STAR Helen Caines The Ohio State University March 2001 Crossing a New Threshold First Results from the Relativistic Heavy Ion Collider Science is a wonderful thing if one does not have to earn one's living at it – Einstein (1879—1955)

Helen Caines OSU – March 2001 STAR Motivation Why Relativistic Heavy Ion Collisions? To study a hadronic matter at high energy density Early universe Center of stars To study the deconfined state of QCD Where is the phase transition? What order is it? To study the Vacuum – Chiral symmetry restoration Origin of (hadronic) mass

Helen Caines OSU – March 2001 STAR The Phase Space Diagram TWO different phase transitions at work! – Particles roam freely over a large volume – Masses change 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

Helen Caines OSU – March 2001 STAR most dangerous event in human history: - ABC News – Sept ‘99 Don’t Panic!!! "Big Bang machine could destroy Earth" -The Sunday Times – July ‘99 the risk of such a catastrophe is essentially zero. – B.N.L. – Oct ‘99 - New Scientist Will Brookhaven Destroy the Universe? – NY Times – Aug ‘99 No… the experiment will not tear our region of space to subatomic shreds. - Washington Post – Sept ‘99 Apocalypse2 – ABC News – Sept ‘99

Helen Caines OSU – March 2001 STAR Welcome to BNL- RHIC!

Helen Caines OSU – March 2001 STAR The Collisions The End Product

Helen Caines OSU – March 2001 STAR The STAR Detector (Year-by-Year) 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

Helen Caines OSU – March 2001 STAR How a TPC works 420 CM Tracking volume is an empty volume of gas surrounded by a field cage Drift gas: Ar-CH 4 (90%-10%) Pad electronics: amplifier channels with 512 time samples –Provides 70 mega pixel, 3D image

Helen Caines OSU – March 2001 STAR 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)

Helen Caines OSU – March 2001 STAR 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

Helen Caines OSU – March 2001 STAR Au-Au Event at 130 A-GeV Peripheral Event From real-time Level 3 display.

Helen Caines OSU – March 2001 STAR Au- Au Event 130 A-GeV Mid-Central Event From real-time Level 3 display.

Helen Caines OSU – March 2001 STAR Au -Au Event 130 A-GeV Central Event From real-time Level 3 display.

Helen Caines OSU – March 2001 STAR STAR Pertinent Facts 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

Helen Caines OSU – March 2001 STAR 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 !

Helen Caines OSU – March 2001 STAR Particle ID Techniques - Topology Decay vertices K s   + +  -   p +  -   p +  +  -   +  -  +  +  +    + K -   “kinks”: K     + VoVo

Helen Caines OSU – March 2001 STAR STAR STRANGENESS! K0sK0s  K+K+ (Preliminary) ̅̅   ̅̅ 

Helen Caines OSU – March 2001 STAR Physics Measurements dN/d  for h- (|  |<= ~1.5) particle density, entropy 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

Helen Caines OSU – March 2001 STAR The Serious Predictions >factor 2 variation in yields Radii increase from SPS R 0 /R s >= 1.6 (long lifetime) Little Stopping – Net proton yield = 4 – 20 Transverse flow – Same a SPS - much higher Heavier particles not see flow

Helen Caines OSU – March 2001 STAR Negative Hadrons:  Distribution and Multiplicity h-h- Full efficiency corrections h-h- Increased particle production per participant pair: 43% compared to 17.2 GeV 30% compared to  200 GeV dN(h-)/d  = 264  1  18 (extrap. to all p t ) At low end of predictions – Kills many models More than just pp happening

Helen Caines OSU – March 2001 STAR 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!!

Helen Caines OSU – March 2001 STAR Is there Thermalization? Almond shape overlap region in coordinate space Origin: spatial anisotropy of the system when created and rescattering of evolving system Look at “Elliptic” Flow

Helen Caines OSU – March 2001 STAR Hydro Calculation of Elliptic Flow P. Kolb, J. Sollfrank, and U. Heinz Equal energy density lines Elliptic flow observable sensitive to early evolution of system Large v 2 is an indication of early thermalization First time in Heavy-Ion Collisions a system created which approaches hydrodynamic model predictions Flow: A pressure build up -> Explosion with azimuthal asymmetry zero for central events Hydrodynamics: Assumes continuum matter with local equilibrium Locally equilibrated or “thermalized”. |  | < < p t < 2.0 Hydro Calculations STAR PRL 86 (2001) 402

Helen Caines OSU – March 2001 STAR OK Have a high enough energy density to cause transition Have a source that is consistent with being thermalized and has a large elliptic flow But what did we create?

Helen Caines OSU – March 2001 STAR Baryon Stopping/Transport Anti-baryons - all from pair production Baryons - pair production + transported B/B ratio =1 - Transparent collision B/B ratio ~ 0 - Full stopping, little pair production Measure p/p,  / , K - /K + (uud/uud) (uds/uds) (us/us) _ _ __

Helen Caines OSU – March 2001 STAR p/p Ratio _ Phys. Rev. Lett March 2001 Ratio = 0.65 ±0.03(stat) ±0.03(sys) Ratio is flat as function of p t and ySlight fall with centrality

Helen Caines OSU – March 2001 STAR Strange Baryon Ratios Ratio = 0.73 ± 0.03 (stat) ~0.84  /ev, ~ 0.61  /ev Reconstruct: _ STAR Preliminary ~0.006   /ev, ~0.005   /ev Ratio = 0.82 ± 0.08 (stat)

Helen Caines OSU – March 2001 STAR ¯ _ _ _ _ _ _ _ Anti-baryon/Baryon Ratios versus  s STAR preliminary Baryon-pair production increases dramatically with  s – still not baryon free 2/3 of protons from pair production, yet pt dist. the same – Another indication of thermalization Pair production is larger than baryon transport

Helen Caines OSU – March 2001 STAR Simple Model Assume fireball passes through a deconfined state can estimate particle ratios by simple quark-counting models D=1.12 No free quarks so all quarks have to end up confined within a hadron Predict D=1.08± 0.08 Measure System consistent with having a de-confined phase

Helen Caines OSU – March 2001 STAR 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 fo + 0.5m o 2 ) Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

Helen Caines OSU – March 2001 STAR 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.

Helen Caines OSU – March 2001 STAR Radial Flow: m t - slopes versus mass Naïve: T = T freeze-out + m   r  2 where   r  = averaged flow velocity  Increased radial flow at RHIC ß r (RHIC)  ß r (SPS/AGS) = 0.6c = c T fo (RHIC)  T fo (SPS/AGS) = GeV = GeV

Helen Caines OSU – March 2001 STAR Particle Ratios and Chemical Content  j = Quark Chemical Potential T = Temperature E j – Energy of quark  j – Saturation factor Use ratios of particles to determine  T ch and saturation factor

Helen Caines OSU – March 2001 STAR Chemical Fit Results Not a 4  -yields fit!  s  1  2  1.4 Thermal fit to preliminary data: T ch (RHIC) = 0.19 GeV  T ch (SPS) = 0.17 GeV  q (RHIC) = GeV <<  q (SPS) = GeV

Helen Caines OSU – March 2001 STAR P. Braun-Munzinger, nucl-ex/ Chemical Freeze-out Baryonic Potential  B [MeV] Chemical Temperature T ch [MeV] AGS SIS LEP/ SppS SPS RHIC quark-gluon plasma hadron gas neutron stars early universe thermal freeze-out deconfinement chiral restauration Lattice QCD atomic nuclei

Helen Caines OSU – March 2001 STAR OK (2) Shown that the collision region: Some evidence that source is thermalized Particles kinetically freeze-out with common T Large transverse flow - common to all species Particles chemically freeze out earlier (higher T) Near y axis on phase diagram Relative particle production consitant with having had free quarks

Helen Caines OSU – March 2001 STAR KK R out R side Measuring the Source “Size” (HBT) ~5 fm x1x1 x2x2 y1y1 y2y2 ~1 m 1D: overall rough “size” 3D decomposition of relative momentum provides handle on shape and time as well as size

Helen Caines OSU – March 2001 STAR 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

Helen Caines OSU – March 2001 STAR Two-particle interferometry (HBT) 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

Helen Caines OSU – March 2001 STAR 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)

Helen Caines OSU – March 2001 STAR Pion HBT Excitation Function Central AuAu (PbPb) Decreasing parameter Decreased correlation strength More baryon resonances ? Saturation in radii Geometric or dynamic (thermal/flow) saturation No jump in effective lifetime No significant rise in size of the  emitting source Lower energy running needed! STAR Preliminary Compilation of world 3D  -HBT parameters as a function of  s

Helen Caines OSU – March 2001 STAR STAR Preliminary Tomášik, Heinz nucl-th/  =0.0  =0.5 opaqueness The R Out /R Side Ratio Emission duration for transparent sources : Small radii + short emission time + opaqueness  short freeze-out

Helen Caines OSU – March 2001 STAR K 0 s -K 0 s Correlations  = 0.7 ±0.5 R = 6.5 ± 2.3 No coulomb repulsion No 2 track resolution Few distortions from resonances K 0 s is not a strangeness eigenstate - unique interference term that provides additional space-time information K 0 s Correlation will become statistically meaningful once we have ~10M events

Helen Caines OSU – March 2001 STAR Hard Probes in Heavy-Ion Collisions a) formation phase parton scattering b) hot and dense phase Quark Gluon Plasma Hadron Gas c) freeze-out emission of hadrons “hard” probes: c  c, b  b and jets –during formation phase parton scattering processes with large Q 2 –create high mass or high momentum objects –penetrate hot and dense matter –sensitive to state of hot and dense matter color screening:  J/  suppression dE/dx  jet quenching QGP vacuum

Helen Caines OSU – March 2001 STAR Negative Hadrons: p t - distributions Power Law A (1 + p t /p 0 ) - n p 0 = 2.74 ± 0.11 GeV/c n = ± 0.42 STAR = ± GeV/c NA49 = ± GeV/c UA1 = ± GeV/c STAR preliminary Mean pt higher than SPS and pp

Helen Caines OSU – March 2001 STAR Au+Au/pp: Compare p t - distributions “Hard” Scaling Nuclear Overlap Integral T AA = 26 mb -1 for 5% most central N AA / N pp = N bin coll = 1050 “Soft” Scaling N AA / N pp = ( 344 / 2 ) Jet Quenching: First hint for QGP formation at RHIC ? STAR preliminary

Helen Caines OSU – March 2001 STAR Conclusions Mapping out “Soft Physics” Regime  Net-baryon  0 at mid-rapidity! (  y = y 0 -y beam ~ 5 )  Chemical parameters Chemical freeze-out appears to occur at same ~T as SPS Strangeness saturation similar to SPS  Kinetic parameters Higher radial flow than at SPS Thermal freeze out same as at SPS  Unexpected: small HBT radii  Strong elliptic flow  Pion phase-space density at freeze-out seems to be universal Promising results from “Hard Physics”  p t spectra from central collisions show clear deviation from p-p extrapolation  high-p t data are consistent with “jet quenching” predictions ! More than we ever hoped for after the first run !!!

Helen Caines OSU – March 2001 STAR 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 Group Profs: PostDocs: Students: T.Humanic Me S.Bekele M.Lisa B.Neilson M.Lopez- Noriega E.Sugarbaker R.Wells R.Wilson The STAR Collaboration

Helen Caines OSU – March 2001 STAR NA49 STAR Preliminary STAR Radius Fits  f BE;no flow  T 0 =99.5 MeV T 0 =94.3 MeV T 0 =89.7 MeV  f BE;flow  T 0 =94.3 MeV T 0 =89.7 MeV The  Phase Space Density “Universal” phase space density observed at SPS appears to hold at RHIC as well Consistent with thermal distribution (T  94MeV) and strong collective flow (   0.58) Fundamental phase space saturation may relate increases in geometry, temperature, multiplicity pion occupation of cell in coordinate  momentum space:

Helen Caines OSU – March 2001 STAR Calibration – Cosmic Rays Determine momentum resolution  p/p < 2% for most tracks

Helen Caines OSU – March 2001 STAR Calibration - Lasers Using a system of lasers and mirrors illuminate the TPC Produces a series of >500 straight lines criss-crossing the TPC volume Determines: Drift velocity Timing offsets Alignment

Helen Caines OSU – March 2001 STAR QGP prediction: Enhancement  >  >  > h Evidence for Strangeness Enhancement WA97

Helen Caines OSU – March 2001 STAR What about the Chemical Freeze-out?  Yields of hadrons characterised by a few simple parameters T, V,  q (or   exp  q /T),  S Absolute abundances require more sophisticated descriptions including such details as flow effects and the fact that the fire-ball isn’t at rest. Perform a least-squared fit to the data with T, V,  q /T and  S as free parameters Made simpler by taking particle ratios.

Helen Caines OSU – March 2001 STAR Energy Density Estimate What is the energy density reached? Is it high enough to cause phase transition? Is there thermalization? Bjorken formula for thermalized energy density R2R2 2c  0 Measure E t at y=0 Assume  0 = 0.5 fm/c Assume full overlap

Helen Caines OSU – March 2001 STAR Elliptic Flow of Pions and Protons Hydro calculations: P. Huovinen, P. Kolb and U. Heinz Mass dependence of v 2 (p t ) shows a behavior in agreement with hydro calculations

Helen Caines OSU – March 2001 STAR Elliptic Flow Excitation Function STAR, PRL 86 (2001) 402

Helen Caines OSU – March 2001 STAR v 2 (p t ) for high p t particles M. Gyulassy, I. Vitev and X.N. Wang, nucl-th/

Helen Caines OSU – March 2001 STAR Before After In case you thought it was easy…

Helen Caines OSU – March 2001 STAR Particle ID Techniques Combinatorics K s   + +  -   K + + K -   p +  -   p +  + Combinatorics  from K + K - pairs K + K - pairs m inv same event dist. mixed event dist. background subtracted dn/dm Breit-Wigner fit Mass & width consistent w. PDG K* combine all K + and  - pairs (x ) m inv (GeV)

Helen Caines OSU – March 2001 STAR Charged particle anisotropy 0< p t < 4.5 GeV/c 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