# Presentation: 815519

## Spin alignment of vector mesons (K*0, φ) in Au+Au and p+p collisions at s-NN = 200 GeV 

***                  ******Jinhui Chen***
***        ******For STAR Collaboration***

- Summer School @ Berkeley

- 5/24/2007

## System orbital angular momentum

***System orbital angular momentum***

- Spin and polarization phenomena are important part of QCD understanding of hadron structure and particle formation dynamics: e.g., the nucleon spin and the hyperon polarization. The origin of nucleon spin is a mystery: *~25% from quarks**[1]** (DIS).*  *The gluon helicity distribution and **orbital angular momentum** are thus essential to the understanding of the nucleon spin.*

*[**1] EMC Col. Nucl. Phys. B 328, 1 (1989); B.W.Filippone and X.Ji, Adv. Nucl. Phys. 26, 1 (2001)*

-      Large orbital angular momentum
-       possessed in non-central Au+Au
-       collisions[2,3]:
*  “**The local angular momentum for the *
*two neighboring parton is larger than the *
*spin of a quark”*
-   *A clear b dependence.*

*[2] Z.T. Liang and X.N. Wang, Phys. Rev. Lett. 94 (2005) 102301*

***[3] Z.T. Liang nucl-th/0705.2852***

*“**Global polarization is essentially a local *
*manifestation of the global angular momentum of the colliding system through interaction of spin-orbit coupling in QCD”*

## Global polarization (1)

***Global polarization (1)***

*** ******Z.T. Liang nucl-th/0705.2852***

-  Features of global polarization:
* **For non-central collisions, it should have a finite value, at small p**T*
*   **in central rapidity;  *
* **It should increase with b; *
* **It should vanish in central collisions*

***At RHIC:***

## Global polarization (2)

*Global spin alignment is sensitive to different hadronization scenarios in different kinematic region**[1]*
- Coalescence *(**ρ**00**<1/3)*
- Fragmentation *(**ρ**00**>1/3)*

*[1] Z.T. Liang and X.N. Wang, Phys. Lett. B 629 (2005) 20.*

*** ******Global vector meson spin alignment and global hyperon polarization***
*** ******Measured through decay products angular distribution w.r.t. reaction plane***

##  Technically -- vector meson spin alignmentcomplementary to hyperon polarization

*** ******Sensitivity: It’s a P******q******2****** effect for vector meson.****** ***
*Z.T. Liang nucl-th/0705.2852** *

## Analysis details (φ-meson)

##       pT dependence

- Within current sensitivity, we cannot probe the proposed pT dependence on different hadronization mechanism as the deviation from ρ00 = 1/3 are predicted to be small in the model.

*K***0** *(0.8<pT<5.0 GeV/c): ρ00 = 0.33 +- 0.04 +- 0.12
*φ*   (0.4<pT<5.0 GeV/c): ρ00 = 0.34 +- 0.02 +- 0.03

## Centrality dependence

- Within current sensitivity, our measurement exhibits no strong spin alignment for vector meson at all collision centralities, presumably because the spin-orbit coupling for quark polarization is not large enough to be manifested in our measurement.

## Spin alignment w.r.t. the production plane

-  Hyperons are transversely polarized
-    *G. Bunce et. al. PRL 36, 1113 (1976); P.T. Cox et. al. PRL 46, 877 (1981); C. Wilkinson et. al. PRL 58, 855 (1987); B.E. Bonner et. al. PRL 62, 1591 (1989); J. Duryea et. al. PRL 67, 1193 (1991)...*
-  It’s related to particle formation dynamics or to intrinsic quark transverse spin distribution
*  **B. Andersson et. al. PLB 85, 417 (1979); J. Szwed PLB 105, 403 (1981); R. Barni et. al. PLB 296, 251 (1992); J. Soffer et. al. PRL 68, 907 (1992)*
-  It may be correlated with the global polarization convoluted with an azimuthal angular anisotropy (v2 )
*   **S. Voloshin, nucl-th/0410089*
-  It may be closely related to the AN
-    *Z. Liang and C. Boros PRL 79, 3608(1997); Q.Xu and Z. Liang PRD 68, 034023 (2003).*

## pT dependence

- pT<2.0 GeV/c
-      ρ00(K*) = 0.43 +- 0.04 +- 0.08 ; ρ00(φ) = 0.42 +- 0.02 +- 0.04
- pT>2.0 Gev/c
-      ρ00(K*) = 0.38 +- 0.04 +- 0.06 ; ρ00(φ) = 0.38 +- 0.03 +- 0.05
- In p+p, ρ00(φ) = 0.40 +- 0.04 +- 0.06

## Comparison between different system

***PRL 95, 202001 (2005)***

-  At small xp region, ρ00 are consistent with 1/3 from ee, pp, to AuAu collisions
-  It’s consistent with the null AN observation at central rapidity from RHIC

## Conclusions

*With respect to the reaction plane, **ρ**00**(p**T**) are consistent within 1/3 within statistical and systematic uncertainty for vector meson in the measured p**T** up to 5 GeV/c and are independent of collision centrality and transverse momentum.*
*      **** **Vector meson in the measured kinematic region appear not to be produced with a strong global spin alignment despite the presence of a large orbital angular momentum for the system created in non-central Au+Au collisions.*
*With respect to the production plane, **ρ**00**(p**T**) is less than 2 standard deviation above 1/3 and is similar to the results from p+p collisions.*
*      **** **Vector mesons in the measured p**T** region at mid-rapidity don’t seem to carry a significant polarization through production dynamics. *

_***Thanks!***_