# Possible decay modes of Pion

I am aware that the following decay mode may happen: $$\pi^- → e^-\overline\nu$$

My question is why is the following decay mode not possible

$$\pi^- → e^-\nu$$

What anna v said is correct, but as far as I can tell, we still don't know the nature of the neutrino (maybe it's a Majorana particle?). We pair ( $$l~\overline{\nu}_l$$ ) and ( $$\overline{l}~\nu_l$$ ) to conserve lepton number, but it may turn out that the neutrino is its own anti-particle - see, e.g. neutrinoless double beta decay. So, maybe the answer to your question is that you wrote the same allowed decay twice.