The particle dependence of v 2 at moderate p T in Au+Au collisions

1. SQM2003–Atlantic Beach, North Carolina March 11–17, 2003 The particle dependence of v2 at moderate pT in Au+Au collisions — Paul Sorensen — University of California–Los Angeles

2. SQM2003–Atlantic Beach, North Carolina March 11–17, 2003 Objective Outline Measurements of the event–wise azimuthal anisotropy of charged particle production indicate that the elliptic anisotropy parameter (v2) is large and nearly pT independent for 2.0 < pT < 8.0 GeV/c. I will present v2(pT) for KS and Λ from three centrality intervals and for a minimum–bias data set reaching pT of nearly 6 GeV/c. • Analysis method overview. • Theories related to our measurements. • The minimum-bias v2(pT) for KS and Λ to pT ~ 6.0 GeV/c. • The v2 for KS and Λ from 0–5, 5–30, 30–70% of the collisions cross-sections. • Discussion. Paul Sorensen 2

3. SQM2003–Atlantic Beach, North Carolina March 11–17, 2003 The products of the decays KSπ+π- (i/  69%) and Λpπ- (i/  64%) are detected in the TPC. Topology cuts are made on the vertices and the yield is extracted from the invariant mass distributions. Paul Sorensen Analysis overview 3

4. Centrality classes and the event plane The centrality is estimated from the number of charged tracks. The reaction-plane is estimated using the event-plane defined by the anisotropy in the azimuthal distribution of tracks. The event-plane is an imperfect estimator of the reaction-plane so the v2 is corrected for the resolution estimated from a sub-events analysis. Paul Sorensen Analysis overview 4

5. The azimuthal anisotropy parameters Paul Sorensen Analysis overview 5

6. Azimuthal anisotropy from energy loss more opaque In an energy loss (dE/dx) scenario, high pT partons lose energy as they traverse the hot nuclear matter. As such, high pT hadron production in central collisions may be suppressed relative to binary nucleon-nucleon collision scaling of peripheral collisions (RAA). The dE/dx may also lead to the loss of away–side correlations from jet fragmentation. less opaque In this picture, dE/dx can also lead to non-zero v2 at high pT. more opaque: fewer high pT particles less opaque: more high pT particles The very large dE/dx limit: Emission is only seen from near the surface. Surface emission should lead to a pT and √sNN independent v2: E. V. Shuryak, Phys.Rev. C66 (2002) Physics scenarios 6

7. A particle dependence from hadronization (from parton transport) (Molnar & Voloshin: nucl-th/0302014) (Bass et al.: nucl-th/0301018) Physics Scenarios 7

8. A particle dependence from the pT scale pT scale This sketch illustrates the possible effects of pQCD overtaking non-pQCD effects (baryon junctions, hydro, etc.) at different pT scales for protons and pions. In this scenario, a particle type dependence will be seen in v2 with proton v2 higher than pion v2 and the saturation of charged particle v2 is caused by the superposition of the two. What will the particle dependence be for RAA? Physics scenarios 8

9. SQM2003–Atlantic Beach, North Carolina March 11–17, 2003 Preliminary sNN = 200 GeV QM 2002 Transverse Momentum pT (GeV/c) QM 2002 KS and Λ v2 results at 130 GeV: “Azimuthal Anisotropy of KS and Lambda + Anti-lambda Production at Midrapidity from Au+Au Collisions at sNN = 130 GeV” Published September 23, 2002, Phys. Rev. Lett. 132301-1. Paul Sorensen Background 9

10. The particle dependence of v2 v2 appears to saturate at approximately 0.16 for KS and 0.23 for Λ (v2 Λ =v2 Λ-bar). The mass dependence is hydro-like at low pT (a smaller mass gives a larger v2). The pT onset of the saturation in v2 is different for KS and Λ. The phenomenology seems better described in mT – m0 than pT ; Why (kinetic energy)? What’s drives the different pT scales for KS and Λ v2? 10

11. Centrality dependence of v2 for KS and Λ The v2 as a function of particle type and event centrality. The mT – m0 plot makes it easier to study the deviations between particle type. The approximate maximum v2 from surface emission for this centrality is well below v2 for both particles but, the systematic errors must be considered. STAR preliminary (Au+Au; 200 GeV; |y|<1.0) Paul Sorensen 11

12. Particle/centrality dependence of v2 for KS and Λ STAR preliminary (Au+Au; 200 GeV; |y|<1.0) Is the Λ v2 to KS v2 ratio smaller in central collisions? The errors–systematic and statistical–must be studied further and reduced, particularly for 0–5% central. Paul Sorensen 12

13. Au+Au collision geometry; Scaling by eccentricity ε Paul Sorensen 13

14. Centrality dependence of v2/ε for KS and Λ v2/ε is increasing monotonically from peripheral to central collisions for both particles. The accuracy of our estimates of ε need to be considered. STAR preliminary (Au+Au; 200 GeV; |y|<1.0) Paul Sorensen 14

15. v2/ε; central/peripheral (species and mT dependence) C. Adler et.al. Phys. Rev. C66, 034904 • While the increase in v2/ε with √s happens at low pT,the increase in v2/ε with centrality happens primarily at high pT. • v2/ε is independent of centrality for hydrodynamics with a constant speed of sound. • v2/ε demonstrates how effective the system is at converting geometric to momentum anisotropy. Central collisions are more effective than peri. in creating anisotropy in high pT. charged hadron v2: Kirill Filimonov QM2002 15

16. SQM2003–Atlantic Beach, North Carolina March 11–17, 2003 Preliminary sNN = 200 GeV From v2 to RAA For the first time the particle dependence of v2 has been measured for high and low pT (and vs. centrality). RAA is an observable that may be intimately related to v2. See Hui Longs talk for the particle dependence of RAA. Charged hadron RAA Paul Sorensen 16

17. The pT scale of KS and Λ production • What physics could be behind the pT scales of the saturation in v2 and the suppression in RAA? • How does the particle type influence the pT scale? • pTmeson≈2·pTparton? • pTbaryon≈3·pTparton? • The saturation of v2 and the drop of RAA seem to be correlated. 17

18. SQM2003–Atlantic Beach, North Carolina March 11–17, 2003 Summary v2 and RAA for KS and Λ show a strong particle type dependence: Compared to KS, Λ production shows a larger azimuthal anisotropy but a smaller suppression for 2.0 < pT < 4.0 GeV/c. v2/ε rises monotonically from peripheral to central collisions: Suggestion of saturation is seen in all centralities. Central collisions are more effective than peripheral at converting spatial anisotropy to momentum anisotropy in high pT KS and Λ and production. For central collisions RAA for Λ is larger than for KS and coincides with our estimate of Nbin scaling expectations for 1.8 < pT < 3.5 GeV/c. While an extreme energy loss scenario predicts a maximum v2 of ~15%, Λ v2 reaches as high as ~24% and KS v2 reaches as high as ~16%. Paul Sorensen 18

19. Outlook In a dE/dx scenario–where opacity from partonic dE/dx leads to a finite, pT independent v2, an RAA below one, and an absence of back-to-back, jet-like correlations–the larger Λ v2 (for 2 < pT < 4) contradicts the smaller Λ suppression manifested in RAA. What are the origins of the particle type dependence; A particle type dependence in the pT scale of a non-pQCD (hydro, baryon-junctions, etc.) to pQCD transition? → Below 4 GeV/c Λ’s are not from jets? A mass dependence (from hydro, cronin or fragmentation)? A 2 vs. 3-quark dependence (coalescence after a partonic v2 is established? For moderate to high pT, the hadronization process may be a crucial component to understanding heavy ion collisions (see, for example, the following slides on coalescence). As we continue the systematic study of identified particles (φ, Ξ, Ω, etc.) STAR will resolve many of these open issues. See for example Hui Long’s work presented at this meeting. Paul Sorensen 19

20. A comparison of v2 for KS and Λ to qualitative predictions from coalescence In this picture v2 of a hadrons at pT is the partonic v2 at pT/n scaled by the # of quarks (n). STAR preliminary (Au+Au; 200 GeV; |y|<1.0) • In this scenario we must assume that some v2 is built up during a partonic stage. • We can then infer the value of the quark v2 in the relevant pT region (~8%). 20

21. This parton coalescence rescaling seems to work for each of our centrality intervals Paul Sorensen 21

22. Supplementary Slides -Lambda vs. Anti-lambda v2- -Reaction plane distribution- -Nbin and Npart scaling- -v2 for 130 GeV identified particles- -Modified blast wave fits- -And much more- Paul Sorensen for the STAR Collaboration 22

23. SQM2003–Atlantic Beach, North Carolina March 11–17, 2003 Paul Sorensen 23

24. v2/ε vs. charged particle density (1/SdN/dy) S is the area of the overlap region; dN/dy is the rapidity density. The most central events approach the hydro limit (a mean free path much smaller than the collision overlap region). v2/ε is increasing with charged particle density. What is the detailed √s and centrality behavior? charged hadron v2√s ratio: Kirill Filimonov QM2002 C. Adler et.al. Phys. Rev. C66, 034904 Paul Sorensen 24

25. v2/ε; mid-central to peripheral C. Adler et.al. Phys. Rev. C66, 034904 charged hadron v2: Kirill Filimonov QM2002 Paul Sorensen 25

26. STAR preliminary (Au+Au; 200 GeV; |y|<1.0) v2(pT) particle dependence At low mT-m0 the Λ and KS follow a hydro like mass dependence. For mT-m0 ≥ 1.0 GeV/c Λ v2 continues to increase while KS v2 begins to saturate; so that the ratio deviates from hydro model expectations. 26 Paul Sorensen

27. Monte Carlo Glauber Calculation sNN = 200 GeV Woods-Saxon nuclear geometry parameters: 0 = .16935 nucl./fm3 r0 = 6.38  0.06 fm c0 = 0.535  0.027 fm Cross-sections: NN = 42  1 mb geo = 7.2  0.4 b 27

28. v2 vs mt – m0 for 130 GeV identified particles C. Adler et al. Phys. Rev. Lett. 87, 182301 (2001). 28

29. Modified blast wave fits A simultaneous fit to both particles at all pT fails. A simultaneous fit to both particles for pT less than 1.0 GeV/c gives results similar to those reported by STAR for 130 GeV identified particle v2*. 29 * C. Adler et al. Phys. Rev. Lett. 87, 182301 (2001).

30. 1.6 1.2 0.8 How do we maintain an azimuthal anisotropy in the number of particles produced at a given pT while the azimuthal anisotropy of <pT> disappears? 30

31. Surface N-binary Scaling for High pT (Model Calculations by An Tai) Surface width=1.6 fm RAA Au+Au at sNN=200 GeV Centrality<5% Nbin_v=1016,Npart=347 5%<Centrality<10% Nbin_v=816,Npart=293 10%<Centrality<20% Nbin_v=592,Npart=226 30%<Centrality<40% Nbin_v=237,Npart=107 1.0 Surface Binary Scaling 1.0 1.0 STAR Preliminary Au+Au 200 GeV pT (GeV/c) • Woods-Saxon distribution. • A binary collision occurs if dmin (/). • Pythia is used to handle particle production for each binary collision with sqrt(s) > 4 GeV, otherwise, particles are produced through resonance,eg NN N. 31

32. v2 for mT – m0 between 1.5 – 4.0 GeV/c2 The relative probability to produce mesons or baryons depends on the density of the quarks. Hadronization Schemes: • Parton-hadron duality • Independent fragmentation • Whatever works? The hadronization scheme may be important for understanding the particle dependence of RAA and v2. Coalescence is qualitatively consistent with the particle dependence of v2 and RAA. Paul Sorensen 32

33. The spectra for KS and Λ + Λ STAR preliminary (Au+Au; 200 GeV; |y|<1.0) Paul Sorensen 33

34. RAA for KS and Λ For pT from 1.8-3.5 GeV/cΛ+ Λ production coincides with Nbin scaling while KS and h+/- production is suppressed in this region (RAA Λ =RAA Λ-bar). At pT ~ 5.5 GeV/c KS, Λ+Λ and h+/- production are suppressed by a similar amount. The pT scales for KS and Λ RAA are different. Paul Sorensen 34

35. STAR preliminary (Au+Au; 200 GeV; |y|<1.0) 35

36. The pT scale of KS and Λ production • What physics is behind the pT scales of the saturation in v2 and the suppression in RAA? • How does the type influence the pT scale? • pTmeson≈2·pTparton? • pTbaryon≈3·pTparton? • The saturation of v2 and the drop of RAA seem to be correlated. 36

37. The pT scale of v2& RAA for KS, Λ & h+/- What physics is behind the pT scales of the saturation in v2 and the suppression in RAA? How does the particle type influence the pT scale? The v2 saturation and the the decrease in RAA appear to be loosely correlated for both KS and Λ. 37

38. Event plane resolution correction factor 200 GeV 130 GeV Maximum resolution correction factor from random sub-events Lambda v2 (130 GeV) : 0.58 KS v2 (130 GeV) : 0.68 Lambda v2 (200 GeV) : 0.80 KS v2 (200 GeV) : 0.80 Track–wise and event–wise cuts • Nhits > 15 • Nhits/ Nmax > 0.52 • |eta| < 1.5 • 0.1 < pT < 2.0 GeV/c • DCA < 2.0 cm • z–vertex < 25 cm 38

39. FFF B RFF B RFF C FFF C Paul Sorensen 39