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Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
PDDr.KlausReygersPhysikalischesIns5tutUniversitätHeidelberg
1
QGPandHigh‐pTPhysicsLecture2:JetsandJetQuenching
HelmholtzGraduateSchoolofFundamentalPhysicsWinterSchoolObergurgl2010January29‐February2,2010
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Outline
• Introduc*on:HardSca0eringandJetQuenching
• TheDiscoveryPhase(ca.2000‐2003)
• FurtherExperimentalResultsRelatedtoJetQuenching
• FutureJetQuenchingMeasurements
2
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers3
Introduc*on:HardSca0eringandJetQuenching
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Theore5calDescrip5onofHigh‐pTPar5cleProduc5on:
Perturba5veQCD
• Sca0eringofpointlikepartonsdescribedbyQCDperturba*ontheory(pQCD)
• SoPprocessesdescribedbyuniversal,phenomenologicalfunc*ons‣ Partondistribu*onfunc*onfromdeepinelas*csca0ering‣ Fragmenta*onfunc*onsfrome+e‐collisions
Leading hadron:
z = phadron / pparton
≈ 0.25
factoriza5on:
4
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
HadronProduc5oninLeadingOrderQCD
Fragmenta5onfunc5onInv.Crosssec5on
Phenomenologicalfactorwhichtakeshigherordercontribu5onsintoaccount
Partondistribu5ons(func5onsofxBjorkenandmomentumtransferQ2)
ElementaryQCDparton‐partoncrosssec5on
E d3σ
dp3= K dxa
xa ,min
1
∫ dxbxb ,min
1
∫ fa A xa ,Q2( ) fb B xb ,Q2( )dσab→cd
dt̂1πzc
Dh c zc ,Q2( )a,b,c ,d =q,q , g∑
zc = pT,Hadron / pT,c
5
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
PartonDistribu5ons:HighPrecisionDatafromHERA
Fitting procedure
proton pdf
Bjorken-x6
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Example:Gluonandu‐QuarkFragmenta5onFunc5ons
Albino,Kniehl,Kramer,Nucl.Phys.B725(2005),181
Fragmenta*onfunc*ons:Numberdensityfortheproduc*onofahadronhwithfrac*onalenergyzinthefragmenta*onofaparton(e.g.determinedfrom) e
+e− → Z 0 → qq
7
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
NLOpQCDWorksatRHIC
p + p→ π0 + X
sNN = 200GeV
AgreementwithpQCDinp+pshowsthathigh‐pTpionsareacalibratedprobeofthemediumcreatedinA+Acollisions
8
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Modifica5onoftheStructureFunc5onsinNuclei
x<0.1:“shadowingregion”
0.1<x<0.3:“an5‐shadowing”
0.3<x<0.7:“EMCeffect”
0.7<x<1.0:Fermi‐mo5onofnucleonsinnuclei
9
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
AnExample:NuclearPDF‘sinPb(EPS09NLO)
Valencequarks Seaquarks gluons
Largeuncertain5esforgluonPDF’satsmallx Eskola et al., arXiv:0902.4154v2 [hep-ph]
10
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetQuenching:BasicIdea
Expecta*on:Simplescalingfromp+ptod+Au
Expecta*on:PionsuppressioninA+A
Expecta*on:Simplescalingfromp+ptoA+Afordirectphotons
11
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetsasAuto‐GeneratedProbesoftheQGP
• Thermaliza*onofthedeconfinedquark‐gluonma0erexpectedatτ0<∼1fm/c
• Hardprobes(jets,high‐pTdirectphotons,c‐andb‐quarks)producedinearlyhardpartonsca0eringspriortoQGP⇒idealQGPprobes
Earlyhardparton‐partonscalerings(Q2>> Λ2
QCD)
Thermalizedmedium(QGP!?)(T0>Tc,Tc≈150–200MeV)
Transi5onQGP → hadrongas
Freeze‐out
%me
12
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
WhatCanWeHopetoLearnfromJetQuenching?
• Objec*vesofheavy‐ionphysics:‣ LearnsomethingaboutQCDintheregimeofhightemperaturesand
densi*es(QCDthermodynamics)‣ Studythedeconfinementtransi*onatTc=150‐200MeVpredictedby
lacceQCDcalcula*ons
• ObservablesrelatedtoJetQuenchingmayhelpto‣ characterizethenewstateofma0eraboveTc‣ understandthemechanismofpartonenergyloss
• Basiclogic
13
QGP SuppressionofhadronsathighpT⇒⇐
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
HowCanWeStudyJetQuenching?
• Measurementofpar*clemul*plici*esathighpT
• Measurementoftwo‐par*cleangularcorrela*ons
• Jetreconstruc*ononanevent‐by‐eventbasis‣ Challengingincentralnucleus‐nucleuscollisionsatRHICduetolarge
par*clemul*plicityfromtheunderlyingevent‣ Situa*onimprovessignificantlyforPb+PbattheLHCduetothe
increasedcrosssec*onforjetproduc*on
14
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetQuenchingHistory
15
• Energylossviaelas*csca0eringwaslaterbelievedtohaveonlyaminoreffectonjets
• Radia*veenergylosswasdiscussedintheliteraturefrom1992onbyGyulassy,Pluemer,Wang,Baier,Dokshitzer,Mueller,Pegne,Schiff,Levai,Vitev,Zhakarov,Wang,Salgado,Wiedemann,…
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Analogy:EnergylossofChargedPar5clesinNormalMaler
• μ+onCu:Radia*onalenergyloss(„bremsstrahlung“)startstodominateovercollisionalenergyloss(„Bethe‐Blochformula“)forp>>100GeV/c
• Forenerge*cquarksandgluonsinQCDma0er,radia*veenergylossviainducedgluonemissionis/wasexpectedtobethedominantprocess
μ CuE E´
16
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
PartonEnergyLoss–ExpectedProper5es
ΔE∝αsCFq̂L2 EnergylossΔEinasta5cmediumoflengthLforE→∞(BDMPSresults)
Radia5veenergylossdominant(?): µ
2 : Typicalmomentumtransferfromthemediumtotheparton
λ : Meanfreepath
q̂ =
µ2
λMediumparameter
Energylossforgluonjetslargerthanforquarkjets
CF =
34 / 3
for gluon jets for quark jets
⎧⎨⎪⎪⎩⎪⎪
L2dependence:Non‐abeliannatureofQCD+quantenm.interference
Nucl.Phys.B483:291‐320,1997
17
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
PartonEnergyLoss:WhyΔE∝L2?
Probabilityforradia*ngagluon:
Coherentgluonwavefunc*onaccumulatestransversemomentumkT.Numberofsca0eringswithmomentumtransferkTun*litdecoheres:
Totalenergyloss:
∝ L
ΔE∝ L2
∝ L
18
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Rela5onbetweenTransportCoefficientandEnergyDensityoftheMedium
• increasessmoothlywithenergydensity
• Nuclearma0er
ideal qgphot, massless pion gas
cold pion gas• QGP(andhothadrongas):
Mediumcharacterizedby(Momentumtransferpermeanfreepath)
q̂nuclear matter < 0.5−1 GeV2 / fm
q̂qgp q̂nuclear matter
q̂ = µ 2 / λ
energydensity(GeV/fm3)
q̂ (G
eV2/fm
)
19
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Medium‐ModifiedFragmenta5onFunc5ons
energylossparton
hadron h, energy
Prob.Distr.forpartonenergylossε(“Quenchingweight”)
Considerfixedpartonenergylossε:
Averageoverenergylossprobability:Hadronsresul5ngfromgluonbremsstrahlungneglected
z =Eh
(1−ε)Eq
, x =Eh
Eq
=x
(1−ε)
dndx
=dndz⋅
dzdx
= Dh / q (z,Q2 ) ⋅ 11−ε
(1−ε)Eq Eq
P(ε) Dh / q (z,Q2 )
Dh / q
med(x,Q2 ) = dεP(ε) Dh / q ( x1−ε
,Q2 ) 11−ε
0
1
∫
PartonenergylosscanbeconvenientlyincludedinapQCDcalcula5onviamodifiedfragmenta5onfunc5ons
20
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
EnergylossintheGLVFormalismforPb+PbattheLHC
energy loss # radiated gluons
CentralPb+Pbat√sNN=5500GeV: L≈6fm,dNg/dy=2000,3000,4000
I. Vitev, Phys.Lett.B639:38-45,2006
∆Egluon/∆Equark=9/4onlyinthelimitE→∞
21
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Radia5vevs.Collisional(i.e.,Elas5c)EnergyLoss
• ∆Eradia*ve>∆Ecollisonalforu,daswellascquarkswithE>10GeV
• ∆Eradia*ve≈∆Ecollisonalforbquarks
Wicks,Horowitz,DjordjevicGyulassy,Nucl.Phys.A784,426‐442
22
High‐EnergyCollisionswithAlice:JetsinNucleus‐NucleusCollisionsKlausReygers
TheNuclearModifica5onFactor(I):Digression:LuminosityofaCollider
Rateofeventsforagivenphysicsprocess:
Luminosity [(s⋅cm2)-1]
Cross section [cm2]
Event rate [s-1]
Iftwobunchesofpar5clescollidewithfrequencyfthen:
Transverse area of the beam
ni: number of particles in bunch i
Example:Au+Au at RHIC: L = 2 x 1026 cm-2 s-1
N = L ⋅σ
L = f
n1n2
Abeam
= fn1n2
4πσxσ y
23
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
• Calculateincreaseoftheeffec*veluminosityofnucleons(orpartons,respec*vely)basedonknownnucleargeometry
• Result:Par*cleyieldsscalewiththeaveragenumber〈Ncoll〉ofinelas*cnucleon‐nucleoncollisionsintheabsenceofnucleareffects
Only changecompared to p+p
TheNuclearModifica5onFactor(II):Par5cleYieldsfromHardScaleringinA+A
dNhartA+B
dpT
= TAB ⋅dσhart
p+ p
dpT
TAB = Ncoll / σNNunel.
24
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
• TABistheeffec*venucleonorpartonluminosityperA+Acollision
• Inprac*ce:〈Ncoll〉fromGlauberMonteCarlocalcula*on
• Intheabsenceofnucleareffects:RAB=1athighpT(pT>2GeV/c)
• Thisfollowsimplicitlyfromthefactoriza*ontheorem
“no medium effects”RAB = 1
RAB
RAB < 1
pT (GeV)
TheNuclearModifica5onFactor(III):RAA(pT)
RAB ( pT ) =d2 N / dpTdy
A+B
Ncoll ×d2 N / dpTdyp+ p
=d2 N / dpTdy
A+B
TAB ×d2σ / dpTdyp+ p
25
TAB = Ncoll / σinelNN
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers26
Thediscoveryphase(ca.2000‐2003)
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
• Hadronsaresuppressed,directphotonsarenot
• Nosuppressionind+Au(notshownhere)
• Evidenceforpartonenergyloss
DiscoveryofJetQuenchingatRHIC(ca.2000‐2003)(I)
27
energy lossfor q and g
No energyloss for γ‘s
RAB =dN / dpT A+B
TAB ×dσinv / dpT p+p
,
where TAB = Ncoll / σinelNN
PHENIX: Phys.Rev.Lett.88:022301, 2002PHENIX: Phys.Rev.Lett.91:072301, 2003PHENIX: Phys.Rev.Lett.94:232301, 2005
STAR: Phys.Rev.Lett.89:202301,2002STAR: Phys.Rev.Lett.90:082302,2003STAR: Phys.Rev.Lett.91:172302,2003
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
DiscoveryofJetQuenchingatRHIC(ca.2000‐2003)(II)
28
DirectphotonsfollowTABscalingasexpectedforahardprobenotaffectedbythemedium
CentralityDependenceoftheπ0anddirectγRAA:
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
DiscoveryofJetQuenchingatRHIC(ca.2000‐2003)(III)
29
• Nojetcorrela*onaround180°incentralAu+Au• Consistentwithjetquenchingpicture
Au+Au peripheral Au+Au central
background and ellip. flow subtracted
Trigger particle: pT > 4 GeV/cAssociated particle: pT > 2 GeV/c trigger particle
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers30
FurtherExperimentalResultsRelatedtoJetQuenching
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
RAAwithHigherSta5s5cs(2004Run)
31
RAA approximately constant up pT = 20 GeV/c
PHENIX: Phys.Rev.Lett.101:232301, 2008
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
SimpleInterpreta5onoftheConstantRAA
Inthissimplis*cviewtheconstantRAA≈0.25impliesaconstantfrac*onalenergylossofabout20%incentralAu+Auat200GeV
π0spectraatRHICenergy(√sNN=200GeV)describedwithn≈8
π0spectrumwithoutenergyloss:
Constantfrac*onalenergyloss:
Thisleadsto:
32
RAA = (1− Sloss )n−2 ⇒ Sloss = 1− RAA
1/(n−2) ≈ 0.2 for RAA ≈ 0.25
withoutenergyloss
withenergyloss(Sloss=0.2)
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
PathLengthDependence:π0RAAasaFunc5onoftheAnglew.r.t.theReac5onPlane
NPart
R AA
• Longerpathlengthforout‐of‐planejets• Hence,strongersuppressionforout‐of‐
planejetsinqualita*veagreementwithpartonenergyloss
33
in-plane
out-of-plane
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
DependenceontheSizeoftheNucleus:√sNNDependenceoftheπ0RAAforCu+Cu(A=63)
34
Phenix, Physical Review Letters 101,162301 (2008)
62.4and200GeVπ0produc*onlesssuppressedthaninAu+Au
22.4GeV• Nosuppression• Enhancementconsistentwithacalcula*onthatdescribesCronineffectinp+A
Sameconclusionasforheaviernuclei:PartonenergylossstartstoprevailoverCroninenhancementbetween√sNN=22.4GeVand62.4GeV
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
HierarchyExpectedforDifferentTypesofPartons
35
ΔEGluon >ΔEQuark ,m=0 >ΔEQuark ,m≠0
largercolorfactorforgluons:
CF =
34 / 3
for gluon jets for quark jets
⎧⎨⎪⎪⎩⎪⎪
Deadconeeffect:Heavyquarks(c,b)areslowerandradiatefewergluons
Dokshitzer&Kharzeev,PLB519(2001)199
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
PionRAAvs.ProtonRAA
36
RA
A A
u+
Au
ce
ntr
al
STAR QM08 JPG35(2008)104006
pRAA> π RAA eventhoughprotonsresultmostlyfromgluonfragmenta*on:
→ Differencebetweenquarkandgluonenergylossnotobserved
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
RAAforElectronsfromc‐andb‐QuarkDecays
37
e± from heavy flavor
e+ande‐fromcandbdecaysasstronglysuppressedaspions:
ΔEGluon >ΔEQuark ,m=0 >ΔEQuark ,m≠0 notobserved!
Phys.Rev.Lel.98,172301(2007)
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
RAAforElectronsfromHeavyQuarks:NotUnderstoodwithCurrentEnergyLossModels
PHENIX
GLV Parton energy loss calculation
• Radia*veenergylossnotsufficienttodescribeexcesselectronRAA• Includingelas*csca0eringimprovesthesitua*ononlyslightly
Radiative energy loss
Radiative energy loss +elastic scattering
38
Au+Auat√sNN=200GeV
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
FurtherResultsfromTwo‐Par5cleCorrela5ons(I):Away‐SideJetsVisibleAgainForHigherJetpT
• Chargedhadroncorrela*on
• Triggerpar*cle:pT>8GeV/c
• Associatedpar*cle:pT>6GeV/c
triggerpar5cle
Forhigherjetenergiesthecorrela*onatΔφ=180°incentralAu+Auisnotfullysuppressedanymore
39
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
FurtherResultsfromTwo‐Par5cleCorrela5ons(IIa):StrikingModifica5onoftheAway‐SideatLowpT
Trigger > 2.5 GeVpartner > 1 GeV
40
Au+Auat√sNN=200GeV
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
FurtherResultsfromTwo‐Par5cleCorrela5ons(IIb):Away‐SideCorrela5onatLow‐pTNotFullyUnderstood
D D
PHENIX preliminary
Possibleexplana*onsofthesplicngoftheaway‐sidepeakinclude
•Machcones
•flowinducedjetdeflec*on•andmanymore….
Mach cone?
41
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
CurrentStatusofJetQuenchingModels(I)
• Currentlyfourmajortheore*calpartonenergylossschemes:(HT,BDMPS‐Z‐ASW,AMY,GLV)
• Schemesmakedifferentapproxima*onsandmodelthemediumdifferently
• AllschemesbasedonpQCDfactoriza*onapproach
• Thefinalhadroniza*onisalwaysassumedtooccurinthevacuum(aPersomeenergyloss)
42
Dh / q
med(x,Q2 ) = dεP(ε) Dh / q ( x1−ε
,Q2 ) 11−ε
0
1
∫Forexample:
energylossprobability
vacuumfragmenta5onfunc5onmediummodifiedfragmenta5onfunc5on
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
CurrentStatusofJetQuenchingModels
• Allschemescanessen*allybereducedto1‐parametermodels(parametere.g.fixedbyficngthepionRAA(pT))
• Noschemedescribesalloftheobservedhigh‐pTobservables(RAAforlightandheavyquarks,RAAvs.reac*onplane,RAAfordifferentpar*clespecies,two‐par*clecorrela*ons)
• Largedifferences(uptoafactor4)betweenextractedmediumparameterslike
• Sofar:„AdvantageData“
43
q̂
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers44
JetAlgorithms
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetEventinap+pCollisionat√s =63GeV
Legoplotshowsenergyvs.pseudorapidityηandazimuthalangleφ
CERN ISR, ca. 1982
45
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetEventattheTevatron
46
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Evolu5onofaJetEvent
HardProcess→PartonCascade→Hadroniza5on
describablewithpQCD
notdescribablewithpQCD(onlyphenomenologicalmodels)
47
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Jet‐FindingAlgorithms
• Objec*ve:reconstructenergyanddirec*onofini*alparton
• Mustbeunambiguouslyapplicableatthelevelofexperimentaldata(tracks/towers)andinperturba*veQCDcalcula*on(partonlevel)
• Star*ngpoint:listofcalorimetertowersand/orchargedhadrontracks
• Twoclassesofalgorithms:‣ Conealgorithm:tradi*onalchoiceinhadron‐hadroncollisions
‣ kTalgorithm:tradi*onalchoiceine+e‐collisions
Cone algorithm:
Sumcontentinconewithradius
kT algorithm:
Successivelymerge“par5cles”inorderofrela5vetransversemomentum.
Termina5onofmergingcontrolledbyaparameterD
Typicalchoice:R=0.7
48
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
ConeAlgorithm
• (Normally)startwithseed(e.g.calorimetermodulewithhighenergy)• Considerallpar*clesinconearoundseedandcalculate
• Repeatthisprocedurewithnewconecenter(ηC,φC)• Terminatewhen“flow”ofconecenterstops• Calculatejetenergyas
• Apar*clemaybelongtotwocones:splitenergyamongjets
• Subtractbackgroundenergyfromunderlyingevent
Fermilab Run II jet physics:hep-ex/0005012
49
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
kTAlgorithm(I)
• Alorithmsstartswithalistofpreclusters(calorimetercells,par*cles,orpartons)
• CalculatepTandrapidityyforeachprecluster
• Foreachpreclusterdefine
• Foreachpair(i,j)ofpreclustersdefine
• ForD=1and∆Rij2<<1,dijistheminimaltransversemomentumkT(squared)ofonevectorwithrespecttotheother
• Findminimumdminofalldianddij
• Mergepreclustersiandjifdminisadij
• Else:Removepreclusteriwithdmin=difromlistofpreclustersandaddittothelistofjets
• Repeatun*llistofpreclustersisempty50
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
kTAlgorithm(II):ASimpleExample
openarrows:preclusters closedarrows:
jets
51
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
WhythekTAlgorithmisTypicallyOnly
usedine+e‐Collisions?
kTjethasnofixedshape/area→Difficulttosubtractbackgroundfromunderlyingevent
52
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetFindingAlgorithms:TypicalRequirements(I)
Thereconstructedjetsshouldnotchangeincaseof
• collinearspli|ng,i.e.,ifonepartonisreplacedbytwopartonsatthesameplace
• so}emission,i.e.,ifalowenergypartonisadded
Twotypesofdivergences:
collinear infrared
53
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetFindingAlgorithms:TypicalRequirements(II)
• Infraredsafety
Exampleofinfraredsensi5vity:So}radia5on(rightplot)causesmergingofjetswhichwouldhavebeenseparatedotherwise
• Collinearsafety
Exampleofcollinearsensi5vity:Seedandthereforejetnotfoundinle}picture
Exampleofcollinearsensi5vity:Reconstructedjetdependsonseed
FermilabRunIIjetphysics:hep‐ex/0005012
54
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetCrossSec5onsattheTevatron
Pre0ygoodagreementbetweendataandNLOpQCDpredic*ons
hlp://www‐d0.fnal.gov/Run2Physics/displays/presenta5ons/klima_lecture_jan2003/Lecture_1.pdf
ET = E sinϑ
55
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers56
FutureJetQuenchingMeasurements
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
ProblemswithRelyingonHadronSpectra
• Energylossbias‣ Hadrons biased to jets that lose
the least energy‣ Geometry („surface bias“)‣ Radiation fluctuations
• Averaging‣ Hadron measurements average
over jet energies‣ Indirect measurement of jet
quenching
• Solu*ons‣ Direct γ - hadron correlation‣ Full jet reconstruction
57
Surfacebias:Survivingjetsbiasedtowardsthesurfaceoftheoverlapregion:DifficulttoprobethehotcoreoftheQGP
Wicks,Horowitz,DjordjevicGyulassy,Nucl.Phys.A784,426‐442
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
WhyisJetReconstruc5onDifficultinCentralAu+AuCollisionsatRHIC?
Au+Auat√sNN=130GeV
CentralAu+Aucollisionat√sNN=130GeV:
ConsiderjetconewithradiusR:
Totaltransverseenergyinthiscone:
• BackgroundenergylargecomparedtojetenergyinA+AatRHIC.
• Increasedjetcrosssec*onhelpsatLHCenergies
ETcone =
d 2 ET
dηdϕ⋅πR2
=1
2πdET
dη⋅πR2 ≈ 40 GeV
R = Δη( )2
+ Δϕ( )2= 0.4
dET
dηη=0
≈ 500 GeV
ET = Ei sinϑ i
i∑ , dET / dη ≈ mT ⋅ dNch / dη
58
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
SinglePar5cleCrossSec5onattheLHC
p+p
Hardsca0eringcrosssec*onsincreasesignificantlyattheLHC
59
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
ChoiceoftheOp5malConeRadius
Smallconeradius:Smallbackgroundfromtheunderlyingeventattheexpenseofsomelossofjetpar*cles
Largeconeradius:Largebackgroundfromtheunderlyingeventbutnearlyalljetpar*clesincluded
Duetohigherbackgroundstheconeradiusinheavy‐ioncollisionstypicallysmallerthaninp+p(R~0.3‐0.4isagoodcompromiseinA+A)
60
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
HowCanOneStudyPartonEnergyLosswithReconstructedJetsattheLHC?
• MeasureJetRAAfordifferentconeradiiR
• Studymediuminducedmodifica*onoflateraljetprofile
• Studymodifica*onofthefragmenta*onfunc*on
61
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetRAAforDifferentConeRadiiR
• LargeconeradiusR:Allenergylostwillberecovered:RAAjet=1
• Outofconeradia*onwillreduceRAA
• Studyenergylossbyreconstruc*ngjetswithdifferentR
62
Calcula5onbyVitev,Wicks,andZhang(JHEP11,(2008),093):
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
LateralJetProfile
Broadeningoftheenergydistribu*onexpectedinPb+Pb
63
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Medium‐ModifiedFragmenta5onFunc5ons
• Reconstruc*onofthefulljetenergyallowstomeasurefragmenta*onfunc*on
• PartonenergylosswillshiPpar*clestolowz(andthushigherξ)
• Moreover,themediumisexpectedtochangethepar*clecomposi*onofthejet,e.g.,theK/πra*o
• PromisingmeasurementforAliceduetoitsexcellentlowpTpar*cleID
Sapeta, Wiedemann, Eur.Phys.J.C55:293-302,2008.
64
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
PointstoTakeHome
• RHICresultshaveestablishedjetquenchingasanexperimentalfact
• Heavyquarksareasstronglysuppressedaslightquarkswhichisnotfullyunderstoodincurrentjetquenchingmodels
• Theul*mategoal,theconsistentcharacteriza*onofthemedium,isnotyetpossiblesincedifferentmodelsyielddifferentresults
• PHENIXandSTARareworkingonfulljetreconstruc*oninA+Acollisions
• Increasedjetcrosssec*onmakesfulljetreconstruc*oneaseratLHCenergies
65
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers66
ExtraSlides
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
SearchforJetQuenchinginthe1990‘s:S+SandPb+Pbat√sNN≈20GeVattheCERNSPS
67
X.N. Wang, PRL 98, 2655, 1998
• pQCD+Croninenhancementdescribesdata:apparentlynopartonenergyloss
• Conclusions‣ NoQGPatCERNSPS
(orveryshort‐lived)
‣ Nopar*clesuppressionathighpTduetointerac*onsinthehadronicphase(advantageousforRHIC!)
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
√sNNDependence:π0RAAforHeavyNucleiat√sNN=17.3,62.4,and200GeV
68
Onsetofsuppressionbetween√sNN=~20GeVund62.4GeV
central collisions
CERNSPSdata:WA98experiment,Phys.Rev.Lel.100:242301,2008
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
Direct‐γ‐TriggeredAway‐sideCorrela5ons:JetFragmenta5onFunc5oninp+pandAu+Au
• Nosurfacebiasforγastriggerpar*cle
• Jetenergyknown:Ejet≈Eγ
• Effec*vefragmenta*onfunc*onD(zT)canbemeasured:
69
zT =
pTh
pTγ
D(zT ) = 1
N trig
dN (zT )dzT
• DifferentslopeofD(zT)inp+pandAu+Aureflectsinfluenceofthemedium
• Datawithhigherprecisionneededtodrawquan*ta*veconclusions
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
JetsinPb+PbattheLHC
JetswithET>~50GeVinPb+Pbat√s=5500GeVattheLHCcanbeiden*fiedabovethebackgroundonanevent‐by‐eventbasis
ALICEsimula5on
Conesizeforjetreconstruc5on
background
InfluenceofbackgroundfromtheunderlyingeventminimizedwithconesizeR~0.3–0.4
70
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
FirstResultsfromFullJetReconstruc5onatRHIC:JetRAAinAu+Auat200GeVfromSTAR
• EvidenceforjetbroadeninginAu+AuatRHIC
• Situa*onmuchmorefavorableattheLHC(increasedjetcrosssec*on)71
STAR, arXiv0908.1799v3
Obergurgl 2010 - QGP and High-pT Physics Klaus Reygers
FirstResultsfromFullJetReconstruc5onatRHIC:JetRAAinCu+Cuat200GeVfromPHENIX
• PHENIXhasdevelopedacone‐likealgorithm(Gaussianfilter)wheretheconeradiusisreplacedbyaGaussianwidthσ
• JetsuppressionobservedincentralCu+Cuat200GeVforσ=0.3
72
Y.-s. Lai (PHENIX collaboration), arXiv:0911.3399