Charged Particle Multiplicities from Cu+Cu, Au+Au and d+Au Collisions at RHIC
Spin alignment of vector mesons (K* 0 , φ ) in Au+Au and p+p collisions at RHIC
description
Transcript of Spin alignment of vector mesons (K* 0 , φ ) in Au+Au and p+p collisions at RHIC
6/24/2007 SQM07 @ Levoca 1
Spin alignment of vector mesons (K*0, φ) in Au+Au and p+p collisions at RHIC
Jin Hui Chen
Shanghai Institute of Applied Physics and UCLA
For STAR Collaboration
STAR
• Motivations
• Analysis details
• Results
– spin alignment w.r.t. reaction plane
– spin alignment w.r.t. production plane
• Conclusions
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Global Angular Momentum and Hadron Polarization
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reaction plane
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The participant matter may have a large angular momentum to begin with in non-central A+A collisions –1) are quarks from the dense matter created polarized (with a
preferred spin direction)? spin-orbital coupling
2) are vector mesons from polarized dense quark blob polarized? hadronization dynamics
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A Theoretical Estimate on Orbital Angular Momentum
• 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.
Z.T. Liang and X.N. Wang, Phys. Rev. Lett. 94 (2005) 102301
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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 hyperon polarization and global vector meson spin alignment
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Spin-Orbital Coupling Quark Polarization
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
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Hadronization does not wash out quark polarization
• 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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Global hyperon polarization and global vector meson spin alignment
– Measured through decay products angular distribution w.r.t. reaction plane
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Hyperon Polarization vs Vector Meson Spin AlignmentSTAR
Hyperon spin ½, -- is the hyperon spin up or down (sz=+1/2 or -1/2) ?-- define an absolute direction (up or down)-- feed down from other multi-strange hyperons
Vector mesons spin = 1, -- do the three components (sz=+1,0,-1) havethe same probabilities (1/3) – spin alignment ?
-- sz = +1, -1 states are degenerate (cos2 dependence)-- phi mesons do not have decay feeddown contributions
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Data SetAu+Au Year 2004 Running, ~23 M Min-Bias Events
p+p Year 2001 Running, ~6 M Min-Bias Events
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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
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Using mixed event method to get the raw dN(pT)/dcos(θ*) distribution.
φK+K-
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pT dependence
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There is no significant spin alignment observed for vector mesons –
model prediction for spin alignment is also small – difficult to observe !
K*0 (0.8<pT<5.0 GeV/c): ρ00 = 0.33 +- 0.04 +- 0.12φ (0.4<pT<5.0 GeV/c): ρ00 = 0.34 +- 0.02 +- 0.03
STAR Preliminary
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Centrality dependenceSTAR
No strong centrality dependence has been observed
-- presumably the spin-orbital coupling may be too small
to polarize the quarks with strong centrality dependence !
STAR Preliminary
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Spin alignment w.r.t. the production plane
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• 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 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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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
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STAR Preliminary
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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
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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 mesons 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.
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