Polarized tau decay and CP violation in ultraperipheral heavy-ion collisions

Abstract

We investigate the role of $τ$-lepton polarization in ultraperipheral heavy-ion collisions (UPCs) as a novel application of the intense electromagnetic fields generated in such processes. In particular, we analyze the decay distributions of polarized $τ$-leptons produced via photon-photon fusion, focusing on both leptonic and semi-leptonic channels. We show that the external magnetic field present in UPCs induces a preferred spin quantization axis, which modifies the angular and energy distributions of $τ$ decay products relative to the standard helicity frame. By formulating the spin polarization along the magnetic field direction, we derive modified polarization-sensitive observables and demonstrate how kinematic selections can retain nonvanishing polarization signals even after ensemble averaging. Furthermore, we propose that the relative polarization of $τ^-$ and $τ^+$, accessible through complementary angular ranges of their decay products, serves as a sensitive observable for potential CP-violating effects. This framework provides a pathway for future experimental studies at the LHC and future colliders to exploit polarized $τ$ decays in UPCs as an application of the strong electromagnetic fields to probe new sources of CP violation.