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

Abstract

We investigate the role of $\tau$-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 $\tau$-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 $\tau$ 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 ${\tau }^{-}$ and ${\tau }^{+}$, 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 $\tau$ decays in UPCs as an application of the strong electromagnetic fields to probe new sources of CP violation.

8 pages, published version

References in corpus (10)

• The Physics of Ultraperipheral Collisions at the LHC
• Performance of missing transverse momentum reconstruction in proton-proton collisions at $\sqrt{s}=$ 13 TeV using the CMS detector
• Observation of the $\gamma \gamma \to \tau \tau$ process in Pb+Pb collisions and constraints on the $\tau$-lepton anomalous magnetic moment with the ATLAS detector
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• CP violation and electric-dipole-moment at low energy $\tau$ production with polarized electrons
• Next-to-Next-to-Leading Order QCD Corrections to Polarized Semi-Inclusive Deep-Inelastic Scattering
• Dimuon and ditau production in photon-photon collisions at next-to-leading order in QED
• Triple-product asymmetry in the radiative two-pion tau decay
• Automated next-to-leading order QCD and electroweak predictions of photon-photon processes in ultraperipheral collisions
• Measurement of the Tau Lepton Polarisation at LEP2