# Interaction - pion to tau and antineutrino

$$\pi^{-} \to \tau^{-}+\overline{\nu}_{\tau}$$

So, this interaction (reaction) is supposed to be forbidden. But, can not see what should be wrong here.

Charge is conserved. Baryon number is fine. Even if I check the lepton numbers, everything works.

Do I see something wrong, or I forgot some conservation law?

There simply isn't enough energy. A tau lepton is heavier than a proton ($$1777\,\mathrm{MeV}$$), so it can't be a decay product of a pion which only has a mass of about $$140 \,\mathrm{MeV}$$.
• Thanks for the reply.@dmckee I see that I need to pay more attention to energies. I thought that barion, charge and lepton numbers could always tell me surely is the interaction possible, Is it then the same if there is kaon going to pion? $$K^{-} \to \pi^{-}$$ Then, here it should be the same? Kaon has too much energy to decay only to negative pion? – solidbastard Jul 3 at 23:56