Particle production at RHIC

1. Particle production at RHIC Aneta Iordanova for the STAR collaboration

2. Outline • Motivation • Analysis Technique • Freeze-out • Chemical • Kinetic • Baryon/meson and strangeness production • Scaling properties Aneta Iordanova University of Illinois at Chicago

3. 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 Tkin at kinetic freeze-out, transverse radial flow (β) Chemical freeze-out properties from particle ratios Tch at chemical freeze-out, strangeness and baryon production Aneta Iordanova University of Illinois at Chicago

4. 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) q q Heavy ion collision evolution the current view Collision Hot, Dense Matter, QGP? Inelastic Scattering Hard Partonic Scattering Hadronization Phase Elastic Scattering time: the system cools and expands Chemical Freeze-out Kinetic Freeze-out Aneta Iordanova University of Illinois at Chicago

5. J.Stachel (Trento 2004) Mapping the QCD Phase Diagram • Lattice QCD predicts critical temperature for QGP phase boundary: • Tc ~ 170 MeV ec ~ 1 GeV/fm3 • Experimentally derived freeze-out parameters from different experiments • Extend the measurements at RHIC over a broad range of • Energy • Centrality • System size Aneta Iordanova University of Illinois at Chicago

6. STAR Experiment Time Projection Chamber Measures charged particle momenta and energy loss within |h|<1 Full azimuthal acceptance Aneta Iordanova University of Illinois at Chicago

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

8. Chemical freeze-out Statistical model approach: particle ratios described by 4 fit parameters: chemical freeze-out temperature,Tch baryon chemical potential, μB strangeness chemical potential, μS strangeness suppression, gS STAR Preliminary 10% central, Cu+Cu @ 62.4 GeV 10% central, Cu+Cu @ 200 GeV Phys.Lett.B465(1999)15, arXiv:nucl-th/0405068 Fits use only data from p, K and p Aneta Iordanova University of Illinois at Chicago

9. Chemical freeze-out • mB 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 (gs) approaches unity with increasing Nch Approaching ‘net baryon free’ Universal Tch~160MeV Chemical equilibrium Aneta Iordanova University of Illinois at Chicago

10. Preliminary Transverse momentum spectra Spectra Mass dependence • Pressure build-up in the center, large pressure gradient  collective expansion • Common expansion velocity? Preliminary Aneta Iordanova University of Illinois at Chicago

11. Kinetic freeze-out PRC48 (1993) 2462 • 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, Tkin • Studies show that resonance decays (at RHIC energies) do not affect the fit results significantly 10% central, Cu+Cu @ 200 GeV STAR Preliminary Aneta Iordanova University of Illinois at Chicago

12. Freeze-out properties STAR Preliminary • Extracted freeze-out parameters • similar for the same number of produced charged-particles, Nch • For all systems • For all colliding energies Tch Tkin Aneta Iordanova University of Illinois at Chicago

13. Freeze-out properties STAR Preliminary TC • Tkin , ‹b› with centrality • information on system expansion • Explosive system, higher b • higher pressure gradients in central events • Tch insensitive to centrality • Insensitive to expansion and cooling • Close to TC • QCD predicted TC~160-170 MeV • Coincides with hadronization • Thus, chemical freeze-out may probe the phase boundary Tch Tkin Aneta Iordanova University of Illinois at Chicago

14. 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) Aneta Iordanova University of Illinois at Chicago

15. Baryon/meson production R.Hollis WWND07 Phys. Rev. Lett. 97 (2006) 152301 Phys. Rev. Lett. 92 (2004) 112301 • Low-pT • Baryon < meson • Centrality independent • Ratio similar to p+p • Intermediate pT ratio enhanced relative to p+p • Maximal at pT~2GeV/c • Strong centrality dependence • centrality independent for pT>5GeV/c (Cu+Cu) and pT>7GeV/c (Au+Au) Aneta Iordanova University of Illinois at Chicago

16. Baryon/meson production • Energy dependence • Centrality dependence of Anti-proton/p enhancement versus pT is similar in 200 and 62.4GeV Phys. Lett. B 655 (2007) 104 Aneta Iordanova University of Illinois at Chicago

17. L/K0S Baryon/meson production • Enhancement also evident in strangeness sector • L/K0 shows the same systematic dependencies Aneta Iordanova University of Illinois at Chicago

18. Nucl. Phys. A715 (2003) p474 NA49 (SPS/CERN) PR C60 044904 (1999) E802 (AGS/BNL) Strangeness production • Small system versus large system: • Strangeness enhancement reported at the AGS and SPS • Relative to p+p • K/p ratio Aneta Iordanova University of Illinois at Chicago

19. Charged kaon enhancement • Charged kaons are enhanced in the Cu+Cu system compared to the Au+Au • At the same Npart Kaon dN/dy STAR Preliminary Npart Aneta Iordanova University of Illinois at Chicago

20. STAR Preliminary K/p - Cu+Cu versus Au+Au • K-/p- versus Npart • no apparent strangeness enhancement • Relative to the p spectra reference • p are “enhanced” in the same way as kaons • Question: if pions (and perhaps all species) are “enhanced”, is Npart the relevant variable for comparison? • Look at Nch as the comparison variable • Nch ~ p STAR Preliminary Aneta Iordanova University of Illinois at Chicago

21. Systematic study • System size • Center-of-mass energy similar values for all systems vs Nch Aneta Iordanova University of Illinois at Chicago

22. mean-pT vs Nch STAR Preliminary Aneta Iordanova University of Illinois at Chicago

23. mean-pT vs Npart STAR Preliminary Aneta Iordanova University of Illinois at Chicago

24. mean-pT vs Npart STAR Preliminary Aneta Iordanova University of Illinois at Chicago

25. mean-pT vs Nch STAR Preliminary All studied systems and energies are described better when using Nch, Aneta Iordanova University of Illinois at Chicago

26. 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 (Tch, Tkin, gS, b) 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, • Tch coincides with the LGT predicted Tc which is around 160 MeV. • In both Cu+Cu and Au+Au collisions, mid-rapidity baryon production at intermediate pT is enhanced compared to that of mesons indicating the coalescence process for hadronization. Aneta Iordanova University of Illinois at Chicago

27. High-pT and Nch Phys. Lett. B 655 (2007) 104 STAR Preliminary STAR Preliminary p++p- p++p- Au+Au 200GeV Au+Au 62.4GeV Au+Au 200GeV Au+Au 62.4GeV • similar scaling trends? • Ncoll relevant at higher pT • scaling with Nch (at mid-rapidity) for high-pT data Aneta Iordanova University of Illinois at Chicago