# How do we know what the flavour of the neutrino from a beta decay is?

I have read that because of the conservation of the leptonic number, a neutron should decay into $p + e^- + \overline{\nu}_e$. I don't understand this argument because I have also learnt that the leptonic number may not be conserved.

Since the flavour eigenstates of a neutrino are not the same as the propagating eigenstates, it seems to me that we should also consider muonic or tauic neutrinos for this decay (or rather the mass eigenstates), because when one computes the cross section, one assumes that the neutrino is observed a long time after the interaction had taken place, so that flavour oscillating neutrino may occur.

What do you think ?

• The usual diagnostic reaction is charged-current quasi-elastic scattering: $\nu_l + n \rightarrow l^- + p$ where $l$ represents a charged lepton ($e$, $\mu$, or $\tau$). – dmckee Nov 9 '14 at 19:57