1. System Size and Energy Dependence of -meson Production at RHIC
X.Z. Caia) for the STAR Collaboration
a) Nuclear Physics division, Shanghai Institute of Applied Physics,
Motivation
-meson analysis and results
-meson from Cu+Cu 200 GeV and 62.4 GeV
System size and energy dependence of -meson production at RHIC
Summary

2. -meson production from Au+Au
<pT>
f/K-
1. <pT>, -meson decoupled early;
2. N()/N(K), ruled out the K-coalescence;
3. Evolution in the centrality dependence;
 Cu+Cu collisions provide unique data to further probe these features.
STAR Col. Phys. Lett. B 612, 181 (2005),
Phys. Rev. Lett. 99, (2007).
April 26-28, Nanjing

3. W and f mostly from bulk s quarks
And bulk s quarks have collective elliptic flow !
Since -mesons are made via coalescence of seemingly thermalized s quarks in central Au+Au collisions, the observations imply hot and dense matter with partonic collectivity has been formed at RHIC.
STAR Col. Phys. Rev. Lett. 99, (2007).
April 26-28, Nanjing

4. The STAR experiment
STAR TPC used to identify Kaon via dE/dx in TPC gas
STAR Detector
We used the high-statistics 200 GeV and 62.4 GeV Cu+Cu data to measure
the -meson production at STAR:
~ 10.6M 200 GeV events (0-54%);
~ 8.8M 62.4 GeV events (0-60%).
Event-mixing method used to estimate combinatorial background from uncorrelated K+K- pairs;
Final subtracted minv distribution fitted with Breit-Wigner + straight line.
April 26-28, Nanjing

5.  mass
0-10%
10-19%
19-28%
28-36%
36-45%
45-54%
STAR Preliminary
0-10%
10-19%
19-28%
28-36%
STAR Preliminary
The  mass is consistent with PDG value at pT>0.7 GeV/c while a drop of ~2.5 MeV at lower pT
The  mass value from data and simulation are consistent and the drop at low pT is understood within detector effects
April 26-28, Nanjing

6. spectra
STAR Preliminary
Levy function describe well the data (exponential in central and power-law-like in peripheral)
Similar trend in Cu+Cu and Au+Au at the similar Npart at same collision energy
April 26-28, Nanjing

7. dN/dy and <pT>
STAR Preliminary
The dN/dy and <pT> are similar for Cu+Cu and Au+Au at similar Npart bin for the same collisions energy.
f yields from Au+Au and Cu+Cu collisions depend on the number of participant nucleons only, unlike Kaon and hyperons.
see STAR Xiaobin Wang – parallel session VII
[1] NA49 Col. Phys. Rev. Lett. 96, (2005);
[2] E-802 Col. Phys. Rev. C 60, (1999).
April 26-28, Nanjing

8. Nuclear Modification factor
STAR Preliminary
Comparing central Au+Au and central Cu+Cu with peripheral Au+Au Collisions—
1) the yield and pT shape depend on number of participants only
2) in terms of number of binary collisons the Cu+Cu data is higher indicating given
the same top 10% centrality Cu+Cu has less number of collisions per participant
April 26-28, Nanjing

9. Strangeness enhancement
STAR Preliminary
Strangeness enhancement: yield relative to p+p
-meson enhancement:
-- between K/L and Xi
GeV data > 62.4 GeV, unlike hyperons
-- could not be solely due to the canonical suppression, there could be dynamics effect X
K, L
200 GeV
62.4 GeV
April 26-28, Nanjing

10. Summary
We present a study of the system size and energy dependence of -meson production at RHIC.
The transverse momentum spectra in both shape and yields are similar in Cu+Cu and Au+Au for collisions with similar Npart
-meson enhancement shows a distinct collision centrality and energy dependence. The enhancement factor of the -meson production (yield per Npart) lies between those of K/L and X, and decreases from 200 GeV to 62.4 GeV data unlike hyperons.
The -meson enhancement cannot solely due to canonical suppression of strangeness production, there could be dynamics effect.
April 26-28, Nanjing

11. N()/N(p)
A decreasing trend in N()/N(p) vs. pT would be an unambiguous signature for early decoupling of  mesons from the hadronic rescattering dynamics[1]
Fitting with Hirano’s model[1] yield:
p+p, ChiSquare/ndf = 6.01/8
Au+Au w rescattering, ChiSquare/ndf = 11.42/8
Au+Au w/o rescattering, ChiSquare/ndf = 29.14/8
Au+Au, Tth=100 Mev, ChiSquare/ndf = 7.75/8
[1] Hydro calculation: arXiv: v1
April 26-28, Nanjing