In the near future, experimental constraints on charged current (CC) lepton universality can be expected to improve dramatically. First, the PIENU experiment at TPIUMF and the PEN experiment at PSI aim to improve the bound on the ratio

by almost an order of magnitude. Second, by analyzing existing data, the Belle experiment aims to improve the bound on the ratio

by a factor of three. Further improvements can be expected by analyzing the τ decays in existing Belle data. There also exist a large amount of unanalyzed W decay data from the Tevatron.

These new bounds will place very tight constraints on possible new physics beyond the Standard Model. For example, the 95% bounds on R-parity violating couplings will be improved as follows:
These limits will be very difficult to improve further, even with future high statistics neutrino experiments such as NuSOnG [1]. However, for certain types of new physics, the combination of CC lepton universality data and NuSOnG will have a dramatic effect. I demonstrate this using a model in which neutrino mixing with heavy gauge singlet states suppress both the CC and NC neutrino couplings.