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

TL;DR

This work investigates symmetry tests and new-physics searches through fermion dipole moments by exploiting polarization observables in polarized photon fusion, focusing on $\gamma\gamma\to \tau^+\tau^-$ at a future $e^+e^-$ collider like STCF. It employs transverse-momentum-dependent (TMD) factorization to express the cross section in terms of photon TMDs and helicity amplitudes, uncovering azimuthal modulations $\cos(2\phi)$, $\sin(2\phi)$, and $\cos(4\phi)$ that encode CP-even and CP-odd information. Three polarization-based asymmetries are proposed—$A_{c2\phi}$, $A_{s2\phi}$, and $A_{c4\phi}$—leading to a $2\sigma$ reach on the tau dipole form factors: $-4.6\times10^{-3}<\mathrm{Re}(a_\tau)<7.0\times10^{-3}$ and $|\mathrm{Re}(d_\tau)|<2.8\times10^{-16}\ e\,\mathrm{cm}$, approaching the SM expectation for $a_\tau$. The paper also connects these collider-based probes to supersymmetric scenarios with RPV couplings, showing that current muon and tau EDM bounds are still far from perturbativity targets but outlining the EDM sensitivities required to constrain representative couplings toward perturbative values, thereby highlighting the complementary role of dipole measurements across fermion species in constraining high-scale new physics.

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 $γγ\to τ^+ τ^-$ 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 $τ^+ τ^-$ kinematics, enabling polarization-based observables that enhance sensitivity to the $τ$ 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 $τ$ dipole moments with improved precision at future lepton colliders. As an illustration, we obtain an improved $2σ$ reach on the anomalous magnetic dipole moment, $-4.6 \times 10^{-3} < \mathrm{Re}(a_τ) < 7.0 \times 10^{-3}$, 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.