IPN-Orsay, Nov 14th, 2005 David d'Enterria (LLR, Paris) 1/39 Jet quenching au RHIC: rsultats...

IPN-Orsay, Nov 14th, 2005 David d'Enterria (LLR, Paris)1/39

Jet quenching au RHIC:Jet quenching au RHIC:résultats expérimentauxrésultats expérimentaux

Journée thématique “Jet quenching”IPN-Orsay, November 14th, 2005

David d’EnterriaLLR – Ecole Polytechnique, IN2P3, Paris


OverviewOverview “Jet physics” results @ RHIC (w/o full jet reco): (1) Inclusive high pT spectra, (2) di-hadron φ,η high-pT triggered correlations.

confronted to non-Abelian radiative energy loss in QGP “paradigm”.

High pT (leading) hadron suppression data in central AA: 1. Magnitude ÞVery dense medium: dNg/dy ~ 1000 (~ dNch/dη). OK 2. Transverse momentum dependence: flat pT . OK 3. Centrality dependence. OK 4. Light-meson species dependence (π0 vs. η). OK 5. Center-of-mass energy dependence (SPS-20 GeV, RHIC-62,200 GeV). OK 6. Non-Abelian radiation. OK 7. Path-length dependence. OK ? 8. System-size (CuCu vs. AuAu) dependence. OK ? 9. Heavy vs. light quark suppression. OK ?

Modified high pT di-hadron (φ,η) correlations in central AA: 1. Disappeareance of away-side azimuthal dijet peak. OK 2. Enhanced (“volcano”-like) away-side associated yield at lower pT. (?) 3. Broadening of near-side pseudo-rapidity correlations. OK

IPN-Orsay, Nov 14th, 2005 David d'Enterria (LLR, Paris)

3/36

“Jet quenching” as a QGP signal

Multiple final-state non-Abelian (gluon) radiation off the produced hard parton induced by the dense QCD medium

Parton energy loss µmedium properties:

Flavor dependent energy losses:

Energy is carried away by gluons emittted inside (broader) jet cone:

“gluonsstrahlung”

Prediction I: Suppression of high pT leading hadrons: dN/dpT

Prediction II: Modification of (di)jet correlations: d2Npair/dφdη Predition III: Modified energy- & particle- flow within full jet

dE/dx ~ s k2T

DEloss(g) > DEloss(q) > DEloss(Q) (color factor) (mass effect)

SPS,RHIC,LHC

RHIC,LHC

LHC

µ(g density, L)

µ (q coeffic., L2)

GLV:

BDMPSWiedemann: ˆ


Jet physics at RHIC: experimental limitsJet physics at RHIC: experimental limits

Full jet reconstruction w/ standard algorithms is unpractical at RHIC due to huge soft background (large “underlying event”):

Feasible at LHC for Ejet >~ 50 GeV

p+p ®jet+jet [Ös = 200 GeV] Au+Au ® X [ÖsNN = 200 GeV]


““Jet physics” at RHIC (I): Single inclusive high pJet physics” at RHIC (I): Single inclusive high pTT spectra spectra

Alternative I : Study the energy modifications suffered by the highest pT hadron in the event (“leading” hadron of the jet) in AA (compared to pp):

Many interesting results obtained from this “first-order” approach !

p+p ® h+ X [Ös = 200 GeV] Au+Au ® h+X [ÖsNN = 200 GeV]


““Jet physics” at RHIC (II): Di-hadron azimuthal correlationsJet physics” at RHIC (II): Di-hadron azimuthal correlations

Alternative II : Study the azimuthal correlations in AA relative to pp between highest pT hadron (“trigger”) & any other “associated” hadron:

Many interesting results obtained from this “2nd-order” approach !

p+p ® h1+h2+ X [Ös = 200 GeV] Au+Au ® h1+h2+ X [ÖsNN = 200 GeV]


““Jet physics” at RHIC (II): Di-hadron azimuthal correlationsJet physics” at RHIC (II): Di-hadron azimuthal correlations

Alternative II : Study the azimuthal correlations in AA relative to pp between highest pT hadron (“trigger”) & any other “associated” hadron:

Many interesting results also obtained from this “2nd-order” approach !




High pT leading hadron spectra at RHIC & jet-quenching models:

Good agreement data theory


Inclusive single spectra at high pT (AA, dA, pp)

High quality large-pT data (up to ~20 GeV/c) available in pp, dA and AA collisions:

Au+Au – 200 GeV

d+Au – 200 GeV

p+p – 200 GeV


Leading hadron spectra in free space: pp Leading hadron spectra in free space: pp @ @ 200 GeV200 GeVHigh pT p0,h± spectra up to ~15 GeV/c. Good theoret. (NLO pQCD) description

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.PRL 91, 241803hep-ex/0304038

High quality data: sensitive to different parametrizations of gluon FF

p+p p0 X p+p h± X

Well calibrated (experimentally & theoret.) p+p baseline spectra at hand.


Leading hadron spectra in AuAuLeading hadron spectra in AuAu @ 200 GeV@ 200 GeV

Au+Au p0 X (peripheral) Au+Au p0 X (central)

Peripheral data agree well with Strong suppression in p+p (data & pQCD) plus “Ncoll-scaling” central Au+Au collisions !


Suppressed high pSuppressed high pTT hadroproduction in central AuAu hadroproduction in central AuAu

Discovery of

high pT suppression(one of most significant results @ RHIC so far)

Ncoll scaling(“hard” production)

x5 suppression

Npart scaling (surface emission)

Very strong suppression (RAA ~ 0.2 !) up to pT ~ 10 GeV/c for π0 h±, well below incoherent scattering expectations for hard cross-sections

PHENIX Collab.PRL 88, 022301 (2002)nucl-ex/0109003[Run-2 data]


13/36

Hadrons are suppressed. Photons are not.Hadrons are suppressed. Photons are not.

Colorless hard probes (direct g insensitive to final-state) are unsuppressed.

Confirms that AuAu collision = incoherent sum of pp collisions (i.e. “Ncoll scaling” expectation is valid) for perturbative probes.

D.d'E, HP'04EJP C 43 (2005)295

PHENIX.PRL 94, 232301


14/36

Hadrons are suppressed in AuAu. Not in dAu.Hadrons are suppressed in AuAu. Not in dAu.

Initial-state cold nuclear matter effects (shadowing, Cronin) are small at RHIC mid-rapidity.

High pT suppression in central AuAu is due to final-state effects (absent in “control” dAu experiment)

D.d'E.,nucl-ex/0401001

PHENIX.PRL 91, 072303 (2003)


High pHigh pT T suppression: “Universal” for all light mesonssuppression: “Universal” for all light mesons

Common suppression pattern (magnitude, pT, centrality, ... dependence) for p0 and :

Same flat RAA ~ 0.2 up to 10 GeV/c

Universal suppression for light mesons indicates it is at partonic level prior to q,g fragmentation into leading meson according to vacuum FFs.


Magnitude of the suppression: medium propertiesMagnitude of the suppression: medium propertiesData vs. models (pQCD+ non-Abelian parton energy loss) comparison:

Initial gluon densities:

Opacities: <n> = L/ ≈ 3 – 4 [Levai et al.]

Transport coefficients:

Medium-induced radiative energy losses:

dE/dx ≈ 0.25 GeV/fm (expanding) dE/dx|eff ≈ 14 GeV/fm (static source) [X.N.Wang]

Very large gluon densities: dNg/dy~1000 consistent w/ measured dNch/dη ~700All medium properties imply energy densities >> ecrit QCD (assuming thermalizat.)

[BDMPS ] [Salgado-Wiedemann] [Dainese, Loizides ...] [Arleo]

dNg/dy ~ 1000 [Vitev & Gyulassy]

<q 0> ~ 14 GeV2/fm


High pHigh pT T suppression: psuppression: pTT-dependence-dependence

Flat pT- dependence described by parton energy loss models:

Underlying LPM interference for single gluon bremsstrahlung would give: DEloss ~ log(pT)

Combination of different effects (convolution w/ realistic gluon energy distribution, local parton pT slope, ...) yields constant suppression factor.

PQM – A. Dainese, C. Loizides, G. PaicEPJ C 38, 461(2005)

GLV – I. Vitev 2005


High pHigh pT T suppression: suppression: ÖÖs-dependences-dependence

Ös- dependence in agreement with parton energy loss in increasingly dense (expanding) medium:

D.d'E., HP'04EJP C 43 (2005)295

RAA ~ 1 @ Ös ~ 20 GeV Þ dNg/dy ~ 400 , <q0> ~ 3.5 GeV/fm2

RAA ~ 0.3 @ Ös = 62 GeV Þ dNg/dy ~ 650 , <q0> ~ 7 GeV/fm2

RAA ~ 0.2 @ Ös = 200 GeV Þ dNg/dy ~ 1100 , <q0> ~ 14 GeV/fm2

[Note: RAA @ SPS uses “revised” pp ref.]

Initial gluon density: Medium transport coeff.:

SPSRHICRHIC


High pHigh pTT meson suppression in AA meson suppression in AA @@ 17.3 GeV ? 17.3 GeV ?Revised pp reference: high pT π

0 production in (0-10%) central PbPb at SPS is slighted suppressed or consistent w/ “Ncoll-scaling” :

D.d'E. PLB 596, 32 (2004)

NA57, PLB62, 317 (2005)

Confirmed by NA57 (& NA49) recent high pT results in central PbPb at SPS:

NA49 (A. Lazslo, QM'05)


High pHigh pT T suppression: non-Abelian naturesuppression: non-Abelian nature

Excitation function (Ös-dependence) & non-Abelian nature of energy loss in agreement w/ parton energy loss calculations: (i) rising initial parton density with Ös (ii) increasing relative fraction of hard-scattered gluons (at fixed pT) with Ös

“Jet quenching” model + 2-D longitudinal plasma expansion

D.d'E. EJP C 43 (2005)295

Gluon: CA = Nc = 3Quark: CF =(Nc

2-1)/2Nc = 4/3

Relative fraction of q,g at pT = 5 GeV/c:

QCD radiation probability:

CA /CF=2.25}


High pHigh pT T suppression: centrality dependencesuppression: centrality dependence

Increasing centrality (Npart) Þ increased L, r Þ increased suppression

Agreement data models as expected for diff. suppressions at different (geometrical) parton production points.

PQM – A. Dainese, C. Loizides, G. PaicEPJ C 38, 461(2005)X.N. Wang, PLB 2003


High pT leading hadron spectra at RHIC & jet-quenching models:

Less good agreement data theory ?


High pHigh pT T suppression: ssuppression: system-size dependenceystem-size dependence

Smaller CuCu system adds significant precision at intermediate Npart~100:Theory predicts: ln(RAA) µNpart

-2/3

Both PHENIX & STAR preliminary data seem to exclude = -2/3Fit to STAR Npart

prefers ~ -1/3 (circumf./area ~ A-1/3 ~ Npart-1/3 ?)

PHENIX data seems to indicate a “steeper” slope at low Npart.Differences STAR PHENIX and PQM GLV still unclear at this point.

M. van Leeuwen, QM'05

C.Klein-Boesing, QM'05


High pHigh pT T suppression: path-length dependencesuppression: path-length dependence

2 times more suppression out-of-plane (“long” direction)

than in-plane (“short” direction).

Glauber parton energy loss model predicts only ~50% increased “out-of-plane” vs “in-plane” π0 emission

Azimuthal anisotropy stronger than “canonical” L2 (or L) path-length dependence.Source of extra azim. anisotropy above pT ~ 4 GeV/c ?

D = 0°

D = 90°

PRELIMINARY[B. Cole, S. Mioduszewski HP'04][D.d'E, EJP C 43 (2005) 295]

RAA vs φ w/ respect to reaction plane :

PQM – Dainese, Loizides, Paic EPJ C 38, 461(2005)


Heavy quark suppression viaHeavy quark suppression via non-photonic electrons (I)non-photonic electrons (I)

Semi-leptonic decays of open charm and bottom mesons = main source of high pT (“non-photonic”) electrons.

proton-proton baseline: Au+Au suppression

(Note: state-of-the-art theory underpredicts data)


Heavy quark suppression viaHeavy quark suppression via non-photonic electrons (II)non-photonic electrons (II)

Latest single e± RAA indicates large suppression in central AuAu:

Note: STAR – PHENIX RAA agrees, but the pp refs are different by ~ 50%.

Peripheral AuAu

Central AuAu


Heavy quark suppression viaHeavy quark suppression via non-photonic electrons (III)non-photonic electrons (III)

Theory expectations :

Quantitative predictions:

Models need larger medium densities: dNg/dy=3500, <q> = 14 GeV2/fm than for light mesons RAA to reproduce data ! Unclear consistency w/ dNch/dy ~ 600

M. Djordjevic et al., nucl-th/0507019

Armesto, Salgado, Wiedemann, QM'05

• Charm RAA = 0.2 - 0.3• Beauty RAA = 0.4 - 0.6

DEloss(g) > DEloss(q) > DEloss(Q) (color factor) (mass effect)

22QQ

2 ])/([1Em

Gluonsstrahlung probability

“Dead cone”: g rad. suppressed at < mQ/EQ


Heavy quark suppression viaHeavy quark suppression via non-photonic electrons (IV)non-photonic electrons (IV)

Models need too dense medium to account for observed suppression in data:

Possible resolutions of the disagreement (or a combination of them ?):(1) Larger suppression of beauty …or charm dominance up to electron pT ≈ 10 GeV?(2) Extra gluon-fragmentation production of charm affected by energy loss ? (would also

explain PHENIX proton-proton data ?)

(3) Hadronic (+ partonic) energy loss ?(4) Radiative + collisional energy loss ? Other ... ?


High pT di-hadron φ,η correlations in high-energy AuAu collisions


““Jet physics” at RHIC (II): di-hadron azimuthal correlationsJet physics” at RHIC (II): di-hadron azimuthal correlations

Study the azimuthal correlations in AA relative to pp between the highest pT hadron (“trigger”) & any other “associated” hadron:




3-5 GeV/c d+Au

Near-side

away-side

Dijets via high pDijets via high pTT di-hadron di-hadron φφ correlations: pp, dAu correlations: pp, dAu

Two-particle correlations: h±- – h±, p0,± – h±. Trigger: highest pT (leading) hadron.

Associated D distribution (e.g. ”assorted”: 2 GeV/c < pTassoc < pT

trigger)

Normalized to number of triggers:

d+Au (cent, MB) p+p

2-3 GeV/c

p+p

Preliminary d+Au

p+p

pTassoc = 2-3 GeV/c pT

assoc = 3-5 GeV/c

Clear near- (D ~ 0) and away- (D ~ p) side jet signals


Dijets via high pDijets via high pTT di-hadron di-hadron φφ correlations: AuAu correlations: AuAu

Same dNpair/dφ analysis as in pp (dAu) but 2 extra “complications”: (1) Increased “underlying event” background (2) Collective elliptic flow (harmonic) contribution

Ajitanand, ICPAQGP'04and nucl-ex/0501025

Delicate subtraction procedure (esp. in finite acceptances).


Di-hadron AuAu Di-hadron AuAu ΔφΔφ correlations: Results at high p correlations: Results at high pTT

Centrality dependence of away-side disappearance globally described by parton energy loss models (increasing medium traversed):

STAR, PRL 90, 082302 (2003) pT trigg = 4 – 6 GeV/cpT assoc > 2 GeV/c

Away-side peak disappearance: “monojet”-like topologies in central AuAu.

Near-side jet-like Gaussian peak unmodified (AuAu ~ dAu ~ pp)

PQM – A. Dainese, C. Loizides, G. PaicEPJ C 38, 461(2005)


Di-hadron AuAu Di-hadron AuAu ΔφΔφ correlations: Results at lower p correlations: Results at lower pTT

W.Holtzman, RIKEN-Corrs. Workshop '05

“Lost” away-side energy is recovered at lower pT values. Strongly modified away-side D correlations in central AuAu:

“peak”“dip”

Std. away-side Gaussian peak

Preliminary

Away-side (p) “dip” and excess of energy (“double peak”) at: PHENIX: p 1.3 rad STAR: p 1.1 rad

Au+Au periph

Au+Au central

PHENIX, PRLsubmitted, nucl-ex/0507004

“peaks”

pT trigg = 4 – 6 GeV/cpT assoc = 0.15 – 4 GeV/c

STAR, PRL95,152301(05)nucl-ex/0501016


33.0)(1

sf

avs cdc

““Double peak” = Mach wave cone ?Double peak” = Mach wave cone ?

Double peak structure at at p 1.2 rad reminiscent of Mach wave conical shock (“sonic boom”) Þ speed of sound accessible

Note: gluon Cerenkov-like emission also proposed [access to medium index refrac. n=1/cos(c)]

Caveats:

cos M sc

M Trigger

Stoecker, Satarov, Mishutin, hep-ph/0505245.Casalderrey, Shuryak, Teaney, hep-ph/0411315.

Mach cone:

= arccos(cavs) = 1.2 rad = 71º

c2s ¹ constant:

QGP (1/ Ö3) ® phase transition (0.) ® HRG (Ö0.2) :

~ exp


d+Au, 40-100%

Au+Au, 0-5%

STAR preliminary

3 < pT(trig) < 6 GeV2 < pT(assoc) < pT(trig)[D. Magestro, HP'04]

Di-hadron Di-hadron DD correlations: AuAu, dAu, pp (I) correlations: AuAu, dAu, pp (I)

Significant broadening of near-side pseudorapidity correlations in AuAu compared to pp, dAu. (“stretching” of jet cone along η).

STAR preliminary p+p

Au+Au, 0-5%

near-side width


d+Au, 40-100%

Au+Au, 0-5%

STAR preliminary

3 < pT(trig) < 6 GeV2 < pT(assoc) < pT(trig)

Armesto, Salgado, WiedemannPRL 93, 242301 (2004)

Di-hadron Di-hadron DD correlations: AuAu, dAu, pp (II) correlations: AuAu, dAu, pp (II)

Significant broadening of near-side pseudorapidity correlations in AuAu compared to pp,dAu. (“stretching” of jet cone along η).

Coupling of g radiation w/ longitudinal expanding medium ?


““Cartoon” Summary: Jet-quenching at RHIC Cartoon” Summary: Jet-quenching at RHIC

Jet profile in pp (dAu) collisions: Jet profile in AuAu central collisions:

D

D ° D

D °

Near-side width: <jT> ~ 600 MeV/cunmodified in pp,dAu

Away-side width and acoplanarityunmodified in pp and dAu

Factor ~5 suppression of leading hadron(very large initial parton densities: dNg/dy~1000)

Disappearance of back-to-back peak (“monojets”)“Double peak” structure at lower pT in away-side(“sonic boom” in medium ?)

Dijet broadening in η (coupling of g radiation w/ expanding medium ?)

Strong QCD medium effects at work !

“QCD vacuum” & “cold QCD medium” “hot & dense QCD Medium”


SummarySummary

Large amount of precision high-pT hadron production data after 5 years of operation at RHIC allows to quantitatively address jet physics in QCD

medium (w/o full jet reco).

Differential observ. of suppressed hadro-production in central Au+Au provide: - stringent constraints on underlying quenching mechanism. - direct access to the density & transport properties of QCD medium.

Are “jet quenching” data due to radiative energy loss in a QGP ?

Good agreement with calculations on: Leading hadrons: Magnitude, √s, pT, centrality, (light) species dependence Dihadron correlations: disappeareance of away-side azimuthal dijet peak,

Broadening of near-side pseudo-rapidity correlations. Some tests are weak at present:

Few details missing in system-size dependence No sharp test of L2 dependence yet. Heavy quark energy loss larger than expected

New insights of medium properties from new observables (cs via “Mach cone”) ?LHC = enormous reach, qualitatively new observ. (full jet reco, jet-jet, jet-g,Z ...)


backup slides ...


Di-jet properties: experimental observablesDi-jet properties: experimental observables

Spread of hadrons around jet axis, relative orientation of the 2 jets: jT, kT, pout

Multiplicity of assoc. hadrons (area under peaks): “fragmentation function” D(z)

hadron

hadron

Δφ

Jet “width” jT

Di-jet acoplanarity kT

: total of parton pairTy Tk p

Tk : of hadron to jet axisTy Tj pTj

parton

parton

near-side away-sideD

nearfar

µTy nearj

2 2µ -T fary neark

[see e.g. J.Jia, nucl-ex/0409024]

D(z), z=p/ptot “Fragmentation function”


““Fragmentation functions”: Central AuAuFragmentation functions”: Central AuAu

syst. errorAway side:

softening of spectra from pp to central Au+Au

energy from initial parton converted to

low pT particles.

energy loss inmedium!

Associated (pT assoc = 0.15 – 4 GeV/c) near- and away- side hadron pT spectra:

Associated near-side jet yields overall enhanced (enhanced underlying evt.)Associated away-side jet yields “shifted down” in pT: spectra closer to pure “soft” inclusive hadron production (“thermalized”)

Near-side Away-side

pT trigg = 4 – 6 GeV/cpT assoc = 0.15 – 4 GeV/c

STAR, submitted to PRLnucl-ex/0501016


Jet production in AA : (a few) theoretical expectationsJet production in AA : (a few) theoretical expectations

Mono-jets:Jet broadening in η:

X.N.Wang&M.Gyulassy PRL 68, 1480 (1992)

Armesto et alhep-ph/0405301

Leading hadron suppression:

Valuable diagnostic tools of QCD medium properties (dNg/dy, <q0>, cs, ...)

Medium-modified FFs:

"Mach cone":

X.N.Wang;A. Majumder,Salgado&Wiedem.Arleo, ...

Bjorken, 1982

dNpair/dφ dNpair/dη

dN/dpT

M Trigger

dN/dzT

cos M sc

Stoecker et al. hep-ph/0505245.Casalderrey, Shuryak, hep-ph/0411315

nucl-th/0302077

Nchjet increases

<zjet> decreases


Intermediate pIntermediate pT T mesons suppressed. Baryons are notmesons suppressed. Baryons are not

Strongly enhanced baryon (p, L) production at pT ~ 2 – 4 GeV/c

Strong centrality dependent baryon/meson: ratio well above “perturbative” (pp) ratios.

Clear deviation from std. vacuum fragmentation functions (large non-pQCD effects) calls for extra baryon production mechanism: recombination.

Above pT ~ 6 GeV/c: Recovery of “vacuum” fragmentation ratio. Baryons suppressed too.

L/K

0 s

Au+Au 0-5%

p+p

Au+Au 0-10%

p+p

Lamont, QM'05

O.Barannikova, QM'05


Baryon vs. meson “fragmentation functions”Baryon vs. meson “fragmentation functions”

However ... Associated yields similar for meson & baryon triggers.

Magnitude and centrality-dependence of associated (near- & away- side) hadron pT spectra for baryon & meson triggers show small differences.

Jet-like production but different suppression for leading baryons and mesons !?

PRC 71, 051902 (2005)

Near-side

Away-side

pT trigg= 2.5 – 4 GeV/cpTassoc = 1.7 – 2.5 GeV/c


How to compare high pT spectra in AA and pp ?

High pT particles issue from hard scatterings describable by pQCD:

A

B

AB

dAB → hard = A·B·dpp → hard

dNAB → hard (b) = TAB(b)·dpp → hard

Nuclear Modification Factor:

ABAt impact parameter b:

geom. nuclear overlap at b production is “shadowed”TAB ~ # NN collisions (“Ncoll scaling”)

“Factorization theorem”:

Independent scattering of “free” partons:

A+B = “simple superposition of p+p collisions”


High pHigh pT T suppression: ssuppression: system-size dependenceystem-size dependence

RAA for Cu+Cu @ √sNN = 200 GeV

• Suppression observed for central Cu+Cu• Models scale density from central Au+Au

All models show reasonable to good agreement

STAR preliminaryCharged hadrons

M. van Leeuwen (STAR), QM'05M. Shimomura (PHENIX), QM'05


PHENIX Preliminary

Di-hadron AuAu: Di-hadron AuAu: ΔφΔφ correlations (III) correlations (III)

Same dNpair/dD result in polar coords. now:

Strongly non-Gaussian away-side “peak”.

Standard back-to-back di-jet topology.

Au+Au peripheral

Au+Au central

PHENIX Collab. PRL to be submitted


Jet production in the “QCD vacuum” (pp collisions)Jet production in the “QCD vacuum” (pp collisions)

Jet : Collimated spray of hadrons in a cone ( ~ 0.7) with 4-momentum of original fragmenting parton

Leading hadron takes away large fraction (<z> ~0.6 –0.8 @ RHIC) of parent parton pT

Jet balanced back-to-back by other hard-scattered "parton" (jet, direct g, ...) A (small) acoplanarity appears due to intrinsic kT (parton Fermi motion).


Jet production in “QCD media” (pA, AA collisions)Jet production in “QCD media” (pA, AA collisions)

kT broadening (Cronin enhancement)

(Leading-twist) shadowing or gluon saturation (CGC)

Final-state effects

Parton energy loss due to medium-induced gluon-strahlung in hot & dense environment

Initial-state effects (accessible via pA colls.):

(accessible in AA colls.):


““Fragmentation functions”: xFragmentation functions”: xEE distributions pp,dAu distributions pp,dAu

Þ Dqp(z)~e-6z

• independent of pTt

<ztrig>=0.85 measured*Away-side associated hadron pT spectra:

xE ~ z/<ztrig> represents away jet fragmentation z

xE variable: Two-particle equivalent of fragmentation variable z

At high pT, i.e. when di-jets are nearly back-to-back.

)(1

assoctrigassoctrig

trig

Etrig

zDzdzdN

Nz

dxdN

N

IPN-Orsay, Nov 14th, 2005 David d'Enterria (LLR, Paris) 1/39 Jet quenching au RHIC: rsultats...

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