Jet Quenching at RHIC

Saskia Mioduszewski

Brookhaven National Laboratory

28 June 2004

Outline

• Introduction to high pT (hard scattering)

• RHIC (Relativistic Heavy Ion Collider)

• Physics goals of heavy ion collisions

• Hard processes in heavy ion collisions– Expected behavior if A+A is an incoherent sum

of individual p+p collisions– In-medium effects

• Summary

Discovery of high pT production in p+p (CERN-ISR)

Spectrum becomes “harder” at high pT – deviates from exponential

(Note the log scale on the y-axis!)

cone of hadrons “jet”

pp“hard scattering” high pT

Jets & proton-antiproton collisions

• International conference on high-energy physics, Paris, 1982

• Results from CERN experiment UA2 really convinced everyone that jets in hadron-hadron collisions had been seen

1984 BNL note about RHIC physics

Jets in nuclear collisions

Subsequent hadron measurements at high pT

show same effect

Production cross section of 0 measured by PHENIX

Thermally-shaped Soft Production

Hard Scattering

• Good agreement with NLO perturbative QCD calculations

• High pT particle yields serve as a calibrated probe of the nuclear medium in nucleus+nucleus (A+A) and deuteron+nucleus (d+A) collisions

The RHIC Experiments

STAR

On the Scale of Downtown Boston….

Fundamental Puzzles of Hadrons

• Confinement– Quarks do not exist as free particles

• Large hadron masses– Free quark mass ~ 5-7 MeV

– Quarks become “fat” in hadrons, constituent mass ~ 400 MeV

• Complex structure of hadrons– Sea anti-/quarks

– Gluons

These phenomena must have occurred with formation of hadrons

nuclear matter p, n

Energy Density of Nuclear Matter

• normal nuclear matter 0 :0 ~ 0.15 GeV/fm3

• critical density c :

c ~ 0.7 GeV/fm3

distance of two nucleons:

2 r0 ~ 2 fm

nuclear matter p, n

Quark-Gluon Plasma q, g

density or temperaturesize of nucleon

rn ~ 0.8 fm

Lattice QCD at Finite Temperature• Coincident transitions: deconfinement and chiral symmetry restoration

F. Karsch, hep-ph/010314

Critical energy density:4)26( CC T

TC ~ 175 MeVC ~ 0.7 GeV/fm3

Ideal gas (Stefan-Boltzmann limit)

B=0)

Chiral symmetry spontaneously broken in nature. Quark condensate is non-zero:

At high temperature and/or baryon density

Constituent mass current mass Chiral Symmetry (approximately) restored.

MeVqq 3)250(

0qq

Schematic Phase Diagram of Strongly Interacting MatterSchematic Phase Diagram of Strongly Interacting Matter

Baryonic Potential B [MeV]

T

em

pera

ture

T [

MeV

]

0

200

250

150

100

50

0 200 400 600 800 1000 1200

AGS

SIS

SPS

RHIC quark-gluon plasma

hadron gas neutron stars

early universe

thermal freeze-outdeconfinementchiral restoration

Lattice QCD

atomic nuclei

P. Braun-Munzinger, nucl-ex/0007021

Test QCD under extreme conditions and in large scale systems

Search for deconfined QGP phase

SISAGS SPS RHICLHC

From high baryon density regime to high temperature regime

RHIC Physics Program

• RHIC was proposed in 1983• One of the main emphases is study of properties of

matter under extreme conditions– large energy densities

– high temperatures

• To achieve these conditions we collide heavy nuclei at very high energies

• Extremely useful to have probes with known properties

Detecting the QGP “matter box”• Rutherford experiment atom discovery of nucleus

SLAC electron scattering e proton discovery of quarks

• “ideal” experiment

• Experiments with QGP not quite that simple

– QGP created in nucleus-nucleus collisions can not be put in “box”

– Thousands of particles produced during collision

vacuum

QGP

penetrating beamabsorption or scattering pattern

cone of hadrons “jet”

p p

hard-scattered

parton in p+p

hadron distributionsoftened, jets broadened?

hard-scatteredparton during Au+Au

increased gluon-radiationwithin plasma?

Jets in heavy ion collisions

Hard scattering

SppS Collisions

proton anti-protons = 200, 546, 900 GeV

UA1, 900 GeV

10’s of particles

RHIC Collisions

Gold GoldsNN = 130, 200 GeV

(center-of-mass energy per nucleon-nucleon collision)

1000’s of particles

Jets in Heavy Ion CollisionsAu+Au peripheral

Phys Rev Lett 90, 082302

15 fm b 0 fm

0 N_part 394

Spectators

Participants

For a given b, Glauber model predicts Npart (No. participants)and Nbinary (No. binary collisions)

Not all A+A collisions are the same -- “Centrality”

Yield of 0 measured by PHENIX

p+p collisions Au+Au collisions

+A DIS (1973) AGS Point-like Scaling

E. Gabathuler, Proc. 6th Int. Symposium on Electron and Photon Interactions at High Energies (1973), Bonn.

DIS scales with A

Scaling from p+p to A+A

• For hard-scattering processes, expect point-like scaling. For inclusive cross sections :

• For semi-inclusive yields, expect :

2

pp

AA A sources like-point ofnumber the of ratio the σ

σ

class centralityA A for theN

collisionsbinary Nucleon -Nucleon ofnumber Yield

Yield

binary

pp

AA

“Binary-Scaling” and RAA

• Define Nuclear Modification Factor RAA

Effect of nuclear medium on yields

pp

centralbinarycentral

Yield

NYield /

peripheralbinaryperipheral

centralbinarycentral

NYield

NYield

//

pp

peripheralbinaryperipheral

Yield

NYield /

• The probability for a “hard” collision for any two nucleons is small

• The total probability in A+A collision is multiplied by the number of times we try, i.e. – the cross-section scales with the number of binary collisions - Nbinary

Systematizing Our Expectations

• Describe in terms of scaled ratio RAA

= 1 for “baseline expectations”

> 1 “Cronin effect”

• Will present most of Au+Au and d+Au data in terms of this ratio

“no effect”

pp

AuAubinaryAuAuAA Yield

NYieldR

/

Motivation

Effect of collision medium on hadron pT spectra:

• Parton scattering with large momentum transfer Hard-scattered partons (jets) present in early stages of

collisions

• Hot and dense medium Hard-scattered partons sensitive to hot/dense medium

Theory predicts radiative energy loss of parton in QGP

• Emission of hadrons High pT hadrons (jet fragments)

Dense medium (QGP) would cause depletion in spectrum of leading hadron at high pT - “jet quenching”

High pT in Au+Au collisions

Investigate hadron pT spectra for evidence of parton energy loss (“jet quenching”) induced by dense medium

X-N. Wang, Phys. Rev. C58 (1998) 2321

Theoretical prediction

Yield of 0 in Au+Au compared to p+p collisions

• Peripheral Au+Au* p+p scaled by Nbinary(peripheral)

• Central Au+Au* p+p scaled by Nbinary(central)

Nuclear Modification Factor

RHIC 200 GeV central -

Suppressionperipheral –

Nbinary scaling

pp

peripheralbinaryperipheral

Yield

NYield /

Comparison of peripheral to central

binary scaling

pp

centralbinarycentral

Yield

NYield /

RAA for 0 and charged hadrons

pp

AuAubinaryAuAuAA Yield

NYieldR

/

PHENIX AuAu 200 GeV0 data: nucl-ex/0304022, submitted to PRL.charged hadron (preliminary) : NPA715, 769c (2003).

• RAA is well below 1 for both charged hadrons and neutral pions.

• The neutral pions fall below the charged hadrons since they do not contain contributions from protons and kaons.

Strong Suppression!

RAA as a Function of Collision Energy

*

* Re-analysis of WA98: d’Enterria nucl-ex/0403055

• Previous measurement from CERN-SPS observed no suppression (pT reach limited to 4 GeV/c)• RHIC measurement shows suppression up to 10 GeV/c (how far in pT will it extend?)

• Latest RHIC measurement at s=62 GeV shows suppression at high pT

Azimuthal distributions in Au+Au

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

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

Au+Au peripheral Au+Au central

pedestal and flow subtracted

Phys Rev Lett 90, 082302

?

d+Au Control Experiment

• Collisions of small with large nuclei were always foreseen as necessary to quantify cold nuclear matter effects.

• Recent theoretical work on the “Color Glass Condensate” model provides alternative explanation of data:– Jets are not quenched, but are a priori made in fewer numbers.– Color Glass Condensate hep-ph/0212316; Kharzeev, Levin, Nardi, Gribov,

Ryshkin, Mueller, Qiu, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu

• Small + Large distinguishes all initial and final state effects.

Nucleus- nucleuscollision

Proton/deuteron nucleuscollision

Is The Suppression Always Seen at RHIC?• NO!• Run-3: a crucial control measurement via d+Au collisions

d+Au results from

presented at a press conference at BNL on June, 18th, 2003

ConclusionThe combined data from Runs 1-3 at RHIC on p+p, Au+Au,

and d+Au collisions establish that a new effect (a new state of matter?) is produced in central Au-Au collisions

Au + Au Experiment d + Au Control Experiment

Preliminary DataFinal Data

Theoretical Understanding?Both

– Au-Au suppression (I. Vitev and M. Gyulassy, hep-ph/0208108)– d-Au enhancement (I. Vitev, nucl-th/0302002 )

understood in an approach that combines multiple scattering with absorption in a dense partonic medium (15 GeV/fm3 ~100 x normal nuclear matter)

Our high pT probeshave been calibratedand are now being used to explore the precise propertiesof the medium

Au-Au

d-Au

Direct Photons in AuAuMany sources, different pT regions

– Thermal Sources (pT < 3-4 GeV)-Partonic (QGP!) , Hadronic Gas (new resonance diagrams

theoretical uncertainties)

-Largest Backgrounds, PHENIX systematics still under investigation in this momentum region

– Hard Scattering (pT > 3-4GeV)-In central AuAu,

/meson background suppressed

-”Cleanest” region (pQCD dominates)

-PHENIX has good sensitivity here

High pT photons provide alternative to high pT hadrons, but Photons do not interact strongly in medium

PHENIX Direct ’s: Step 0) Measure Background

• We are looking for the signal over a large background

• Requires precise knowledge of the ’s

Calculated from

p+p->0 + X

PHENIX Run2 200 GeV p-p

Phys. Rev. Lett. 91, 241803 (2003)

Vogelsang calculation reference: JHEP 9903 (1999) 025/ Private Comm.

Direct Photon Result in p+p Collisions

•Excess Above Background Double Ratio:

[]measured / [background measured/background

•The excess above 1 is the direct photon signal

•Small direct signal found in 200 GeV p+p

Ratio

PHENIX Preliminary PHENIX Preliminary

expected bkg

measured

Central Au+Au Direct Photon Result

0-10% Central 200 GeV AuAu

PHENIX Preliminary PbGl / PbSc Combined

[]measured / []background = measured/background

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

PHENIX PreliminaryPHENIX Preliminary

Summary of High pT Physics at RHIC

Summary• Goal of colliding heavy ions at high energies is to detect

and study the properties of QCD phase transition (QGP)• One possible signature of the QGP is energy loss of

“hard-scattered” partons in the dense medium• Have measured charged particle and neutral pion yields

up to pT ~10 GeV/c• Spectra exhibit significant suppression in yield at high pT

in central collisions relative to binary-scaled p+p collisions, which requires a very dense medium

• Confirmed that it is a final-state effect with d+Au data• Consistent with parton energy loss in dense, strongly

interacting medium• Suppression of hadrons at high pT allows for “easier”

measurement of pQCD photons

RHIC Performance

Run Year Species s1/2 [GeV ] Ldt Ntot tot. data

01 2000 Au - Au 130 1 μb-1 10M 3 TB

02 2001/2002 Au - Au 200 24 μb-1 170M ~20 TB

p- p 200 0.15 pb-1 3.7G ~10 TB

03 2002/2003 d - Au 200 2.74 nb-1 5.5 G 46 TB

p - p 200 0.35 pb-1 4.0G 35 TB

Centrality Dependence of Direct Photon Signal

70-80% Central AuAu 200 GeV60-70% Central AuAu 200 GeV50-60% Central AuAu 200 GeV40-50% Central AuAu 200 GeV30-40% Central AuAu 200 GeV20-30% Central AuAu 200 GeV10-20% Central 200 GeV AuAu0-10% Central 200 GeV AuAu

PHENIX Preliminary