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Fluctuation Studies at STAR Supriya Das* (for STAR Collaboration)

Explore mean pt fluctuation and correlation phenomena in nuclear collisions, featuring STAR data analysis and ongoing research.

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Fluctuation Studies at STAR Supriya Das* (for STAR Collaboration)

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  1. Fluctuation Studies at STAR Supriya Das* (for STAR Collaboration) Variable Energy Cyclotron Center, Kolkata, India * Present address : Gesellschaft für Schwerionenforschung mbH (GSI) Darmstadt, Germany Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  2. SolenoidalTracker At RHIC Tracking Detectors: TPC, FTPC, SVT Calorimeters: BEMC, EEMC, ZDC Trigger Detectors: CTB, BBC, MWPC Photon Detector: PMD Others:TOF, FPD Goal : to track the hadrons (and photons) in each collision Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  3. Features of STAR data (shown in this talk) M. Anderson et al. for STAR Collaboration, NIMA499 (2003) • Au + Au @ sNN1/2 = 20, 62, 130, 200 GeV • TPC data with || < 1.0, -p < f <p • PID from TPC dE/dx • Collision centrality measured by charged particle multiplicity in ||<0.5 • Centrality slices 0-5%, 5-10%, 10-20 %, … • Use Glauber MC model to estimate number of participants. Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  4. Fluctuation Analyses from STAR so far • Published papers : • Net Charge / Multiplicity fluctuation : PRC 68 (2003) 044905 PLB 634 (2006) 347 • Mean pt fluctuation / correlation : PRC 71 (2005) 064906 PRC 72 (2005) 044902 J. Phys G 32 (2006) L37 • Balance function : PRL 90 (2003) 172301 Submitted to arXive : • Mean pt fluctuation / correlation: nucl-ex/0408012 nucl-ex/0605022 Many interesting presentations by others on these topics Recent ongoing analyses within the working group: • Charge fluctuation (energy dependence, higher moments) • Balance function (mass dependence) • Long range correlation • Event by event K/p ratio Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  5. Mean pt fluctuation CI CD G distribution Results for hadrons for pt < 2GeV/c • Deviations from statistical reference • Excess in charge independent fluctuation • - varies smoothly and monotonically • with centrality • RHIC events are not fully equilibrated – • even for low pt and most central events • No evidence of critical fluctuation PRC 71 (2005) 064906 Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  6. pt correlation (energy dependence) PRC 72 (2005) 044902 • Non-zero pt correlations in Au+Au collisions from 20 to 200 A Gev • <Dpt,iDpt,j> decreases with increasing centrality • dN/dh <Dpt,iDpt,j> Dspt:n shows similar behavior as earlier • HIJING shows little or no centrality dependence • Saturation attributed to onset of thermalization or jet suppression Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  7. pt correlation (energy dependence) PRC 72 (2005) 044902 • Scaled value of correlation show a little or no dependence on incident energy. • HIJING calculations under predict the data. Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  8. pt angular correlation (scale dependence) • pt variance difference increases with increasing collision energy. • Same side peak and away-side ridge in autocorrelation attributed to minijets. nucl-ex/0605022 Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  9. pt angular correlation (scale dependence) Rapid rise within dh < 0.2 SSC Slow linear rise after that LSC --- Scale dependence • pt variance difference is centrality dependent • saturation in energy dependence for peripheral collisions Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  10. pt angular correlation (scale dependence) pt variance difference increases with increasing collision energy Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  11. Autocorrelations after subtracting monopole, dipole and quadrupole components pt correlation on h, f Structures attributed to minijets variance difference auto-correlation Scale dependence of variance difference and corresponding autocorrelations J. Phys G 32 (2006) L37 Au + Au @ 200 GeV Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  12. confined: few d.o.f. deconfined: many d.o.f. • Charged multiplicity: nch = n+ + n– • Net charge: Q = n+ - n– • Charge ratio: R = n+ / n- • (1) v(Q)  Var(Q)/<nch> • (for stochastic emission, v(Q) = 1) • (2) v(R)  Var(R) * <nch> • (for stochastic emission, v(R) = 4) • (3) F(Q) • ndynamic • Moments of Net charge distributions Net Charge fluctuation (motivation) • Prediction:A drastic decrease in the EbyE fluctuations of net charge in local phase space regions in the deconfined QGP phase compared to that of the confined case hadronic gas. • QGP:4 and pion gas: 1-2 • Jeon, Koch: PRL (2000) 2076 • Asakawa, Heinz & Muller: PRL (2000) 2072 • Evolution of fluctuation • Shuryak & Stephanov: PR C63 (2001) 064903 • Heiselberg & Jackson: PR C63 (2001) 064904 • Mohanty, Alam & Nayak: PR C67 (2003) 024904 Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  13. Net Charge fluctuation Charge conservation limit Charge conservation limit Resonance gas Au+Au @ 130 GeV PRC 68 (2003) 044905 • Fluctuation for inclusive non-identified charge particles exceed by factor of 2 compared to charge conservation limit • Results agree with expectation based on measurement from p+p collisions at ISR • Two particle correlation may be modified in central collisions relative to the peripheral collisions • Fluctuation measure closer to the prediction with resonance gas • Pions show little larger fluctuation compared to inclusive measurement • Kaons and protons show 2 to 4 times the value of fluctuation compared to inclusive measurement Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  14. Net Charge fluctuation (energy dependence) STAR: 5% Central Au+Au PHENIX ||<0.35, =/2 CERES 2.0<  <2.9 C. Pruneau - nucl-ex/0401016 C. Pruneau QM 05 Dynamical net charge fluctuation are invariant of beam energies starting from top SPS onwards. Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  15. Moments of net charge distributions (motivation) (similar to kurtosis) Lattice calculations Calculation of Non-linear susceptibilities(higher order derivatives of pressure with respect to chemical potentials): • Ejiri, Karsch and Redlich: hep-ph/0510126 • Gavai, Gupta: hep-lat/0510044 4th moment • Net charge • Isospin • Strangeness 2nd moment 6th moment • Interesting structure close to T=TC • Is it possible to make precise measurement of higher moments of net charge? • bins in centrality • bins in pT Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  16. Net charge (Q) distributions MEAN of Q distributions Q distributions for AuAu 200GeV at 4 different centralities and 6 bins in pT <Q> low pT high pT <Q>/Npart Q (net charge) T. Nayak – SQM ‘06 <Q>/Npart is independent of centrality. Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  17. Variance and kurtosis of net charge distributions n(Q) with pT binned AuAu 200GeV Kurtosis (4th moment) Centrality & pT T. Nayak – SQM ‘06 • n(Q) is low at low pT and increases with increase of pT. Could be an effect of more resonance production at low pT. • First analysis of the 4th moment of net charge distribution is performed. Detailed comparison in terms of lattice calculations is expected soon. Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  18. Balance function (motivation) • Bass-Danielewicz-Pratt, PRL 85, 2000 • D. Drijard et al, NP B(155), 1979 Z=0 • Opposite charged particles are created at the same location of space–time. • Charge–anticharge particles created earlier (early stage hadronization) get further separated in rapidity. • Particle pairs that were created later (late stage hadronization) are correlated at small Δy. • The Balance Function quantifies the degree of this separation and relates it with the time of hadronization. Early Hadronization  Large Late Hadronization  Small Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  19. Balance function • Width of balance function is narrower for more central collision. • HIJING matches with data only for peripheral collisions. Inclusive charged pairs Au+Au @ 130 GeV Identified pions PRL 90 (2003) 172301 Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  20. Balance function (mass dependence) 1.3-1.4 • The balance function width for pions get narrower with increasing centrality, remains constant for kaons. • HIJING reproduces results for kaons, but not for pions. • The ratio of widths of pions to kaons is consistent with delayed hadronization picture. Gary Westfall, J.Phys.G30, S345-S349 (2004) Heavier particles are characterized by narrower bf distributions: Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  21. Long-range multiplicity correlations Correlation strength: Terence J Tarnowsky Nuclear Dynamics, San Diego March 2006 nucl-ex/0606018 • Study of correlations among particles produced in different rapidity regions. • The long-range correlations are expected to be much stronger in p-A and A-A, compared to p-p at the same energy. • STAR: forward region of 0.8<h<1.0 & backward of -1.0<h<-0.8. • Increase in correlation strength observed for central collisions compared to peripheral for Au+Au collisions at 200GeV. Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  22. Long-range multiplicity correlations nucl-ex/0606018 For central collision, long range correlation is less than PSM calculation Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  23. Strangeness fluctuation (motivation) C. Roland QM 2004 S. Afanasiev for NA49 PRL 86 (2001) • Amount of fluctuation decreases with increase in collision energy. • Data disagree with UrQMD model • Dynamical fluctuation is positive Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  24. Strangeness fluctuation : K/p distribution [ Data not corrected for efficiency] S. D. : SQM’06 Symposium Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  25. Strangeness fluctuation (energy and centrality dependence) • s = rms/mean • sdyn = sqrt(sdata2 – smixed2) S. D. : SQM’06 Symposium Fluctuation in K/p decreases with increasing energy till the top SPS energy and remains flat above it. The amount of fluctuation decreases with increasing centrality and is similar for 62GeV as well as 200GeV AuAu collisions. Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  26. Summary • STAR has the capability to study the event by event fluctuation for many different observables. • Mean pt fluctuation results show non-zero fluctuation which scales with centrality. • Mean pt analysis in finer (h,f) subspace shows interesting structures which could be attributed to production of minijets. • Net charge fluctuation is greater than predicted value from charge conservation conjecture but closer to resonance gas picture. • Dynamical net charge fluctuation is essentially flat for energies more that top SPS. • Results from higher moments of net charge are promising, but needs more work. • Balance function results support delayed hadronization picture. • Long range multiplicity correlation reduces (compared to model calculation) in central collisions. • Event by event fluctuation in K/p ratio is positive and centrality dependent but show no dependence on collision energy at RHIC. Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

  27. Outlook STAR has a shining future in the proposed low energy scan at RHIC With wider momentum range and K, p separation, it will play a key role in strangeness fluctuation study STAR will also give more insights in the energy dependence of Balance function at lower energies Correlation and Fluctuation in Relativistic Nuclear Collisions - Florence, July 2006

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