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**Decay of tauon into pion and neutrino...**

Hello all.

I have a question.

One of the weak decays of a tauon(energy 20GeV) is to a charged pion(rest mass 139.6Mev/c^2) and a neutrino(take the neutrino to be massless).

What condition results in the maximum energy of the pion after the decay?

Sketch how the decay appears in this case, and calculate the energy.

Attempt at solution.

Ok so I got that in the lab frame, the energy of the pion would be maximised if the neutrino remained stationary. Is this correct? The reason I am questioning is that if I am taking the neutrino as massless, can a massless particle be stationary? Because then by the relativistic relation

E^2=p^2 +m^2 since m is zero and p is zero as it's stationary then it has...well nothing.

Is there something obvious I am missing?

Thanks in advance,

Shroom