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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