9 th June 2008 Seminar at UC Riverside Probing the QCD Phase Diagram Aneta Iordanova

2 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Outline Motivation Analysis Technique Freeze-out Chemical Kinetic Baryon/meson and strangeness production Scaling properties

3 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Motivation Identified particle spectra in heavy-ion collisions at different center-of-mass energies and system size provide: a unique tool to explore the QCD phase diagram. system size dependence of the freeze-out parameters at RHIC. Bulk particle production provide: Kinetic freeze-out properties from spectral shapes T kin at kinetic freeze-out, transverse radial flow (β) Chemical freeze-out properties from particle ratios T ch at chemical freeze-out, strangeness and baryon production

4 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Heavy ion collision evolution Experimental Observation: the bulk of produced matter is “soft” (99%) to study the QGP → study hadronization and properties of produced particles species abundances (Chemical freeze-out and equilibration) momentum distributions (Kinetic freeze-out) Collision Hard Partonic Scattering Hot, Dense Matter, QGP? Hadronization Phase Inelastic Scattering Elastic Scattering the current view Chemical Freeze-out Kinetic Freeze-out q q time: the system cools and expands

5 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Mapping the QCD Phase Diagram Lattice QCD predicts critical temperature for QGP phase boundary: T c ~ 170 MeV  c ~ 1 GeV/fm 3 Experimentally derived freeze- out parameters from different experiments Extend the measurements at RHIC over a broad range of Energy Centrality System size J.Stachel (Trento 2004)

6 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside STAR Experiment Time Projection Chamber Measures charged particle momenta and energy loss within |  |<1 Full azimuthal acceptance

7 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Time Projection Chamber: provides charged particle information trajectories momentum Particle signatures in detector side view pzpz z pTpT pseudorapidity: rapidity: beam view x colors ~ ionization: low high y  

8 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Exploit the ionization energy loss (dE/dx) Distribution normalized by the theoretical expectation for different particle types. Normalized distribution sliced into  p=50MeV/c for |y|<0.1 (mid-rapidity) 6 centrality bins (60% of the cross- section) √s NN =62.4GeV and 200 GeV. Raw yields: extracted from multi- Gaussian fits. Consistent analysis technique for Different center-of-mass energies Colliding systems. Cu+Cu 200 GeV 0-10% central 0.50<p T <0.55 GeV/c Cu+Cu 200 GeV Minimum Bias pion kaon electron proton normalized STAR Preliminary Low-p T particle identification

9 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Corrections From MC Single particle efficiency acceptance and tracking inefficiency hadronic interactions and particle decays Energy loss (low momentum) multiple Coulomb scattering tracking corrects assuming only pion masses Feed-down / mis-identification  yields are corrected for weak decays  contamination and background pions from detector material Total contribution: ~15% at 0.3 GeV/c, ~5% at 1 GeV/c p(pbar) are not corrected for weak decays. Estimated contribution (max) ~30%.

10 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Corrections From data Proton background Accounts for protons whose origin is not from collision products. Primary source: protons kicked-out from dead material (beam-pipe) predominantly at low-p T. Correction: The ratio of "data" protons to total protons is independent of centrality

11 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Chemical freeze-out Statistical model approach: particle ratios described by 4 fit parameters: chemical freeze-out temperature, T ch baryon chemical potential, μ B strangeness chemical potential, μ S strangeness suppression,  S STAR Preliminary 10% central, 62.4 GeV10% central, 200 GeV Fits use only data from , K and p Phys.Lett.B465(1999)15, arXiv:nucl-th/

12 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Chemical freeze-out  B decreases with increasing collision energy Approaching a net baryon free system Freeze-out temperature independent of initial conditions Collision energy Energy density Net baryon density Strangeness suppression (  s ) approaches unity with increasing N ch Chemical equilibrium Approaching ‘net baryon free’ Universal T ch ~160MeV

13 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Transverse momentum spectra Spectra Mass dependence Pressure build-up in the center, large pressure gradient  collective expansion Common expansion velocity? Preliminary

14 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside PRC48 (1993) 2462 Kinetic freeze-out Hydro-dynamically motivated “blast-wave” model fits Gain insight into the dynamics of the collision. Model assumes a boosted thermal source in transverse and longitudinal directions. Can describe the data with a common set of fit parameters Transverse flow velocity, β Kinetic freeze-out temperature, T kin Studies show that resonance decays (at RHIC energies) do not affect the fit results significantly 10% central, 200 GeV STAR Preliminary

15 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside STAR Preliminary T kin T ch Freeze-out properties Extracted freeze-out parameters similar for the same number of produced charged-particles, N ch For all systems For all colliding energies

16 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside STAR Preliminary T kin T ch Freeze-out properties T kin , ‹  ›  with centrality information on system expansion Explosive system, higher  higher pressure gradients in central events T ch insensitive to centrality Insensitive to expansion and cooling Close to T C QCD predicted T C ~ MeV Coincides with hadronization Thus, chemical freeze-out may probe the phase boundary TCTCTCTC

17 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Probing the phase boundary Current systematics: Increasing collision energy Using different collision systems and centrality Next step: RHIC low-energy scan J.Stachel (Trento 2004)

18 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Baryon/meson production Low-p T Baryon < meson Centrality independent Ratio similar to p+p Intermediate p T ratio enhanced relative to p+p Maximal at p T ~2GeV/c Strong centrality dependence centrality independent for p T >5GeV/c (Cu+Cu) and p T >7GeV/c (Au+Au) R.Hollis WWND07 Phys. Rev. Lett. 92 (2004) Phys. Rev. Lett. 97 (2006)

19 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Baryon/meson production Energy dependence Centrality dependence of Anti-proton/  enhancement versus p T is similar in 200 and 62.4GeV Phys. Lett. B 655 (2007) 104

20 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Enhancement also evident in strangeness sector  /K 0 shows the same systematic dependencies  /K 0 S Baryon/meson production

21 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Strangeness production PR C (1999) E802 (AGS/BNL) Nucl. Phys. A715 (2003) p474 NA49 (SPS/CERN) Small system versus large system: Strangeness enhancement reported at the AGS and SPS Relative to p+p K/  ratio

22 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Charged kaons are enhanced in the Cu+Cu system compared to the Au+Au At the same N part Kaon dN/dy N part Charged kaon enhancement STAR Preliminary

23 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside K/  - Cu+Cu versus Au+Au K - /  - versus N part no apparent strangeness enhancement Relative to the  spectra reference   are “enhanced” in the same way as kaons Question: if pions (and perhaps all species) are “enhanced”, is N part the relevant variable for comparison? Look at N ch as the comparison variable N ch ~  STAR Preliminary

24 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Systematic study similar values for all systems vs N ch System size Center-of-mass energy

25 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside mean-p T vs N ch STAR Preliminary

26 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside mean-p T vs N part STAR Preliminary

27 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside mean-p T vs N part STAR Preliminary

28 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside mean-p T vs N ch All studied systems and energies are described better when using N ch, STAR Preliminary

29 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Charged particle measurements at midrapidity Grow with energy Grow with centrality (geometry) Cu+Cu exhibit similar features Central Cu+Cu data is higher than Au+Au for the same N part Systematic studies of N ch

30 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Ratios show a factorization in centrality (geometry) and energy independent of collision species Saturation model (KLN) dN ch /d  ~ initial gluon density Freeze-out properties most probably determined at the initial stages of the collision Driven by the initial energy density 200/ / /130 Cu+Cu preliminary Au+Au HIJING Saturation KLN Systematic studies of N ch

31 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Summary STAR has enlarged the variety of hadron spectra measurements at RHIC by providing new results for Cu+Cu at two different center-of- mass energies, √s=200 and 62.4 GeV. The freeze-out properties (T ch, T kin,  S,  at RHIC energies seem to scale by the number of produced charged hadrons at mid-rapidity determined at the initial stages of the collision and driven by the initial energy density, T ch coincides with the LGT predicted T c which is around 160 MeV. In both Cu+Cu and Au+Au collisions, mid-rapidity baryon production at intermediate p T is enhanced compared to that of mesons indicating the coalescence process for hadronization.

32 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside High-p T and N ch similar scaling trends? N coll relevant at higher p T scaling with N ch (at mid-rapidity) for high-p T data STAR Preliminary STAR Preliminary Au+Au 200GeV Au+Au 62.4GeV Au+Au 200GeV Au+Au 62.4GeV ++-++- ++-++- Phys. Lett. B 655 (2007) 104

33 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Statistical Thermal Model Statistical Thermal Model (THERMUS) * was used fitting T ch, μ B, μ S, and γ S (strangeness saturation factor). Particles used in the fit: π, K, p, Λ, Ξ, Ω and . Particles were corrected for weak decays. * Thermus, A thermal Model Package for Root S. Wheaton & Cleymans, hep-ph/

34 9 th June 2008 Aneta Iordanova University of Illinois at Chicago UC Riverside Including the multi-strange baryons in the Thermal fit Does it make a difference if we include the strange hyperons in the thermal fit? Values change slightly and fit errors are reduced. T=150 MeV  165 MeV. Temperature and μ B increase slightly.