1. φ-meson production and partonic collectivity at RHIC
Jin-hui Chen for the STAR Collaboration
Shanghai Institute of Applied Physics (SINAP), CHINA
Outline
Motivation & introduction
Dataset & analysis method
φ-meson production (spectra & RCP)
Elliptic flow v2 of φ-meson from RHIC
Conclusion & outlook
Thanks to: J. G. Ma, H.Z. Huang (UCLA), S. Blyth, N. Xu (LBL), X.Z. Cai, G.L. Ma,
Y.G. Ma(SINAP), H.B. Zhang (BNL) et al.
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

2. Importance of 
The medium produced in HI collisions is very short-lived → we need probes which carry information from the early stage to find out about the medium constituents: K+ K- φ
QGP
The φ-meson (ss) is a clean probe from early time:
Small σ for interactions with non-strange particles[1]
Relatively long-lived (41 fm/c) →decays outside the fireball
Previous measurements have ruled out K+ + K- coalescence as φ production mechanism[2] → info not “diluted” by hadronic phase
The φ can provide info on particle production mechanism :
The φ is a meson but as heavy as Λ,p baryons
Differentiate between mass-type or meson/baryon- type dependencies in the RHIC measurement of grouping RCP and v2 [3]
[1] A. Shor, Phys. Rev. Lett. 54 (1985) 11
[2] J. Adams et al., Phys. Lett. B 612 (2005) 181
[3] J. Adams et al. Phys. Rev. Lett. 92 (2004)
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

3. Dataset and analysis method
We used the high-statistics 200 GeV Au+Au data to measure the φ observables at STAR:
~14.2 M minbias (0-80%) events
~13 M central triggered (0-12%) events
Measured decay channel:
φ → K++K- (BR = 49.1%)
STAR TPC used to identify K via dE/dx in TPC gas
STAR Detector
Event-mixing method used to estimate background from uncorrelated K+K- pairs
Final subtracted minv distribution fitted with Breit-Wigner + straight line
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

4. Methods to extracted φ-meson v2
Standard event plane method[1,2]
1)In each pT bin, extract φ yield in each (Ф-Ψ2) bin.
2)Fitted φ yield(pT) vs. (Ф-Ψ2) to obtain v2obs(pT).
3)Corrected the v2obs (pT) with reaction plane resolution
Invariant mass method[3]
v2=v2obs/res
[1]A.M. Poskanzar and S.A. Voloshin Phys. Rev. C 58 (1998) 1671
[2]J.Adams et al. Phys.Rev.Lett 95 (2005)
[3]Borghini et al. nucl-th/
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

5. φ-meson production (1)
STAR Preliminary
The shape of the φ pT spectra provide info on the mechanisms of particle production:
φ pT spectra show a systematic centrality-dependent evolution in shape
For peripheral collisions, a pQCD power-law tail is evident
Peripheral spectra favor a Levy function description
For central collisions, exponential and Levy functions fit spectra equally well
The power-law tail is suppressed by the medium produced in central collisions
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

6. φ-meson production (2)
STAR Preliminary
For both centrality groupings, nuclear modification factor RCP of φ < 1 : φ yield suppressed in central compared to peripheral collisions:
Particle grouping behaviour:
φ follows the same trend as K0S in RCP
Confirmation of meson-baryon dependence of RCP rather than mass-type dependence
Described by recombination/ coalescence models[1]
[1]R. J. Fries et al., Phys. Rev. C 68 (2003)
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

7. φ-meson production (3)
Comparison with model expectations on particle production can give insight on the constituents of the medium produced in heavy-ion collisions:
R. Hwa's recombination model[1]:
φ and Ω (sss) spectra (pT<8GeV/c) mainly due to recombination of thermal quarks (TT)
Seems to match data well
BUT... Ω/φ ratio has similar shape to other baryon/meson measurements
Model matches data for pT < 4 GeV/c
STAR Preliminary
STAR Preliminary
[1] R. Hwa & C-B Yang, nucl-th/
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

8. Elliptic flow v2
The φ experiences significant elliptic flow! (result is mean of 2 different, but consistent methods):
STAR Preliminary
Early time information:
For pT<2 GeV/c, φ flows as much as other ID'd particles, consistent with hydro. mass-ordering
For pT>2 GeV/c, φ v2 is more consistent with K0S than Λ (favors NCQ[1] =2)
Consistent with v2 of other multi-strange hadrons (Ξ, Ω)[2] i.e. s-quarks flow!
Particle production mechanisms:
Further evidence of species-type dependence of v2 at intermediate pT (described by recombination/ coalescence models[3])
[1] X. Dong et al., Phys. Lett. B, 597 (2004) 328
[2] STAR Collab. Phys. Rev. Lett. 95 (2005)
[3] C. Nonaka et al., Phys. Lett. B 583 (2004) 73
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

9. Elliptic flow v2
The φ experiences significant elliptic flow! (result is mean of 2 different, but consistent methods):
Early time information:
For pT<2 GeV/c, φ flows as much as other ID'd particles, consistent with hydro. mass-ordering
For pT>2 GeV/c, φ v2 is more consistent with K0S than Λ (favors NCQ[1] =2)
Consistent with v2 of other multi-strange hadrons (Ξ, Ω)[2] i.e. s-quarks flow!
STAR Preliminary
Particle production mechanisms:
Further evidence of species-type dependence of v2 at intermediate pT (described by recombination/ coalescence models[3])
[1] X. Dong et al., Phys. Lett. B, 597 (2004) 328
[2] STAR Collab. Phys. Rev. Lett. 95 (2005)
[3] C. Nonaka et al., Phys. Lett. B 583 (2004) 73
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

10. Elliptic flow v2
The φ experiences significant elliptic flow! (result is mean of 2 different, but consistent methods):
Early time information:
For pT<2 GeV/c, φ flows as much as other ID'd particles, consistent with hydro. mass-ordering
For pT>2 GeV/c, φ v2 is more consistent with K0S than Λ (favors NCQ[2] =2)
Consistent with v2 of other multi-strange hadrons (Ξ, Ω)[2] i.e. s-quarks flow!
STAR Preliminary
Particle production mechanisms:
Further evidence of species-type dependence of v2 at intermediate pT (described by recombination/ coalescence models[3])
[1] X. Dong et al., Phys. Lett. B, 597 (2004) 328
[2] STAR Collab. Phys. Rev. Lett. 95 (2005)
[3] C. Nonaka et al., Phys. Lett. B 583 (2004) 73
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

11. Conclusions Large elliptic flow (despite small σ) at low pT
STAR Preliminary
Large elliptic flow (despite small σ) at low pT
NCQ-scaling of v2 for pT > 2 GeV/c (similar to Ω (sss) )
Reco. models describe data well[1]
RCP critical confirmation of baryon-meson dependence of RHIC observables
Scaling described by reco. models
Central data well-described (intermediate pT) by reco. model[2]
pQCD power-law tails suppressed in central compared to peripheral spectra
Central Ω/φ ratio well-described by thermal quark reco. model up to pT~4 GeV/c[2]
[1] C. Nonaka et al., Phys. Lett. B 583 (2004) 73
[2] R. Hwa & C-B Yang, nucl-th/
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

12. PARTONIC COLLECTIVITY & DECONFINEMENT
Conclusions
STAR Preliminary
Large elliptic flow (despite small σ) at low pT
NCQ-scaling of v2 for pT > 2 GeV/c (similar to Ω (sss) )
Reco. models describe data well[1]
PARTONIC COLLECTIVITY & DECONFINEMENT
RCP critical confirmation of baryon-meson dependence of RHIC observables
Scaling described by reco. models
Central data well-described (intermediate pT) by reco. model[2]
pQCD power-law tails suppressed in central compared to peripheral spectra
Central Ω/φ ratio well-described by thermal quark reco. model up to pT~4 GeV/c[2]
[1] C. Nonaka et al., Phys. Lett. B 583 (2004) 73
[2] R. Hwa & C-B Yang, nucl-th/
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

13. PARTONIC COLLECTIVITY & DECONFINEMENT ...hints of THERMALIZATION?
Conclusions
Large elliptic flow (despite small σ) at low pT
NCQ-scaling of v2 for pT > 2 GeV/c (similar to Ω (sss) )
Reco. models describe data well[1]
STAR Preliminary
PARTONIC COLLECTIVITY & DECONFINEMENT
RCP critical confirmation of baryon-meson dependence of RHIC observables
Scaling described by reco. models
Central data well-described (intermediate pT) by reco. model[2]
pQCD power-law tails suppressed in central compared to peripheral spectra
Central Ω/φ ratio well-described by thermal quark reco. model up to pT~4 GeV/c[2]
...hints of THERMALIZATION?
[1] C. Nonaka et al., Phys. Lett. B 583 (2004) 73
[2] R. Hwa & C-B Yang, nucl-th/
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai

14. Outlook : φ puzzle? With Full TOF
Full TOF MRPC modules construction has been approved by NSF of China. NSFC, CAS and MoST of China will jointly support the project. It will be finished in 2.5 years.
φ reconstruction with Full TOF:
φ -> K+K-
φ -> e+e-
φ -> +-
is there a φ puzzle in RHIC energies, esp. in lower pT region as NA49 and NA50 shows?
NA50
NA49
Open symbols: φ -> ++-
Solid symbols: φ -> K++K-
Pal, Ko, Lin, NPA707(02)505
J.H. Chen
6th China-Japan Joint Nuclear Physics Symposium, May 16-20, 2006, Shanghai