First measurement of D^*+ vector meson spin alignment in Pb–Pb collisions at TeV

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 ρ₀₀ 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 TeV. Evidence of spin alignment ρ₀₀> 1/3 has been found for p_T> 15 GeV/c and 0.3 < |y| < 0.8 with a significance of 3.1σ. 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).