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Jet Structure of Baryons and Mesons in Nuclear Collisions

Why jets in nuclear collisions? Initial state What happens in the nuclear medium? Medium modification of jet fragmentation

PHENIX CollaborationBarbara JacakStony Brook UniversityAug. 16, 2004

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pressure builds up

why jets in nuclear collisions?

PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

Kpnd,Reflect (thermal)

properties when collisions cease

Quark gluon plasma?

use short wavelength probes. Fast q, g nearly ideal

Hard scattering, heavyquark production. Rate calculable from QCD + nuclear geometry.

System expands & cools

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Expected interaction with the medium

Hard scattering happens early

affected by initial state nucleus

Hard partons traverse the interesting stuff

Energy loss by induced gluon radiation

Modification of fragmentation outside the medium??

recombination with medium partons

radiated gluons nearby!

coneRFragmentation:

z hadron

parton

p

p

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Initial state: p+p collisions

p-p PRL 91 (2003) 241803

Good agreementwith NLO pQCD

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/( , )

a Nf x Q

2

/( , )ch a

D z Q

Parton distribution functions

Fragmentation functions

0

0 well described by pQCD and usual fragmentation functions

To generalize for nuclei:fa/N(xa,Q2,r) fa/N(xa,Q2) .

Sa/A(xa,r) .

tA(r)

Nuclear modification to structure function (shadowing, saturation, etc.)

Nuclear thickness function

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pQCD in Au+Au? direct photons

[w/ the real suppression]

( pQCD x Ncoll) / background Vogelsang/CTEQ6

[if there were no suppression]

( pQCD x Ncoll) / ( background x Ncoll)

Au+Au 200 GeV/A: 10% most central collisions

[]measured / []background = measured/background

Preliminary

Probe calculation works!

pT (GeV/c)

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Nuclear medium modifies initial state

Cronin effect for baryons > mesonsBut shouldn’t initial state scattering and fragmentation factorize?!

Probe response of cold nuclear matter with increased number of collisions.

Initial state multiple soft or semi-hard scatteringkT broadening

d+Au: Cronin Effect (RdA>1): Multiple Collisions broaden pT spectrum

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Turn to nuclear collisions: single particles

h/0 ratio shows baryons enhanced for pT < 5 GeV/c

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The baryons scale with Ncoll !

Greco, Ko, Levai: PRC 68 (2003)034904

But observed enhancement can be explained by recombination of thermal quarks from an expanding quark gluon plasma. NOT Jet-like!

Jet-likeNot suppressed!!?

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So, do jet analysis in Au+Au

Trigger:hadron with pT > 2.5 GeV/cIdentify as baryon or mesonBiased, low energy, high z jets!

Plot of associated partnersCount associated lower pT

particles for each trigger “conditional yield”Near side yield: number of jet associated particles from same jet in specified pT binAway side yield: jet fragments from opposing jet

trigger2 particle correlationsnear side < 90°Partner from same jet

away side > 90° opposing jet

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Subtract the underlying event

CARTOON

flow

flow+jet dN

Ntrig d

includes ALL triggers(even those with no

associated particles inthe event)

jetUnderlying event isbig! Collective flow causes another correlation in them:

B(1+2v2(pTtrig)v2(pT

assoc)cos(2))

associated particles with non-flow angular

correlations -> jets!

Treat as 2 Gaussians

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combinatorial background large in Au+Au!

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2 particle correlations

Select particles with pT= 2.5-4.0GeV/c

Identify them as mesons or baryons viaTime-of-flight

Find second particle with pT = 1.7-2.5GeV/c

Plot distribution of the pair opening angles

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jet partner equally likely for trigger baryons & mesons

Same side: slight decrease with centrality for baryonsLarger partner probability than pp, dAu

Away side: partner rate as in p+p confirms jet source of baryons!“disappearance” of away-side jet for both baryons and mesons

intermediate pT baryons ARE from jets

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What’s going on?

Thermal quark recombination

Radiated gluons are collinear (inside jet cone)

Increases partner yield

Fries, Bass & Muellernucl-th/0407102

Meson triggerbaryon

Dilutes jet partner yield

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Jet partner distribution on trigger side

Corrected to full jet yieldPartner spectrum flatter, indicates jet sourcePartners soften in most central collisions

Fragmentation fn. modified!

Jet partners

Inclusive

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Baryon formation is not outside medium

formation time from hadron size, Rh and mass, mh

In laboratory frame: f ~ Rh (Eh /mh)consider 2.5 GeV pT hadrons f, ~ 9-18 fm/c (Rh~0.5-1 fm); f,p~ 2.7 fm/c (Rh~1 fm)

So, could expect hard-soft recombination (C.M. Ko)

Partner spectrumHwa & Yangnucl-th/0407081

Soft-hard recomb.can also explainbaryon Cronineffect!

(4-6 GeV/c)

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Conclusions

Baryon excess has a significant jet componentDilution becoming visible in most central collisions

Jet fragmentation is modified by the mediumAu+Au jets richer in soft hadrons than p+p or d+Au

effect of induced radiation?Au+Au jets baryon yield increases with medium volume

effect of prevalent soft quarks?

See some evidence that jet fragments are beginning to thermalize in the medium, even on near side

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Jets in PHENIX

Large multiplicity of charged particles--solution: find jets in a statistical manner using angular correlations of particles

mixed events give combinatorial background 2 x 90 degree acceptance in phi and ||<0.35

--solution: correct for azimuthal acceptance,

but not for acceptance Elliptic flow correlations

--solutions: use published strength values and subtract (could integrate over 90° to integrate all even harmonics to zero)

PHENIX PRL 91 (2003) 182301

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Compare p+p and d+Au to PYTHIA

d+Au

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Hydro. expansion at low pT + jet quenching at high pT.

Coalesce (recombine) boosted quarks hadrons enhances mid pT hadrons baryons especially

pQCD spectrum shifted by 2.2 GeV

Teff = 350 MeV

R. Fries, et al

Are extras from the (soft) underlying event?

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Phase space filled with partons:coalesce into hadrons

ReCo of hadrons: convolution of Wigner functions

Where does ReCo win?

),()2

,2

;2

,2

()2()2( 3

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,3

3

3qrq

PrRq

PrRW

rqddRd

Pd

dNMab

ba

M Wab(1;2) = wa(1)wb(2)

fragmenting parton:ph = z p, z<1

recombining partons:p1+p2=ph

Power law:

Exponential: TpTAew /~

DAeDwN TzPT /frag ~

TPTeAwwN /2reco ~

Tpw ~

TPN ~frag

2reco ~

TPN

Use lowest Fock state, i.e. valence quarks

R. Fries

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Coalescence Model results

Fries et al: Phys.Rev. C68 (2003) 044902Greco, Ko, Levai: PRC 68 (2003)034904

•particle ratios and spectra OK

•intermediate pT hadrons from

coalescence of flowing partons NOT from jets, so no jet-like associated particles

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kT, jT at RHIC from p+p Data

J. Rak, Wed.

J. Rak, DNP03

s=200 GeV

di-hadron

Statistical Errors Only

near-side away-side

nearfar

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Pions in 3 detectors in PHENIX

Charged pions from TOF

Neutral pions from EMCAL

Charged pions from RICH+EMCAL

Cronin effect gone at pT ~ 8 GeV/c

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Does Cronin enhancement saturate?

A different approach:

Intrinsic momentum broadening in the excited projectile proton:

hpA: average number of collisions:

X.N.Wang, Phys.Rev.C 61 (2000): no upper limit.

Zhang, Fai, Papp, Barnafoldi & Levai, Phys.Rev.C 65 (2002): n=4 due to proton d dissociation.