Investigation of charm hadronisation and early magnetic field in ultrarelativistic heavy-ion collisions via D ^∗+ -meson spin alignment with ALICE

Heavy quarks, i.e. charm and beauty, are produced in the early stages of heavy-ion collisions and are sensitive to the large initial orbital angular momentum and strong magnetic field. Under these conditions, charm quarks can be polarised. This polarisation is expected to be further transferred to the final-state hadrons during the hadronisation process and can be probed by measuring the spin density matrix element (ρ 00 ) of spin-1 mesons, such as D *+ mesons. In these proceedings, we report the first measurement of prompt D *+ -meson spin alignment in Pb–Pb collisions with respect to the direction orthogonal to the reaction plane. A deviation from the unpolarised value, ρ 00 > 1/3, is observed 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 to that of inclusive J/ψ mesons measured at forward rapidity. Additionally, the first measurement of the ρ 00 parameter of D *+ mesons in pp collisions based on the first data samples collected during LHC Run 3 is also reported.