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Description
In ultrarelativistic heavy-ion collisions, a quark–gluon plasma (QGP) is formed. Charm and beauty quarks are produced predominantly in early hard partonic scatterings, on timescales shorter than the medium formation time. In non-central collisions, they are sensitive to the fireball’s initial angular momentum and to the strong magnetic field generated perpendicular to the reaction plane. These conditions can induce charm-quark polarisation, which is expected to be transferred to charm hadrons during hadronisation. Experimentally, charm polarisation can be studied via the spin density matrix element $\rho_{00}$ of spin-1 mesons (e.g. D$^{*+}$) and the longitudinal polarisation $P_{\rm z}$ of spin-1/2 baryons (e.g. $\Lambda_{\rm c}^{+}$). Measurements in pp collisions provide an essential baseline and help constrain possible polarisation sources unrelated to the magnetic field and system angular momentum.
This contribution reports a new measurement of the D$^{*+}$ $\rho_{00}$ parameter in Pb–Pb collisions at $\sqrt{s_\mathrm{NN}}=5.36$ TeV, obtained with significantly improved precision compared with the previous Run 2 result. In addition, measurements of D$^{*+}$ $\rho_{00}$ and of the longitudinal polarisation $P_{\rm z}$ of $\Lambda_{\rm c}^{+}$ baryons in pp collisions at $\sqrt{s}=13.6$ TeV are presented, separating the prompt and non-prompt components in the latter case.
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