STAR Physics Highlights Huan Zhong Huang Department of Physics and Astronomy University of California Los University Oct , 2003

Outline Quark-Gluon Plasma (QGP) and Relativistic Heavy Ion Collider (RHIC) Salient Features of Particle Production at High, Intermediate and Low p T Spin Physics Program Outlook

Quark-Hadron Phase Transition

STAR Relativistic Heavy Ion Collider --- RHIC Au+Au 200 GeV N-N CM energy Polarized p+p up to 500 GeV CM energy

The STAR Collaboration: 49 Institutions, ~ 500 People England: University of Birmingham France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes Germany: Max Planck Institute – Munich University of Frankfurt India: Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC Netherlands: NIKHEF Poland: Warsaw University of Technology Russia: 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 (1982) X.N. Wang and M. Gyulassy, PRL 68, 1480 (1992)) 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/ . cc Charm quarks c-c may not bind Into 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/ )

Nucleus-Nucleus Collisions and Volcanic Eruption Volcanic high p T -- Strombolian eruption Volcanic mediate p T – Spatter (clumps) Volcanic low p T – Bulk matter flows

Jet Quenching in QCD Color Matter leading particle q q q q qq Mono-Jet No back-to-back jet correlation High p T 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!

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

Naïve Expectation for Au+Au Use number of binary nucleon-nucleon collisions to gauge the colliding parton flux: N-binary Scaling  R AA or R CP = 1 simple superposition of independent nucleon-nucleon collisions ! High p T particles are from hard scattering of partons --

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 !

Intermediate p T Region Volcanic mediate p T – Spatter (clumps) At RHIC intriguing experimental features: multi-quark clustering  enhanced baryon over meson production strangeness equilibration  increased multi-strange hypeons

Elliptic Flow Parameter v 2 y x pypy pxpx coordinate-space-anisotropy  momentum-space-anisotropy Initial/final conditions, dof, EOS

STAR PHENIX Particle Dependence of v 2 Baryon Meson Why saturation at intermediate p T ? Why baryon and meson difference ?

Multi-Parton Dynamics K S – two quark coalescence  – three quark coalescence from the partonic matter surface?! Particle v 2 may be related to quark matter anisotropy !! p T < 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 R AA R CP Multi-parton dynamics predict baryon yield increases with centrality FASTER than mesons! Yield ~  n and n  >n K  a feature not present in single parton fragmentation ! Multi-parton dynamics: coalescence, recombination and gluon junctions. R CP R CP = [yield/N-N] central [yield/N-N] peripheral

Intermediate p T Dynamics Multi-parton dynamics – clustering of quarks – could be responsible for -- increased baryon production -- strange baryon enhancement -- strong elliptic flow at intermediate p T !!! Hadronization of bulk partonic matter -- different phenomenon from e+e- collisions !

Low p T Phenomenon at RHIC Volcanic mediate p T – Spatter (clumps) Volcanic low p T – Bulk matter flows Prominent features at low p T : bulk matter flows ! 1) Thermal statistical models can describe the yield of most particles. 2) Particle p T spectra and elliptic flow v 2 – hydrodynamics.

 = 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  p T  1.4 GeV/c |y|  0.5 STAR Preliminary K *0   *(1520) STAR preliminary p+p at 200 GeV , f 0,  *(892), , ,  *(1385),  *(1520) ρ 0 f 0 K 0 S ω K *0 f 2  0 & f 0  ++ 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/  STAR Preliminary 

Transition from Soft to Semi-Hard Dynamics P T GeV/c  TOF PID Region !

Non-Trivial Dynamics at Intermediate p T Enhanced Baryon Yield at Intermediate p T 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 p T 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 p T – 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-baryons H – ( , uuddss) [  ] (ssssss) strange quark matter meta-stable Parity/CP odd vacuum bubbles disoriented chiral condensate ……

Spin Physics Program The Spin Structure of the Proton: ½ = ½   q +  G + q  up, down and strange quarks G  gluons L  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 and how to measure

RHIC Spin Physics At RHIC we use polarized p+p collisions to study 1)Gluon spin structure function q+g  q+  2)Sea quark spin structure function q+q  W boson 3)Quark transverse spin distribution Syst. Uncer. = ±0.05 STAR FPD Preliminary Data Assuming A N (CNI)= p T = GeV/c  s=200 GeV p  + p   0 + X A N xFxF First RHIC spin result on single 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 Approaches Emphasis on Observables Sensitive to Early Partonic Stages: 1) High p T 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 effect 6)Direct and thermal photon production Precision Measurements to Map Out Hadronic Evolution Dynamics 1)Resonances ( , ,  f 0, K *, ,  (1385) and  (1520)) – Sensitive to hadronic evolution between chemical and kinetic freeze-out 2)Momentum-space-time relations at the kinetic freeze-out thru correlations of identical, non-identical pairs, and light clusters. We need TOF upgrade !!

A Pictorial View of Micro-Bangs at RHIC Thin Pancakes Lorentz  =100 Nuclei pass thru each other < 1 fm/c Huge Stretch Transverse Expansion High Temperature (?!) The Last Epoch: Final Freezeout-- Large Volume Au+Au Head-on Collisions  40x10 12 eV ~ 6 micro-Joule Human Ear Sensitivity ~ erg = 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 = p T 2 (GeV 2 ) 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.  ~ |A 1 - A 2 e ip·b | 2 - At y=0  =  0 [1-cos(p  b)] - p T - For  0 w/ XnXn ~ 20 fm - 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 p T 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 Preliminary Au+Au 200 GeV V 2 in the high p T region: should large parton energy loss lead to surface emission pattern ?! Particle Dependence of v 2 ? Collective Pressure High pressure gradient Large expansion velocity Small expansion velocity p T dependent ! Surface Geometrical Phase Space Surface Emission Pattern High particle density Low particle density p T independent ! or p T dependence may come from surface thickness (p T ) x

Energy Scale and Phase Transition Entity Energy Dimension PhysicsBulk PropertyP/T Atom10’s eV mIonizatione/Ion PlasmaNo Nucleus 8 MeV mMultifrag.Liquid-GasY(?) QCD200 MeV mDeconfine.QGPY(?) EW100 GeV mP/CP Baryon AsymmetryY(?) GUT GeVSupersymmetry TOE10 19 GeVSuperstring

A Magnificent Collision in the Universe Collision of two galaxies: the Antennae; Hubble Space Telescope

Quarks and Quantum ChromoDynamics Baryon Density:  = baryon number/volume normal nucleus  0 ~ 0.15 /fm 3 ~ 0.25x10 15 g/cm 3 Temperature, MeV ~ 1.16 x K second after the Big Bang T~200 MeV Three family of quarks UpCharmTop DownStrangeBeauty Strong interaction is due to color charges Color Interaction mediated by gluons (EM by photons) Gluons carry color charges (photons electric neutral) Ordinary Matter: proton (uud) and neutron (udd)