First measurement of D$^{*+}$ vector spin alignment in Pb$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}} = 5.02}$ TeV

Abstract

The first measurement of prompt D$^{*+}$-meson spin alignment in ultrarelativistic heavy-ion collisions with respect to the direction orthogonal to the reaction plane is presented. The spin alignment is quantified by measuring the element $\rho_{00}$ of the diagonal spin-density matrix for prompt D$^{*+}$ mesons with $4 1/3$ has been found for $p_{\rm T}>15$ GeV/$c$ and $0.3<|y|<0.8$ with a significance of $3.1\sigma$. The measured spin alignment of prompt D$^{*+}$ mesons is compared with the one of inclusive J$/\psi$ mesons measured at forward rapidity ($2.5 < y < 4$).