Improved Search for Heavy Neutrinos in the Decay $\pi\rightarrow e\nu$

Understand

A search for massive neutrinos has been made in the decay $\pi\rightarrow e^+ \nu$.

• No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos ($\pi\rightarrow e^+ \nu_h$).
• Upper limits (90 \% C.L.) on the neutrino mixing matrix element $|U_{ei}|^2$ in the neutrino mass region 60--135 MeV/$c^2$ were set, which are %representing an order of magnitude improvement over previous results.