5-12 April 2008 Winter Workshop on Nuclear Dynamics STAR Particle production at RHIC Aneta Iordanova for the STAR collaboration

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics Outline Motivation Analysis Technique Freeze-out Chemical Kinetic Baryon/meson and strangeness production Scaling properties

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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)

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics STAR Experiment Time Projection Chamber Measures charged particle momenta and energy loss within |  |<1 Full azimuthal acceptance

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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/

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics Transverse momentum spectra Spectra Mass dependence Pressure build-up in the center, large pressure gradient  collective expansion Common expansion velocity? Preliminary

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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)

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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)

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics Enhancement also evident in strangeness sector  /K 0 shows the same systematic dependencies  /K 0 S Baryon/meson production

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics Systematic study similar values for all systems vs N ch System size Center-of-mass energy

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics mean-p T vs N ch STAR Preliminary

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics mean-p T vs N part STAR Preliminary

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics mean-p T vs N part STAR Preliminary

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics mean-p T vs N ch All studied systems and energies are described better when using N ch, STAR Preliminary

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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.

STAR April 2008 Aneta Iordanova University of Illinois at Chicago Winter Workshop on Nuclear Dynamics 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