Figure2(Above):Thejet-energyspectrumwithUEenergysubtractedviamethods2and3. AlsoshownaretheGaussianfitstothenegativejetenergiestoestimatethecombinatorialjetcontributions.

• [1]EPJC(2012)72:1896• [2]Z.Chang’sthesis,“InclusiveJetLongitudinalDouble-SpinAsymmetryAllMeasurementsin

510GeVPolarizedppCollisionsatSTAR”(2016)andALICE(perpendicularconesmethod)PRD91,112012(2015)

• All3methodsofsubtractingbackgroundinthereconstructedchargedjetspectrumagreewithinafewpercentforenergiesgreaterthan1.5GeVandwithin~10%forenergiesgreaterthan1GeV.• Methods2and3,whichfirstsubtractUEenergy,andsubsequentlythecombinatorialjetcomponent,agreetowithin~25%tothelowestjetenergiesmeasured.• SinceMethod1assumesnoUEenergy,subtractingonlycombinatorialjetsasabackground,wecanconcludethatthisassumptionisgoodforR=0.3andjetenergygreaterthan1GeV.

• .

SubtractingtheUnderlying-EventEnergyinReconstructedJetsin√𝐬𝐍𝐍 =200GeVProton+ProtonCollisionsatSTAR

C.OstbergDepartmentofPhysics,SanFranciscoStateUniversityREUStudentattheCyclotronInstitute,TexasA&M

MentoredbyProfessorSaskiaMioduszewski

TheAnti-kt algorithmisusedtoclusterthejets.Thisalgorithmpicksaparticleinaregion,andclustersotherparticlesinnearvicinity,intermsofazimuth(Δφ)andpseudorapidity (Δη),to

formaroughlyconicaljet.

𝑑%& =1

max 𝑝-%. , 𝑝-&.∆𝑅%&.

𝑅. 𝑑%3 =1𝑝-%.

π-3π/4

π+3π/4

π-π/2

π+π/2

Recoil Side

Uncorrelated jet candidates

Uncorrelated jetcandidates

[UE]

[UE]

Figure3(Above):Thebackground-subtractedjet-energyspectraforall3methods.Theinsertshowsazoomed-inviewofthelowjet-energy

q

Collimated spray of Hadrons

q

Proton

Proton

two back to back jets

Collimated spray of Hadrons

Inultra-relativisticcollisions,theenergyishighenoughfor“hard”scatteringstooccur. Thesearecharacterizedbyalargemomentumtransferbetweenpartons ofthecollidingbaryons/nuclei.Thehard-scatteredpartons thenfragmentintocollimatedspraysofhadrons,called“jets”.These

jetsareusedtoprobetheQGPinheavyioncollisions.

Notalloftheenergyfromthecollisiongoestowardsthehardscatteredjets.Uncorrelatedsoftparticlesarealsoproduced,andcontributetotheobservedhardjetenergy.Theseparticlesarewhatthebackgroundiscomposedof,and

whatmustbesubtracted.

Thefigurebelowdefinesthe“recoil”region(Δφ =π± π/4withrespecttothetriggerparticle)andthe“UE”region(underlying-eventregionwhereΔφ =±π/2−π/4withrespecttothetriggerparticle).Therearetwotypesofbackground:1.)“combinatorial”jets– thoseclusteredbythejet-finder,butnotoriginatingfromhardscatterings,and2.)underlying-eventenergyincludedintheclusteringofthetrue“hard”jets. ForR=0.3(relativelysmalljets),thelatterisexpectedtobesmall,buthowsmallwasstudied

here.

Method3isalsoajetbyjetsubtraction,butismorespecificwithhowitcalculatestheaverage𝑃5.CircularareaswithjetparameterRarecreatedat𝜑 +/- 𝜋/2,withrespecttotherecoiljet’s𝜑,andatthesameη astherecoiljet.There,theaverage𝑃5 perareaisfoundforeachregion(𝜎; and𝜎<).Thesequantitiesarethenaveraged(𝜎=>?)andmultipliedbythearea

oftherecoiljetinordertoscaleittotheindividualjetssize(𝑃5@A).

Combinatorialjetsarethensubtractedasfollows.

• Thebackground-jetspectrainFig.1fallmuchfasterthantherecoil-regionjetspectrum,showingthattheoverallbackgroundenergyissmall,andthecombinatorialjetsaretypicallymadeupofasmallnumberofsoft(lowenergy)particles.

• Methods2and3resultinsimilarUE-energysubtractedspectra(Fig.2).ThecombinatorialjetsaresubtractedassumingtheGaussiandistribution(fittothenegativejetenergies).• Forjetenergiesgreaterthan1.5GeV,allthreemethodsagreeinthebackground-subtractedresultsshowninFig.3.

Introduction

WecontroltheareaofthejetbychangingthejetparameterR(orjetradius)intheequationabove.𝑅 ≈ ∆𝜑. + ∆𝜂.� (η =ln(𝑡𝑎𝑛<L M

.) ,θ

istheanglebetweenthejetandbeamaxis,𝜑 =istheazimuthalangleinphasespace).Inthis

studyR=0.3radians,andonlychargedparticlesareincludedinjetreconstruction.

DataUsedtoReconstructJets:• STARExperimentdatafrom2009RHICRun.• √𝒔𝑵𝑵 =200GeVproton-protoncollisions• Eventswithtriggerparticleof9<𝑃5 <30GeV• Chargedtrackswith0.2<𝑃5 <20GeV

RecoilJet𝑃5AverageUEJet𝑃5(Method2)OffCone𝑃5 (Method3)UEJet𝑃5 (Method1)

𝑃5,&?-Q?RS [GeV]

Jet-Reconstruction

Jetsresultingfromhardscatterings(i.e.scatteringswithlargemomentumtransfer)provideinsightintoparton energylossinthehot,densemediumproducedbyultra-relativisticheavy-ioncollisions.Complementarymeasurementsinthemedium-freeproton-protonenvironmentestablishavacuumfragmentationreference.Inacollision,energythatgoesintotheproductionofparticlesnotoriginatingfromthehardscatteringisbackground(referredtoastheunderlyingevent)thatmustbesubtractedfromthemeasuredjet-energy.Wepresentastudyofdifferentmethodstosubtracttheunderlying-eventenergyin200GeVproton-protoncollisionsrecordedattheSTARexperimentatRHIC.

Abstract

Results

Acknowledgements References

Summary

Methods

< 𝑃5,&?-UV >=∑ 𝑃5,%UVY%Z[

𝑁&?-]

[1]

[2]

𝑃5,&?-^_@ [GeV]

Method3(AfterGaussianSub)

Method1

Method2(AfterGaussianSub)

Figure1(Above):Therawrecoiljetenergydistribution(blue)beforesubtractingthebackgrounds,aswellastheenergydistributionsofthethreebackgroundestimates

Method1usesreconstructedjetsintheUEregion(thetwoUncorrelatedjetregionsshownabove)andsubtractstheir𝑃5 (transversemomentum)statisticallyfromtherecoiljets’𝑃5 .Thisproceduresubtractsanestimateofcombinatorialjets,assumingtheUEbackgroundissmallenoughto

neglect.

𝑃5,&?-^_@ [GeV]

Method3(Off-Cone Sub)Method3GaussianFitMethod2(AvgUESubtraction)Method2GaussianFit

Method2usestheaverage𝑃5 (< 𝑃5,&?-UV >)ofthesameUEjets,butinsteadsubtractsitonajet-by-jetbasisfromtherecoiljets.ThisproceduresubtractsanestimateoftheUEenergy,leavingcombinatorialjetsasaremainingbackground. Thesewillbesubtractedusingaseparate

proceduredescribedbelow.

• NSFGrant(PHY– 1659847)• DOEGrant(DE-FG02-07ER41485)• ThankyoutoDr.Nihar Sahoo andProfessorSaskiaMioduszewskiformentoringme

andallowingmetoexperiencewhatitsliketobeanuclearphysicistforasummer.

PptrkT

Acon

= ��

PptrkT

Acon

= �+

�ave =1

2(�� + �+)

pbgT = �ave ⇥Ajet

CombinatorialJetSubtractionforMethods2and3Sincenegativejetenergiesareunphysical,wetakeentriesinthesebinstobeanestimateofthecontributionfromcombinatorialjets. Assumingthe combinatorialjetsshowupasasymmetricdistributionaroundzeroenergy(afterUEenergysubtraction),wefitaGaussianfunctiontothe

negativesideofthedistribution,reflectittothepositiveside,andsubtractthefullGaussianfromthejetenergydistribution.

p+p 200GeV(Run9)

p+p 200GeV(Run9)

p+p 200GeV(Run9)