31 12 2007 12
HIGH ENERGY PHYSICS AND NUCLEAR PHYSICS
Vol.31,No.12
Dec.,2007
Spin Alignment of Vector Mesons K∗0(892)andφ(1020)in Au+Au and p+p Collsions at√
s NN=200GeV.The values ofρ00with respect to reaction plane in Au+Au collisions are0.36±0.02(stat)±0.13(sys)for K∗0(892)and
0.38±0.01(stat)±0.04(sys)forφ(1020).No evident global spin alignment with respect to reaction plane is
observed in the measured p T region up to5GeV/c with current sensitivity.ρ00with respect to the production plane of the vector meson is also measured for K∗0(892)andφ(1020)in Au+Au collisions,and forφ(1020)in p+p collisions.No significant difference for theρ00between Au+Au and p+p collisions is observed with our data sample.
Key words spin alignment,global polarization,vector meson
1Introduction
The system created in non-central relativistic nucleus-nucleus collisions carries large angular orbital momentum.Due to the spin-orbital coupling,quarks and anti-quarks produced in such a system could be-come globally polarized along the direction of the ini-tial system angular momentum[1].The degree of the polarization of thefinal hadrons is argued to be sen-sitive to the hadronization mechanism[2].Since the direction of the initial system angular momentum is perpendicular to the impact parameter vector,one can measure such global quark polarization via global spin alignment of vector meson with respect to reac-tion plane.It has also been pointed out that this global polarization is correlated with transverse po-larization measured with respect to the particle’s pro-duction plane[3].
To measure the hyperon polarization,one needs
to know not only the orientation but also the sign(or
direction)of the reaction plane which can only be de-
termined by the directedflow in each event class.We
measure the global spin alignment of vector meson
rather than the global polarization of hyperon.
The angular distribution of its decay products is
analyzed to measure the spin alignment of a vector
meson.However,the angular distribution of theλ=1
andλ=−1stats are degenerate thus the value of the
ρ11andρ−1−1elements in the spin alignment ma-
trix can not be distinguish and only one
reaction massρ00=1−ρ11−ρ−1−1,is accessible.ρ11,ρ00andρ−1−1 should be equal to1/3in the unpolarized case and
a deviation ofρ00from1/3manifests the spin align-
ment of vector meson.For the case of a vector meson
12
TANG Ze-Bo Spin Alignment of Vector Mesons K∗0(892)andφ(1020)in Au+Au and p+p Collsions at√
decaying to two pseudoscalar mesons,the angular dis-tribution can be written[4]
W(cosθ∗)=
3
s NN=200GeV.About12M mid-central(20%—60%)Au+Au events and4M p+p events are analyzed.
K∗0(892)andφ(1020)are reconstructed through their hadronic decay channels:K∗0→K+π−,
K∗0(892)were added together and the term K∗0(892)in this analysis refers to the average of K∗0(892)and
s NN=200GeV. Rawφ(1020)and K∗0(892)yields are extracted from fitting to the combinatorial background subtracted invariant mass distribution with a Breit-Wigner func-tion and a residual background polynomial curve. The raw yields are then corrected for detection ac-ceptance and efficiency.The correction factors are determined by embedding simulated tracks into real events at the raw data level,reconstructing the full events and comparing the simulated input to the re-constructed o
utput.The lower panels in Figure1 show the corrected cos(θ∗)distribution forφ(1020) and K∗0(892).Theρ00are extracted viafitting with Eq.(1).The polarization direction is taken as the normal of reaction plane to determine the parameter θ∗.The reaction plane is estimated with the second order event plane reconstructed using the charged pri-mary tracks from STAR TPC[9,10].Finally,theρ00 is corrected byfinite reaction plane resolution.The systematic errors include contributions from different methods of yield extraction,variations of the charged particles selection cuts and from uncertainty in detec-tor acceptance
analysis.
Fig.1.(a)φ(1020)and(b)K∗0(892)invariant
mass distribution after combinatorial back-
ground subtraction in mid-central(20%—
60%)Au+Au collisions.Dashed lines repre-
sent the residual background.(c)φ(1020)and
(d)K∗0(892)are the cos(θ∗)distribution with
respect to reaction planefitted with Eq.(1).
2.1Spin alignment with respect to reaction
plane
Figure2shows the preliminary STAR measure-ments of global spin alignment from mid-central Au+Au collisions at√
1194 (HEP&NP) 31
s NN=200GeV with respect to reaction plane.Caps and
boxes show the systematic errors while bars
are the statistical errors.
Figure3shows the preliminary STAR results on collision centrality dependence ofρ00with respect to reaction plane from Au+Au collisions at√
s NN=200GeV with respect to reaction plane.Caps and boxes
show the systematic errors while bars are the
statistical errors.
2.2Spin alignment with respect to produc-
tion plane
Usually,hyperon polarization and vector meson spin alignment are measured with respect to the pro-duction plane.Hyperon produced in high energy un-polarized hadron-hadron collisions have been found to be transversely polarized with respect to the pro-duction plane[11,12].Leading vector meson K∗+(892) from neutron-carbon interactions has also been ob-served to be significantly aligned in its transverse frame[13].Thus,it’s of great interest to measure the vector meson spin alignment in the transverse frame at RHIC.Possible correlations in vector meson spin alignment between reaction plane and produc-tion plane can be explored to study particle produc-tion and spin dynamics.
Figure4shows the results ofρ00with respect to the production plane as a function of p T in mid-central Au+Au collisions at√
s NN=200GeV with respect to the vector mesons’productions
plane.Caps and boxes show the systematic
errors while bars are the statistical errors.
The spin alignment ofφ(1020)with respect to production plane is also measured in p+p collisions at√
12
TANG Ze-Bo Spin Alignment of Vector Mesons
K ∗0(892)
and φ(1020)in Au+Au and p+p Collsions at
√
difference is observed with current statistics.Fur-ther exploration (higher statistics in p+p collisions)is
warranted.
Fig.5.φ(1020)ρ00as a function of p T com-pared between mid-central Au+Au (open cross)and p+p (filled circle)collisions at √
s NN =
200mid-central (20%—60%)Au+Au and p+p col-lisions GeV at RHIC were presented.
The values
of ρ00with respect to reaction plane in mid-central Au+Au collisions are 0.36±0.02(stat)±0.13(sys)for K ∗0(892)and 0.38±0.01(stat)±0.04(sys)for φ(1020).No evident global spin alignment for K ∗0(892)and φ(1020)was observed in the measured p T region up to 5GeV/c with current sensitivity.The spin alignment
measurements with respect to the vector mesons’pro-duction plane for K ∗0(892)and φ(1020)in Au+Au collisions and for φ(1020)in p+p collisions were also discussed.No significant difference in φ(1020)spin alignment between Au+Au and p+p collisions has been observed.
We thank the RHIC Operations Group and RCF at BNL,and the NERSC Center at LBNL for their support.
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