Direct observation of dijets in Direct observation of dijets in central Au+Au collisions at RHIC central Au+Au collisions at RHIC Dan Magestro, The Ohio State University
for the STAR Collaboration
Quark Matter 2005
• Jets and azimuthal correlationsJets and azimuthal correlations
• Raising the pRaising the pTT scale scale
• Dijet tomography of QCD matterDijet tomography of QCD matter
• Modified picture of jet suppressionModified picture of jet suppression??
August 5, 2005
QM 2005 Budapest Dan Magestro, STAR 2
The key theoretical questionThe key theoretical question
What is the nature of high-pWhat is the nature of high-pTT suppression? suppression?
• How is energy loss reflected in away-side How is energy loss reflected in away-side suppression?suppression?
• What is the What is the relationshiprelationship between jet between jet “associated” yields and inclusive spectra?“associated” yields and inclusive spectra?
• Is there really a surface bias? Is there Is there really a surface bias? Is there near-near-sideside energy loss? energy loss?
• Can data distinguish between Can data distinguish between different different energy loss scenariosenergy loss scenarios? ? (e.g. collisional vs. radiative)(e.g. collisional vs. radiative)
• What is the What is the path length dependencepath length dependence of of energy loss?energy loss?
• ..........
Drees et al, PRC 71 (2005) 034909
QM 2005 Budapest Dan Magestro, STAR 3
Evolution of Evolution of correlations at RHIC correlations at RHIC
Trigger
• ““Trigger-associated” technique valuable for tagging jets Trigger-associated” technique valuable for tagging jets in high-multiplicity environment (vs. jet-cone algorithms)in high-multiplicity environment (vs. jet-cone algorithms)
• Probes the jet’s interaction with the QCD mediumProbes the jet’s interaction with the QCD medium
• Provides stringent test of energy-loss modelsProvides stringent test of energy-loss models
correlationscorrelations Associated
Associated
Higher pHigher pTT →→ Away-side suppression Away-side suppression
Pedestal&flow subtracted
STAR, PRL 91 (2003) 072304 STAR, nucl-ex/0501016
Lower pLower pTT →→ Away-side enhancement Away-side enhancementpT(assoc) > 2 GeV/c pT(assoc) > 0.15 GeV/c
4 < pT(trig) < 6 GeV/c
QM 2005 Budapest Dan Magestro, STAR 4
Probing fragmentation region w/ pProbing fragmentation region w/ pTT
• Successful RHIC Run 4 (2004) Successful RHIC Run 4 (2004) → → ~10x more central, minbias Au+Au~10x more central, minbias Au+Au
• STAR event reconstruction ongoingSTAR event reconstruction ongoing
• Raising pRaising pTT thresholds allows to: thresholds allows to:
• Extend into fragmentation regionExtend into fragmentation region
• Significantly reduce uncorrelated Significantly reduce uncorrelated background, relative vbackground, relative v22 contribution contribution
• Look for leading away-side hadrons Look for leading away-side hadrons
• How does How does correlation evolve with increasing p correlation evolve with increasing pTT??
• Does away-side jet “punch through”?Does away-side jet “punch through”?
• Is the fragmentation modified? If so, how?Is the fragmentation modified? If so, how?
• Does away-side suppression disappear at very high pDoes away-side suppression disappear at very high pTT??
~pQCD
??
QM 2005 Budapest Dan Magestro, STAR 5
Emergence of dijets with increasing Emergence of dijets with increasing ppTT(trig)(trig)
Au+Au, 0-5%
correlations (not background subtracted)correlations (not background subtracted)
• Hint of narrow back-to-back peak for higher pHint of narrow back-to-back peak for higher pTT(trig)(trig)
• Higher pHigher pTT(trig) reflects higher-Q(trig) reflects higher-Q22 hard scattering hard scattering
pT(trig)
pT(assoc) > 2 GeV/c
preliminary
QM 2005 Budapest Dan Magestro, STAR 6
Emergence of dijets w/ increasing Emergence of dijets w/ increasing ppTT(assoc)(assoc)
correlations (not background subtracted)correlations (not background subtracted)
• Narrow peak emerges cleanly above vanishing backgroundNarrow peak emerges cleanly above vanishing background
8 < pT(trig) < 15 GeV/c
pT(assoc) > 2 GeV/cpT(assoc) > 3 GeV/cpT(assoc) > 4 GeV/cpT(assoc) > 5 GeV/cpT(assoc) > 6 GeV/cpT(assoc) > 7 GeV/cpT(assoc) > 8 GeV/c
QM 2005 Budapest Dan Magestro, STAR 7
Centrality dependence of near, away-side yieldCentrality dependence of near, away-side yield
• Near-side yields consistent within errorsNear-side yields consistent within errors
• Away-side yields decrease monotically with increasing NAway-side yields decrease monotically with increasing NPartPart
• Suppression pattern similar for two pSuppression pattern similar for two pTT(assoc) ranges!(assoc) ranges!
Fit scaledby x2
8 < pT(trig) < 15 GeV/c
QM 2005 Budapest Dan Magestro, STAR 8
Initial physics thought on dijets Initial physics thought on dijets
• Are we seeing “tangential” dijets?Are we seeing “tangential” dijets?
Red lines determined from constraining ratio of b+c to total to measured RAA
C. Loizides: Poster #78 (Thur-Fri)
a
b
c
• Hard scattering takes place at Hard scattering takes place at surface, with neither parton surface, with neither parton traversing mattertraversing matter
• Dainese Dainese et alet al: use inclusive : use inclusive suppression to estimate probability suppression to estimate probability for parton(s) to escapefor parton(s) to escape
• From fit to RFrom fit to RAAAA, probability is vanishingly small for both partons , probability is vanishingly small for both partons
to escapeto escape
L1 L2
L1 L2
L1 L2
A. Dainese et al, EJP 38 (2005) 461
QM 2005 Budapest Dan Magestro, STAR 9
Hadron-triggered fragmentation functionsHadron-triggered fragmentation functions
• Xin-Nian Wang: Modified di-hadron Xin-Nian Wang: Modified di-hadron fragmentation functionfragmentation function
• Quantify energy loss in terms of away-Quantify energy loss in terms of away-side particle production per trigger side particle production per trigger particleparticle
• Without knowledge of Without knowledge of parton energyparton energy, , DDhhhh(z(zTT) is next best thing) is next best thing
zT = pT(assoc)/pT(trig)
pT(trig)
pT(assoc)
QM 2005 Budapest Dan Magestro, STAR 10
Hadron-triggered fragmentation functionsHadron-triggered fragmentation functions
• Away-side D(zAway-side D(zTT) suppressed, but shape unchanged) suppressed, but shape unchanged
~0.54
~0.25
Scalingfactors
8 < pT(trig) < 15 GeV/c
QM 2005 Budapest Dan Magestro, STAR 11
Dijet assoc. yields (IDijet assoc. yields (IAAAA) vs. R) vs. RAAAA
STAR, Phys. Rev. Lett. 91 (2003) 072304
• Near-side yields consistent with unityNear-side yields consistent with unity
• Away-side associated yields similar to RAway-side associated yields similar to RAAAA values values
IAA = Yield(0-5% Au+Au) Yield(d+Au)
Note: Very different quantities are being compared
= Near-side IAA
= Away-side IAA
8 < pT(trig) < 15 GeV/c
QM 2005 Budapest Dan Magestro, STAR 12
Confronting predictions - 1Confronting predictions - 1
Majumder Majumder et al:et al: Near-side energy lossNear-side energy loss affects trigger bias affects trigger bias
• Trigger carries smaller <z> of higher-E parton Trigger carries smaller <z> of higher-E parton →→ more energy for more energy for associated production, Iassociated production, IAAAA >> 1 as p >> 1 as pTT(assoc) increases(assoc) increases
? X-N Wang: X-N Wang: Away-side fragmentationAway-side fragmentation altered relative to p+p altered relative to p+p
• Drops steeply below zDrops steeply below zTT~0.4, roughly constant above z~0.4, roughly constant above zTT~0.5~0.5
Majumder, Wang, Wang, nucl-th/0412061 X-N Wang, PLB 595 (2004) 165
• Majumder Majumder et alet al: Near-side energy loss affects trigger bias: Near-side energy loss affects trigger bias
• Trigger carries smaller <z> of higher-E parton Trigger carries smaller <z> of higher-E parton →→ more energy for more energy for associated production, Iassociated production, IAAAA >> 1 as p >> 1 as pTT(assoc) increases(assoc) increases
• X-N Wang: Away-side fragmentation altered relative to p+pX-N Wang: Away-side fragmentation altered relative to p+p
• Drops steeply below zDrops steeply below zTT~0.4, roughly constant above z~0.4, roughly constant above zTT~0.5~0.5
= STAR preliminary = STAR preliminary
No evidence for near-side E-loss; away-side data more suppressed
QM 2005 Budapest Dan Magestro, STAR 13
Similarity of away-side widthsSimilarity of away-side widths
• Widths similarWidths similar
• No evidence for away-side No evidence for away-side broadening that is broadening that is observed at lower pobserved at lower pTT
8 < pT(trig) < 15 GeV/c4 < pT(trig) < 6 GeV/cpT(assoc) > 1.5 GeV/c
4 < pT(trig) < 6 GeV/cpT(assoc) > 1.5 GeV/c
J. Ulery, STAR (Parallel 3c)
QM 2005 Budapest Dan Magestro, STAR 14
Confronting predictions - 2Confronting predictions - 2
• Vitev: Away-side broadening due to gluon radiation, to high pVitev: Away-side broadening due to gluon radiation, to high pTT
• Broadening due (entirely?!) to radiated gluonsBroadening due (entirely?!) to radiated gluons
Vitev, hep-ph/0501225
Calculation with realistic path-length distribution needed
However, data show no significant broadening (unlike lower-pHowever, data show no significant broadening (unlike lower-pTT))
A. Dainese et al, EJP 38 (2005) 461
QM 2005 Budapest Dan Magestro, STAR 15
Combining the observationsCombining the observations
Dijets more suppressed from Dijets more suppressed from d+Au to central collisionsd+Au to central collisions
Away-side fragmentation Away-side fragmentation pattern unchangedpattern unchanged
Away-side widths similar for Away-side widths similar for central, noncentralcentral, noncentral
QM 2005 Budapest Dan Magestro, STAR 16
vacuum fragmentation
radiated gluon(s)fragmentation
parent partonfragmentation
Theoretical basis for noninteracting dijetsTheoretical basis for noninteracting dijets
PRC 71 (2005) 014903
• E-loss models contain noninteracting termE-loss models contain noninteracting term
1)1) Wang, Wang: Modified fragmentation functionsWang, Wang: Modified fragmentation functions
2)2) Salgado, Wiedemann: Discrete term in Salgado, Wiedemann: Discrete term in quenching probabilityquenching probability PRL 89 (2002) 092303
no medium-induced energy loss
medium-induced energy loss
• First observation of First observation of unmodifiedunmodified fragmentation? fragmentation?
QM 2005 Budapest Dan Magestro, STAR 17
The beginnings of dijet tomographyThe beginnings of dijet tomography
8 < pT(trig) < 15 GeV/c6 < pT(trig) < 8 GeV/c
preliminary preliminary
• What will they teach us about energy loss?What will they teach us about energy loss?
• What is the What is the naturenature of jet quenching? of jet quenching?
• What are the properties of induced What are the properties of induced radiationradiation??
• Does Does collisional energy losscollisional energy loss need to be reconsidered? need to be reconsidered?
• These are the first precision measurements of dijets in the These are the first precision measurements of dijets in the fragmentation regime in Au+Au collisions at RHICfragmentation regime in Au+Au collisions at RHIC
• Dijet suppression driven by Dijet suppression driven by geometrygeometry (system, system size) (system, system size)
• Near-side jet fragmentation in Au+Au Near-side jet fragmentation in Au+Au consistentconsistent with p+p with p+p
• Away-side fragmentation Away-side fragmentation suppressedsuppressed; shape, peak widths ; shape, peak widths unchangedunchanged
• Away-side IAway-side IAAAA quantitatively consistent with R quantitatively consistent with RAAAA
• What will they teach us about the medium?What will they teach us about the medium?
• Mechanisms leading to partonic equilibration?Mechanisms leading to partonic equilibration?
• Determine significant Determine significant upper boundupper bound on the gluon density? on the gluon density?• Idea: Constrain degrees of freedom? Idea: Constrain degrees of freedom? Müller, Rajagopal, hep-ph/0502174
18
Potential back-up slides Potential back-up slides
QM 2005 Budapest Dan Magestro, STAR 19
An upper bound on energy density?An upper bound on energy density?
• MMüller and Rajagopal: From entropy and jet quenching to üller and Rajagopal: From entropy and jet quenching to deconfinement deconfinement hep-ph/0502174
• The number of degrees of freedom can be determined independently of The number of degrees of freedom can be determined independently of direct measurements of temperaturedirect measurements of temperature
Energy density:
Entropy:
Degrees of freedom:
• A lower bound on A lower bound on (T) is constrained by an upper bound on energy (T) is constrained by an upper bound on energy densitydensity
QM 2005 Budapest Dan Magestro, STAR 20
Reminder: charge ordering in b2b paper Reminder: charge ordering in b2b paper
• Charge ordering showed jetlike nature of near-side correlationCharge ordering showed jetlike nature of near-side correlation
• Comparison to Pythia showed agreementComparison to Pythia showed agreement
STAR, PRL 90 (2003) 082302
QM 2005 Budapest Dan Magestro, STAR 21
Dijets: Dijets: Like vs. unlike sign (first look)Like vs. unlike sign (first look)
• Strong near-side charge ordering; away-sides consistentStrong near-side charge ordering; away-sides consistent
• Comparison to Pythia Comparison to Pythia → generic information about parton?→ generic information about parton?
Like signUnlike sign
3 < pT(assoc) < 4 4 < pT(assoc) < 5
5 < pT(assoc) < 6 pT(assoc) > 6
preliminary
8 < pT(trig) < 12 GeV/c
QM 2005 Budapest Dan Magestro, STAR 22
Hadron-triggered F.F. vs. PythiaHadron-triggered F.F. vs. Pythia
• Pythia agrees in magnitude with d+Au on away-sidePythia agrees in magnitude with d+Au on away-side
0.54
0.25
Scalingfactors
preliminary
QM 2005 Budapest Dan Magestro, STAR 23
correlations at 62 GeVcorrelations at 62 GeV
• High-pHigh-pTT rate reduced rate reduced →→ far fewer pairs in dataset far fewer pairs in dataset
• Away-side peak emerges in central events, Away-side peak emerges in central events, notnot present for similar trig/assoc present for similar trig/assoc thresholds at 200 GeV !thresholds at 200 GeV !
• Weak near-side peakWeak near-side peak due to harder fragmentation due to harder fragmentation
preliminary
QM 2005 Budapest Dan Magestro, STAR 24
PRL 90 (2003) 082302
correlations: a solid foundationcorrelations: a solid foundation
• Suppression of back-to-back Suppression of back-to-back correlations in AAcorrelations in AA
• Away-side correlation weakens Away-side correlation weakens with increasing centralitywith increasing centrality
STAR, PRL 90 (2003) 082302
Pedestal&flow subtracted
STAR, PRL 91 (2003) 072304
• Suppression is not an Suppression is not an initial-state effectinitial-state effect
• d+Au: Away-side jet alive and d+Au: Away-side jet alive and well, similar yield as p+pwell, similar yield as p+p
• Fits into jet-Fits into jet-quenching picturequenching picture
STAR, PRL 93 (2004) 252301
• Amount of suppression Amount of suppression sensitive to geometrysensitive to geometry
• Path length dependence Path length dependence of energy loss of energy loss →→jet quenchingjet quenching
• Away-side spectra softer in Away-side spectra softer in central Au+Au central Au+Au
• Larger away-side Larger away-side ppTT, N, Nchch
• Softer particle spectra on away-Softer particle spectra on away-side side → equilibration with medium→ equilibration with medium
STAR, nucl-ex/0501016, sub. to PRL