Spin Alignment Measurements of Vector Mesons in ALICE at the LHC

Spin alignment of K$^{*0}$ and $\phi $ vector mesons produced in non-central heavy-ion collisions could occur due to the presence of large initial angular momentum. This phenomenon leads to a non-uniform angular distribution of the decay daughters of vector mesons with respect to the quantization axis in the rest frame of vector mesons. We present the recent spin alignment measurements of K$^{*0}$ and $\phi $ vector mesons at mid-rapidity ($|y|<$ 0.5) in Pb–Pb collisions at $\sqrt{s_{\mathrm {NN}}}$ = 2.76 TeV with respect to the production plane and the second-order event plane. The measured value of the second diagonal spin density matrix element ($\rho _{00}$) is lower than 1/3 for both the vector mesons at low transverse momentum ($p_{\mathrm {T}}$) and consistent with 1/3 (a value implying no spin alignment) at high $p_{\mathrm {T}}$ in mid-central Pb-Pb collisions. The results are also compared with the spin alignment of vector mesons in pp collisions at $\sqrt{s}$ = 13 TeV and spin alignment of K$_{\mathrm {S}}^{0}$ (spin zero hadron) in Pb–Pb collisions at $\sqrt{s_{\mathrm {NN}}}$ = 2.76 TeV. In both cases the extracted $\rho _{00}$ values are consistent with 1/3 within the uncertainty.