1. for STAR Collaboration
Spin alignment of vector mesons (Φ, K*0) in Heavy-Ion Collisions at RHIC
Jinhui Chen
for STAR Collaboration

2. Outline Introduction Analysis method and results
1) Spin alignment with respect to reaction plane
2) Spin alignment with respect to production plane
Summary and outlook
Jinhui Chen
QM06 Nov Shanghai China

3. Introduction
Spin alignment of vector mesons is a unique probe of particle production and dynamics[1,2]
1. Orbital angular momentum  spin?
2. Coalescence/Fragmentation of polarized quarks?
3. Contributions to the significant collective flow observed at RHIC?
Spin alignment of vector mesons is described by
We measure ρ00 through decayed products angular distribution in the rest frame of vector mesons:
spin density matrix (s=1)
 Deviation of ρ00 from 1/3
manifests the alignment of
vector mesons
1. Z.T. Liang and X.N. Wang, PRL (05), PRL (06), PLB 629 (05) 20-26;
2. S.A. Voloshin nucl-th/
Jinhui Chen
QM06 Nov Shanghai China

4. Previous measurements
Λ polarization Fermi-lab 1976
G. Bunce et al. PRL 36 (1976) 1113
OPAL Col. PLB 412 (1997) 210
ρ00
J. Duryea et al. PRL 67 (1991) 1193
E896 Col. NPA 698 (2002) 499 PΛ
Jinhui Chen
QM06 Nov Shanghai China

5. Measurement from RHIC-STAR
STAR Detector
Features at RHIC:
Formation of bulk partonic matter
Hadronization from bulk partonic matter is different from fragmentation scheme
 Constituent quark degree of freedom ?
STAR Collaboration, NPA 757 (05) 102 x y
beam
reaction plane
Measurements for:
Global hyperon polarization
Global vector meson spin alignment
Jinhui Chen
QM06 Nov Shanghai China

6. Hyperon polarization vs vector meson spin alignment
27%
Yi-Jin Pei, hep-ph/ x y
beam
secondary vertec
reaction plane
Jinhui Chen
QM06 Nov Shanghai China

7. Analysis details (φ-meson)
Using mixed event method to
get the raw dN(pT)/dcos(θ*)
distribution.
Correct the dN(pT)/d(cosθ*) with detector acceptance.
Fitting dN(pT)/d(cosθ*) with
to get the ρ00obs.
Correct ρooobs with “reaction plane
resolution” to obtain ρ00.
Jinhui Chen
QM06 Nov Shanghai China

8. ρ00 vs pT
STAR Preliminary
|y|<1.0
With statistical and systematical uncertainties, φand K*0 spin are not aligned w.r.t. reaction plane.
Jinhui Chen
QM06 Nov Shanghai China

9. ρ00 vs Npart
STAR Preliminary
|y|<1.0
With the data sample, we have not observed global spin alignment for φand K*0 at different centrality.
Centrality 10-40%
Jinhui Chen
QM06 Nov Shanghai China

10. What about Λ global polarization?
200GeV (20-70%)
62GeV (0-80%)
(GeV/c)
STAR Preliminary
I. Selyuzhenkov’s talk
Section 3.4, Nov. 19
With statistical and systematical uncertainties, we have not observed global spin alignment for φ, K*0 and global polarization for Λ at mid-central collisions.
Jinhui Chen
QM06 Nov Shanghai China

11. Spin alignment w.r.t. production plane
Hyperons H produced in unpolarized p+p  H+X or p+A  H+X
are transversely polarized w.r.t. production plane.
What about vector mesons?
transversely polarized?
correlated with spin alignment w.r.t. reaction plane?
production plane
Jinhui Chen
QM06 Nov Shanghai China

12. ρ00 vs pT
STAR Preliminary
|y|<1.0
Φand K*0 spin may be slightly aligned at low pT w.r.t. production plane.
0.8<pT<1.2GeV/c
Jinhui Chen
QM06 Nov Shanghai China

13. And p+p collisions ?
STAR Preliminary
|y|<1.0
φ-meson spin alignment in p+p collisions needs more statistics to be conclusive.
Jinhui Chen
QM06 Nov Shanghai China

14. Compared with Au+Au STAR Preliminary
We are interested in possible difference in φ and K*0 spin alignment between Au+Au and p+p collisions due to different dynamics.
In current statistics, the spin alignment of φ-meson between Au+Au and p+p collisions are similar.
Jinhui Chen
QM06 Nov Shanghai China

15. Time of Flight detector upgrade
One tray will enhance the S/B ratio by a factor of 3.5 at intermediate pT.
Au+Au 200GeV
(0-80%)
STAR Preliminary
Full barrel ~ a factor of 10.
J. Wu’s talk
Section 1.4, Nov.15
In our current analysis, We used kaons from TPC only.
STAR Preliminary
Au+Au 200GeV
(0-80%)
Jinhui Chen
QM06 Nov Shanghai China

16. Summary and Outlook
Reaction plane: within statistics, we have not observed a global polarization for Λ and global spin alignments for  and K*0.
Production plane:  and K*0 spin may be slightly aligned in low pT region.
No significant difference in  spin alignments, w.r.t. production plane, between p+p and Au+Au collisions have been observed.
STAR Full Barrel Time-of-Flight detector upgrade will significantly enhance our capability for a conclusive measurement in the future.
Thanks!
Jinhui Chen
QM06 Nov Shanghai China