SMandjetmeasurementsattheLHC

AndyPilkington–IPPPandManchesterPresentedattheYoungExperimentandTheoryIns7tute,Durham,January2011

PartI–jetcharacteris.cs1)Introduc?ontoATLASandtheLHC2)Jetreconstruc?onandperformance3)Inclusivejetmeasurements

PartII–eventstructure4)Mul?‐jetfinalstates5)Vectorbosonproduc?on(with/withoutjets)6)Whattoexpectfrom2011

TheLHCin2010

•  45pb‐1ofdatatakenintotal.•  Averagenumberofinterac?onsperbunchcrossingwasabout3.5forthefinalruns.

•  Theplanistotakeatleast1U‐1ofdatain2011at√s=7TeV.–  Likelythattheluminositywillbehigherthanthis,possiblyevenincreaseinenergy

to8‐9TeV(underdiscussion)

TheATLASdetector–ageneralmul?purposedetector

JetsatATLAS:reconstruc?onandperformancein2010

TheATLAScalorimeter

Thean?‐kTalgorithm

•  DefaultalgorithmforstudyingjetsatbothATLASandCMS.

•  Foreachobject,‘i’,andeachpairofobjects,‘ij’,definethemergingparameters:

where

•  Ifdijissmallest,mergeobjects‘i’and‘j’•  IfdiBissmallest,defineobject‘i’asajetand

removefromthelist.•  Repeatprocedureun?lnoobjectsremaining.

•  Algorithmisbothinfra‐redandcollinearsafe.•  Generallyproducesjetsofdefinitesize

JHEP04(2008)063

Jetinputs

•  Topo‐clustersare3Dobjects:

–  Clusteringacrossmanylayersofcalorimeter–  Seedcelliden?fiedby|E|>4σini?atesthe

cluster–  Neighbourcellsitera?velyaddedtoclusterif

|E|>2σ–  RingofGuardcellsadded(|E|>0σ)

•  Towersareconstructedbyusingcellsthatarealignedalongagivenη‐ϕdirec?on–  6400towersinATLAS,–  Noisesuppressedtowersarethoseconstructed

fromcellsthatbelongtotopo‐clusters

Jetcalibra?onschemes

Defaultjetenergyscalecalibra?onanduncertainty

•  Thedefaultcalibra?onschemecurrentlycorrectsEM‐scalejets.

•  Response,R,ofajetinthecalorimeterisstudiedforjetsinPYTHIA:

•  Responseparameterizedandinvertedtogivethejetenergyscale(JES)correc?on

•  UsingtheMCderivedresponseintroducesuncertaintydueto:

–  Detectorsimula?on(e.g.geometry,deadmaterial,noise,hadronicshowermodels…)

–  Eventgeneratormodeling(hadroniza?on,partonshowerandunderlyingevent)

pEMT = R(ptruth

T , η) ptruthT

Theproblemwithforwardjets

•  Therela?veresponseasafunc?onofpseudo‐rapidityismeasuredin‐situ:

–  Dijeteventsselected–  ISR/FSRsuppressedbyselec?ngback‐to

backjets(|Δϕ|>2.6)andcuongonthirdjetpT

–  Asymmetryofdijetsmeasuredtogivetherela?veresponse

•  DataandMCdonotagreewellintheforwardregion!Couldbe:

–  Detectorsimula?onproblem(shouldcalibrate).

–  Aphysicsmodelingproblem(shouldnotcalibrate

Thedifferencebetween‘quark’and‘gluon’jets

•  Jetsstudiedsofarincludejetsorigina?ngfrombothhardgluonsandhardquarksintheMC:–  jetsfromgluonstypicallysoserandsospar?clestypicallydepositlessenergy

•  Thiscausesaproblemifthesampleofjetswearelookingatdonothavethesamemixtureofquarks/gluonsastheinclusiveMCsample–  Addi?onaluncertaintyneededthatisanalysisspecific(no‘onecalibra?onfitsall’approach)

Theproblemofnear‐byjets

•  Thejetcalibra?onwascalculatedforjetsthatarewellseparatedfromanyotherjetac?vity

•  Responseismuchlowerinjetsifthereare‘nearby’jets•  Addi?onaluncertaintyinmul?‐jetanalyses

–  dependsonthetopologyoftheeventinques?on.

Howgoodisthiscalibra?on?–currentATLASview

•  Valida?onoftheenergyscalewasperformedwiththesinglehadronresponse,measuringtheE/pforchargedpar?cles.Liuledevia?onbetweendataandMC.

•  Propaga?ngthisuncertaintytothejet‐level[usingpseudo‐experiments],yieldsamuchsmallerintrinsicuncertaintythanthedefaultMC‐basedapproach.

Howgoodisthiscalibra?on?–somepre‐dataexpecta?ons

•  Themeasurementoftheproduc?oncross‐sec?onofVBFHiggsiscrucialtotheextrac?onoftheHiggscouplings.DominantuncertaintyfromJES.

–  TheCSCstudyassumed7%(15%)JESuncertaintyforcentral(forward)jets.–  Wealreadyhavethatprecision!

•  Themeasurementofthetop‐quarkmassinsemi‐leptonicandhadronicchannelsisdominatedbyJES

–  0.7GeVuncertaintyperpercentJESuncertainty.–  Alongwaytogofortop‐studies.

Howgoodisthiscalibra?on?–comparisonwithCMS

•  TheCMSbasicjetenergyscaleissmallerthanthecurrentATLASoneinthecentralregion–  Thismakessense,astheyhaveused2.9pb‐1ofdataandphoton‐jetbalancingtogetan

in‐situhandleontheJES.TheATLASJESwillbesimilar.•  Intheforwardregion,theCMSuncertaintyisMUCHsmallerthantheATLASone.

–  MC/datadiscrepancyiscalibratedawayaserextrapola?ngthediscrepancytopT,3=0–  ThisassumesthatthediscrepancyisNOTduetophysicseffects.ATLASdoesnotauempt

toclaimthisyet.

Howgoodisthiscalibra?on?–comparisonwithD0

•  Itshouldn’tcomeasasurprisethattheJESatD0isbeuerthanthatatATLASandCMS

•  Centraluncertaintydeterminedbyphoton‐jetbalancetobe<2%forjetsinthecentralregion.

•  Uncertaintygrowstoabout4%by|η|~3

•  TheATLAS/CMSgoalofa1%uncertaintyintheJESisrealis?c(atleastforcentraljets)

InclusivejetmeasurementsatATLAS

Inclusivejetmeasurements–thebenefits

•  Keyobservableinpar?clephysics–  previouslymeasuredine+e‐,ep,ppandcolliders

•  Theinclusivemeasurementisanidealsteppingstonetomorecomplicatedjettopologies.–  Simplestpossiblejetmeasurement

–  NLOpredic?onknownforover20years,withsmalluncertainty

•  Oncetheinclusivejetmeasurementhasbeenmadetohighaccuracy,canuseto–  provideprecisemeasurementofαs

–  determinethestructureoftheproton(PDFfits)–  studyBSMphysics(seetalkbyC.Issever)

pp̄

Eventselec?on

•  An?‐kTalgorithmusedwithR=0.4andR=0.6andcomparedtoQCDcalcula?ons

–  Differentsosphysicseffectsfordifferentjetradii

•  Inclusivecross‐sec?onmeasuredasafunc?onofjetpTandyforalljetswithpT>60GeVand|y|<2.8

–  Alljetsinregionofcentraljet‐triggerplateau–  Noforwardjets(MC/dataJESdiscrepancy)

•  Dijetcross‐sec?onmeasuredasafunc?onofmjjandχ=exp(|y1‐y2|)–  Leadingjetsa?sfiespT>60GeVand|y|<2.8–  Secondjetsa?sfiespT>30GeVand|y|<2.8–  Highpurityandefficiencyofjetreconstruc?on

forbothjets–  StabilityofNLOimprovedbyasymmetriccuts

Also:1)  Vertexrequiredwith5+tracks2)  Jetscleanedtoremovejetsfrom

‘noise’andpoorlyreconstructedregionsofdetector

Unfoldingdetectoreffects

•  Bin‐by‐binunfoldingtoremoveeffectofdetectorinefficienciesandresolu?ons.•  Unfoldingfactor(σtruth/σreco)determinedusingPYTHIA6,uncertaintyonthisevaluated

by(i)usingdifferentgenerators(ii)changingtheresolu?on(iii)changingtheshapeoftheinputdistribu?ons.

•  Typically5%(orless)uncertaintyinunfoldingfactors.

Inclusivejets:comparisontoLOeventgenerators

•  ShapeiswelldescribedbytheLO2‐>2matrixelement+partonshowerapproach•  Cross‐sec?onsdeterminedatleadingorder–normaliza?onrequiredtogettheMCto

matchthedata

Inclusivejets:comparisonwithNLOpredic?ons

•  NLOpredic?onatparton‐levelfromNLOJet++usingCTEQ6.6PDF(checkedwithJETRAD)•  Sos(non‐perturba?ve)correc?onsappliedtopartonlevelpredic?onstoallowcomparison

todata

ComparisonwithNLOpredic?ons,inabitmoredetail

Approximately5%uncertaintyinNLOtheory,comparedto30%experimentaluncertainty‐thisisJESdominated,shouldbeabletodras?callyimproveonthisinthenearfuture

Dijetmassdistribu?ons

Dijetangulardistribu?ons

Inclusivejetcross‐sec?oncomparisonwithPOWHEG

•  POWHEG‐Boxrecentlyimplementeddijetproduc?on(publishedinarXiv:1012.3380)•  NLOmatrixelementinterfacedtoPYTHIA/HERWIGtobenefitfrompartonshower,

hadroniza?onandunderlyingevent.–  Allowsout‐of‐boxpar?clelevelcomparison(shownabove)

•  But,fortheexperiments,POWHEGcouldbeusedto–  improvetheuncertaintyintheunfoldingcorrec?ons

–  ReducephysicsmodelingdependenceofJESdetermina?onusingdijetbalancing

Summary–partI

Themeasurementofjetsinacolliderexperiment1)  Alotofworkisneededtocalibratetheenergyofjetsinacolliderexperimenttothe

accuracyrequiredforprecisionmeasurements2)  ThecurrentpublicATLAS(CMS)uncertaintyisabout7%(4%)forcentraljetsandis

improvingsteadilythroughin‐situmeasurements.

Theinclusivejetcross‐sec.onmeasurement1)  Thesimplestmeasurementofjetproduc?onattheLHC‐asteppingstonetothemore

complicated,exclusivefinalstatesdiscussedinthenextlecture2)  DominatedbyJESuncertainty(>30%)3)  NLOpredic?onsaccuratetoabout5%intheinclusivesinglejetmeasurement

Eventwithdijetmass,mjj=3.1TeV–theSMatunprecedentedenergies

Extras

90 91 92 93 94 95 96 97 98 99 100

MUON-CSC

MUON-DT

MUON-RPC

HCAL BARREL

HCAL ENDCAP

HCAL FORWARD

ECAL BARREL

ECAL END-CAP

PRE-SHOWER

STRIP TRACKER

PIXEL TRACKER

MUON-CSC

MUON-DT

MUON-RPC

HCAL BARREL

HCAL ENDCAP

HCAL FORWAR

D

ECAL BARREL

ECAL END-CAP

PRE-SHOWE

R

STRIP TRACKE

R

PIXEL TRACKE

R Series1 98.5 99.8 98.8 99.9 100 99.9 99.3 98.9 99.8 98.1 98.2

StatusoftheCMSdetectors

Thelimita?onsoftheinclusivejetmeasurements

•  Lowsta?s?cs:measurementperformedwithlessthan1/1000thofthecurrentavailabledata.

•  Conserva?veJESes?mate–resultsinexperimentaluncertainty6xlargerthanNLOuncertainty

•  CoversalimitedpTrange(60<pT<500GeV)andlowrapidityrange(|y|<2.5).

•  Futureextensions(probablyforthewinterconferences):–  ImprovedJESonitsway!

–  Fulldatasetof2010,allowinglargerpTandmjjtobeprobed–  AswellasprobingsmallerpT(aslowas20GeV)andlarger|y|(upto4.5)