High p T Probes of QCD Matter Huan Zhong Huang 黄焕中 Department of Physics and Astronomy University of California, Los Angeles Department of Engineering Physics Tsinghua University

Characteristics of Interactions

Non-Abelian Nature of QCD QEDQCD

Salient Feature of Strong Interaction Asymptotic Freedom: Quark Confinement: 庄子天下篇 ~ 300 B.C. 一尺之棰，日取其半，万世不竭 Take half from a foot long stick each day, You will never exhaust it in million years. QCD qq qq q q Quark pairs can be produced from vacuum No free quark can be observed Momentum Transfer Coupling Strength Shorter distance  (GeV)

e+e-  qq p+p  jet+jet Quarks are Real ! Discovery of the Gluons ?!

Geometry of Nucleus-Nucleus Collisions N part – No of participant nucleons N binary – No of binary nucleon-nucleon collisions cannot be directly measured at RHIC estimated from Woods-Saxon geometry

Expectations for High p T from Au+Au Use number of binary nucleon-nucleon collisions to gauge the colliding parton flux: N-Binary Scaling  R AA = 1 N-Binary Scaling works for very rare processes i.g., Drell-Yan and direct photon production with some caveats (parton F 2 and G change in A). R AA can also be measured using central/peripheral ratios !

Collision Dynamics (high p T processes !)

Collisions at high p T (pQCD) At sufficiently large transverse momentum, let us consider the process: p + p  hadron + x 1) f(x,  2 ) – parton structure function 2)  ab->cd – pQCD calculable at large  2 3) D(z h,  2 ) – Fragmentation function

Kinematic Variables in 2  2 process

Parton Distribution Function

Fragmentation Function from e+e collisions  charged hadron

PQCD LO Parton-Parton Cross Sections

Jet prominent at high energy collisions !

Jet total energy  Avoid Fragmentations !

PQCD Works for p+p at RHIC

You can ‘see’ jets clearly!! High Energy p+p collisions! Four Jets event possible !

Jet Energy Reconstruction NOT at RHIC !

Hard Scattering and Jet Quenching back-to-back jets disappear leading particle suppressed Hard Scattering in p+p Parton Energy Loss in A+A Reduction of high p T particles Disappearance of back-to-back high p T particle correlations

Disappearance of back-to-back correlation ! Disappearance of back-to-back angular correlations x y p trig p ss p os P trig – p ss same side  correlation P trig – p os opposite side  corr. p trig > 4 GeV/c, p ss p os 2<p T <p trig

Suppression of high p T particles p T Spectra Au+Au and p+p p+p Au+Au 0-5% R AA =(Au+Au)/[N binary x(p+p)] Strong high p T suppression by a factor of 4-5 in central Au+Au collisions ! The suppression sets in gradually from peripheral to central Au+Au collisions !

Two Explanations for High p T Observations Energy Loss: Particles lose energy while traversing high density medium after the hard scattering. Energy loss quenches back-to-back angular correlations. J. Bjorken, M. Gyulassy, X-N Wang et al…. Parton Saturation: The parton (gluon) structure function in the relevant region (saturation scale) is modified. Not enough partons available to produce high pT particles. Parton fusion produces mono-jet with no back-to- back angular correlations. D. Kharzeev, L. McLerran, R. Venugopalan et al…..

d+Au Collisions q q q q Au+Au Geometry d+Au Geometry d+Au collisions: Little energy loss from the dense medium created, But Parton saturation from Au nuclei persists!

Data from d+Au collisions No high p T suppression ! No disappearance of back-to-back correlations!

High p T Phenomena at RHIC Very dense matter has been created in central Au+Au collisions! This dense matter is responsible for the disappearance of back-to-back correlation and the suppression of high pT particles !

The Suppression is the Same for   and  – parton level effect No suppression for direct photons – photons do not participant !

No Significant Difference Between Quarks and Gluons at High p T Baryons more likely from gluon fragmentations in the pQCD region

STAR PRELIMINARY Energy Loss and Soft Particle Production Leading hadrons Medium

High p T Physics 1)Energy Balance in Jet Production and Trigger pT Effect -- Mach Shockwave Phenomenon --  and  Correlations 2)Heavy Quark Energy Loss  -jet Correlations 4)Di-jet Correlations -- k T Smearing -- Nuclear A Dependence of k T Scale 5)Can High pT Probes Be Sensitive to the DOF of the Dense Medium? 6)Color Glass Condensate

STAR data motivated sonic-boom prediction F. Wang (STAR), QM’04 talk, nucl-ex/ Now published: STAR, PRL 95, (2005). p T trig =4-6 GeV/c, p T assoc = GeV/c Many recent studies: H. Stoecker, nucl-th/ Muller, Ruppert, nucl-th/ Chaudhuri, Heinz, nucl-th/ Y.G. Ma, et al. nucl-th/ Casalderrey-Solana, Shuryak, Teaney, hep-ph/ Actually sonic-boom was first predicted in the 70’s by the Frankfurt school.

Sonic Boom MM Trigger Casalderrey-Solana, Shuryak and Teaney Linearize disturbance (1)Can hydro equation be applied to a few particles? (2) Transverse expansion? Dumitru

Jet-Medium Interactions how does a fast moving color charge influence the medium it is traversing? can Mach-shockwaves be created?  information on plasma’s properties is contained in longitudinal and transverse components of the dielectricity tensor two scenarios of interest: 1.High temperature pQCD plasma 2.Strongly coupled quantum liquid (sQGP) H. Stoecker, Nucl. Phys. A750 (2005) 121 J. Ruppert & B. Mueller, Phys. Lett. B618 (2005) 123 J. Casalderrey-Solana, E.V. Shuryak, D. Teaney, hep-ph/

1.High temperature pQCD plasma: Calculation in HTL approximation color charge density wake is a co-moving screening cloud 2.Strongly coupled quantum liquid (sQGP): subsonic jet: analogous results to pQCD plasma case supersonic jet: emission of plasma oscillations with Mach cone emission angle: ΔΦ=arccos(u/v) [v: parton velocity, u: plasmon propag. velocity] Wakes in the QCD Medium J. Ruppert & B. Mueller, Phys. Lett. B618 (2005) 123

High p T Low p T The Structure at  Correlations in Central Au+Au At Low pT !

In order to discriminate Mach-cone from deflected jets, one needs three-particle correlation. away near Medium mach cone Medium away near deflected jets  1  2   0 0  1  2  0 0 

ppAu+Au 80-50%Au+Au 30-10% d+AuAu+Au 50-30% Au+Au 10-0% STAR preliminary 3-particle correlation results Au+Au ZDC central (12%) data: x10 more statistics.

 : System, Centrality Dependence at 200 GeV 3 < p T (trig) < 6 GeV 2 < p T (assoc) < p T (trig) |  | < 0.5 STAR preliminary Au+Au: peak broadens, height drops with centrality

 : System, Centrality Dependence at 200 GeV   increases from p+p to central Au+Au at lower p T (trig) –Higher p T (trig) flat across all centralities –Systematic error not assigned (fit range,  projection window) 2 < p T (assoc) < p T (trig) |  | < < pT(trig) < 12 GeV 3 < pT(trig) < 6 GeV

Observed baryon to meson ratio is higher for away-side jets Centrality Dependence Jet-yield Ratios for baryons and mesons

Partonic Matter Hadronization 1)Definition of Nuclear Modification Factors and v 2 2)p T Scale for Fragmentation Processes 3)Degree of Freedom at Hadron Formation 4)More Identified Particles and Higher pT 5)Model Dependence – Recombination/Coalescence

The Field & Feynman picture of cascade fragmentation

Baryon Production from pQCD  K K p p  e + e -  jet fragmentation from SLD Normal Fragmentation Cannot Produce the Large Baryon Yield

Too Many Baryons at Intermediate p T

p T Scales and Physical Processes R CP Three P T Regions: -- Fragmentation -- multi-parton dynamics (recombination or coalescence or …) -- Hydrodynamics (constituent quarks ? parton dynamics from gluons to constituent quarks? )

Multi-Parton Dynamics for Bulk Matter Hadronization Essential difference: Traditional fragmentation  particle properties mostly determined by the leading quark ! Emerging picture from RHIC data (R AA /R CP and v 2 )  all constituent quarks are almost equally important in determining particle properties ! v 2 of hadron comes from v 2 of all constituent quarks ! The fact that in order to explain the v 2 of hadrons individual constituent quarks (n=2-meson,3-baryon) must have a collective elliptic flow v 2 and the hadron v 2 is the sum of quark v 2  Strong Evidence for Deconfiement !

Recombination+Fragmentation Model basic assumptions: at low p t, the quarks and antiquark spectrum is thermal and they recombine into hadrons locally “at an instant”:  features of the parton spectrum are shifted to higher p t in the hadron spectrum at high p t, the parton spectrum is given by a pQCD power law, partons suffer jet energy loss and hadrons are formed via fragmentation of quarks and gluons shape of parton spectrum determines if recombination is more effective than fragmentation baryons are shifted to higher p t than mesons, for same quark distribution  understand behavior of baryons!

Reco: Single Particle Observables  consistent description of spectra, ratios and R AA

Recombination model (Hwa+Yang) p Parton distribution p 1 +p 2 pq (recombine)(fragment) hadron momentum Mesons(2 quarks): Baryons(3 quarks): F: joint distribution of partons T: thermal parton(low pt) S: shower parton(high pt)  The traditional hadronization: high momentum partons fragment into hadrons  Recombination as a hadronization process: lower momentum partons recombine to a hadron. May cause higher yield at some p T region.

Recombination model on d+Au data proton 0-20%/60-90%  p

dE/dx at higher p T Momentum: GeV/c dE/dx of  K,p) separation: 2  Log10(p) Log10(dE/dx)

Identified Particles at High p T STAR:TPC dE/dx + TOF; Topological Identified Particles

Open Issues 1)Is there a MACH cone effect and how the energy loss of partons is distributed in the medium? 2) Light quark (u,d,s) versus heavy quark (c,b), and quarks versus gluons 3) Multi-parton correlations and coalescence recombination models

The END

Comparison with calculations Any of pQCD calculations describe data well –Adding kT broadening makes factor of ~2 difference Around same factor as E706 –Calculation suggests that slopes of the spectra at RHIC and E706 are same Jet Photon included calculation ( Fries et al., PRL 90, (2003) ) is also shown –Fits very well above 4GeV! –Assuming existence of hot dense medium Prompt partons scatter with thermal partons –The line approaches to simple pQCD calculation in high pT

Photon-hadron correlations STAR preliminary  +jet correlation in Au+Au in run4? More accurate determination of initial Et

Jet Photon overwhelms QGP? Break-up of Fries prediction Jet Photons overwhelms all the other contributions below 7GeV/c Jet production rate calculated by LO pQCD with K factor compensation of 2.5 pQCD photon calculation from LO with no K factor Fitting too good! –In Peripheral, the calculation should fit the data as well R AA and spectra themselves tell you what happens –Calculation is assuming existence of hot dense medium, which is not the case in peripheral!

Results for p-p NLO-pQCD calculation –CTEQ6M PDF. –Gluon Compton scattering + fragmentation photon –Set Renormalization scale and factorization scale pT/2,pT,2pT Systematic Error: –20(high pT)-45(low pT)% The theory calculation shows a good agreement with our result. (Subtraction) Bands represent systematic errors. Errors on the backgrounds result in enlarged errors on the signal, especially at low-pT region.

Implication of the Experimental Observation 1) At the moment of hadronization in nucleus-nucleus collisions at RHIC the dominant degrees of freedom is related to number of constituent (valence) quarks. 2) These ‘constituent quarks’ exhibit an angular anisotropy resulting from collective interactions. 3) Hadrons seem to be formed from coalescence or recombination of the ‘constituent quarks’, and the hadron properties are determined by the sum of ‘constituent quarks’. Is this picture consistent with recent LQCD on spectral function calculations near T c ?