Spin alignment measurement of vector mesons produced in high energy collisions

This paper covers the recent experimental development on spin alignment measurements of K∗0 and ϕ 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 is studied by measuring the second diagonal element ρ00 of spin density matrix. The spin density matrix element ρ00 is obtained by measuring the angular distribution of vector meson decay daughter with respect to the quantization axis in vector meson rest frame. Measured ρ00 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 ρ00 values are found to be smaller than 1/3 (ρ00 = 1/3 implies no spin alignment) for K∗0 and ϕ vector mesons at low transverse momentum in Pb–Pb collisions at s NN = 2.76 TeV. These 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 noncentral heavy-ion collisions and its subsequent hadronization by the process of recombination. No significant spin alignment effect is observed for KS0 (spin = 0) in mid-central Pb–Pb collisions and for vector mesons in pp collisions. However, the preliminary results of ρ00 for ϕ mesons are larger than 1/3 at intermediate pT in Au–Au collisions at RHIC energies and can be attributed to the presence of ϕ 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.