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NuclearPhysicsA00(2011)1–4
Tagged jets and jet reconstruction as a probe of the quark-gluon plasma
R.B.Neufeld
LosAlamosNationalLaboratory
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1
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2 Abstract
n The large center-of-mass energies available to the heavy-ion program at the LHC and recent experimental ad-
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vances at RHIC will enable QCD matter at very high temperatures and energy densities, that is, the quark-gluon
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plasma (QGP), to be probed in unprecedented ways. Fully-reconstructed inclusive jets and the away-side hadron
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showersassociatedwithelectroweakbosons,thatis,taggedjets,areamongtheseexcitingnewprobes. Fulljetrecon-
] structionprovidesanexperimentalwindowintothemechanismsofquarkandgluondynamicsintheQGPwhichis
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notaccessiblevialeadingparticlesandleadingparticlecorrelations. Theoreticaladvancesintheseexcitingnewfields
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ofresearchcanhelpresolvesomeofthemostcontroversialpointsinheavyionphysicstodaysuchasthesignificance
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p oftheradiative,collisionalanddissociativeprocessesintheQGPandtheapplicabilityofstrongversusweakcoupling
e regimestodescribejetproductionandpropagation. Inthisproceedings,Iwillpresentresultsontheproductionand
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subsequentsuppressionofhighenergyjetstaggedwithZbosonsinrelativisticheavy-ioncollisionsatRHICandLHC
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energiesusingtheGyulassy-Levai-Vitev(GLV)partonenergylossapproach.
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v
6 The observation of the suppression of energetic partons in the QGP, that is jet quenching, is one of the most
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importantresultsfromtheheavy-ionprogramatRHIC[1,2]. Experimentalmeasurementsthusfarhavemostlybeen
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restrictedtoleadingparticlesuppressionrelativeto p+pcollisions[3]becauseofthelimitedcenter-of-massenergies
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. available. However, in order to constrain the underlying quantum chromodynamic (QCD) theory of jet quenching,
1 new and more differential observables are needed. Leading particle suppression alone cannot discriminate between
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partonic energy loss formalisms or extract quantitatively the jet quenching properties of the QGP [4]. Particularly
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1 promising new observables are jets tagged with electromagnetic [5, 6] or weakly interacting probes and jet shapes
: [7,8].
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i Tagged jets refer to high energy partons produced in association with the tagging particle. Because they have
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negligible medium induced modifications, electroweak bosons are ideal tags to study partonic energy loss. Within
r leadingorder(LO)kinematics,thetaggingparticleservesasanexactcalibrationoftheinitialassociatedjetenergy.
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Inthisway,onehopestoobtaintheamountanddistributionofpartonicenergylossbyreconstructingthetaggedjet.
TheleptonicfinalstatesoftheZ0 bosonareanespeciallyattractivejettagbecausethelargeinvariantmassoftheZ0
bosonmakesiteasytodistinguishfromthebackgroundgeneratedinaheavy-ioncollision. Theheavy-ionprogramat
theLHCwillenabletheexperimentalmeasurementofthisfeatureforthefirsttime.
As mentioned previously, at LO the kinematics ensure that the tagging Z0 boson serves as an exact measure of
the associated jet p (magnitude of momentum transverse to the beam line). This is on display in Fig. 1 where I
T √
present results [5] for jets associated with Z0/γ∗ → µ+µ− in p+p collisions at s = 4 TeV. In the plot, the tagging
Z0/γ∗ isrequiredtohave7< p <13GeVandcurvesarepresentedfortheleadingandnext-to-leadingorder(NLO)
T (cid:112)
results,aswellasforthreedifferentvaluesofjetreconstructionradius,R = (∆φ)2+(∆y)2. TheNLOcalculations
are performed using the publicly available Monte Carlo for FeMtobarn processes (MCFM) developed by Campbell
andEllis[9]. Inthefigure,oneindeedseesthatatLOtheZ0/γ∗ servesasanexacttagoftheassociatedjetenergy,as
R.B.Neufeld/NuclearPhysicsA00(2011)1–4 2
Figure1:WhengoingfromLOtoNLO,thetaggingpoweroftheZ0bosoniscompromisedbythepossibilityofanextrajetinthefinalstate,as
seenfromthefigure.TheLOresultisconstrainedexactlybythepT cutonthetaggingparticle.However,thisisnolongertrueatNLO.
thejet p distributionisentirelyinsideofthetaggingwindowof7−13GeV.However,atNLOthiscorrespondence
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issmearedduetothepossibilityofanextrajetinthefinalstate. ThisrelativelylargesmearingeffectatNLOsuggests
greatcaremustbetakeninexperimentallytaggingtheinitialassociatedjetenergyinAAcollisions.
√
InextconsiderresultsforjetsassociatedwithZ0/γ∗ →µ+µ− inPb+Pbcollisionsat s=4TeV/nucleonpair. In
thisproceedingsIwillnotconsidercoldnuclearmattereffects[12],butwillleavethatforafuturework. InPb+Pb
collisions it is essential to include medium-induced radiative jet energy loss. The partonic energy loss in medium
fortheresultsthatfollowwasobtainedfromtheGuylassy-Levai-Vitev(GLV)energylossformalism[11]. TheNLO
results for tagging Z0/γ∗ required to have 7 < p < 13 GeV is shown in Fig. 2 [5]. The result is shown for two
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Figure2: TheNLOpT differentialcrosssectionpernucleonpairforjetstaggedwithZ/γ∗ →µ+µ−inp+pandPb+Pb. ThetaggingZ/γ∗is
requiredtohave7<pT <13GeV.TheresultsareshownfortwodifferentvaluesofjetconeradiusR=0.2(leftpanel)R=0.8(rightpanel).The
mediuminducedenergylossshiftsthecurvetolowervaluesofpT.Asthejetconeradiusisincreased,themediummodifiedcurvesapproachthe
p+presult,asmoreandmoreofthemediuminducedradiationisrecoveredinthejet.
differentvaluesofjetconereconstructionradius,R = 0.2,0.8(leftandrightpanel,respectively). The p differential
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crosssectionpernucleonpairforPb+Pbissuperimposedwiththeresultfromp+pwhichwasshowninFig. 1. Itis
clear from the figure that the effect of medium induced energy loss is to shift the curve to lower values of p . It is
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alsoclearthatasthejetconeradiusisincreased,themediummodifiedcurvesapproachthe p+presult,asmoreand
moreofthemediuminducedradiationisrecoveredinthejet. Thisoccursregardlessofthestrengthofthemedium
coupling,α . Asthejetconeradiusisdecreased,thesensitivitytoα becomesmoreimportant,aslessofthemedium
s s
R.B.Neufeld/NuclearPhysicsA00(2011)1–4 3
induced radiation is recovered. This sensitivity of the result in Pb+Pb to jet cone radius is directly related to the
angulardistributionofthemedium-inducedradiation,whichwillbediscussedfurtherbelow.
FulljetreconstructionandjetshapesarealsopromisingnewobservablestohelpconstraintheunderlyingQCD
theory of jet quenching, and in particular, to obtain an experimental handle on the large angle medium-induced ra-
diation spectrum [10]. To understand why this is true, consider Fig. 3 from Ref. [7], which shows a schematic
(cid:112)
Figure3: Aschematicillustrationofthejetreconstructionconeradius,R = (∆φ)2+(∆y)2,andtheparticletowerenergyselection,ωmin. As
theseparametersarevaried,theamountofenergyrecoveredinjetreconstruction(andhencethejetcrosssection)alsovaries.
(cid:112)
illustrationofthejetreconstructionconeradius,R = (∆φ)2+(∆y)2,andtheparticletowerenergyselection,ω .
min
Asonevariestheseparameters,theamountofenergyrecoveredinjetreconstruction(andhencethejetcrosssection)
likewise varies. This variation can be exploited to reveal the structure of the underlying medium induced radiation
spectrum,asisdemonstratedinFig. 4,whichistakenfromRef. [8]. TheFigureshowspredictionsforthesensitivity
(cid:112)
Figure4:ThefiguregivespredictionsforjetRAAasafunctionofjet√reconstructionconesizeR= (∆φ)2+(∆y)2usingtheGLVpartonicenergy
lossformalism.TheplotisforthemostcentralCu+Cucollisionsat s=200GeVpernucleon.ThesensitivityofthesuppressiononconesizeR
reflectstheangulardistributionofmedium-inducedradiation.TheinsertshowstheratiosofjetcrosssectionsforselectedvaluesofR.
of jet R on jet reconstruction cone size R using the GLV partonic energy loss formalism. The plot was done for
AA √
themostcentralCu+Cucollisionsat s = 200GeVpernucleon. Thedependenceofthesuppressiononconesize
Rdirectlyreflectstheangulardistributionofmedium-inducedradiation. Theheavy-ionprogramattheLHCandthe
RHICupgradeswillenableahighstatisticsexperimentalmeasurementofthisfeatureforthefirsttime.
R.B.Neufeld/NuclearPhysicsA00(2011)1–4 4
In summary, recent experimental advances at RHIC and the heavy-ion program at the LHC have ushered in a
neweraofexperimentalcapabilitiestoprobethequark-gluonplasma. Amongthesenewprobes,jetshapesandjets
taggedwithweaklyinteractingprobesareespeciallypromising, particularlyinlightofthefactthatleadingparticle
suppressionaloneisnotsufficienttodiscriminatebetweenpartonicenergylossformalismsortoextractquantitatively
thejetquenchingpropertiesoftheQGP.
Ihaveherediscussedthepowerofjetobservablestorevealthespectrumofmediuminducedradiationandthereby
shed light on the applicability of the commonly used energy loss formalisms. I presented results on the production
and subsequent suppression of high energy jets tagged with Z bosons in relativistic heavy-ion collisions using the
GLVpartonenergylossapproach. Theresultssuggestthatcaremustbeappliedinexperimentallytaggingtheinitial
associatedjetenergywithelectroweakbosonsbecauseofsmearingduetohigherorderproductionprocesses.Itisalso
observedthatthedependenceofthesuppressioninPb+Pbrelativetop+ponjetreconstructionradiusRisasensitive
probeofthespectrumofmediuminducedradiation. Iwouldherementionthattaggedjetsmayprovideanexciting
opportunity to probe the medium response to hard partons [13], especially if the NLO smearing effects are brought
undersufficienttheoreticalcontrol.
Acknowledgements
I wish to thank my collaborators Ivan Vitev and Ben-Wei Zhang. This work was supported in part by the US
DepartmentofEnergy,OfficeofScience,underContractNo. DE-AC52-06NA25396.
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