Energy and system size dependence of strangeness production, from SPS to RHIC

1. Energy and system sizedependence of strangeness production, from SPS to RHIC Jun Takahashi & Marcelo Munhoz for the STAR collaboration

2. Motivation • In A+A, With a QGP scenario strangeness production is expected to be enhanced. Strangeness enhancement has been observed from various data from SPS and RHIC. • In p+p, Strangeness production may be limited due to canonical suppression. This suppression should scale with the strange quark content of the particles. • Energy dependence of the yields: Can we see any dramatic variation in the excitation function of different parameters? • Chemical Equilibrium: Is strangeness equilibrated? • Baryon to Meson enhancement and RCP behavior in the intermediate pT region.

3. The STAR experiment • STAR is a large acceptance collider detector. • We have accumulated a large amount of data that allows a systematic study of the strangeness production, both as a function of energy and as a function of system size. • Strange particles are identified through various analysis methods, such as: V0 and Cascade reconstruction, event mixing and dE/dx. • In this presentation, I will mainly concentrate on the weak decaying strange particles: K0s, Λ, Ξ, Ω.

4. Data RHIC data: STAR • p+p • 200 GeV • d+Au • 200 GeV • Au+Au • 200 GeV • 130 GeV • 62.4 GeV • 19.6 GeV • Cu+Cu • 200 GeV • 62.4 GeV (some very preliminary results) SPS data: NA57 & NA49 • Pb+Pb

5. System Size: • Event centrality classes are defined based on the measured charge particle multiplicities. • The equivalent number of particles that participate in the reaction Npart is calculated using Glauber Model, that also provides the equivalent number of binary collisions NColl or NBin. Phys. Rev.C 70, 054907 (2004)

6. System Size:

7. CuCu200 spectra: K0s and Λ´s From A. Timmins • 55M min-bias events analyzed. • High statistic data, with spectra extended to high pT coverage. • Fully corrected spectra, with feed-down corrections.

8. CuCu200 spectra: Ξ´s and Ω´s STAR Preliminary STAR Preliminary • Statistics is high enough to separate even the Ωspectra in different event centrality classes. • After efficiency, acceptance and feed-down correction, total yield is obtained from a fit using a Boltzmann function.

9. ΛSpectra Comparison between Au+Au and Cu+Cu From A. Timmins Talk on sunday STAR Preliminary • Λ spectra measured from Cu+Cu(0-10%) has same shape as spectra measured in Au+Au(20-40%), with equivalent Npart. • Shows deviation from Maxwell-Boltzmann behavior at high pT.

10. ΞSpectra Comparison between Au+Au and Cu+Cu STAR Preliminary • Ξ spectra shows same shape and slope parameter for Cu+Cu(0-10%) and Au+Au (20-40%). • Ξ error bars are larger but, similar trend as seen in the Lambda spectra can be observed with a deviation from exponential at the high pt region.

11. Mean Transverse momentum of Ξs and Ωs. • Spectra measured from Au+Au seems to be harder than p+p. • For Au+Au collisions, <pT> does not seem to vary with Npart, in the measured range. • Ω spectra shows higher <pT> than Ξ´s. • New Cu+Cu data seems to be consistent with Au+Au values. STAR Preliminary

12. CuCu200 & AuAu200 Yields: K0s, and Λ´s • Au+Au: particle yields increase with system size. • Cu+Cu: On a first look, data seems to be consistent with Au+Au yields. • Central Λ yields from Cu+Cu data seems to be slightly higher than the equivalent centrality region in Au+Au. • Red symbols are for Au+Au 200GeV • Black symbols are for Cu+Cu 200 GeV • Solid symbols are for the particles • Open symbols are for anti-particles.

13. Strangeness Enhancement: Au+Au 200 GeV nucl-ex/0705.2511 • Clear enhancement of hyperon production in Au+Au compared to pp. • Strangeness enhancement over p+p data is already observed at most peripheral Au+Au bin. • Difference between baryon and Anti-baryon is expected due to non-zero net baryon number. • Clear increase of strangeness enhancement with strange quark content, indicating enhancement hierarchy that is in accordance to GC Thermal production. • Strangeness enhancement show dependence with volume (Npart) which disagrees with GC thermal production. STAR Preliminary A. Tounsi, A. Mischke and K. Redlich, Nucl. Phys. A 715, 565 (2003).

14. Strangeness Enhancement: AuAu200 & CuCu200 • Still see clear hyperon enhancement in Cu+Cu data. • On a first look, Cu+Cu data seems to be consistent with enhancement observed at Au+Au. • Λ enhancement from most central Cu+Cu data seems to be higher than equivalent Au+Au data, should try different scaling. STAR Preliminary

15. Statistical Thermal Model: Cu+Cu 200 GeV • Statistical Thermal Model (THERMUS)* was used fitting Tch, μB, μS, andγS (strangeness saturation factor). • Particles used in the fit: • π, K, p, Λ, Ξ, Ω. • Particles were corrected for weak decays. • Measured particle ratios are reasonably well fit with statistical thermal model. * Thermus, A thermal Model Package for Root S. Wheaton & Cleymans, hep-ph/0407174

16. Statistical Thermal Model: Au+Au 200 GeV • Statistical Thermal Model (THERMUS)* was used fitting Tch, μB, μS, andγS (strangeness saturation factor). • Particles used in the fit: • π, K, p, Λ, Ξ, Ω. • Particles were corrected for weak decays. • Measured particle ratios are reasonably well fit with statistical thermal model. * Thermus, A thermal Model Package for Root S. Wheaton & Cleymans, hep-ph/0407174

17. Statistical Thermal Model: Au+Au 62.4 GeV • Statistical Thermal Model (THERMUS)* was used fitting Tch, μB, μS, andγS (strangeness saturation factor). • Particles used in the fit: • π, K, p, Λ, Ξ, Ω. • Particles were corrected for weak decays. • Measured particle ratios are reasonably well fit with statistical thermal model. • Ratio K0/π was excluded to improve chi2. * Thermus, A thermal Model Package for Root S. Wheaton & Cleymans, hep-ph/0407174

18. Cu+Cu 200 GeV Statistical Thermal Model: Fit parameters vs. system size • Baryon chemical potential μB is small for Au+Au 200 GeV. • Small variation with system size. • μS is consistent with zero. • Au+Au 62 GeV data shows higher value of baryon chemical potential. • Au+Au 62 GeV shows larger variation with system size when compared to Au+Au 200 GeV. • Cu+Cu 200 GeV baryon chemical potential seems to be in good agreement with Au+Au 200 GeV. * Thermus, A thermal Model Package for Root S. Wheaton & Cleymans, hep-ph/0407174

19. Cu+Cu 200 GeV Statistical Thermal Model: Fit parameters vs. system size • Temperature seems constant with system size for Au+Au 200 GeV. • Au+Au 62 GeV data shows same temperature values of Au+Au 200 GeV and also no system size dependence can be observed within error bars. • Cu+Cu 200 GeV temperature shows a smaller value then compared to Au+Au data, but is in agreement within error bars. * Thermus, A thermal Model Package for Root S. Wheaton & Cleymans, hep-ph/0407174

20. Cu+Cu 200 GeV Statistical Thermal Model: Fit parameters vs. system size • Strangeness saturation constant, shows an increase with system size, reaching saturation around Npart~150. • Au+Au 62 GeV data shows same values and behavior of Au+Au 200 GeV. • Cu+Cu 200 GeV data fits seems to yield a strangeness saturation constant consistent with 1. * Thermus, A thermal Model Package for Root S. Wheaton & Cleymans, hep-ph/0407174

21. Statistical Thermal Model: Excitation function

22. Particle ratios • Smooth rise of ratio with energy from SPS to RHIC, indicating an evolution from baryon transport regime to pair production dominated regime. • Ratio approaching baryon free environment at RHIC energies. • The higher the strangeness content, ratio is closer to unity. • New Cu+Cu data seems to follow systematic. • Ratio seems to be the same, independent of the system size. STAR preliminary SPS data from NA49 AGS data from E896 & E802

23. Baryon Excitation function • Above AGS energies baryon yield is relatively constant with energy. • Λ yield variation with energy seems to follow proton dependence. • For LHC: Λ ~ 10-30 • Ξ ~ 3-6 • Ω ~ 0.4-0.7 STAR Preliminary

24. Anti-Baryon Excitation function • Anti-baryon yield increase continuously and smoothly with energy, indicating smooth increase of pair-production. STAR Preliminary

25. Baryon to Meson ratio: Au+Au 62 GeV & Au+Au 200 GeV • Baryon enhancement in the intermediate pT region observed in p/πratio, consistent with recombination model. • Λ/K0Sratio shows that strange baryon production is also enhanced over strange meson production in the intermediate pT region. • Strange baryon enhancement is higher than enhancement observed with p/πratio. • Au+Au 62 GeV also shows baryon to meson enhancement at intermediate pT. Values seems to be higher than Au+Au 200 GeV data. Lambdas are not feed-down corrected

26. Baryon to meson ratio: Cu+Cu 200 GeV & Au+Au 200 GeV • Baryon to meson enhancement is also observed in Cu+Cu data, but seems to be higher then in Au+Au data when comparing equivalent centrality classes. STAR preliminary

27. Baryon to meson enhancement • Higher enhancement of Λ to K0s in the intermediate pT region in Cu+Cu 200 GeV when comparing to Au+Au 200 GeV. • These seems to be also a energy dependence of the baryon to meson enhancement.

28. Nuclear Modification factor: Au+Au 200 GeV • Baryon meson difference in the intermediate pT region is also observed in Rcp and RAA plots for strange particles. • RAA of protons seems to be different from the RAA of strange particles, where higher values of RAA are observed for strange particles. • Maybe canonical suppression of strangeness production in p+p collisions observed in low pt region extends to the intermediate pT region causing the increase of RAA ratio, something that cannot be observed with Rcp plots. This is different from Cronin effect. STAR preliminary nucl-ex/0705.2511 0 1 2 3 4

29. Rcp - Nuclear Modification factor: Cu+Cu 200 GeV & Au+Au 200 GeV • Similar suppression observed in Cu+Cu at high pT for same centrality ratios. • Similar baryon meson suppression difference seen in the intermediate pT region.

30. Rcp - Nuclear modification factor: Au+Au 62 GeV & Au+Au 200 GeV • Similar suppression observed in Au+Au 62.4 GeV at high pT. • Similar baryon meson suppression difference seen in the intermediate pT region.

31. Difference between mesons & baryons Rcp • This ratio describes how different is the baryon Rcp from the meson Rcp in the intermediate pT region. • Remarkable consistency is seen between different systems and different energies.

32. Summary / Conclusions • New Cu+Cu data was compared with Au+Au data scaled by Npart to study system size dependence. • Overall yields and spectra shape seems to be consistent with the equivalent peripheral Au+Au collision. • Lambdas produced at most central event centrality classes seems to show higher yield compared to the equivalent Au+Au peripheral collision. Perhaps a higher thermalization degree is achieved for Cu+Cu central collisions? • Ξ and Ωare consistent within error with Au+Au data, but, error bars are large and may not be sensitive to an enhancement over Au+Au data. • We are currently analyzing Cu+Cu 62.4 GeV data that should enrich this systematic study.

33. Summary / Conclusions • Statistical Thermal model fits reasonably well the particle ratios measured in STAR, indicating that data is consistent with a thermalized system. • Au+Au 200 and 62 GeV: Centrality dependence of thermal fits show increase of the strangeness saturation parameter, and it reaches 1 only after Npart ~150. • Cu+Cu200 GeV: yields the same temperature and baryon chemical potential values obtained from the fit to Au+Au data, but, the strangeness saturation parameter is already at 1, even in the peripheral events. • Excitation functions of strange baryon production don’t show any surprises. • At intermediate pt, baryon to meson enhancement is observed. • Λ/π ratio shows an enhancement higher than the enhancement observed with p/π. • The enhancement increases with centrality and are different for Au+Au 62 and 200 GeV and Cu+Cu 200 GeV data. • Nuclear modification factor Rcp data also consistent with baryon to meson enhancement in the intermediate pT. • Even in a small system like Cu+Cu, we still see a suppression at high pT region and the baryon to meson difference in the intermediate pT region.

34. Thank you The STAR Collaboration University of Illinois at Chicago - Argonne National Laboratory Institute of High Energy Physics - University of Birmingham Brookhaven National Laboratory - California Institute of Technology - University of California, Berkeley - University of California, Davis - University of California, Los Angeles - Carnegie Mellon University - Creighton University – Nuclear Physics Inst., Academy of Sciences - Laboratory of High Energy Physics - Particle Physics Laboratory - University of Frankfurt - Institute of Physics, Bhubaneswar - Indian Institute of Technology, Mumbai - Indiana University Cyclotron Facility - Institut de Recherches Subatomiques de Strasbourg - University of Jammu - Kent State University - Institute of Modern Physics - Lawrence Berkeley National Laboratory - Massachusetts Institute of Technology - Max-Planck-Institut fuer Physics - Michigan State University - Moscow Engineering Physics Institute - City College of New York - NIKHEF and Utrecht University - Ohio State University - Panjab University - Pennsylvania State University - Institute of High Energy Physics - Purdue University – Pusan National University - University of Rajasthan - Rice University - Instituto de Fisica da Universidade de Sao Paulo - University of Science and Technology of China - Shanghai Institue of Applied Physics - SUBATECH - Texas A&M University - University of Texas, Austin - Tsinghua University - Valparaiso University – Variable Energy Cyclotron Centre, Kolkata - Warsaw University of Technology - University of Washington - Wayne State University - Institute of Particle Physics - Yale University - University of Zagreb -UNICAMP

35. Extras

36. p/π - Baryon to meson ratio (PHENIX) • Baryon to meson ratio show baryon enhancement in the intermediate pt region. Au+Au 200 GeV M. Konno, JPS2007 PHENIX Preliminary Cu+Cu D. Morrison, SQM2006

37. Rcp - Nuclear modification factor: AuAu62 & AuAu200

38. Scaling with charge particle production (dNch/dy)

39. Strangeness Enhancement: RHIC & SPS • SPS NA57 data also shows clear enhancement behavior. • Enhancement rate at RHIC seems to be lower than the enhancement observed at SPS data. • Need to check if the enhancement reduction is in agreement with GC Thermal model predictions. NA57 data from nucl-ex/0601021

40. STAR Preliminary Total Yield • After efficiency, acceptance and feed-down correction, total yield is obtained from a fit using a Boltzmann function.

41. Scaling with Nbin

42. Lambda/K0s from SPS