New Physics and Symmetry Tests with Polarized Photon Fusion and Dipole Moments

TL;DR

This paper explores how polarized photon fusion at future colliders can improve measurements of tau lepton dipole moments, providing new methods to test fundamental symmetries and search for new physics beyond the Standard Model.

Contribution

It introduces polarization observables in photon fusion processes to enhance sensitivity to tau dipole moments and systematically disentangles CP-even and CP-odd interactions.

Findings

Improved 2σ sensitivity to tau anomalous magnetic dipole moment.

Azimuthal asymmetries reveal polarization effects in tau pair production.

Complementary constraints on new physics from dipole moments across fermionic systems.

Abstract

We discuss new-physics searches and symmetry tests with dipole moments, emphasizing the role of polarization observables. As a primary benchmark, we consider polarized photon fusion in the $e^+ e^-$ environment of the Super Tau-Charm Facility (STCF) and study $\gamma \gamma \to \tau^+ \tau^-$ in the nearly back-to-back region, where a transverse-momentum-dependent (TMD) description provides a convenient framework for organizing polarization effects. We show that linearly polarized photons induce characteristic azimuthal asymmetries in the $\tau^+ \tau^-$ kinematics, enabling polarization-based observables that enhance sensitivity to the $\tau$ electromagnetic dipole form factors. Moreover, $CP$-even and $CP$-odd dipole interactions can be disentangled through distinct angular structures, offering a systematic path to probe $\tau$ dipole moments with improved precision at future lepton colliders. As an illustration, we obtain an improved $2\sigma$ reach on the anomalous magnetic dipole moment, $-4.6 \times 10^{-3} < \mathrm{Re}(a_\tau) < 7.0 \times 10^{-3}$, reaching a precision level close to the Standard Model expectation. To place these prospects in a broader context, we briefly summarize the experimental status of dipole-moment measurements across different fermionic systems and highlight their complementarity in constraining new physics. We illustrate this interplay with supersymmetric scenarios featuring $R$-parity violation, in which loop-induced dipole moments provide correlated probes of $CP$-conserving and $CP$-violating interactions. Taken together, polarized photon fusion and precision dipole measurements constitute a coherent program for testing fundamental symmetries and exploring physics beyond the Standard Model.