STAR Tom Humanic The Ohio State University FNAL --Feb 2003 Physics at RHIC Results from the RHIC STAR Experiment

Outline Motivation for studying Relativistic Heavy Ion Collisions RHIC and the STAR experiment Soft Physics from STAR Hard Physics from STAR Summary

Why heavy ion collisions? Study bulk properties of nuclear matter The “little bang” Extreme conditions (high density/temperature) expect a transition to new phase of matter… Quark-Gluon Plasma (QGP) partons are relevant degrees of freedom over large length scales (deconfined state) believed to define universe until ~  s Heavy ion collisions ( “little bang”) the only way to experimentally probe deconfined state Study of QGP crucial to understanding QCD low-q (nonperturbative) behaviour confinement (defining property of QCD) nature of phase transition

The “little bang” Stages of the collision pre-equilibrium (deposition of initial energy) rapid (~1 fm/c) thermalization (?) high-p T observables probe this stage QGP formation (?) hadronic rescattering hadronization transition (very poorly understood) Chemical freeze-out: end of inelastic scatterings Kinetic freeze-out: end of all scatterings low-p T hadronic observables probe this stage Does “end result” look about the same whether a QGP was formed or not??? time temperature

The Phase Space Diagram TWO different phase transitions at work! – Quarks and gluons roam freely over a large volume – Quarks behave as though they are massless Calculations show that these occur at approximately the same point Two sets of conditions: High Temperature High Baryon Density Lattice QCD calc. Predict: T c ~ MeV  c ~ GeV/fm Deconfinement transition Chiral transition Quark-Gluon Plasma Hadrons

Beam energy up to 100 GeV/A (250 GeV for p); Two independent rings (asymmetric beam collisions are possible); Beam species: from proton to Au; Six interaction points: STAR, PHENIX, PHOBOS and BRAHMS

RHIC Data-Taking Year 2000:Au GeV 2 weeks Year 2001:Au GeV15 weeks Au + 20 GeV 1 day p GeV 5 weeks Year 2003: 1 st of January d GeV 10 weeks p GeV (5) + 3 weeks 

Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP- Protvino U.S. Labs: Argonne, Berkeley, Brookhaven National Labs U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, MSU, CCNY, Ohio State, Penn State, Purdue,Rice, Texas A&M, UT Austin, Washington, Wayne State, Yale Brazil: Universidade de Sao Paolo China: IHEP - Beijing, IPP - Wuhan England: University of Birmingham France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes Germany: Max Planck Institute – Munich University of Frankfurt Poland: Warsaw University, Warsaw University of Technology Institutions: 36 Collaborators: 415 The Ohio State U. Group Profs: PostDocs: Students: T.Humanic D.Majestro S.Bekele M.Lisa B. Nilsen M.Lopez- Noriega E.Sugarbaker I. Kotov R.Wells R.Willson The STAR Collaboration

The STAR Detector

Year 2000, year 2001, year-by-year until 2003, installation in 2003 ZCal Silicon Vertex Tracker * Central Trigger Barrel + TOF patch FTPCs (1 + 1) Time Projection Chamber Vertex Position Detectors Magnet Coils RICH * yr.1 SVT ladder Barrel EM Calorimeter TPC Endcap & MWPC Endcap Calorimeter ZCal

STAR Time Projection Chamber (TPC) Active volume: Cylinder r=2 m, l=4 m –139,000 electronics channels sampling drift in 512 time buckets –active volume divided into 70M 3D pixels On-board FEE Card: Amplifies, samples, digitizes 32 channels

Spectators – Definitely going down the beam line Participants – Definitely created moving away from beamline Triggering/Centrality Impact Parameter Spectators Zero-Degree Calorimeter Participants Several meters

Spectators – Definitely going down the beam line Participants – Definitely created moving away from beamline Triggering/Centrality Impact Parameter Spectators Zero-Degree Calorimeter Participants Several meters “Minimum Bias” ZDC East and West thresholds set to lower edge of single neutron peak. REQUIRE: Coincidence ZDC East and West “Central” CTB threshold set to upper 15% REQUIRE: Min. Bias + CTB over threshold ~30K Events |Z vtx | < 200 cm

Au-Au Event at 130 A-GeV Peripheral Event From real-time Level 3 display.

Au- Au Event 130 A-GeV Mid-Central Event From real-time Level 3 display.

Au -Au Event 130 A-GeV Central Event From real-time Level 3 display.

STAR Pertinent Facts (130 GeV) Field: 0.25 T (Half Nominal value)  worse resolution at higher p lower p t acceptance TPC: Inner Radius – 50cm (p t >75 MeV/c) Length – ± 200cm ( -1.5  1.5) Events: ~300,000 “Central” Events –top 8% multiplicity ~160,000 “Min-bias” Events

Needle in the Hay-Stack! How do you do tracking in this regime? Solution: Build a detector so you can zoom in close and “see” individual tracks Good tracking efficiency Clearly identify individual tracks high resolution P t (GeV/c)

Particle ID Techniques - dE/dx

dE/dx PID range: ~ 0.7 GeV/c for K /  ~ 1.0 GeV/c for K/p dE/dx

Particle ID Techniques - dE/dx dE/dx PID range: ~ 0.7 GeV/c for K /  ~ 1.0 GeV/c for K/p dE/dx 6.7%Design 7.5%With calibration 9 %No calibration Resolution: Even identified anti- 3 He !

Particle ID Techniques - Topology Decay vertices K s   + +  -   p +  -   p +  +  -   +  -  +  +  +    + K -   “kinks”: K     + VoVo

Physics Measurements (ones in red will be shown) dN/d  for h- (|  |<= ~1.5) particle density, entropy Elliptic flow early dynamics, pressure p/p,  /  stopping Particle spectra temperature, radial flow Particle ratios  chemistry Particle correlations geometry, collective flow High P t jet quenching _ _ Neutral particle decays ,K 0 s,  strangeness production

Transverse Energy PHENIX Preliminary Phenix Electromagnetic Calorimeter measures transverse energy in collisions Central Events: Lattice predicts transition at  ~ 5.0 GeV/fm 3  critical ~ GeV/fm 3 Have the Energy Density!!

Soft Physics (p T < 2 GeV/c)

99.5% The majority of produced particles are low p T. Do they interact and exhibt collective behaviour? What are the bulk dynamics ?

Is there Thermalization? Almond shape overlap region in coordinate space Origin: spatial anisotropy of the system when created and rescattering of evolving system  probe of the early stage of the collision Look at “ Elliptic ” Flow

Elliptic Flow of Pions and Protons from STAR (130 GeV) Hydrodynamic calculations: P. Huovinen, P. Kolb and U. Heinz Mass dependence of v 2 (p t ) shows a behavior in agreement with hydro calculations, which assumes a system in equilibrium

Charged particle elliptic flow 0< p t < 4.5 GeV/c from STAR(130 GeV) Around p t > 2 GeV/c the data starts to deviate from hydro. However, v 2 stays large. Only statistical errors Systematic error 10% - 20% for p t = 2 – 4.5 GeV/c

Kinetic Freeze-out and Radial Flow Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

Kinetic Freeze-out and Radial Flow Look at m t =  (p t 2 + m 2 ) distribution A thermal distribution gives a linear distribution dN/dm t  e -(mt/T) mtmt 1/m t d 2 N/dydm t Slope = 1/T Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

Kinetic Freeze-out and Radial Flow If there is transverse flow Look at m t =  (p t 2 + m 2 ) distribution A thermal distribution gives a linear distribution dN/dm t  e -(mt/T) mtmt 1/m t d 2 N/dydm t Slope = 1/T Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

Kinetic Freeze-out and Radial Flow If there is transverse flow Look at m t =  (p t 2 + m 2 ) distribution A thermal distribution gives a linear distribution dN/dm t  e -(mt/T) mtmt 1/m t d 2 N/dydm t Slope = 1/T Slope = 1/T meas ~ 1/(T freeze out + 0.5m o  flow  2 ) Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

First RHIC spectra - an explosive source data: STAR, PHENIX, QM01 model: P. Kolb, U. Heinz various experiments agree well different spectral shapes for particles of differing mass  strong collective radial flow

First RHIC spectra - an explosive source data: STAR, PHENIX, QM01 model: P. Kolb, U. Heinz various experiments agree well different spectral shapes for particles of differing mass  strong collective radial flow mTmT 1/m T dN/dm T light heavy T purely thermal source

First RHIC spectra - an explosive source data: STAR, PHENIX, QM01 model: P. Kolb, U. Heinz various experiments agree well different spectral shapes for particles of differing mass  strong collective radial flow mTmT 1/m T dN/dm T light heavy T purely thermal source explosive source T,  mTmT 1/m T dN/dm T light heavy

First RHIC spectra - an explosive source data: STAR, PHENIX, QM01 model: P. Kolb, U. Heinz various experiments agree well different spectral shapes for particles of differing mass  strong collective radial flow mTmT 1/m T dN/dm T light heavy T purely thermal source explosive source T,  mTmT 1/m T dN/dm T light heavy good agreement with hydrodynamic calculations

T  = 190 MeV T  = 300 MeV T p = 565 MeV mid-rapidity m t slopes vs. Centrality Increase with collision centrality  consistent with radial flow: T freeze out =0.12 GeV,  flow =0.6c

We’ve shown so far that for RHIC collisions: Some evidence that source is thermalized Particles kinetically freeze-out with common T Large transverse flow - common to all particle species

“HBT 101” - probing source geometry 5 fm 1 m  source  (x) r1r1 r2r2 x1x1 x2x2 p1p1 p2p2 q = p 2 – p 1  T = U(x 1,p 1 )exp{i(r 1 -x 1 )p 1 }U(x 2,p 2 )exp{i(r 2 -x 2 )p 2 } + U(x 1,p 2 )exp{i(r 2 -x 1 )p 2 }U(x 2,p 1 )exp{i(r 1 -x 2 )p 1 } Integrate  *  over  (x) e.g.  ~ exp(-r 2 /2R 2 )  C = 1 + exp(-q 2 R 2 )

“HBT 101” - probing source geometry 5 fm 1 m  source  (x) r1r1 r2r2 x1x1 x2x2 p1p1 p2p2 1-particle probability 2-particle probability q = p 2 – p 1  T = U(x 1,p 1 )exp{i(r 1 -x 1 )p 1 }U(x 2,p 2 )exp{i(r 2 -x 2 )p 2 } + U(x 1,p 2 )exp{i(r 2 -x 1 )p 2 }U(x 2,p 1 )exp{i(r 1 -x 2 )p 1 } Integrate  *  over  (x) e.g.  ~ exp(-r 2 /2R 2 )  C = 1 + exp(-q 2 R 2 )

“HBT 101” - probing source geometry Measurable! F.T. of pion source 5 fm 1 m  source  (x) r1r1 r2r2 x1x1 x2x2 p1p1 p2p2 1-particle probability 2-particle probability q = p 2 – p 1  T = U(x 1,p 1 )exp{i(r 1 -x 1 )p 1 }U(x 2,p 2 )exp{i(r 2 -x 2 )p 2 } + U(x 1,p 2 )exp{i(r 2 -x 1 )p 2 }U(x 2,p 1 )exp{i(r 1 -x 2 )p 1 } Integrate  *  over  (x) e.g.  ~ exp(-r 2 /2R 2 )  C = 1 + exp(-q 2 R 2 )

“HBT 101” - probing the timescale of emission KK R out R side Decompose q into components: q Long : in beam direction q Out : in direction of transverse momentum q Side :  q Long & q Out (beam is into board) beware this “helpful” mnemonic! R O 2 = R S 2 = R L 2 =

HBT and the Phase Transition without transition “”“” with transition cc Rischke & Gyulassy NPA 608, 479 (1996) Generic prediction of 3D hydrodynamic models Primary HBT “signature” of QGP ~ emission timescale Phase transition  longer lifetime; R out /R side ~ 1 + (  )/R side

Two-pion interferometry (HBT) from STAR Correlation function for identical bosons: 1d projections of 3d Bertsch- Pratt 12% most central out of 170k events Coulomb corrected |y| < 1, < p t < q out STAR preliminary q long

Radii dependence on centrality and k t Radii increase with multiplicity - Just geometry (?) Radii decrease with k t – Evidence of flow (?) low k T central collisions   “multiplicity” STAR preliminary x (fm) y (fm)

Hydro attempts to reproduce data R out R side R long : model waits too long before emitting K T dependence approximately reproduced  correct amount of collective radial flow Right dynamic effect / wrong space-time evolution???  the “RHIC HBT Puzzle” generic hydro model emission timescale too long

HBT excitation function STAR Collab., PRL (2001) decreasing parameter partially due to resonances saturation in radii geometric or dynamic (thermal/flow) saturation the “action” is ~ 10 GeV (!) no jump in effective lifetime NO predicted Ro/Rs increase (theorists: data must be wrong) Lower energy running needed!? midrapidity, low p T  - from central AuAu/PbPb

time Evolution of a heavy-ion collision Before collision (heavy nuclei) After collision: QM formation?? Hadronization Strong hadronic rescattering “Freezeout” (hadrons freely stream to detectors) In order to study QM/hadronization stage of collision from freezeout hadrons, need to understand rescattering stage first!

Hadronic rescattering model (T. J. Humanic, Phys.Rev.C 57, 866, (1998)) 1) Assume a simple hadronization picture to set the initial geometry and momenta. 2) Put in a bunch of hadrons whose multiplicities are consistent with RHIC experiments (or predictions). 3) Let hadrons undergo strong binary collisions until the system gets so dilute (since it is expanding) that all collisions cease. 4) Record the time, mass, position, and momentum of each hadron when it no longer scatters.  freezout condition. 5) Calculate hadronic observables  p T distributions, elliptic flow, HBT, … 1/m T dN/dm T = m T exp(-m T /T) T = 300 MeV  parameters: initial temperature (T), hadronization proper time (  ) (cylindrical) (thermal)   K, N,  ’…..   (i,j) z r z =  sinh y ; t =  cosh y  = 1 fm/c

Comparison of the Rescattering model with RHIC data for p T distributions As seen above, the qualitative shapes are the same for p T < 3 GeV/c !

Comparison of Recattering model with RHIC data for m T distributions The slopes are seen to agree !

Elliptic Flow vs. p T from rescattering model compared with STAR flattening at high pT as in data

STAR  HBT vs Rescattering Model Rescattering qualitatively describes the centrality and momentum dependences of the pion HBT data!!

* * * * * * * * * * * * Comparison of Rescattering model with SPS and RHIC data for pion HBT ( rescattering model * ) Model is seen to describe the beam energy dependence of the HBT parameters well!

Conclusion for “soft” (i.e. low p T ) RHIC physics: Hadronic rescattering with a short hadronization time (  = 1 fm/c) describes dynamic features well!

Hard Physics: p T > 2GeV/c Goal: Use jets to probe properties of medium Some Basic Observables: - Inclusive Spectra and R AA - Azimuthal Anisotropy, v 2 - Statistical  &  Correlations STAR p+p  Di-Jet hadrons leading particle suppressed q q ?

The Experimental Challenge p+p  dijet Central Au+Au Event Find this ………………………………………………….in here

Inclusive Charged Hadron Production STAR, PRL 89, (2002)  s = 130 GeV  s = 200 GeV nucl-ex/

Leading Particle Suppression: Theory leading particle Wang and Gyulassy: partonic energy loss  proportional to gluon density,  glue  effective softening of fragmentation  suppression of leading hadron yield Nuclear Modification Factor: /  inel p+p Partonic Energy loss in high density matter hadrons q q leading particle hadrons leading particle suppressed leading particle suppressed q q (Nuclear Geometry)

Leading Hadron Suppression: Data STAR p+p reference in the works… Suppression saturates at 3~5 for p T > 6 GeV/c R CP  Central/Peripheral R AA using UA1 NN Reference  s = 200 GeV Preliminary Suppression similar at 130 GeV (PRL 89, (2002)) nucl-ex/

Azimuthal Correlations Px (GeV/c) Py (GeV/c) Pt   Correlation with respect to leading particle (p T >4 GeV/c) Consider only particles above 2 GeV/c Small difference in relative pseudorapidity |     

Peripheral Au + Au Central Au + Au Ansatz: Au+Au = p+p + Elliptic Flow High p T Azimuthal Correlations nucl-ex/  Near-side correlation shows jet-like signal in central/peripheral Au+Au  Away-side correlation suppressed in central Au+Au

Surface Emission of Jets ? ? This is in accordance with 1) the measured suppresion of the inclusive spectra with respect to binary collisions, and 2) high-p T azimuthal correlations. –We only ‘see’ jets emitted from the surface?

Suppression of away-side jet consistent with strong absorption in the bulk, with emission dominantly from the surface

Summary of Au+Au Collisions at RHIC Hard physics: Strong suppression of inclusive yields Azimuthal anisotropy at high p T Suppression of back-to- back hadron pairs Soft physics: System appears to be thermalized Rapid hadronization, strong rescattering Large radial flow, elliptic flow, and HBT results all explainable as resulting from hadronic rescattering Large parton energy loss with surface emission? ?

STAR STRANGENESS! K0sK0s  K+K+ (Preliminary) ̅̅   ̅̅ 

The Collisions The End Product