Spin alignment measurement of vector mesons produced in high energy collisions

Abstract

This review covers the recent experimental development on spin alignment measurements of $K^{*0}$ and $\phi$ vector mesons in heavy-ion and pp collisions at RHIC and LHC energies. Measurements in $e^+e^-$ collisions at LEP energies are also discussed. Spin alignment of vector mesons are studied by measuring the second diagonal element $\rho_{00}$ of spin density matrix. The $\rho_{00}$ is obtained by measuring the angular distribution of vector meson decay daughter with respect to the quantization axis in vector meson rest frame. Measured $\rho_{00}$ values for vector mesons are found to be larger than 1/3 at high momentum in $e^+e^-$ collisions at LEP energies, suggesting the preferential production of vector meson with helicity zero state from the fragmentation process. The $\rho_{00}$ values are found to be smaller than 1/3 ($\rho_{00}$ = 1/3 implies no spin alignment) for $K^{*0}$ and $\phi$ vector mesons at low transverse momentum in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. This observations are qualitatively consistent with the expectation from models which attribute the spin alignment effect due to polarization of quarks in the presence of large initial angular momentum in non-central heavy-ion collisions and its subsequent hadronization by the process of recombination. No significant spin alignment effect is observed for $K^0_S$ (spin = 0) in mid-central Pb--Pb collisions and for vector mesons in pp collisions. However, the preliminary results of $\rho_{00}$ for $\phi$ mesons are larger than 1/3 at intermediate $p_{\mathrm{T}}$ in Au--Au collisions at RHIC energies and can be attributed to the presence of $\phi$ meson field. Although there is evidence of spin alignment effect of vector mesons in heavy-ion collisions but the measured effect is surprisingly larger in context of hyperon polarization. Therefore these results will trigger further theoretical study.