Speaker
Description
Polarization and spin alignment have emerged as important tools for investigating the dynamical properties of the strongly interacting matter produced in relativistic nuclear collisions. Spin-dependent observables provide access to different aspects of the collision depending on the particle species and the choice of the spin-quantization axis. Correlations with the event or reaction plane can probe the vortical structure, global angular momentum, and early electromagnetic fields of the system, while measurements in production-related reference frames constrain the spin dynamics of particle production and its possible modification in the medium. Experimentally, the spin information is inferred from the angular distributions of decay products through weak, strong, or electromagnetic decays.
This presentation gives an overview of recent polarization measurements performed with the ALICE detector at the LHC. Longitudinal polarization measurements of hyperons in different collision systems are presented as probes of the local vortical and shear structures associated with anisotropic expansion. Measurements of the global polarization of ($\Lambda$) and ($\overline{\Lambda}$) hyperons are discussed in connection with the transfer of the initial orbital angular momentum to the spin degrees of freedom of the produced medium. The overview is then extended to the charm sector through measurements of inclusive (J/$\psi$) and prompt ($D^{*+}$) spin alignment with respect to the direction perpendicular to the reaction plane in Pb–Pb collisions. These results test possible correlations between heavy-quark spin and the global collision geometry, with sensitivity to charm-quark transport, early electromagnetic fields, quarkonium evolution, and hadronization. Finally, (J/$\psi$) polarization measurements in the helicity and Collins–Soper frames in pp and Pb–Pb collisions are presented to investigate production-related polarization and its possible modification in the nuclear medium. Together, these measurements illustrate how complementary choices of particle species, collision system, and reference axis provide access to distinct aspects of spin dynamics in high-energy nuclear collisions.
