Universal lepton universality violation in exclusive processes

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 $α$. 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 $B^-\to l^-\barν_l$, where the resummed result in the ratio gives significant deviations from the naive expectation from lepton universality.

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