First measurement of D*+ vector meson spin alignment in Pb–Pb collisions at √sNN=5.02 TeV

A bstract 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 ρ 00 of the diagonal spin-density matrix for prompt D *+ mesons with 4 < p T < 30 GeV /c in two rapidity intervals, | y | < 0 . 3 and 0 . 3 < | y | < 0 . 8, in central (0–10%) and midcentral (30–50%) Pb–Pb collisions at √sNN=5.02 TeV. Evidence of spin alignment ρ 00 > 1 / 3 has been found for p T > 15 GeV /c and 0 . 3 < | y | < 0 . 8 with a significance of 3 . 1 σ . The measured spin alignment of prompt D *+ mesons is compared with the one of inclusive J / ψ mesons measured at forward rapidity (2 . 5 < y < 4).