Jets at RHIC Jiangyong Jia

Outline n Jet and it’s importance in RHIC u Jets and hard scattering u soft vs hard u pQCD approach F pp F pA F AA n Angular correlation u some examples u Phenix experiment u my angular correlation study u outlook n Summary

Relativistic Heavy-Ion Collider (RHIC) n Goal u Searching for the quark gluon plasma(QGP) u exploring the physics of this new state of matter. n Jet quenching as probe Au Au at

Jet quenching: a QGP signature n What is a Jet ? n QCD energy loss in deconfined dense matter n Theory prediction dE/dx = 2-4GeV/fm n QGP suppresses high pt particles and angular correlations b a c d

Ref, K.Geiger

Jet and hard scattering at n Jet production rate (K.J.Eskola,hep-ph/ ) n Hard scattering contribution to particle production is important( )

d  /p T dp T string model hydrodynamic model pQCD (Hijing,PCM) pTpT soft Semi-hard hard 1GeV5GeV soft process vs hard process Jet

Soft/hard boundary? n At SPS ( ) u Hydrodynamic models. u pQCD(X.N.Wang, hep-ph/ ) u at SPS : both works. u Small jet cross-section, measure only to n At RHIC ( ) u increase, larger jet cross-section,measure to high pt ( ). u Angular correlation

Hydrodynamic Model for SPS Plot created by Jane Burward Hoy

pQCD calculation X.N.Wang,nucl-th/

Jet production (X.N.Wang,nucl-th/ ) n pQCD approach Schematic representation of a high-pt reaction factorized into parton distribution function (f), parton fragmetation function D and a hard-scattering subprocess parton-parton collision convolution Cross-section n Cross-section

pp--Initial Kt n Need to take into account intrinsic pt of partons u Uncertainty principle u high order pQCD process(initial state radiation,etc) n modify parton distribution function u where

pA-- Modification of hard scattering n Assume only one hard parton- parton collision. n Enhancement of high Pt spectra (Cronin effect ) u decrease with 1/p t 2 and  A (b) is number of scattering assume hard sphere distribution  2 is the soft Kt kick, it has following form n Modification of nuclear structure function (Shadowing) u assume parton distribution:

Cronin and Shadowing in pA R pAu p + Au

AA--Jet quenching n If we have QGP, multiple interactions of parton with the medium lead to loss of energy. n Time scale (  h  1/p 0  0.1fm/c, p 0  2GeV/c,  s  1/  QCD  1 fm/c) High pt parton collisions happen in the earliest stage of collision. Then they suffer energy loss in the medium. n Hijing Assume (model dependent): u constant energy loss u incoherent multiple scattering.

R AA (PT)

Angular correlation At RHIC n Back-to-Back correlation: u QGP F (Di-)Jets suppression F enhanced acoplanarity. n Study the centrality (impact parameter) dependence u From peripheral to central n Some examples

CCOR At ISR pp forwardbackward Ref, M.J.Tannenbaum

X.N.Wang,Phys.Rep.280,287(1997).

Phenix Detector n Good Charged particle tracking u high pt resolution, u high interaction rate, L=10 26~27 cm 2 /s (read out 50Hz ) n Phenix detector acceptance u default acceptance: F Pt  0.2 GeV/c; F 70  <  <110  or |  |<0.35 F  <  <56.25 ,  <  <  u 1/2*sin(  /2) = 17  of 4  u acceptance in azimuthal  leads to an intrinsic jet shape Phenix acceptance :

DC  (degree)  (degree)  Detector LayoutDetector Acceptance EMCal TEC RICH

Studies of pp collisions at n Pythia Generator u pp events at u min jet pt = 20GeV/c. n Analysis scheme (as CCOR): u study  distributions: F back to back correlation of jets ( w/o  acceptance) F modification with a HIC background (RQMD) one Pythia event(two jets) mixed with one RQMD event F effect of Phenix  acceptance.

Leading Pt>2.0

1<Pt<2 4<Pt<53<Pt<4 2<Pt<3

Outlook n RQMD (Hadrons + Strings) use other models as background (Hijing, Strings + Minijets) n Determine methods to correct for  and  acceptance of Phenix(limited acceptance) u event by event acceptance correction u investigate the fiducial region( , ,pt) n Study angular correlation function: n Rate estimate(1/2000) n when we have the data…. u Study angular correlation as a function of centrality. Fiducial region Detector acceptance  

Summary n Large pt partons or Jets are good probes of the dense matter in ultra- relativistic heavy-ion collisions. n In RHIC, can use pQCD based models to calculate high pt spectra. u Need to combine soft and hard processes together. n Study back to back jets angular correlation u if QGP is generated, angular correlation will be affected by jet quenching u Phenix detector have limited acceptance, we can: F increase pt and use the fiducial region F learn how to correct for  and  acceptance. n We will further our study on jet angular correlation and look for jet quenching as a probe of QGP formation at RHIC.

References n K.J.Eskola,hep-ph/ u Give the parton production rate. n Also K.J.Eskola,K.Kajantie,and J.Lindfors Nucl.Phys.B323,37(1989) u it calculate the parton production rate, saying: F for RHIC, U + U, have 290 partons Pt>2GeV, and 75 partons Pt>3GeV. F For hard Et,SPS 5%, RHIC 50%, Tevatron 80%. n X.N.Wang,nucl-th/ u Base of this talk. n X.N.Wang,Phys.Rep.280,287(1997). u Base of this talk. n J.F.Owens,Rev.Mod.Phys. 59, 465 (1987). u Gives the jet cross section formula. n X.N.Wang,hep-ph/ u Calculate the jet quenching under simple assumption. n M.Arneodo,Phys.Rep.240,301(1994) u shadowing effect. n K.Geiger. Ultra-Relativistic nuclear collisions in a QCD based space-time description: the parton cascade model u a very good article delving into the history of AA collision, comparing with Hijing, and have a lot of interesting graph. n Nucl.Phys.B u Having UA1 Jet cross section measurement

PP pA AA Single Inclusive cross section

1<Pt<2 4<Pt<5 3<Pt<4 2<Pt<3