First measurement of D

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