Magnetic fields in relativistic heavy-ion collisions: observables and applications

Heavy quarks produced in early hard scatterings serve as sensitive probes of the early electromagnetic fields and geometry of the quark–gluon plasma in relativistic heavy-ion collisions. The speaker discusses the rotational Brownian motion of heavy-quark spins in the QCD medium and its consequences for the polarisation of open heavy-flavour hadrons. Analytical expressions are obtained for vector and tensor polarisation, relevant for baryon spin polarisation and vector-meson spin alignment. Assuming an initial heavy-quark polarisation generated by strong magnetic fields in off-central collisions, we show that interactions with the medium lead to momentum-dependent depolarisation, and compare the results with recent ALICE Collaboration measurements of D*+ spin alignment. Plarisation harmonics of open heavy hadrons is proposed as a new observable sensitive to the initial geometric anisotropies of the fireball. Their azimuthal modulation encodes path-length dependent spin relaxation and provides a complementary probe of the early-time dynamics and geometry of heavy-ion collisions.

Having established the observable signals of magnetic fields in relativistic heavy ion collisions, the speaker discusses the application of this strong field environment to study CP violation in weak interactions. In particular, tau leptons created in ultraperipheral heavy ion collisions (UPCs) and their subsequent weak decay are considered. The strong field environment in UPCs lead to production of spin polarised tau+ and tau- leptons, and the relative anisotropy of their decay distributions provides a novel CP-violating observable that may be accessible at the LHC. This establishes UPCs as a novel arena for testing scenarios of physics beyond the Standard Model.