I'd really appreciate any help you can give me with the following questions: Use lepton universality and lepton-quark symmetry (ignore quark mixing) to estimate the branching ratios for: a) b -> c + e- + anti-electronneutrino b) tau -> e- + anti-electronneutrino + tauneutrino Surely (a) is not possible without quark mixing? Isn't it true that without quark mixing, quarks can change flavour but only within their generation, e.g. can have u -> d but not u -> s? b) I think for this question I just need to look at the ratio of the masses of the mu and the e-, since all differences in their interactions are due to their difference in mass, is this correct? So the answer I would give would be mass(e-)/mass(mu). Why does observation of the process antimuneutrino + e- -> antimuneutrino + e- constitute unambiguous evidence for weak neutral currents, whereas the observation of antielectronneutrino + e- -> antielectronneutrino + e- does not? I've no idea about this, I'd have thought either scattering process could be an electromagnetic interaction; doesn't any interaction involving the Z have an equivalent involving photons?