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STAR. Motivations Analysis details Results spin alignment w.r.t. reaction plane spin alignment w.r.t. production plane Conclusions. 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. - PowerPoint PPT Presentation
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5/24/2007 Summer School @ Berkeley 1
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
STAR
• Motivations
• Analysis details
• Results
– spin alignment w.r.t. reaction plane
– spin alignment w.r.t. production plane
• Conclusions
2
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
STAR
[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”
3
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 pT
in central rapidity;
– It should increase with b;
– It should vanish in central collisions
At RHIC:
STAR
4
Global polarization (2)
• Global spin alignment is sensitive to different hadronization scenarios in different kinematic region[1]
– Coalescence (ρ00<1/3)
– Fragmentation (ρ00>1/3)
[1] Z.T. Liang and X.N. Wang, Phys. Lett. B 629 (2005) 20.
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STAR
Global vector meson spin alignment and global hyperon polarization
– Measured through decay products angular distribution w.r.t. reaction plane
5
Technically -- vector meson spin alignmentcomplementary to hyperon polarization
27% Yi-Jin Pei, hep-ph/9703243.
secondary vertex x
y
beam
reaction plane
STAR
Sensitivity: It’s a Pq2
effect for vector meson.
Z.T. Liang nucl-th/0705.2852
6
Analysis details (φ-meson)
Correct ρooobs for the “reaction
plane resolution” to obtain ρ00.
Correct the dN(pT)/d(cosθ*) with
detector acceptance and tracking efficiency.
]cos)13()1[(4
3 *20000 Norm
Fitting dN(pT)/d(cosθ*) with
to get the ρ00obs.
Au+Au (20-60%) w.r.t. reaction plane
0.8<pT<1.2 GeV/c
STAR
Using mixed event method to get the raw dN(pT)/dcos(θ*) distribution.
φK+K-
7
pT dependence
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STAR
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
8
Centrality dependenceSTAR
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.
9
Spin alignment w.r.t. the production plane
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STAR Col.
nucl-ex/0703033
• 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).
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STAR
10
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
STAR
11
Comparison between different system
K*0 PLB 412 (1997) 210
XKqqZee 0*
DELPHI Col. PLB 406 (1997) 271
GeVpp 200@
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
STAR
12
Conclusions
• With respect to the reaction plane, ρ00(pT) are consistent within 1/3 within statistical and systematic uncertainty for vector meson in the measured pT 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, ρ00(pT) is less than 2 standard deviation above 1/3 and is similar to the results from p+p collisions.
Vector mesons in the measured pT region at mid-rapidity don’t seem to carry a significant polarization through production dynamics.
STAR
Thanks!Thanks!