Direct observation of dijets in central Au+Au collisions at RHIC

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


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


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

??


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


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


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


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



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


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)


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



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



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


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)


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


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


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?


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


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


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


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



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


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


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

Documents

Direct observation of dijets in central Au+Au collisions at RHIC