Measurement of strong magnetic fields in heavy ion collisions via the spin alignment of charmed vector mesons

This report summarizes the results of the summer student project at the ALICE experiment, where the spin alignment of prompt D∗+ mesons (i.e. primary particles coming directly from the collision) was investigated in mid-central Pb–Pb collisions via the measurement of the spin density matrix element ρ00 of the D∗+. The candidate selection was done with the help of a classification algorithm based on machine learning by employing a boosted decision tree (BDT) in order to differentiate between prompt, non-prompt (i.e. feed down, mostly from the decay of beauty hadrons) and combinatorial background D∗+ candidates. Only the prompt D∗+ mesons are expected to experience a spin alignment along the magnetic field created by the spectator protons in the very early stages of the collision. A hint of deviation from ρ00 = 1/3 was found in the 8 GeV/c < pT < 15 GeV/c interval indicating a polarization of the charm quarks due to the magnetic field.