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 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 is studied by measuring the second diagonal element rho(00) of spin density matrix. The spin density matrix element 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 root 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 K-S(0) (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(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.