First measurement of D${}^{\ast +}$ vector spin alignment in Pb$-$Pb collisions at $\sqrt{{\mathbf{s}}_{\mathrm{NN}}}=5.02$ TeV

Abstract

The first measurement of prompt D${}^{\ast +}$-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 $\ddot{I}{}_{00}$ of the diagonal spin-density matrix for prompt D${}^{\ast +}$ mesons with $4<p_{\mathrm{T}}<30$ 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 $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. Evidence of spin alignment $\ddot{I}{}_{00}>1/3$ has been found for $p_{\mathrm{T}}>15$ GeV/$c$ and $0.3<|y|<0.8$ with a significance of $3.1\ddot{I}ƒ$. The measured spin alignment of prompt D${}^{\ast +}$ mesons is compared with the one of inclusive J$/\ddot{I}ˆ$ mesons measured at forward rapidity ($2.5<y<4$).

24 pages, 5 captioned figures, 1 table, authors from page 18, figures at http://alice-publications.web.cern.ch/node/12345