
jet jet
thejetenergyscalethatincreasesfrom7%to22%asNjet in the muon channel is shown in Fig. 2. This observable
increases and from 8% to 12% as the pT increases. The cancels part of the systematic uncertainty and constitutes
totalsystematic uncertaintyincreasesfrom9%to 23%as an improved test of the Standard Model. The data indi-
Njetincreases;andfrom10%atlowpT to13%athighpT. catethatthecrosssectionsdecreasebyafactorofapprox-
imatelyfivewiththerequirementofeachadditionaljetin
thefinalstate.
4 TheoreticalPredictions Theinclusivejetdifferentialcrosssectionasafunctionof
p is presentedin Fig. 3 in eventswith at least one jet in
T
the finalstate fortheelectronchannel.Thecrosssections
ThecrosssectionresultsarecomparedtotheNLOpQCD
aredividedbythecorrespondinginclusiveZcrosssection
predictions,ascomputedwithBlackHat[5]usingCTEQ6.6
timesbranchingratiowiththeaimofcancelingsystematic
PDFs [6] andfactorizationandrenormalizationscale µ =
uncertaintiesrelatedto leptonidentificationandthelumi-
H /2(H isthescalarsumofthep ofallparticles)where
T T T nosity. The cross sections decrease by more than two or-
thepredictionsincludenon-perturbativecorrections.
ders of magnitude as p increases in the explored range.
The measurements are also compared to the LO predic- T
Fig. 4 shows the measured differentialcross section as a
tionsincludingpartonshower,asdeterminedinALPGEN[4],
functionoftherapidityseparationofthejetsforthemuon
Sherpa [7] and PYTHIA [8]. The ALPGEN and Sherpa
channel.
samples are normalizedto the next-to-next-to-leadingor-
The combination of electron and muon results has also
der (NNLO) pQCD inclusive Drell-Yan prediction. The
beenperformedbyextrapolatingthemeasurementsofthe
PYTHIAsamplehasbeennormalizedtodatafromtheav-
twochannelstoacommonleptonkinematicalregion:p
erageofelectronandmuoncrosssectioninthe 1jetbin. T
≥ >20GeVand η <2.5asdefinedatthevertexoftheZbo-
| |
son.Theelectronandmuonresultsarecombinedusingthe
BLUE (Best Linear Unbiased Estimate) [9] method. Fig.
5 Results
5 showsthe dijetcross section a functionof the invariant
massofthetwoleadingjets.
Fig. 1 presents the measured fiducial cross section as a Themeasuredcrosssectionsareingeneralwelldescribed
functionoftheinclusivejetmultiplicityineventswithup byNLOpQCDpredictionsincludingnon-perturbativecor-
HadronColliderPhysicsSymposium2011
+≥µµ→γσ≥--1) N)+ *((Z/)/ N+jetjet 00000..23...23455 pa∫jTne ttL i>- Akd 3ttT j0=eL tG3sA,5e S VRp,b |=-y1 je0t.|4 <, 4.4 DASPBZYLhla/aγPetTc*arGH(k p→2HIEaA0a N µ1t( 0n++µo ( -rH)m+sEj ae=Rlti szW7e TIdGe tV o ) data) jjσφ∆σ(1/| [1/rad]/d|) d+µµ→γ-*Z/ 1100--21 Zpa∫jTn/e γttL i*>- Akd+ 3ttT≥ j0=e L2 tG3s Aj,e5e S t VRsp,,b |=-y1 je0t.|4 <, 4.4 DASPBZLYhaa/γPectT*akrG(H pH→2EIaa0A Nµt1 ( 0+n+µ o( -H)r+msEj ea=Rtl isW7z eTIdGe V t o) data)
+µµ→-) 0.15
γσ*((Z/ 0.1
Data / NLO 00111.....682461 Dthaetoar e2t0ic1a0l u/ nBclaecrtkaHinatties Data / NLO 01..5521 Dthaetoar e2t0ic1a0l u/ nBcaecrktaHiantties
Data / MC 0111....82461 D ata 2010 / ALPGEN Data / MC 1.521 D ata 2010 / ALPGEN
0.6 0.5
Data / MC 0111....82461 D ata 2010 / Sherpa Data / MC 1.521 Data 2010 / Sherpa
0.6 0.5
≥1/≥0 ≥2/≥1 ≥3/≥2 ≥4/≥3 0 0.5 1 1.5 2 2.5 3
Njet |∆φjj| (leading jet, 2nd leading jet) [rad]
Fig.2.RatioofcrosssectionsforZ/γ ( µ+µ )+jetsproduction Fig.4.DijetcrosssectioninZ/γ ( µ+µ )+jetsproductionnor-
∗ − ∗ −
→ →
asafunctionoftheinclusivejetmultiplicity. malizedbyDrell-Yancrosssectionasafunctionoftheazimuthal
separationofthetwoleadingjets.
σσ(1/ [1/GeV]/dp) d+→γ-e e*Z/T 1100--21 pZ∫ajTn/ eγLttA i* >-d+Tk t 3tL ≥ =j0e A 1 3tGs S6j,ee RtpV,b, =-|1y 0je.t4| ,< 4.4 DASPBZ/LYhlaaγePtT*car(GHk p→2HEIa0A aNe1 t(+ 0n+e o-( )Hr m+sE a=jRel iWt7zs eTIdGe Vto) data) jjσ [pb/GeV] / dmd101-1 ap∫njT eLttA>i -dTk3tt0 L =j eGA t3seS6,V p,R b| y-=1je t0|<.44,.4 Z/γ*(→DASNC lhL+aLTleP-tOE)ar+GQ pB2jeaE60lta.Ns16c 0 ,(k+ lµ H=( H ea=s,Et µ H=R c TW 7o/2TmIe GbVi)ned)
10-3
10-4 10-2
Data / NLO 0111....82461 Dthaetoar e2t0ic1a0l u/ nBclaecrtkaHinatties Data / NLO12..2551 Dthaetoar e2t0ic1a0l u/ nBclaecrtkaHinatties
0.5
C 1.6 40 Dat6a0 2010 / 8A0LPGEN100 120 140 160 180 2.5 Data 2010 / ALPGEN
Data / M 011...8241 Data / MC1.251 NNLO uncertainties
C 1.6 40 Dat6a0 2010 / 8S0herpa100 120 140 160 180 0.5
M 1.4 2.5 Data 2010 / Sherpa
Data / 01..821 40 60 80 100 120 140 160 180 Data / MC1.251
0.5
pjTet [GeV] 100 mjj (1le50ading jet,2 020nd leadin2g50 jet) [GeV3]00
Fig.3.InclusivejetcrosssectioninZ/γ ( e+e )+jetsproduc-
∗ −
tionnormalizedbyDrell-Yancrosssection→asafunctionofpT. Fig.5.DijetcrosssectioninZ/γ∗(→ l+l−)+jetsproductionasa
functionoftheinvariantmassofthetwoleadingjets.
rections,aswellas bypredictionsofLO matrixelements
of up to 2 5 parton scatters, supplemented by parton 2. TheATLASCollaboration,[arXiv:hep-ph/1111.2690].
→
showers,asimplementedintheALPGENandSherpaMC 3. M.Cacciari,etal.,JHEP0804,(2008)063.
generators.InthecaseofPYTHIA,theLOpQCD(qq¯ 4. Mangano,M.L.etal.,JHEP0307,(2003)001.
→
Z/γ gandqg Z/γ qprocesses)MCpredictionsunder- 5. C.F.Bergeretal.,Phys.Rev.D78,(2008)036003.
∗ ∗
→
estimatethemeasuredcrosssections. 6. P.M.Nadolskyetal.,Phys.Rev.D78,(2008)013004.
7. T.Gleisberg,etal.,JHEP0902,(2009),007.
8. T.Sjostrandetal.,JHEP0605,(2006)026.
References 9. L. Lyons, et al, Nucl. Instrum. and Methods A
270,(1988)110.
1. TheATLASCollaboration,JINST3,(2008)S08003.