Universal lepton universality violation in exclusive processes

TL;DR

This paper demonstrates that QED corrections in high-energy exclusive processes involving different leptons lead to universal violations of lepton universality, with large logarithmic effects causing significant deviations from naive expectations.

Contribution

It introduces a universal factorization approach to resumming large logarithms in lepton flavor ratios of exclusive processes, revealing significant universality violations.

Findings

Resummed QED corrections cause deviations from lepton universality.

Large logarithms due to lepton mass differences are universally factorized.

Predicted deviations are significant in processes like Z→l+l− and B−→l−ν̄l.

Abstract

In high energy exclusive processes involving leptons, QED corrections can be sensitive to infrared scales like the lepton mass and the soft photon energy cut, resulting in large logarithms that need to be resummed to all order in $\alpha$. When considering the ratio of the exclusive processes between two different lepton flavors, the ratio $R$ can be expressed in terms of factorized functions in the decoupled leptonic sectors. While some of the functional terms cancel, there remain the large logarithms due to the lepton mass difference and the energy cut. This factorization process can be universally applied to the exclusive processes such as $Z\to l^+l^-$ and $B^-\to l^-\bar{\nu}_l$, where the resummed result in the ratio gives significant deviations from the naive expectation from lepton universality.