STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar1 Emergence of a Consistent Picture from First Results of STAR at RHIC? Mike Lisa, Ohio State University STAR Collaboration U.S. Labs: Argonne, Lawrence Berkeley National Lab, Brookhaven National Lab U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, Michigan State, 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 Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP-Protvino

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar2 Overview ~ 1 year from initial data-taking in new energy regime overall picture / underlying driving physics unclear Outline Ultrarelativistic Heavy Ion Collisions and STAR at RHIC First data  Transverse momentum spectra  Momentum-space anisotropy (elliptic flow) Initial quantitative success of hydrodynamics Two-pion correlations (HBT)  STAR HBT and the “HBT Puzzle” Characterization of freeze-out from the data itself  K-  correlations  particle-identified elliptical flow  azimuthally-sensitive HBT: theory and first data Summary Skipped

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar3 Why heavy ion collisions? Study bulk properties of strongly-interacting matter far from ground state 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 Study of QGP crucial to understanding QCD low-q (nonperturbative) behaviour confinement (defining property of QCD) nature of phase transition Heavy ion collisions ( “little bang”): the only way to experimentally probe the deconfined state The “little bang”

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar4 The “little bang” Stages of the collision pre-equilibrium (deposition of initial energy density) rapid (~1 fm/c) thermalization (?) QGP formation (?) hadronic rescattering hadronization transition (very poorly understood) freeze-out: cessation of hard scatterings low-p T hadronic observables probe this stage “end result” looks very similar whether a QGP was formed or not!!!

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar5 Already producing QGP at lower energy? J. Stachel, Quark Matter ‘99 Thermal model fits to particle yields (& strangeness enhancement, J/  suppression)  approach QGP at CERN? is the system really thermal? dynamical signatures? (no) what was pressure generated? what is Equation of State of strongly-interacting matter? warning: e + e - yields fall on similar line!! Must go beyond chemistry:  study dynamics of system well into deconfined phase (RHIC) lattice QCD applies

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar6 uRQMD simulation of  s=200 GeV pure hadronic & string description (cascade) generally OK at lower energies applicability in very high density (RHIC) situations unclear produces too little collective flow at RHIC freeze-out given by last hard scattering

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar7 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 very good agreement with hydrodynamic prediction

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar8 Hydrodynamics: modeling high-density scenarios Assumes local thermal equilibrium (zero mean-free-path limit) and solves equations of motion for fluid elements (not particles) Equations given by continuity, conservation laws, and Equation of State (EOS) EOS: relates pressure, temperature, chemical potential, volume –direct access to underlying physics Works qualitatively at lower energy but always overpredicts collective effects - infinite scattering limit not valid there lattice QCD input

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar9 Hydro time evolution of non-central collisions Equal energy density lines P. Kolb, J. Sollfrank, and U. Heinz correlating observations with respect to event-wise reaction plane allows much more detailed study of reaction dynamics entrance-channel aniostropy in x-space  pressure gradients (system response)  p-space anisotropy (collective elliptic flow) self-quenching effect - sensitive to early pressure

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar10 Azimuthal-angle distribution versus reaction plane v 2 increases from central to peripheral collisions –natural space-momentum connection  particle -  reaction plane or

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar11 Measurements at AGS; E895 and E877 (Protons) At low beam energies negative v 2 (“squeeze- out”) Balancing energy around 4 AGeV, sensitive to EOS E lab (AGeV) v2v2 E895, Phys. Rev. Lett. 83 (1999) 1295 P. Danielewicz, Phys. Rev. Lett. 81 (1998) 2438

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar12 Local thermal equilibrium versus Low Density Limit SPS; Low-Density-Limit and Hydro miss p t dependence RHIC; p t dependence quantitatively described by Hydro p t dependence sensitive to early thermalization?  p Charged particles

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar13 Momentum-space characteristics of freeze-out appear well understood Coordinate-space ? Probe with two-particle intensity interferometry (“HBT”) The other half of the story…

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar14 “HBT 101” - probing source geometry Measurable! F.T. of pion source Creation probability  (x,p) = U * U 5 fm 1 m  source  (x) r1r1 r2r2 x1x1 x2x2 p1p1 p2p2

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar15 “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!

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar16 Large lifetime - a favorite signal of “new” physics at RHIC hadronization time (burning log) will increase emission timescale (“lifetime”) magnitude of predicted effect depends strongly on nature of transition measurements at lower energies (SPS, AGS) observe  <~3 fm/c “”“” with transition cc Rischke & Gyulassy NPA 608, 479 (1996) 3D 1-fluid Hydrodynamics  ~ …but lifetime determination is complicated by other factors…

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar17 First HBT data at RHIC STAR Collab., PRL (2001) Data well-fit by Gaussian parametrization Coulomb-corrected (5 fm full Coulomb-wave) “raw” correlation function projection 1D projections of 3D correlation function integrated over 35 MeV/cin unplotted components

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar18 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

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar19 First STAR HBT data - systematics STAR Collab., PRL (2001)  +,  - HBT parameters similar Grossly similar to AGS/SPS all radii increase with multiplicity R o, R s - geometric effect R l - increase not seen at AGS/SPS With increasing m T increases  fewer resonances radii decrease  x-p correlations stronger effect in Ro than at AGS/SPS systematic errors

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar20 m T dependence at y cm for 2 AGeV central collisions collective flow  dynamical correlation between position and momentum  R(m T ) R’s are “lengths of homegeity”  - from decays  (m T ) x (fm) y (fm)

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar21 Hydro attempts to reproduce data out side long K T dependence approximately reproduced  correct amount of collective flow R s too small, R o & R l too big  source is geometrically too small and lives too long in model Right dynamic effect / wrong space-time evolution?  the “RHIC HBT Puzzle” generic hydro

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar22 “Realistic” afterburner makes things worse pure hydro hydro + uRQMD STAR data Currently, no physical model reproduces explosive space-time scenario indicated by observation R O /R S

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar23 Now what? No dynamical model adequately describes freeze-out distribution Seriously threatens hope of understanding pre-freeze-out dynamics Raises several doubts –is the data consistent with itself ? (can any scenario describe it?) –analysis tools understood? Attempt to use data itself to parameterize freeze-out distribution Identify dominant characteristics Examine interplay between observables Isolate features generating discrepancy with “real” physics models

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar24 Characterizing the freezeout: An analogous situation

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar25 Probing f(x,p) from different angles Transverse spectra: number distribution in m T Elliptic flow: anisotropy as function of m T HBT: homogeneity lengths vs m T,  p

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar26 m T distribution from Hydrodynamics-inspired model E.Schnedermann et al, PRC48 (1993) 2462 R  s Infinitely long solid cylinder  b = direction of flow boost (=  s here) 2-parameter (T,  ) fit to m T distribution

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar27  2 contour maps for 95.5%CL T th [GeV]  s [c] -- K-K- p T th [GeV]  s [c] T th [GeV]  s [c] T th = MeV =0.52 ±0.06[c] tanh -1 ( ) = 0.6 = 0.8  s Fits to STAR spectra;  r =  s (r/R) 0.5 -- K-K- p 1/m T dN/dm T (a.u.) m T - m [GeV/c 2 ] thanks to M. Kaneta preliminary STAR preliminary

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar28 Excitation function of spectral parameters Kinetic “temperature” saturates ~ 140 MeV already at AGS Explosive radial flow significantly stronger than at lower energy System responds more “stiffly”? Expect dominant space-momentum correlations from flow field

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar29 Implications for HBT: radii vs p T Assuming , T obtained from spectra fits  strong x-p correlations, affecting R O, R S differently p T =0.2 p T =0.4 y (fm) x (fm)

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar30 Implications for  HBT: radii vs p T STAR data model: R=13.5 fm,  =1.5 fm/c T=0.11 GeV,  0  = 0.6 Magnitude of flow and temperature from spectra can account for observed drop in HBT radii via x-p correlations, and R o <R s …but emission duration must be small p T =0.2 p T =0.4 y (fm) x (fm) Four parameters affect HBT radii

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar31 Kaon – pion correlation: dominated by Coulomb interaction Static sphere : – R= 7 fm ± 2 fm (syst+stat) Blast wave –T = 110 MeV (fixed) – = 0.62 (fixed) –R = 13 fm ± 4 fm (syst+stat) Consistent with other measurements STAR preliminary

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar32 Initial idea: probing emission-time ordering Catching up: cos  0 long interaction time strong correlation Ratio of both scenarios allow quantitative study of the emission asymmetry Moving away: cos  0 short interaction time weak correlation Crucial point: time-ordering means kaon begins farther in “out” direction purple emitted first green is faster purple emitted first green is slower

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar33 Space-time asymmetry Evidence of a space – time asymmetry –   -  K ~ 4fm/c ± 2 fm/c, static sphere –Consistent with “default” blast wave calculation   p T  = 0.12 GeV/c STAR preliminary K  p T  = 0.42 GeV/c

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar34 Non-central collisions: coordinate- and momentum-space anisotropies Equal energy density lines P. Kolb, J. Sollfrank, and U. Heinz

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar35 More detail: identified particle elliptic flow soliddashed 0.04   0.02  a (c) 0.04  S2S   0.02  0 (c) 100   20 T (MeV) STAR, in press PRL (2001) Flow boost:  b = boost direction Meaning of  a is clear  how to interpret s 2 ? hydro-inspired blast-wave model Houvinen et al (2001)

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar36 Ambiguity in nature of the spatial anisotroy  b = direction of the boost  s 2 > 0 means more source elements emitting in plane case 1: circular source with modulating density RMS x > RMS y RMS x < RMS y case 2: elliptical source with uniform density

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar37 Azimuthal HBT: (transverse) spatial anisotropy Source in b-fixed system: (x,y,z) Space/time entangled in pair system (x O,x S,x L ) U. Wiedemann, PRC 57, 266 (1998) large RHIC induces space-momentum correlations  p-dependent homogeneity lengths  sensitive to more than “just” anisotropic geometry out  b KK x y side

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar38 Reminder: observations for Au(2 AGeV)Au  p (°) R 2 (fm 2 ) outsidelong ol os sl E895 Collab., PLB (2000)  p =0°  p =90° out-of-plane extended source interesting physics, but not currenly accessible in STAR with 2 nd -order reaction plane Lines are global fit Oscillation magnitude  eccentricity Oscillation phases  orientation

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar39 x out x side K Meaning of R o 2 (  ) and R s 2 (  ) are clear What about R os 2 (  )  p (°) R 2 (fm 2 ) outsidelong ol os sl E895 Collab., PLB (2000) R os 2 (  ) quantifies correlation between x out and x side No correlation (tilt) b/t between x out and x side at  p =0° (or 90°) K x out x side K x out x side K x out x side K x out x side K x out x side K x out x side  p = 0°  p ~45° Strong (positive) correlation when  p =45°

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar40 STAR HBT “Out” “Side” “Long” C(Q) Q (GeV/c) Correlation function:  p =45º R O 2 (fm 2 ) R S 2 (fm 2 ) R OS 2 (fm 2 )  - from semi-peripheral events raw corrected for reactionplane resolution datafit only mix events with “same”  RP retain relative sign between q-components HBT radii oscillations similar to AGS curves are not a global fit R S almost flat STAR preliminary

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar41 Out-of-plane elliptical shape indicated case 1 using (approximate) values of s 2 and  a from elliptical flow case 2 opposite R(  ) oscillations would lead to opposite conclusion STAR preliminary

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar42 s 2 dependence dominates HBT signal error contour from elliptic flow data color:  2 levels from HBT data STAR preliminary  s 2  =0.033, T=100 MeV,  0   a  R=10 fm,  =2 fm/c

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar43 A consistent picture

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar44 Summary Spectra Very strong radial flow field superimposed on thermal motion T saturates rapidly ~ 140 MeV  higher at RHIC space-momentum correlations important “stiffer” system response? consistent with hydro expectation Momentum-space anisotropy sensitive to EoS and early pressure and thermalization significantly stronger elliptical flow at RHIC, compared to lower energy indication of coordinate-space anisotropy as well as flow-field anisotropy (v 2 cannot distinguish its nature, however) for the first time, consistent with hydro expectation

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar45 Summary (cont’) HBT radii grow with collision centrality R(mult) evidence of strong space-momentum correlations R(m T ) non-central collisions spatially extended out-of-plane R(  ) The spoiler - expected increase in radii not observed presently no dynamical model reproduces data Combined data-driven analysis of freeze-out distribution Single parameterization simultaneously describes spectra elliptic flow HBT K-  correlations most likely cause of discrepancy is extremely rapid emission timescale suggested by data - more work needed!

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar46 The End

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar47 Very large event anisotropies seen by STAR, PHENIX, PHOBOS v2v2 centrality space-momentum connection clear in multiplicity dependence different experiments agree well finally, we reach regime of quantitative hydro validity  evidence for early thermalization AGS: magnitude described by cascade models RHIC; Hydro description for central to mid-central collisions –26% more particles in-plane than out-of-plane (even more at high p T )!!

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar48 Experimental correlation functions A(q) q (GeV/c) # pairs from same event B(q) q (GeV/c) # pairs from different events most pairs at high q (need statistics!) shape of A(q), B(q) dominated by phasespace and single-particle acceptance (complicated in principle, especially in multiple dimensions) q (GeV/c) C(q) only correlated effects persist in ratio (including residual detector artifacts…) Correlation functions from different experiments (and from theory) can be compared

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar49 A consistent picture main source of discrepancy?

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar50 Geometry of STAR ZCal Barrel EM Calorimeter Endcap Calorimeter Magnet Coils TPC Endcap & MWPC ZCal FTPCs Vertex Position Detectors Central Trigger Barrel or TOF Time Projection Chamber Silicon Vertex Tracker RICH

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar51 Peripheral Au+Au Collision at 130 AGeV Data Taken June 25, Pictures from Level 3 online display.

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar52 Au on Au Event at CM Energy ~ 130 AGeV Data Taken June 25, 2000.

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar53 Summary Spectra, elliptic flow, and HBT measures consistent with a freeze-out distribution including strong space-momentum correlations In non-central collisions, v 2 measurements sensitive to existence of spatial anisotropy, while HBT measurement reveals its nature Systematics of HBT parameters: flow gradients produce pT-dependence (consistent with spectra and v 2 (p T,m)) anisotropic geometry (and anisotropic flow boost) produce  -dependence (average) out-of-plane extension indicated however, distribution almost “round,” --> more hydro-like evolution as compared to AGS  While data tell consistent story within hydro-inspired parameterization, hydro itself tells a different story - likely point of conflict is timescale

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar54 STAR 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

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar55 Joint view of  freezeout: HBT & spectra common model/parameterset describes different aspects of f(x,p) for central collisions Increasing T has similar effect on a spectrum as increasing  But it has opposite effect on R(p T )  opposite parameter correlations in the two analyses  tighter constraint on parameters spectra (  ) HBT STAR preliminary 

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar56 Time-averaged freezeout shape close to RHIC info on evolution duration? STAR preliminary (E895)