1. sevil.salur@yale.edu 1 Baryonic Resonance Why resonances and why  * ? How do we search for them ? What did we learn so far? What else can we do in the near future? Sevil Salur Yale University STAR Collaboration    p   p  Studies with

2. sevil.salur@yale.edu 2 Chemical freeze-out Thermal freeze-out **  * measured       * lost          * measured time Why do we study resonances in heavy-ion collisions? Due to the very short lifetime (  < Δ t ) of resonances: Large fraction of the decays occur inside the reaction zone Possible change in the physical properties: width broadening mass shift change in p T spectra Determination of the hadronic expansion time between chemical and thermal freeze-out Information about strangeness production due the strange quark content and high mass of  * (1385) ΔtΔt    MeV/c 2 Г :35 MeV/c 2 I(J P ) =1(3/2 + )

3. sevil.salur@yale.edu 3 Particle Identification M inv GeV/c 2   * (1385) N ENTRIES  s NN =200 GeV p+p    -  and   ±   ± N ENTRIES A  candidate is combined with a  to get a  *(1385). The background is formed by mixing  mesons from one event with  candidates from another event.

4. sevil.salur@yale.edu 4  * Invariant Mass Spectra *±*±  The masses and widths of  are in agreement with the PDG and the results with other topological reconstruction techniques.    M = 1387 MeV,   39 MeV ) (    M = 1383 MeV,   36 MeV *±*±  s NN =200 GeV @ d+Au  s NN =200 GeV @ p+p  s NN =200 GeV @ Au+Au 0-5% N ENTRIES 

5. sevil.salur@yale.edu 5  * Corrected p T Spectra Exponential Fit Function : |y|<0.5 =1.02 ±0.02±0.07 GeV/c T inv slope = 319±9±16 MeV |y|<0.5 =1.14 ± 0.05 ± 0.08 GeV/c T inv slope = 386 ± 15 ± 27 MeV |y|<0.5 =1.28 ± 0.15 ± 0.09 GeV/c T inv slope = 456 ± 54 ± 23 MeV Similar is measured.

6. sevil.salur@yale.edu 6  p T  vs Particle Mass Parameterization is from ISR data at √s=26 GeV (Not correct for heavy particles. )  p T  values merge for Au+Au and p+p for heavier particles. Are heavier particles produced predominately in more violent p+p collisions? Do heavier particles flow less in Au+Au with respect to  ?

7. sevil.salur@yale.edu 7 Version 6.3 Leading Order pQCD model K factor represents the factorization of next-to-leading order (NLO) processes. A large K factor  Large NLO contributions  * in p+p and Pythia Comparisons

8. sevil.salur@yale.edu 8 Version 6.3  * in p+p and Pythia Comparisons K=3 too hard for the light mesons. Pythia Comparisons: Pythia Comparisons: K=3 is also needed to describe strange baryons  (1385) K=3 ok for strange baryons

9. sevil.salur@yale.edu 9 T171 ± 9 MeV ss 0.53 ± 0.04 r3.49 ± 0.97 fm B and Q = 2, S = 0 T168 ± 6 MeV ss 0.92 ± 0.06 r15 ± 10 fm  B (4.5 ± 1.0) X 10 -2 GeV  S (2.2 ± 0.7) X 10 -2 GeV  Q (-2.1 ± 0.8) X 10 -2 GeV Particle ratios are represented except . Particle ratios are well described except the  */  T is same for pp and AuAu Thermal Model Predictions

10. sevil.salur@yale.edu 10 Resonance Ratios in p+p, d+Au and Au+Au If there is re-scattering then regeneration is needed !  t= 2 ± 1 fm/c from K*/K  t= 10 ± 6 fm/c from  */  K*/K and  */  exhibits a slight suppression  the re-scattering Regeneration σ (K*) > σ (  *)  */    regeneration

11. sevil.salur@yale.edu 11 Nuclear Modification Factor R dAu  (1385) follow h + +h - Cronin Effect might explain R dAu above 1 Less so for mesons than baryons ! d+Au p+p √s NN =200 GeV MESONS BARYONS

12. sevil.salur@yale.edu 12 Conclusions The  (1385) resonance can be clearly identified via combinatorial techniques in all collision environments. No strong increase of  p T  pp  AuAu  Different production mechanisms (jets in p+p)? Pythia with K=3 factor. No suppression or enhancement in the ratios of  * /  in p+p, d+Au and 0-5% Central Au+Au collisions within the uncertainties of the measurement.  Regeneration is needed for the re-scattering picture if the time between chemical and thermal freeze-out is non-zero.  Sequential freeze-outs instead. T is similar for pp and AuAu collisions. Nuclear modification factor (R dAu ) for  (1385) follows the same trend as p.  Species or mass dependence can be further investigated with R AA measurement from Run 4 More data is available from Run 4 !!! Better centrality measurement for  *. Au+Au at √s NN =200 GeV, 50 Million Events taken. More to Come. Keep Tuned !

13. sevil.salur@yale.edu 13 First Resonance Measurement (Y*) Alvarez Nobel Prize 1968 "for his decisive contributions to elementary particle physics, in particular the discovery of a large number of resonance states, made possible through his development of the technique of using hydrogen bubble chamber and data analysis" Resonances are strongly decaying, extremely short lived particles. [~fm/c] Invariant Mass Distribution of Y* =  *(1385) Two possible explanations: Resonant States (Energies at which the cross section is a maximum) Resonance Particles (Real Particles) M. Alston, Phys. Rev. Lett. 5, 520 (1960). 

14. sevil.salur@yale.edu 14 T171 ± 9 MeV ss 0.53 ± 0.04 r3.49 ± 0.97 fm B and Q = 2, S = 0 T168 ± 6 MeV ss 0.92 ± 0.06 r15 ± 10 fm  B (4.5 ± 1.0) X 10 -2 GeV  S (2.2 ± 0.7) X 10 -2 GeV  Q (-2.1 ± 0.8) X 10 -2 GeV Particle ratios are represented except . Particle ratios are well described except the  */  T is same for pp and AuAu J. Cleymans hep-ph 0212335 The relative strangeness production for Pb+Pb at SPS similar to p+p at RHIC.  s is higher in AuAu An enhancement in the K/  ratios ~ 50% Thermal Model Predictions

15. sevil.salur@yale.edu 15 Thermal Model Predictions T168 ± 6 MeV ss 0.92 ± 0.06 r15 ± 10 fm  B (4.5 ± 1.0) X 10 -2 GeV  S (2.2 ± 0.7) X 10 -2 GeV  Q (-2.1 ± 0.8) X 10 -2 GeV Particle ratios are represented except . Particle ratios are well described except the  */  Small  B  No incoming baryon number