Spin polarization as a probe for local parity violation*

Due to non-perturbative QCD topological transitions induced by high temperature, the Quark Gluon Plasma generated by ultra-relativistic heavy ion collisions is expected to be created with a chiral imbalance, whose value fluctuates in a event-by-event basis. In other words, we expect a local breaking of parity symmetry in hot QCD matter. However, this has never been observed. The search for local parity violation is primarily carried out through the Chiral Magnetic Effect that requires an accurate estimate of the magnetic filed generated during the collisions. There is an ongoing dedicated isobar run to search for this effect at Relativistic Heavy Ion Collider. In this talk, I will show that local parity violation due to chiral imbalance can also be revealed, without relying on the knowledge of the magnetic field, by measuring the projection of the polarization vector onto the momentum, i.e. the helicity, of final state baryons. I will show that the polarization of spin 1\2 baryons receives an additional contribution along their momentum proportional to the chiral chemical potential. This additional, parity-breaking, contribution to helicity can be detected by studying the helicity-helicity correlation of two particles with large azimuthal angle difference.