Testing lepton flavor universality in Υ(4S) decays

We propose a novel method to probe the persistent hints of lepton flavor universality violation observed in semileptonic B decays. Relying on the specific properties of the Belle II experiment, it consists in comparing the inclusive rates of Upsilon(4S) -> e(+/-) mu(-/+) X, Upsilon(4S) -> mu(+/-)tau(-/+)(had) X and Upsilon(4S) -> e(+/-) tau(-/+)(had) X. We show that such a measurement can be directly related to the ratio R(X)(tau l) = Gamma(B -> X tau nu)/Gamma(B -> Xl nu) (l = e or mu), once appropriate experimental cuts are applied to suppress the effects of neutral B mixing and leptons emitted through charm or tau decays. Such a measurement would thus constitute an additional and potentially competitive probe of lepton flavor universality in b -> cl nu transitions, complementary to existing exclusive measurements, accessible in the Belle II environment.

Automated summary

A novel method is proposed to probe lepton flavor universality violation in semileptonic B decays by comparing inclusive rates of different decay processes. The measurement is directly related to a specific ratio and can suppress the effects of mixing and other factors through appropriate experimental cuts.