Towards testing $(g-2)_\tau$ in $e^+e^-\to\tau^+\tau^-$: radiative corrections and projections for Belle II

TL;DR

This paper provides a comprehensive one-loop calculation of polarized $e^+e^- o au^+ au^-$ processes, essential for testing the tau's magnetic moment at Belle II, including radiative corrections and future projection strategies.

Contribution

It offers the complete one-loop polarized calculation and implementation in Monte Carlo tools, enabling precise asymmetry measurements at Belle II for new physics searches.

Findings

Complete one-loop polarized process results

Implementation in Monte Carlo integrator McMule

Projections for Belle II measurements with polarization

Abstract

The arguably most promising avenue towards testing physics beyond the Standard Model in the anomalous magnetic moment of the $\tau$ proceeds via suitably constructed asymmetries in $e^+e^-\to\tau^+\tau^-$ in the presence of a polarized electron beam. Such a program, as could be realized at Belle II assuming a polarization upgrade of the SuperKEKB $e^+e^-$ collider, crucially relies on a careful consideration of radiative corrections. In this work, we present the complete one-loop result for the fully polarized $e^+e^-\to\tau^+\tau^-$ process and its implementation in the Monte-Carlo integrator McMule. As an application, we discuss projections relevant for measurements at Belle II, both with and without electron polarization, and outline the necessary steps for a generalization to next-to-next-to-leading order.