Particle correlations at STAR Jan Pluta Heavy Ion Reactions Group (HIRG), Faculty of Physics, Warsaw University of Technology Some results from the STAR HBT group, presented recently by: Z.Chajecki, A.Kisiel, M.Lisa, M.Lopez-Noriega, S.Panitkin, F.Retiere, P.Szarwas. 3-rd Budapest Winter School on Heavy Ion Physics, 10 XII 2003

Outline: The STAR experiment RHIC HBT Puzzle General analysis asHBT Two K 0 - short correlations p-Au, d-Au data Nonidentical particles - emission asymmetry Plans for future

Relativistic Heavy Ion Collider (RHIC) 2:00 o’clock 4:00 o’clock 6:00 o’clock 8:00 o’clock 10:00 o’clock STAR PHENIX RHIC AGS LINAC BOOSTER TANDEMS 9 GeV/u Q = MeV/u Q = +32 HEP/NP  g-2 U-line BAF (NASA) PHOBO S 12:00 o’clock BRAHM S  Beam energy up to 100 GeV/A (250 GeV for p)  Two independent rings (asymmetric beam collisions are possible)  Beam species: from p to Au  Six interaction points  Four experiments: STAR, PHENIX, PHOBOS and BRAHMS  =3.8 km 1740 superconducting magnets

Solenoidal Tracker At RHIC

STAR Detector – side view and STAR Collaboration – face view

STAR Collaboration 500 Collaborators including – ~65 graduate students – ~60 postdocs 12 countries 49 institutions Spokesperson: John Harris Tim Hallman now USA, Brazil, China, Croatia Czech Republich, England, France, Germany, India, Netherlands, Poland, Rusia

HBT+FSI Space-time sizes and dynamics Correlation function Momenta and momentum difference The idea: Quantum statistics and Final-State Interaction Particle correlations

STAR Event 2 Central Event: AuAu 200GeV/A Real-time track reconstruction Pictures from Level 3 Trigger, online display. Typically 1000 to 2000 tracks per event into the TPC 4m

Event and Particle Selection Au+Au Collisions at Sqrt(S NN )=200GeV Particle identification via specific ionization (dE/dx) electron band removed by cuts Optimum performance for  HBT: < p T (GeV/c) < for K 0 s : < p T (GeV/c) < Centrality selection based on number of charged hadrons. three different centralities Midrapidity -0.5 < y < 0.5 STAR PRELIMINARY N ch STAR PRELIMINARY Minbias trigger

Two-particle kinematics Main (approximative) relations: Q out  P t Q side  Q long  K T P t Some base definitions - to be used for results presentation LCMS: (P 1 +P 2 ) z =0

HBT Excitation Function Comparison with lower energies for ~ 10% most central events at midrapidity k T ~ 0.17 GeV/c No significant increase in radii with energy R O /R S ~ 1 Gap in energy that needs to be closed

RHIC HBT Puzzle Most “reasonable” models still do not reproduce RHIC sqrt(S NN) = 130GeV HBT radii “Blast wave” parameterization (Sollfrank model) can approximately describe data …but emission duration must be small    = 0.6 (radial flow) T = 110 MeV R = 13.5  1fm (hard-sphere)  emission = 1.5  1 fm/c (Gaussian) from spectra, v 2 √S NN = 130GeV PHENIX PRL (2002) STAR 130 GeV PHENIX 130 GeV  +  - Hydro + RQMD

Statistical errors only!! raw Coulomb corrected q * (GeV/c) 3Dimensional Pion HBT Pratt-Bertsch Parameterization LCMS frame: (p 1 +p 2 ) z =0 Central Events p T = GeV/c Coulomb correction → spherical Gaussian source of 5fm momentum resolution corrected (~1% effect at 200GeV, due to higher B-field) R out (fm)R side (fm)R long (fm) 0.66 ± ± ± ± 0.11 STAR PRELIMINARY Sqrt(S NN) = 200 GeV  –  –

Centrality and m T dependence at 200 GeV R L varies similar to R O, R S with centrality HBT radii decrease with m T (flow) Roughly parallel m T dependence for different centralities R O /R S ~ 1 (short emission time) Central Midcentral Peripheral 200GeV STAR PRELIMINARY

Longitudinal radius: at 200GeV identical to 130 GeV (fit to STAR Y2 data only) STAR PRELIMINARY Central Midcentral Peripheral PHENIX Central 200GeV GeV Comparison: 200 to 130 GeV. Longitudinal radius

Evolution timescale from R L (fit to STAR Y2 data only) Simple Mahklin/Sinyukov fit (assuming boost-invariant longitudinal flow) Assuming T K =110 MeV (from spectra at 130 GeV) Makhlin and Sinyukov, Z. Phys. C 39 (1988) 69 STAR PRELIMINARY Central Midcentral Peripheral PHENIX Central 200GeV GeV

Comparison: 200 to 130 GeV. Transverse radii * Central Midcentral Peripheral PHENIX Central 200GeV GeV Higher B-field  higher p T Transverse radii : similar but not identical low-p T R O, R S larger at 200 GeV steeper falloff in m T (PHENIX 130GeV) R o falls steeper with m T STAR PRELIMINARY

Azimuthally sensitive HBT (asHBT) sensitive to interplay b/t anisotropic geometry & dynamics/evolution “broken symmetry” for b  0 => more detailed, important physics information another handle on dynamical timescales – likely impt in HBT puzzle P. Kolb and U. Heinz, hep-ph/

HBT respect Reaction Plane out pp b KK side x y Lines: projections of 3D Gaussian fit 1D projections,  =45° √S NN = 130GeV

HBT(φ) Results – 130 GeV Star preliminary Minbias Bolstered statistics by summing results of p - and p + analyses Blast-wave calculation (lines) indicates out-of-plane extended source Data corrected for both event plane resolution and merging systematic T=100 MeV  0   a  R=11.7 fm,  =2.2 fm/c

RYRY RXRX A model of the freezeout - BlastWave BW: hydro-inspired parameterization of freezeout longitudinal direction infinite extent geometrically boost-invariant longitudinal flow Momentum space temperature T transverse rapidity boost ~ r coordinate space transverse extents R X, R Y freezeout in proper time  evolution duration  0 emission duration  00 00 

RYRY RXRX A model of the freezeout- BlastWave BW: hydro-inspired parameterization of freezeout Longitudinal direction infinite extent geometrically boost-invariant longitudinal flow Momentum space temperature T transverse rapidity boost ~ r Coordinate space transverse extents R X, R Y freezeout in proper time  evolution duration  0 emission duration  7 parameters describing freezeout

BlastWave fits to published RHIC data reasonable centrality evolution OOP extended source in non- central collisions centralmidcentralperipheral 74.3 / / / 101  2 / ndf 0.8    1.4  (fm/c) 6.5    0.3  0 (fm/c) 10.1    0.3 R Y (fm) 8.0    0.3 R X (fm) 0.04    0.01 aa 0.81    0.01 00 95  4106  3108  3 T (MeV) PeripheralMidcentra l Central

Estimate of initial vs F.O. source shape estimate  INIT from Glauber from asHBT:  FO =  INIT  FO <  INIT → dynamic expansion  FO > 1 → source always OOP-extended constraint on evolution time

asHBT at 200 GeV in STAR – R(  ) vs centrality 12 (!)  -bins b/t  (k T - integrated) 72 independent CF’s clear oscillations observed in transverse radii of symmetry- allowed * type R o 2, R s 2, R l 2 ~ cos(2  ) R os 2 ~ sin(2  ) centrality dependence reasonable oscillation amps higher than 2 nd - order ~ 0→ (*) Heinz, Hummel, MAL, Wiedemann, Phys. Rev. C (2002)

Pion correlation in d – Au : data selection p-Au selection 1D correlation function 3D correlation function d-Au vs p-Au K T dependence Centrality dependence Pion Correlations d-Au and p-Au Pion Correlations d-Au and p-Au

p-Au selection: Using information from ZDC-d STAR can separate events with neutron spectator from deuteron ZDC-d Au d ZDC-Au FTPC E -Au All trigger events

1D Correlation Function: Gaussian fit: ➢ CF is very wide (rel Au-Au) ➢ Coulomb/merging less important ➢ CF looks reasonable ➢ 1D Gaussian fit is not good ➢ needed more deeply study of fit method STAR preliminary theoretical CF: R inv =6 fm, = 0.5 d*-Au : d-Au without p-Au only statistical error included !

3D Correlation Function: Gaussian parametrization is not perfect but HBT radii characterize the width of CF cut on the others Q's components < 30 MeV/c 3D Gaussian fit : STAR preliminary Fit results: R out, R side sensitive to the number of participants [GeV/c]

K T dependence: p – Au d – Au ● clear K T dependence ● R out and R side - sensitive to the number of participants ● R long – the same K T dependence for dAu and pAu STAR preliminary

K T dependence: d-Au & Au-Au divided by p-p ● the same trend of K T dependence for d-Au and Au-Au as for p-p ● HBT radii are scaled by constant factors STAR preliminary

 for different collisions M T dependence of R long : STAR preliminary R long = const (m T ) -  p-p d-Au Au-Au Au-Au peripheral midcentral STAR preliminary m T    k T 2 + mass   Sinyukov fit:

Centrality definition in d-Au: FTPC-Au: charged primary particle multiplicity in -3.8<  <-2.8 ZDC-d Au d ZDC-Au FTPC E -Au 321 most peripheral most central

Centrality dependence: ● clear centrality dependence ● similar to AuAu ● connection to geometry p – Au d – Au centrality minbias STAR preliminary [*] - Glauber calculations (Mike Miller)

K 0 s K 0 s Correlations

m t scaling violation? Next RHIC HBT puzzle ? inv

The asymmetry analysis Catching up Interaction time larger Stronger correlation Moving away Interaction time smaller Weaker correlation “Double” ratio Sensitive to the space-time asymmetry in the emission Kinematics selection on any variable e.g. k Out, k Side, cos(v,k) R.Lednicky, V. L.Lyuboshitz, B.Erazmus, D.Nouais, Phys.Lett. B373 (1996) 30. Non-identical particle correlations:

Double ratio definitions p1p1 p1p1 p2p2 p2p2 2k* = p 1 – p 2 P = p 1 + p 2 k side < 0 k side > 0 k out > 0 k side k side sign selection arbitrary k out k out sign selection determined by the direction of the pair momentum P Correlation functions Double ratios k long k long is the z component of the momentum of first particle in LCMS 2k* [GeV/c] simulation

What to expect from double ratios Initial separation in Pair Rest Frame (measured) can come from time shift and/or space shift in Source Frame (what we want to obtain) We are directly sensitive to time shift, the space shift arises from radial flow – possibility of a new radial flow measurement observed transverse velocity thermal velocity Flow velocity Out direction Side direction

What do we probe? Source of particle 1 Source of particle 2 Boost to pair rest frame Mean shift ( )  seen in double ratio Sigma (  r* )  seen in height of CF  r* =  pair  r  –  pair  t  Separation between source 1 and 2 in pair rest frame rr r (fm) r* (fm)  r* Separation due to space and/or time shift tt

Correlation functions and ratios Good agreement for like- sign and unlike-sign pairs points to similar emission process for K + and K - Out Side Long CF Clear sign of emission asymmetry Two other ratios done as a double check – expected to be flat Preliminary

STAR preliminary Results for Pion-Proton 130 AGeV Similar preliminary analysis done for pion-proton We observe Lambda peaks at k*~m inv of Λ Good agreement for identical and non-identical charge combinations Λ peaks

Preliminary results for Kaon-Proton Using data from Year2 (200 AGeV) – sufficient statistics No corrections for momentum resolution done No error estimation yet – fit indicates theoretical expectations K + p K - anti-p Best Fit STAR preliminary

Modeling the emission asymmetry Need models producing strong transverse radial flow: –Blast-wave as a baseline –RQMD –UrQMD –T. Humanic's rescattering model What do we measure and how to compare it to the models? Is our fitting method working? And if yes, what does it tell us? Need to disentangle flow and time shift

Understanding models Blast wave = Flow baseline Blast wave –Parameterizes source size (source radius) radial flow (average flow rapidity) and momentum distribution (temperature): –No time shift –Only spatial shift due to flow –Infinitely long cyllinder (neglects long contribution) R tt Rside Rout Kt = pair Pt Parameterization of the final state

Blast wave: how does the flow work Pion p t = 0.15 GeV/c  t = 0.73 Kaon p t = 0.5 GeV/c  t = 0.71 Proton p t = 1. GeV/c  t = 0.73 Average emission points Spatial shifts (r) Particle momentum

Fitting and quantitative comparisons Fits assume gaussian source in PRF r* out distributions have non- gaussian tails Use the same fitting procedure for models and data - correlation functions constructed with “Lednicky's weights” Example of r* out distribution from RQMD

Comparing models to data Rescattering models and blast- wave are consistent with data Blast wave parameters constrained by STAR measurements In models flow is required to reproduce the data More points in β t needed to map and discriminate the flow profile – needs STAR upgrades in PID capability (TOF barrel)

STAR HBT Matrix (circa 2003) “traditional” HBT axis Analysis in progress published 3 Correlations (accepted PRL) asHBT Phase space density Correlations with Cascades dAu, pp Cascades submitted Not shown:

What have we learned so far? RHIC HBT puzzle –Break down of theoretical description of correlations at RHIC –Indication of short source lifetime and freeze-out duration at RHIC –Short lived hadronic phase? Out of plane extended pion source in non-central collisions –Also points to short emission times Weak energy dependence of the HBT radii –Where is the phase transition? Large pion phase space densities (non-universal) –Small entropy per pion? Chaotic pion source from 3p correlations –No multiparticle effects above Pt~200 MeV/c Source asymmetries from non-identical correlations –Consistent with collective flow and short time scales Only systematics measurements may provide answers!

What will affect STAR HBT analysis? RHIC upgrades progress STAR upgrades Various other measurements (e.g. spectra, high Pt, strangeness, flow, etc) New theoretical ideas

Consequences for STAR HBT Large statistics AuAu datasets Plans for 2004: 14 weeks of AuAu “physics” running: –~30M central, ~50M peripheral events What can be done? Many analysis which were statistics limited! –Rare particle correlations W, X,L, etc (identical, non-identical) Early freeze-out, sequence of emission, flow, FSI, etc –Correlations relative to reaction plane Kaons Non-identical –Baryon correlations: ppbar, LLbar, pL, etc –Coalescence, light nuclei and anti-nuclei Large statistics pp (~100M events) 500 GeV –STAR HBT matrix (e.g. non identical correlations) –HBT in Jets? –spin dependent HBT? (with polarized beams) Different energies Different beams Add dependencies on centrality, Kt, reaction plane Event by Event HBT New analyses ideas (S.Pratt, imaging, etc)

Consequences for STAR HBT Better particle identification Extension of HBT systematics to higher Kt: 1-3 GeV/c Region of transition from Hydro to pQCD What is space-time picture in this region? –Correlations of identical particles –Scan in Pt for Non-identical correlations Sensitivity to flow profile, model details –asHBT Higher efficiency of hyperons reconstruction – ~x10 for W compare to TPC alone –High statistics correlations with hyperons

Summary RHIC and STAR future seems to be certain for next 5-10 years Upgrade path is visible The number of available datasets and possible analysis topics will be rapidly increasing Data volumes will be unprecedented (at least for us) –Can we do analysis in a reasonable time? Analyses will be “moving” to rare particles Shall we continue with systematic approach? –Probably yes If new results or theoretical predictions will suggest promising measurement - we will concentrate on it