1carl gagliardi – qnp 2004 recent results from rhic carl gagliardi texas a&m university...

1Carl Gagliardi – QNP 2004

Recent Results from RHIC

Carl GagliardiTexas A&M University

Outline• Introduction to relativistic heavy ion collisions• What have we learned in Au+Au collisions?

– In the “low-pT” regime– In the “intermediate-” and “high-pT” regimes

• What did we see in d+Au collisions?


Lattice QCD at finite temperature

F. Karsch, hep-ph/0103314

Critical energy density: 4)26( CC T

TC ~ 175 MeVC ~ 1 GeV/fm3

Ideal gas (Stefan-

Boltzmann limit)


RHIC: the Relativistic Heavy Ion Collider


Geometry of heavy ion collisions

Number of participants: number of incoming nucleons in the overlap region

Number of binary collisions: number of equivalent inelastic nucleon-nucleon collisions

Both depend on impact parameter or “centrality”

x

z


Experimental determination of centrality

Paddles/BBCZDC ZDC

Au Au

Paddles/BBC Central

Multiplicity Detectors

Paddle signal (a.u.)

5% Central

STAR


pT distribution of charged particles

Systematic Errors not shown

nucl-ex/0401006


Bulk particle production in Au+Aud

Nch

/d

19.6 GeV 130 GeV 200 GeV

PRL 91, 052303

CentralPeripheral

• Central collisions at 200 GeV: 5000 charged particles (!)• Energy density: boost invariant hydrodynamics (Bjorken)

d

dNp

Rdy

dE

Rch

TT

2

31122

(R ~ A1/3, = 1 fm/c)

Central Au+Au at 130 GeV: GeV/fm3 (PHENIX, PRL 87, 052301)


Azimuthal anisotropy: “elliptic flow”

)](2cos[21 2 Rvd

dN

Elliptic term

xy z

Initial coordinate-space anisotropy

py

px

Final momentum-space anisotropy

x

y

p

patan

y 2 x 2

y 2 x 2 22

22

2

yx

yx

pp

ppv


Hydrodynamic fitsKolb and U. Heinz (2002)

Obtain reasonable simultaneous description with the QGP equation of

state predicted by lattice QCD.

Elliptic flowRadial flow


HBT interferometry versus reaction plane

Geometrical analog of v2 – confirms source anisotropy.

Note: Magnitudes of Rl and Ro/Rs smaller than predicted by hydrodynamic calculations.

nucl-ex/0312009STARSTAR


Chemical equilibrium?

Thermal model: Partition fn with params T, B, s, I3

Fit to ratios of antiparticle/particle: , K, p, , , , K*0,

….


Phase diagram at chemical freeze-out


Hard partonic collisions and energy lossin dense matter

Multiple soft interactions:

Strong dependence of energy loss on gluon density gluemeasure color charge density at early hot, dense phase

Gluon bremsstrahlung

Opacity expansion:

glueSmediumT

SR

kq

LqC

E

2

2

ˆ

ˆ4

L

ELogrdCCE jet

glueSaA 23 2

,

Bjorken, Baier, Dokshitzer, Mueller, Pegne, Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang, Wang, Salgado, Wiedemann,…


Jets at RHIC

p+p jet+jet (STAR@RHIC)

Au+Au ??? (STAR@RHIC)

nucleon nucleonparton

jet

Find this……….in this


Partonic energy loss via leading hadrons

Energy loss softening of fragmentation suppression of leading hadron yield

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

Binary collision scaling p+p reference


Au+Au and p+p: inclusive charged hadronsPRL 91, 172302


Suppression of inclusive hadron yield

• central Au+Au collisions: factor ~4-5 suppression • pT > 5 GeV/c: suppression ~ independent of pT

PRL 91, 172302

Au+Au relative to p+p

Au+Au central/peripheral

RAA RCP


PHENIX observes a similar effect with π0

PRL 91, 072301

Binary scaling

Factor of 4-5

PRL 91, 241803

p-pAu-Au


So doPHOBOS ( ~ 0.8) and BRAHMS ( ~ 2.2)

PHOBOS: PL B578, 297 and PRL 91, 072302

BRAHMS: PRL 91, 072305Also have = 0

BRAHMS


Azimuthal anisotropy and partonic energy loss

M. Gyulassy, I. Vitev and X.N. Wang

)](2cos[)(21 2 RTpvd

dN

Anisotropy at high pT is sensitive to the gluon density of the medium.


Anisotropy vs. pT at 200 GeV

Significant anisotropy in Au+Au at least up to 7~8 GeV/c

PreliminarySTARSTAR Preliminary


Jets and two-particle azimuthal distributionsp+p di-jet • trigger: track with pT>4 GeV/c

• distribution: 2 GeV/c<pT<pTtrigger

• normalize to number of triggers

trigger

Phys Rev Lett 90, 082302


Azimuthal distributions in Au+Au

Au+Au peripheral Au+Au central

Near-side: peripheral and central Au+Au similar to p+p

Strong suppression of back-to-back correlations in central Au+Au

pedestal and flow subtracted

Phys Rev Lett 90, 082302

?


Other effects that might change RAA and RCP

• Hadronic re-interactions

• Change in particle production mechanisms

• Initial- or final-state multiple scattering (“Cronin effect”)

• Nuclear modifications of the parton distributions (“shadowing” and “anti-shadowing”)

• Gluon saturation at high energy and low x


Final-state “pre-hadronic” plus hadronic rescattering

May also have difficulty explaining high-pT v2 and near-side angular correlations

Cassing, Gallmeister, and Greiner, hep-ph/0311358


Baryons vs. mesons

In central Au+Au collisions, baryons are substantially overproduced relative to mesons at intermediate pT

PHENIX: PRL 91, 172301


Power law:

Exponental:

Hadronization via constituent quark recombination/coalescence

Coalescence of partons into hadrons:

Where does coalescence win over fragmentation?

),()2

,2

;2

,2

()2()2( 3

33

,3

3

3qrq

PrRq

PrRW

rqddRd

Pd

dNMab

ba

M

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

fragmenting parton:ph = z p, z<1

recombining partons:p1+p2=ph

TpTAew /~

DAeDwN TzPT /frag ~

TPTeAwwN /2reco ~

Tpw ~

TPN ~frag

2reco ~

TPN

From R. Fries, QM’04


Constituent quark scaling at intermediate pT

2 2 2 2an22 3

d 3p pM tt

B tt

pv p vv

pv p

At intermediate pT, different baryon and meson v2 values may reflect a common underlying partonic flow.


Limits of recombination/coalescence

PHENIX: PRL 91, 172301STAR: PRL 92, 052302

The “baryon enhancement” ends around pT ~ 5-6 GeV/c

STARSTAR


PRL 91, 172302RCP

Theory vs. datapQCD-I: Wang, nucl-th/0305010pQCD-II: Vitev and Gyulassy, PRL 89, 252301Saturation: KLM, Phys Lett B561, 93

pT>5 GeV/c: well described by gluon saturation model (up to 60% central) and pQCD+jet quenching

Final state

Initial state


Is suppression an initial or final state effect?

Initial state?gluon

saturation

Final state?partonic energy

loss

How to discriminate? Turn off final state d+Au collisions!


d+Au vs. p+p: Theoretical expectations

If Au+Au suppression is initial state(KLM gluon saturation: 0.75)

1.1-1.5

pT

RA

B

1

Inclusive spectra

If Au+Au suppression is final state

~2-4 GeV/c

pQCD: no suppression, small broadening due to Cronin effect

0 /2

(radians)

0

High pT hadron pairs

saturation models: suppression due to mono-jet contribution? suppression?

broadening?


d+Au inclusive yields relative to binary-scaled p+p

ddpdT

ddpdNR

Tpp

AB

TAB

AB /

/

Suppression of the inclusive yield in central Au+Au is a final-state effect

STAR: PRL 91, 072304

• d+Au : enhancement

• Au+Au: strong suppression

STARSTAR


PHENIX, PHOBOS, BRAHMS find similar results

PRL 91, 072302/3/5

PHENIX

PHOBOS

BRAHMS


Azimuthal distributions

pedestal and flow subtracted

Near-side: p+p, d+Au, Au+Au similarBack-to-back: Au+Au strongly suppressed relative to p+p and d+Au

Suppression of the back-to-back correlation in central Au+Au is a final-state effect

PRL 91, 072304


The strong suppression of the inclusive yield and back-to-back correlations at high pT observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions.


Phys Rev Lett 91,072302/3/4/5


1 + ( pQCD direct x Ncoll) / ( phenix pp backgrd x Ncoll)

1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang NLO

1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang, mscale = 0.5, 2.0

AuAu 200 GeV Central 0-10%

Direct photons

Direct photons aren’t suppressed in central Au+Au collisions. Another demonstration that suppression is a final-state effect

preliminary


Back-to-back correlations vs. reaction plane

STARSTAR preliminary20-60% central

Near-side correlations: independent of orientation

Back-to-back correlations: suppressed more strongly when the path length is longer


Mid-rapidity vs. forward rapidity

s

pxx T

T

2~

Mid-rapidity spectra tend to sample quarks and gluons in the incident nuclei at:

In contrast, forward spectra tend to sample quarks at large x and gluons around:

yT exx ~


Statistical errors dominate over systematic at =2 and 3

Systematic error (not shown) ~15%

RdAu at different pseudo-rapidities

Possible evidence that gluon saturation isimportant for forward physics at RHIC

nucl-ex/0403005and preliminary

BRAHMS


Have we found the Quark Gluon Plasma at RHIC? We now know that Au+Au collisions generate a medium that:

• is dense (pQCD theory: up to ~100 times cold nuclear matter)

• is dissipative

• exhibits strong collective behavior

There is still work to do

We have yet to show:

• hydrodynamics can describe spectra, v2, and HBT simultaneously

• direct evidence the system is thermalized

• chiral restoration

• a transition occurs: can we turn the effects off ?

This represents significant progress in our understanding of QCD matter


Kinematics for colliders

2tanln y

||

||21 ln

pE

pEyRapidity:

TT dydpp

d

pd

dE

2

2

3

3

Invariant cross section:

Pseudo-rapidity: for m/p << 1

Transverse momentum (pT) and (pseudo-)rapidity (y or )provide a convenient description


Why is elliptic flow interesting?

Free-streaming does not convert spatial

anisotropy to momentum anisotropy

Developed early(t < 2 fm/c)

Zhang, Gyulassy, Ko, PL B455 (1999) 45

Sensitive to the physics of constituent interactions at early time


Particle spectra and radial flow

Fit Au+Au spectra to blast wave model:Fit Au+Au spectra to blast wave model:• S S (surface velocity) drops with dN/d(surface velocity) drops with dN/d • T (temperature in GeV) almost constant.T (temperature in GeV) almost constant.

Blue y=0 Red y=1

T

pT (GeV/c)

1 and 3 contours

y= 0

An explosive expansion?

BRAHMS preliminary


Suppression of inclusive yield at 130 GeV

PHENIX, PRL 88, 022301STAR, PRL 89, 202301

Both STAR and PHENIX see significant suppression

Limitation: Ambiguities in the reference spectra at 130 GeV

00 2 4

pT (GeV/c)

1

2

RA

A


RAA different for charged hadrons and 0 ?

STAR PRL 91, 172302PHENIX PRL 91, 072301

For pT less than ~5 GeV/c, charged hadrons are less suppressed in central Au+Au collisions than 0

Charged Hadrons 0


Baryons vs. mesons – yet another view

K* (and maybe ) follow K0S -- Ξ (and maybe Ω) follow Λ


Final state “pre-hadronic” and hadronic scattering

• Calculations based on the idea of “color transparency”

• Hard-scattered parton neutralizes its color, then propagates as a color-neutral object

• The object grows into a well-defined hadron during a “formation time” tf. The interaction cross section grows over the same period.

tf

Calculations use:


Modified gluon density: “Gluon saturation”

MidRapidity

ForwardRapidity CTEQ6M

Saturation sets in when gluons start to overlap

yT es

px 2

~


Relative charge dependenceStrong dynamical charge correlations in jet fragmentation

Compare ++ and -- charged azimuthal correlations to +- azimuthal correlations

STAR PRL 90, 0823020-10% most central Au+Au

p+p minimum bias4<pT(trig)<6 GeV/c

2<pT(assoc.)<pT(trig)

System (+-)/(++ & --)

p+p 2.7+-0.6

0-10% Au+Au 2.4+-0.6

Jetset 2.6+-0.7

Same correlated particle production for pT>4 GeV/cin pp and central Au+Au

Au+Au

p+p


High pT correlations: Au+Au vs p+pPRL 90, 082302 (2003)

Central Au + Au

Peripheral Au + Au

Back-to-back jets are suppressed in central collisions

22 2 2( ) ( ) (1 cos(2 ))D Au Au D p p B v

near side

away side

peripheral central


Collision timescale

0 5 10 15

Evolution time (fm/c)

Hydrodynamical model

Blast-wave fit

Rlong Sinyukov fit

ceq

HBT()

Buda-Lund


Baryon production in d+Au

nucl-ex/0309012 STARSTAR

Central Au+Au

d+Au

p+p

Baryon enhancement in central Au+Au is final state effect

1carl gagliardi – qnp 2004 recent results from rhic carl gagliardi texas a&m university...

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