Strange Hadrons Production in Cu+Cu collisions at  s NN = 62.4 GeV at RHIC

1. Strange Hadrons Production in Cu+Cu collisions at sNN = 62.4 GeV at RHIC Marcelo G. Munhoz Universidade de São Paulo – Brazil for theSTAR Collaboration

2. Motivation • Systematic study of particle production in relativistic heavy ion collisions as a function of energy and system size • Allows to investigate the mechanisms behind strangeness production in these collisions • What is the influence of the system geometry? • How does strangeness production change as a function of energy?

3. In this talk… • First results on K0 short, Λ, Ξ and Ω production from Cu+Cu collisions at √sNN = 62.4 GeV • special credits to Geraldo Magela – UNICAMP and Ulisses Gulart – USP • How does strange hadron production change when one goes from Cu+Cu to Au+Au or from 62.4 to 200 GeV? • What can we learn from this systematic comparisons? • Few comparisons between • different √sNN : 62.4 GeV and 200 GeV • different colliding systems: Cu+Cu and Au+Au

4. Neutral Strange Hadrons Cu+Cu, 62.4 GeV STAR Preliminary Minimum bias 0.5 < pt < 4.5 • Particle identification through the topology of the weak decay • ~10M events analyzed • Clear peak at the invariant mass spectra for |y| < 0.5 and 0.5 < pt < 4.5 GeV/c • Good statistics (less than 1% statistical error) • Polynomial fit of the background K0 Cu+Cu, 62.4 GeV STAR Preliminary Minimum bias 0.5 < pt < 4.5 Λ

5. Cu+Cu, 62.4 GeV Multi-strange Hadrons • Particle identification through the topology of the weak decay • ~10M events analyzed • Clear peak at the invariant mass spectra for |y| < 0.5 and 0.75 < pt < 4.0 GeV/c • Good statistics (less than 1% statistical error for Ξ and less than 7% for Ω) • Polynomial fit of the background Cu+Cu, 62.4 GeV

6. Centrality definition 40 – 60% 30 – 40% 20 –30% 10 – 20% Npart • The data sample was divided in 5 centrality bins according to measured charged particle multiplicities • Each centrality bin is associated to a number of participant nucleons (Npart) using a geometrical Glauber approach 0 – 10%

7. Transverse Mass Spectra • Good statistics for |y| < 0.5 and 0.5 < pt < 4.5 • Corrected for detector efficiency and acceptance • Statistical error only • Λ spectra corrected for feed-down from Ξ weak decay. Feed-down from Ω is negligible STAR Preliminary STAR Preliminary K0 Λ Cu+Cu, 62.4 GeV Cu+Cu, 62.4 GeV

8. Transverse Mass Spectra • Good statistics for |y| < 0.5 and 0.75 < pt <3.5 • Corrected for detector efficiency and acceptance • Statistical error only • For Ω, studied 3 centrality bins: 0-10%, 10-20% and 20-40% STAR Preliminary STAR Preliminary Ξ- Ω- Cu+Cu, 62.4 GeV Cu+Cu, 62.4 GeV

9. Bulk strangeness production STAR Preliminary • Strangeness enhancement: • strange hadrons are enhanced relative to p+p • more strangeness production or canonical suppression in p+p? • relative enhancement seems to be slightly lower than in SPS • dependence with Npart – production volume not proportional to Npart F. Antinori et. al. (NA57 Collab.), J. Phys. G, 32 (2006) J. Takahashi for the STAR Collaboration, nucl-ex/0809.0823

10. Bulk strangeness production STAR Preliminary • Previous observation for strange hadrons production at Cu+Cu 200 GeV: • yield does not follow the same Npart dependence as in Au+Au collisions J. Takahashi for the STAR Collaboration, nucl-ex/0809.0823

11. Bulk strangeness production • Previous observation for strange hadrons production at Cu+Cu 200 GeV: • yield does not follow the same Npart dependence as in Au+Au collisions • this result contradicts the suggested volume dependence with Npart: V = AαV0, where A = Npart/2, V0 = 4/3.πR3 and α = 1 STAR Preliminary , 2/3 or 1/3 M.A.C Lamont for the STAR Collaboration, J. Phys.: Conf. Ser. 110 032011

12. STAR Preliminary Bulk strangeness production • Previous observation for strange hadrons production at Cu+Cu 200 GeV: • yield does not follow the same Npart dependence as in Au+Au collisions • in addition, the meson Φ seems to be an exception, for both Cu+Cu 200 and 62.4 GeV J. H. Chen for the STAR Collaboration, nucl-ex/0804.4363

13. Bulk strangeness production • Similar behavior was already observed for lower energies (SPS - √sNN=17.2 GeV), where the K/π ratio was higher in lighter systems for the same Npart • What about the strangeness production in lighter systems (Cu+Cu) at √sNN=62.4 GeV? C. Höhne for the NA49 Collaboration, Nucl. Phys. A 715 (2003) 474

14. Yield at mid-rapidity × centrality • Integrated pt spectra using an exponential function up to 2.0 GeV/c • Statistical error only • Cu+Cu yield higher than Au+Au for the same Npart • Same behavior for 200 and 62.4 GeV K0 Λ

15. Yield at mid-rapidity × centrality • Integrate the pt spectra using a boltzman function • Statistical error only • Cu+Cu yield slightly higher than Au+Au for the same Npart • Same behavior for 200 and 62.4 GeV Ξ- Ω- + Ω+

16. STAR Preliminary Bulk strangeness production • Strangeness production does not have the same Npart dependence in Cu+Cu and Au+Au collisions at 200 and 62.4 GeV • Is it a unique feature from strange hadrons? • For a quick answer, one can compare strange hadron production with pion yields A. Iordanova for the STAR Collaboration, nucl-ex/0806.0286

17. Bulk strangeness production • Strangeness production does not have the same Npart dependence in Cu+Cu and Au+Au collisions at 200 and 62.4 GeV • Is it a unique feature from strange hadrons? • Npart is not a good parameter to account for geometry differences from lighter (Cu+Cu) to heavier (Au+Au) collision systems

18. Summary • First results on K0 short, Λ, Ξ and Ω production from Cu+Cu collisions at √sNN = 62.4 GeV • Bulk strangeness production: • Cu+Cu yield higher than Au+Au for the same Npart, as observed for other hadrons • Similar behavior in Cu+Cu 62.4 GeV and 200 GeV • Npart is not a good parameter to account for geometry differences in Au+Au and Cu+Cu collisions • Work in progress: once we have the data analyzed, we need to develop a deeper understanding of strangeness production mechanisms comparing all systems and energies available…