2025/10/15 by Hoferichter, Martin, Levati, Gabriele
#FOS: Physical sciences #High Energy Physics - Experiment (hep-ex) #High Energy Physics - Phenomenology (hep-ph) #Nuclear Theory (nucl-th)
paper · doi:10.48550/arxiv.2510.13966
While electron and muon dipole moments are well-established precision probes of physics beyond the Standard Model, it is notoriously challenging to test realistic New-Physics (NP) scenarios for the τ lepton. Constructing suitable asymmetries in e+e-→τ+τ- has emerged as a promising such avenue, providing access to the electric and magnetic dipole moment once a polarized electron beam is available, e.g., with the proposed polarization upgrade of the SuperKEKB e+e- collider. However, this interpretation relies on an effective-field-theory (EFT) argument that only applies if the NP scale is large compared to the center-of-mass energy. In this Letter we address the consequences of the asymmetry measurements in the case of light NP, using light spin-0 and spin-1 bosons as test cases, to show how results can again be interpreted as constraints on dipole moments, albeit in a model-dependent manner, and how the decoupling to the EFT limit proceeds in these cases. In particular, we observe that the imaginary parts generated by light new particles can yield non-vanishing asymmetries even without electron polarization, presenting opportunities for NP searches that can be realized already with present data at Belle II.