Can you help with particle physics?

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?

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 Originally posted by m_ridsdale 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?
agreed.
 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).
shouldn t it be the ratio of the tau to the electron for this reaction?

 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?
photons cannot couple to neutral particles. so photons cannot interact with neutrinos.

but i d say the nu_e reaction is not unambiguous evidence for weak neutral currents, because it this reaction could procede with a W boson. not so in the nu_mu case.

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