Have we seen a QGP at RHIC?

	What we have learned & what we need to do
	Yasuo MIAKE, Univ. of Tsukuba

RHIC and its operation

	New machine at BNL, operational since 2000.
	Series of measurements;
		Au+Au collisions at ÖsNN = 200 GeV as highest & largest (240 mb-1)
		Au+Au at ÖsNN = 19.6, 130, 200 GeV as energy scan
		p+p at ÖsNN = 200 GeV as  comparison data
		d+Au ÖsNN = 200 GeV as controlled comparison

Npart vs Nbinary

	Since nucleus is extended object, centrality of collision plays important role.
	For comparison with pp or dAu also for centrality study, we need scaling variables.
	Npart;
		# of participant nucleons
		Particle production in hA is prop. to Npart, (Wounded-Nucleon Model)
	Nbinary;
		# of binary nucleon-nucleon collisions
		Pass through at high energy.
	Evaluation of Npart & Nbinary by Glauber Model.

Particle production　(h distr.)

	Data taken at RHIC-Phobos
	dn/dh ; wider & larger in higher energies.
	Not very flat even at 200 GeV
	Features of limiting fragmentation seen
		Simple application of Lorentz Invariance cannot be done!

Particle production (total mult.)

	Total multiplicity per Npart stays constant.
		WNM holds in AA.
		Slight deviation at higher collision energies.
	Deviation from Npart scaling more visible at mid-rapidity.

Transverse momentum distr.

	p0 measured in Au+Au
	Second component at high pt region.
		Exponential + Power law
		Importance of hard process at high pt region
		èComparison with pp

Expected scaling behavior

	Total mult; Npart scaling
		Low pt region
	Jets; Nbinary scaling
		High pt region
	èExpected behavior;
		From Npart scaling at low pt to Nbinary scaling at high pt region.

High pt suppression (Jet Quench)

	Taken as a big surprise!
	Clear high pt suppression in Au+Au, while not observed in d+Au.
	Since not seen in dAu, effect is not due to initial state, but final state.

Disappearance of back-to-back corr.

	Direct evidence of loss of ‘jet’
	Azimuthal correlation w.r.t. high pt leading particle (trigger).
		pp ; clean di-jet
		dAu; similar to pp
		Au+Au; Similar on the same side, but b-to-b disappeared
			Effect is not in initial but in final stage
			Energy loss of partons in dense matter created in Au+Au

Energy loss of parton

	Energy loss of charged particle in matter;
		Collisions with atomic electrons, proportional to the electron density
		Radiative energy loss.
			èBether-Heitler Formula
	In QCD, major loss will be radiative
		Energy loss of parton should be proportional to the gluon density

Thermal equilibrium

	Particle yield ratios well parameterized with Tch, mq,ms.
		Chemical Eq. holds
		Strangeness
		Tch ~ 170 - 180 MeV
	Transverse distr well parameterized with Tth, br.
		Thermal Eq.
		Tth ~ 110 - 120 MeV

Azimuthal anisotropy v2

	In non-central col., participant has almond shape at initial stage.
	Emission of particle in azimuth is influenced by l & R relation.
		l >> R ; isotropic
		l << R ; hydro.èelliptic
			Anisotropy of the coordinate space converted to that of the momentum space.
	As the system expands, effects vanishes
		Sensitive to the initial stage

Large azimuthal anisotropy

	Getting larger & larger in higher energies.
		Scaling w. h-ybeam !?

Failure of hadronic scenarios

	Hadronic scenario underestimates v2 at RHIC.
		V2 ~ 1 - 2 %
	System thermalized early with the mechanism other than hadronic rescatterings.

From hydro to jet region

	Low pt region;
		v2(p) > v2(K) > v2(p)
		Good agreement with hydrodynamics
			Very early thermalization (0.6 fm/c) required !
	Deviations from the hydro at higher pt ; (> 2 GeV/c) jet region
		v2(p,K) < v2(p)
			Order Reversed !
	What is the mechanism to create v2 in the jet region?
		èEnergy loss of parton !

Quark recombination model

	Other possible production mechanism of high pt hadrons than the frag.
	Quarks, anti-quarks combine to form mesons and baryons.
		W2 for meson, w3 for baryon
		Bacause of the steep distr., this process wins at mid-pt.
		Characteristic scaling features expected.
		èQuark number scaling

Proton dominance by RECO

	Recombination model explains the proton dominance.

Quark number scaling

	Quark number scaling clearly observed in v2.
	Distinct difference between Baryon Meson also seen in RCP

Summary

	We have seen partonic matter,ie, a QGP!
	Successful description of the system in terms of statistical thermo-dynamics;
		Particle yield ratios in Tch, m
		Kinematical distribution in Tth and b
	Partonic
		Large azimuthal anisotropy cannot be created with hadronic process.
		High pt suppression and disappearance of back-to-back is at parton level.
		Successful description of quark recombination;
			Phenomenological, but universal quark distribution function!

What need to be done

	What we have not seen is the phase transition.
		When and how it happens need to be investigated
	Homeworks
		HBT puzzle
		J/y suppression?
		Direct photon
		Energy loss of charm
			Azimuthal anisotropy of charm in study

J/y in Au+Au collisions

	No clear conclusion yet.
		100 times better statistics recorded on tape.
		Data analysis in progress !

Open charm production & v2

	consistent with Ös systematics and binary scaling.
	Analysis of v2 in progress.

HBT puzzle

	Extended life time of fireball expected if QGP.
	3D analysis of HBT
		Rout/Rside should reflect life time of fireball.
		Measured Rout/Rside ~ 1 !?
	Failure of theories

Backups

Ratios of particle production

	Hadro-chemical eq. holds.

Transverse momentum ditr.

	PID by high res. TOF.
	Flatter pt distr for heavier particle mass
		Known as effects by collective expansion.
	Proton dominance > 2 GeV/c, where the proton/pion ratio ~ 1
		increases with centrality
		most peripheral ~ ee/pp
		peak(?) at 2-3 GeV/c
		frag. process should show np> np as seen in ee/pp
	Clear heavy ion effect

Collective expansion in pt ditr

	Kinematical eq.+Collective expansion velocity
	Fit with low pt region

Energy dependence of v2 (1)

Energy dependence of v2 (2)

RHIC vs SPS