Spin alignment measurements of the $K^{\ast 0}(892)$ and $\varphi (1020)$ vector mesons at RHIC

Abstract

We present the first spin alignment measurements for the $K^{\ast 0}(892)$ and $\varphi (1020)$ vector mesons produced at mid-rapidity with transverse momenta up to 5 GeV/c at $\sqrt{s_{NN}}$ = 200 GeV at RHIC. The diagonal spin density matrix elements with respect to the reaction plane in Au+Au collisions are ${\rho }_{00}$ = 0.32 $\pm$ 0.04 (stat) $\pm$ 0.09 (syst) for the $K^{\ast 0}$ ($0.8<p_T<5.0$ GeV/c) and ${\rho }_{00}$ = 0.34 $\pm$ 0.02 (stat) $\pm$ 0.03 (syst) for the $\varphi$ ($0.4<p_T<5.0$ GeV/c), and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector meson spins. Spin alignments for $K^{\ast 0}$ and $\varphi$ in Au+Au collisions were also measured with respect to the particle's production plane. The $\varphi$ result, ${\rho }_{00}$ = 0.41 $\pm$ 0.02 (stat) $\pm$ 0.04 (syst), is consistent with that in p+p collisions, ${\rho }_{00}$ = 0.39 $\pm$ 0.03 (stat) $\pm$ 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.

7 pages, 4 figures. fig.1 updated; one more reference added, one typo corrected, published in PRC.77.061902

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