Form-factor-independent Test of Lepton Universality in Semileptonic Heavy Meson and Baryon Decays

In the semileptonic decays of heavy mesons and baryons, the lepton-mass dependence factors out in the quadratic cos(2) theta coefficient of the differential cos. distribution. We call the corresponding normalized coefficient the convexity parameter. This observation opens the path to a test of lepton universality in semileptonic heavy meson and baryon decays that is independent of form-factor effects. By projecting out the quadratic rate coefficient, dividing out the lepton-mass-dependent factor, and restricting the phase space integration to the tau lepton phase space, one can define optimized partial rates which, in the Standard Model, are the same for all three (e, mu, tau) modes in a given semileptonic decay process. We discuss how the identity is spoiled by new physics effects. We discuss semileptonic heavy meson decays such as (B) over bar (0) -> D-(*()+)l(-)(nu) over bar (l) and B-0(-) -> J/Psi(eta(C))l(-)(nu) over bar (l) and semileptonic heavy baryon decays such as Lambda(b) -> Lambda(c)l(-)(nu) over bar (l) for each l = e, mu, tau.