Signature and application of magnetic field in relativistic heavy-ion collisions

by Dr Amaresh Jaiswal (NISER)

Thursday 9 Apr 2026, 14:30 → 15:30 Asia/Kolkata

A 304 and On Zoom

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. I discuss the rotational Brownian motion of heavy-quark spins in the QCD medium and its consequences for the polarization of open heavy-flavor hadrons. Analytical expressions are obtained for vector and tensor polarization, relevant for baryon spin polarization and vector-meson spin alignment. Assuming an initial heavy-quark polarization generated by strong magnetic fields in off-central collisions, we show that interactions with the medium lead to momentum-dependent depolarization, and compare the results with recent ALICE Collaboration measurements of D*+ spin alignment. We further propose polarization harmonics of open heavy hadrons 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, I discuss the application of this strong field environment to study CP violation in weak interactions. In particular, I consider tau leptons created in ultraperipheral heavy ion collisions (UPCs) and its subsequent weak decay. The strong field environment in UPCs lead to production of spin polarized 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.