Jet Quenching at RHIC Saskia Mioduszewski Brookhaven National Laboratory 28 June 2004

Outline Introduction to high p T (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 p T production in p+p (CERN-ISR) Spectrum becomes “harder” at high p T – deviates from exponential (Note the log scale on the y-axis!) cone of hadrons “jet” p p “hard scattering”  high p T

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 p T 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 p T 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/fm 3 critical density  c :  c ~ 0.7 GeV/fm 3 distance of two nucleons: 2 r 0 ~ 2 fm nuclear matter p, n Quark-Gluon Plasma q, g density or temperature size of nucleon r n ~ 0.8 fm

Lattice QCD at Finite Temperature Coincident transitions: deconfinement and chiral symmetry restoration F. Karsch, hep-ph/ Critical energy density : T C ~ 175 MeV  C ~ 0.7 GeV/fm 3 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.

Schematic Phase Diagram of Strongly Interacting Matter Baryonic Potential  B [MeV] Temperature T [MeV] AGS SIS SPS RHIC quark-gluon plasma hadron gas neutron stars early universe thermal freeze-out deconfinement chiral restoration Lattice QCD atomic nuclei P. Braun-Munzinger, nucl-ex/ Test QCD under extreme conditions and in large scale systems Search for deconfined QGP phase SIS  AGS  SPS  RHIC  LHC 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 beam absorption or scattering pattern

cone of hadrons “jet” p p hard-scattered parton in p+p hadron distribution softened, jets broadened? hard-scattered parton during Au+Au increased gluon-radiation within plasma? Jets in heavy ion collisions Hard scattering

SppS Collisions proton anti-proton  s = 200, 546, 900 GeV UA1, 900 GeV 10’s of particles

RHIC Collisions Gold  s NN = 130, 200 GeV (center-of-mass energy per nucleon-nucleon collision) 1000’s of particles

Jets in Heavy Ion Collisions Au+Au peripheral Phys Rev Lett 90,

15 fm b 0 fm 0 N_ part 394 Spectators Participants For a given b, Glauber model predicts N part (No. participants) and N binary (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 :

“Binary-Scaling” and R AA Define Nuclear Modification Factor R AA  Effect of nuclear medium on yields 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 - N binary

Systematizing Our Expectations Describe in terms of scaled ratio R AA = 1 for “baseline expectations” > 1 “Cronin effect” Will present most of Au+Au and d+Au data in terms of this ratio “no effect”

Motivation Effect of collision medium on hadron p T 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 p T hadrons (jet fragments) Dense medium (QGP) would cause depletion in spectrum of leading hadron at high p T - “jet quenching”

High p T in Au+Au collisions Investigate hadron p T 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 N binary (peripheral) Central Au+Au * p+p scaled by N binary (central)

Nuclear Modification Factor RHIC 200 GeV central - Suppression peripheral – N binary scaling  Comparison of peripheral to central binary scaling

R AA for  0 and charged hadrons PHENIX AuAu 200 GeV  0 data: nucl-ex/ , submitted to PRL. charged hadron (preliminary) : NPA715, 769c (2003). R AA 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!

R AA as a Function of Collision Energy * * Re-analysis of WA98: d’Enterria nucl-ex/ Previous measurement from CERN-SPS observed no suppression (p T reach limited to 4 GeV/c) RHIC measurement shows suppression up to 10 GeV/c (how far in p T will it extend?) Latest RHIC measurement at  s=62 GeV shows suppression at high p T

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, ?

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/ ; Kharzeev, Levin, Nardi, Gribov, Ryshkin, Mueller, Qiu, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu Small + Large distinguishes all initial and final state effects. Nucleus- nucleus collision Proton/deuteron nucleus collision

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, 18 th, 2003

Conclusion The 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 Experimentd + Au Control Experiment Preliminary DataFinal Data

Theoretical Understanding? Both –Au-Au suppression (I. Vitev and M. Gyulassy, hep-ph/ ) –d-Au enhancement (I. Vitev, nucl-th/ ) understood in an approach that combines multiple scattering with absorption in a dense partonic medium (15 GeV/fm 3 ~100 x normal nuclear matter)  Our high p T probes have been calibrated and are now being used to explore the precise properties of the medium Au-Au d-Au

Direct Photons in AuAu Many sources, different p T regions –Thermal Sources (p T < 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 (p T > 3-4GeV) - In central AuAu,   /meson background suppressed -”Cleanest” region (pQCD dominates) -PHENIX has good sensitivity here  High p T photons provide alternative to high p T 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, (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   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 N coll ) /  phenix backgrd Vogelsang NLO

PHENIX Preliminary Summary of High p T 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 p T ~10 GeV/c Spectra exhibit significant suppression in yield at high p T 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 p T allows for “easier” measurement of pQCD photons

RHIC Performance Run Year Species s 1/2 [GeV ]  Ldt N tot tot. data Au - Au μb -1 10M 3 TB /2002 Au - Au μb M ~20 TB p- p pb G ~10 TB /2003 d - Au nb G 46 TB p - p pb G 35 TB

Centrality Dependence of Direct Photon Signal 70-80% Central AuAu 200 GeV 60-70% Central AuAu 200 GeV 50-60% Central AuAu 200 GeV 40-50% Central AuAu 200 GeV30-40% Central AuAu 200 GeV 20-30% Central AuAu 200 GeV 10-20% Central 200 GeV AuAu 0-10% Central 200 GeV AuAu PHENIX Preliminary