STAR Physics Highlights

Huan Zhong HuangDepartment of Physics and Astronomy

University of California Los Angeles

@ShanDong UniversityOct. 16-18, 2003

Outline

• Quark-Gluon Plasma (QGP) and Relativistic Heavy Ion Collider (RHIC)

• Salient Features of Particle Production at High, Intermediate and Low pT

• Spin Physics Program

• Outlook

Quark-Hadron Phase Transition

STAR

Relativistic Heavy Ion Collider --- RHIC

Au+Au 200 GeV N-N CM energyPolarized p+p up to 500 GeV CM energy

The STAR Collaboration: 49 Institutions, ~ 500 PeopleEngland:

University of BirminghamFrance:

Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes

Germany: Max Planck Institute – Munich University of Frankfurt

India:Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC

Netherlands:

NIKHEFPoland:

Warsaw University of TechnologyRussia:

MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP - Protvino

U.S. Labs: Argonne, Berkeley, and Brookhaven National Labs

U.S. Universities: UC Berkeley, UC Davis, UCLA, Caltech, Carnegie Mellon, Creighton, Indiana, Kent State, MIT, MSU, CCNY, Ohio State, Penn State, Purdue, Rice, Texas A&M, UT Austin, Washington, Wayne State, Valparaiso, Yale

Brazil: Universidade de Sao Paolo

China: IHEP - Beijing, IPP - Wuhan, USTC,Tsinghua, SINR, IMP Lanzhou

Croatia: Zagreb University

Czech Republic: Institute of Nuclear Physics

Ideas for QGP Signatures

Strangeness Production: (J.Rafelski and B. Muller PRL 48, 1066 (1982))

s-s quark pair production from gluon fusions in QGP leads to strangeness equilibration in QGP most prominent in strange hyperon production (and anti-particles).

Parton Energy Loss in a QCD Color Medium:(J.D. Bjorken Fermilab-pub-82-059 (1982) X.N. Wang and M. Gyulassy, PRL 68, 1480 (1992))

Quark/gluon

Quark/gluon dE/dx in color medium is large!

Ideas for QGP Signatures

Chiral Symmetry Restoration: T = 0, m(u,d,s) > 0 – Spontaneous symmetry breaking T> 150 MeV, m=0 – Chiral symmetry restored Mass, width and decay branching ratios of resonances may be different in dense medium .

QCD Color Screening: (T. Matsui and H. Satz, Phys. Lett. B178, 416 (1986))

A color charge in a color medium is screened similar to Debye screening in QED the melting of J/.

c c Charm quarks c-c may not bindInto J/ in high T QCD medium

The J/ yield may be increased due to charm quark coalescence at the final stage of hadronization (e.g., R.L. Thews, hep-ph/0302050)

Nucleus-Nucleus Collisions and Volcanic Eruption

Volcanic high pT -- Strombolian eruption Volcanic mediate pT – Spatter (clumps)

Volcanic low pT – Bulk matter flows

Jet Quenching in QCD Color Matter

leading particle

qq q

q

q q

Mono-JetNo back-to-back jet correlationHigh pT yield suppresion

Tangential jets from surface Complete quenching

of di-jet events

leading particle

Contribute to surface emission pattern !!

Contribute to residual back-to-back particle correlation!

Disappearanceof back-to-back correlation !

Disappearance of back-to-back angular correlations

x

y ptrigpss

pos

Ptrig – pss same side correlation

Ptrig – pos opposite side corr.

ptrig> 4 GeV/c, pss pos 2<pT<ptrig

Naïve Expectation for Au+Au

ddp

Nd

collddpNd

TAA

T

pp

T

AA NpR 2

2

/)(

Use number of binary nucleon-nucleon collisions to gauge the colliding parton flux:

N-binary Scaling RAA or RCP = 1 simple superposition of independent nucleon-nucleon collisions !

Peripheralcoll

T

Centralcoll

TTCP

NddpNd

NddpNd

pR

]/[

]/[

)( 2

2

High pT particles are from hard scattering of partons --

Suppression of high pT particles

pT Spectra Au+Au and p+p

p+p

Au+Au 0-5%

RAA=(Au+Au)/[Nbinaryx(p+p)]

Strong high pT 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 pT 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

qq

qq

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 pT suppression ! No disappearance of back-to-back correlations!

High pT 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 !

Intermediate pT Region

Volcanic mediate pT – Spatter (clumps)

At RHIC intriguing experimental features: multi-quark clustering enhanced baryon over meson production strangeness equilibration increased multi-strange hypeons

Elliptic Flow Parameter v2

y

x

py

px

coordinate-space-anisotropy momentum-space-anisotropy

Initial/final conditions, dof, EOS

1i

Ritttt

))ψcos(i(2v1dydpp

dN

2π

1

dyddpp

dN

STAR

PHENIX

Particle Dependence of v2

Baryon

Meson

Why saturation at intermediate pT ?Why baryon and meson difference ?

Multi-Parton Dynamics

KS – two quark coalescence– three quark coalescence from the partonic matter surface?!

Particle v2 may be related to quark matter anisotropy !!

pT < 1 GeV/c may be affected by hydrodynamic flow !

Hadronization Scheme for Bulk Partonic Matter:

Quark Coalescence – (ALCOR-J.Zimanyi et al, AMPT-Lin et al,Molnar+Voloshin …..)

Quark Recombination – (R.J. Fries et al….)

STAR+PHENIX

Multi-strange Baryons Flow Too

Hydro calcs.: P. Huovinen et al., Phys. Lett. B503, 58(2001).

Nuclear Modification Factor RAA RCP

Multi-parton dynamics predict baryon yield increases with centrality FASTER than mesons! Yield ~ n and n>nK a feature not present in single parton fragmentation !

Multi-parton dynamics: coalescence, recombination and gluon junctions.

RCP

RCP=[yield/N-N]central

[yield/N-N]peripheral

Intermediate pT Dynamics

Multi-parton dynamics – clustering of quarks – could be responsible for -- increased baryon production -- strange baryon enhancement -- strong elliptic flow at intermediate pT !!!

Hadronization of bulk partonic matter -- different phenomenon from e+e- collisions !

Low pT Phenomenon at RHIC

Volcanic mediate pT – Spatter (clumps) Volcanic low pT – Bulk matter flows

Prominent features at low pT: bulk matter flows ! 1) Thermal statistical models can describe the

yield of most particles. 2) Particle pT spectra and elliptic flow v2 – hydrodynamics.

= 0 0.5 Infinite

Nuclear Collision Evolution Epoches

Chemical Freeze-out --- formation of hadrons

Kinetic Freeze-out --- Interaction ceases

Between Two Epoches: Resonance Physics

Au+Au 40% to 80%

1.2 pT 1.4 GeV/c

|y| 0.5

STAR Preliminary

K*0

*(1520)

STAR preliminary p+p at 200 GeV

, f, f00, , *(892), *(892), , , , , *(1385), *(1385), *(1520)*(1520)

ρ0

f0

K0S

ω K*0

f2

0 & f0

++

p+p

Kinetic Freeze-out Condition

Hydrodynamics-inspired model fit most particles decouple at T~ 100 MeV and expansion velocity ~ 0.55c !Some particles decouple at earlier time because of smaller coupling strength with the hadronic medium!

important messengers of partonic matter !

Messenger for Conditions at Phase Boundary

Particles with small hadronic rescattering cross sections can be used to probe phase boundary at the hadron formation: , , , D, J/ ..........

KKSTAR Preliminary

KDD 00 KD

STAR Preliminary

STAR Preliminary

Transition from Soft to Semi-Hard Dynamics

PT 1.5-4 GeV/c TOF PID Region !

Non-Trivial Dynamics at Intermediate pT

Enhanced Baryon Yield at Intermediate pT cannot be due to Cronin Effect Alone !

d+Au

Au+Au

STAR TOFr physics paper (see Jian Wu’s talk)

What has been learned

1)Evidence for high energy density matter produced in central Au+Au collisions – high pT particle suppression and disappearance of back-to-back correlations.

2)Multi-parton dynamics for particle formations – hadronization of bulk partonic matter.

3)Hydrodynamical behavior at low pT – collective partonic dynamics and thermal statistical description of particle production.

Discoveries from Unexpected Areas?!

RHIC -- Frontier for bulk partonic matter formation -- Factory for exotic particles/phenomena

Potential exotic particles/phenomena:penta-quark states (uudds, uudds!)

di-baryonsH – (, uuddss) [] (ssssss)

strange quark matter

meta-stable Parity/CP odd vacuum bubblesdisoriented chiral condensate……

Spin Physics Program

The Spin Structure of the Proton:

½ = ½ q + G + <L>

q up, down and strange quarksG gluonsL angular momentum of quarks and gluons

Experimentally: 1) total spin in quarks ~ 30% 2) sea quarks are polarized too 3) little info about the gluon polarization 4) even less know about <L> and how to measure <L>

RHIC Spin PhysicsAt RHIC we use polarized p+p collisions to study1) Gluon spin structure function q+gq+2) Sea quark spin structure function q+qW boson3) Quark transverse spin distribution

Syst. Uncer. = ±0.05

STAR FPD Preliminary DataAssuming AN(CNI)= 0.013

pT=1.1 - 2.5 GeV/c

s=200 GeV

p+ p 0 + X

A

N

0

0.2

0.4

-0.20 0.2 0.4 0.6 0.8 1.0

xF

First RHIC spin result onsingle spin asymmetry !

RHIC Physics Outlook

Heavy Ion Physics: 1) discovering the Quark Gluon Plasma 2) Properties of high density QCD matter 3) Chiral symmetry at high temperature and density 4) Search for exotic particles/phenomena at RHIC

RHIC Spin Physics Using Polarized p+p Collisions: 1) the gluon spin structure function major milestone to understand the spin of the proton! 2) sea quark spin structure function 3) quark transverse spin distribution

End of Talk

STAR Physics ApproachesEmphasis on Observables Sensitive to Early Partonic Stages:

1) High pT particles – Jet quenching? New baryon dynamics? 2) Particle fluctuations and large scale correlations – probe

conditions near phase boundary.3) Partonic collective flow observables especially for those particles

believed to have small hadronic re-scattering cross sections, D mesons and J/.

4) D meson production for initial gluon flux and structure function of nuclei.

5) J/ for possible color screening effect6) Direct and thermal photon production

Precision Measurements to Map Out Hadronic Evolution Dynamics

1) Resonances (,,f0, K*, , (1385) and(1520)) – Sensitive tohadronic evolution between chemical and kinetic freeze-out

2) Momentum-space-time relations at the kinetic freeze-out thrucorrelations of identical, non-identical pairs, and light clusters.

We need TOF upgrade !!

A Pictorial View of Micro-Bangs at RHIC

Thin PancakesLorentz =100

Nuclei pass thru each other

< 1 fm/c

Huge StretchTransverse ExpansionHigh Temperature (?!)

The Last Epoch:Final Freezeout--

Large Volume

Au+Au Head-on Collisions 40x1012 eV ~ 6 micro-Joule

Human Ear Sensitivity ~ 10-11 erg = 10-18 Joule

A very loud Bang, indeed, if E Sound……

Ultra-Peripheral Collision Physics

STAR Preliminary, 200 GeV Au+Au

dN

/dt

(GeV

2)-1

Data (w/ fit) No Interference Interference

t = pT2 (GeV2)

Au+Au at 200 GeV

1) Two indistinguishable possibilities: A photon from nucleus 1 scatters from 2 A photon from nucleus 2 scatters from 1

2) Negative parity – destructive inter. ~ |A1 - A2eip·b|2

- At y=0 =0[1-cos(pb)] - pT <<1/<b> - For 0 w/ XnXn <b> ~ 20 fm

- Clear signal of interference!Clear signal of interference!

Au beam as virtual photon source Strong Field QED and Spectroscopy

Strangeness from Bulk Partonic Matter

Strangeness enhancement is most prominent at intermediate pT from quark coalescence in an equilibrated bulk matter !

What More Measurements ?

• Experimentally determine the amount of jet energy loss? Where did the energy loss go (increase in soft particle emissions?)

• Is the experimental energy loss consistent with theoretical calculation of dE/dx from a QCD medium, not with a hadronic medium?

• Signatures of QCD deconfinement?• Peoperties of bulk partonic matter at the phase

boundary?

Practically we need Au+Au, Si+Si …… at several beam energies , , J/, open charm mesons, direct photons…..

y

Dynamical Origin of Elliptic Flow

STAR PreliminaryAu+Au 200 GeV

V2 in the high pT region: should large parton energy loss lead to surface emission pattern ?! Particle Dependence of v2 ?

Collective Pressure

High pressure gradientLarge expansion velocity

Small expansion velocity

pT dependent !

Surface Geometrical Phase Space

Surface Emission PatternHigh particle density

Low particle density

pT independent ! orpT dependence may comefrom surface thickness (pT)

x

Energy Scale and Phase Transition

Entity Energy Dimension Physics Bulk Property P/T

Atom 10’s eV 10-10 m Ionization e/Ion Plasma No

Nucleus 8 MeV 10-14 m Multifrag. Liquid-Gas Y(?)

QCD 200 MeV 10-15 m Deconfine. QGP Y(?)

EW 100 GeV 10-18 m P/CP Baryon Asymmetry Y(?)

GUT 1015-16 GeV Supersymmetry

TOE 1019 GeV Superstring

A Magnificent Collision in the Universe

Collision of two galaxies: the Antennae; Hubble Space Telescope

Quarks and Quantum ChromoDynamics

Baryon Density: = baryon number/volumenormal nucleus 0 ~ 0.15 /fm3 ~ 0.25x1015 g/cm3

Temperature, MeV ~ 1.16 x 1010 K10-6 second after the Big Bang T~200 MeV

Three family of quarks

Up Charm TopDown Strange Beauty

Strong interaction is due to color chargesColor Interaction mediated by gluons (EM by photons)Gluons carry color charges (photons electric neutral)

Ordinary Matter: proton (uud) and neutron (udd)