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



Jun Takahashi & Marcelo Munhoz for the STAR collaboration




                                                            1
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.


 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                       2/34
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.


 Jun Takahashi for the STAR collaboration, Levoca, Slovakia   3/34
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



 Jun Takahashi for the STAR collaboration, Levoca, Slovakia          4/34
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)




 Jun Takahashi for the STAR collaboration, Levoca, Slovakia             5/34
System Size:
                     Au+Au                                        Cu+Cu
     Cent           Npart               NBin          Cent        Npart        NBin


    0-5       351.0 ± 3.0         1039 ± 79

    5-10      293 ± 7.            810 ± 58

    10-20     231 ± 3.2           574 ± 42

    20-40     139 ± 5.            278 ± 30          0-10      98.4 ± 1.0   185.7 ±5.9

                                                    10-20     74.8 ± 2.5   126.7 ±6.7
    40-60     59.0 ± 5.           82 ± 12
                                                    20-30     54.4 ± 2.8   81.5 ± 6.0

                                                    30-40     38.5 ± 2.5   51.0 ± 4.8
    60-80     19.0 ± 3.5          19 ± 5
                                                    40-60     21.9 ± 2.6   24.3 ± 3.9




 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                             6/34
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.


 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                     7/34
CuCu200 spectra: Ξ´s and Ω´s
                                                                    STAR Preliminary




                                                      STAR Preliminary
  Statistics is high enough to separate even the Ω spectra in different
  event centrality classes.
  After efficiency, acceptance and feed-down correction, total yield is
  obtained from a fit using a Boltzmann function.
 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                            8/34
Λ Spectra 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.


Jun Takahashi for the STAR collaboration, Levoca, Slovakia                             9/34
Ξ Spectra 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.


 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                      10/34
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
                           STAR Preliminary            consistent with Au+Au values.




 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                           11/34
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.


 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                            12/34
Strangeness Enhancement:
Au+Au 200 GeV
                                     nucl-ex/0705.2511
     STAR Preliminary                                         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.

                                                                A. Tounsi, A. Mischke and K. Redlich,
                                                                Nucl. Phys. A 715, 565 (2003).
 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                      13/34
Strangeness Enhancement:
AuAu200 & CuCu200
                                        STAR Preliminary      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.



 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                        14/34
 Statistical Thermal Model: Cu+Cu 200 GeV

Cu  Cu, S NN  200 GeV                                  Statistical Thermal Model
                                                         (THERMUS)* was used fitting Tch,
                                                         μB, μS, and γS (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

   Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                   15/34
Statistical Thermal Model: Au+Au 200 GeV

Au  Au, S NN  200 GeV
                                                    Statistical Thermal Model (THERMUS)*
                                                    was used fitting Tch, μB, μS, and γS
                                                    (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

  Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                   16/34
 Statistical Thermal Model: Au+Au 62.4 GeV

Au  Au, S NN  62.4 GeV                            Statistical Thermal Model (THERMUS)*
                                                    was used fitting Tch, μB, μS, and γS
                                                    (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

   Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                   17/34
Statistical Thermal Model:
Fit parameters vs. system size
                                              Baryon chemical potential μB is small for
                                              Au+Au 200 GeV.
      Cu+Cu 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

 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                   18/34
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
                      Cu+Cu 200 GeV
                                              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

 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                   19/34
Statistical Thermal Model:
Fit parameters vs. system size
                                              Strangeness saturation constant, shows
                                              an increase with system size, reaching
                                              saturation around Npart~150.

                      Cu+Cu 200 GeV
                                              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

 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                   20/34
Statistical Thermal Model:
Excitation function




 Jun Takahashi for the STAR collaboration, Levoca, Slovakia   21/34
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.
            STAR preliminary
                                                              New Cu+Cu data seems to
                                                              follow systematic.
                                                              Ratio seems to be the same,
SPS data from NA49                                            independent of the system
AGS data from E896 & E802                                     size.

 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                22/34
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



 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                            23/34
Anti-Baryon Excitation function


                                                              Anti-baryon yield increase
                                                              continuously and smoothly
                                                              with energy, indicating
                                                              smooth increase of pair-
                                                              production.




              STAR Preliminary



 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                          24/34
 Baryon to Meson ratio:
 Au+Au 62 GeV & Au+Au 200 GeV
  Baryon enhancement in the intermediate pT region observed in p/π ratio, consistent
  with recombination model.
  Λ/K0S ratio 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/π ratio.
  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


   Jun Takahashi for the STAR collaboration, Levoca, Slovakia                     25/34
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



 Jun Takahashi for the STAR collaboration, Levoca, Slovakia     26/34
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.

 Jun Takahashi for the STAR collaboration, Levoca, Slovakia         27/34
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

    Jun Takahashi for the STAR collaboration, Levoca, Slovakia               28/34
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.
    Cu  Cu, S NN  200 GeV                                   Au  Au, S NN  200 GeV




 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                        29/34
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.

     Au  Au, S NN  62.4 GeV                                 Au  Au, S NN  200 GeV




 Jun Takahashi for the STAR collaboration, Levoca, Slovakia                        30/34
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.

 Jun Takahashi for the STAR collaboration, Levoca, Slovakia           31/34
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 Ω are 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.



    Jun Takahashi for the STAR collaboration, Levoca, Slovakia           32/34
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 p T region
         and the baryon to meson difference in the intermediate pT region.




    Jun Takahashi for the STAR collaboration, Levoca, Slovakia                              33/34
                                           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




    Jun Takahashi for the STAR collaboration, Levoca, Slovakia                                           34/34

				
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