On the decays π0→νν′

Phenomenological aspects of π⁰→νν′ are considered. From the existing experimental information, we deduce an upper limit $\text{Γ(π}{}^0\text{→νν′)}\text{Γ(π}{}^0\text{→}\text{all}\text{)}\text{< 2.4 × 10}{}^{\mathrm{?5}}\text{(90\%}\text{c.l.}\text{)}$. Possible mechanisms which can give rise to this process are discussed. Branching ratios of the order of 10^?6 for decays into the known neutrinos (ν_e, ν_μ) are found possible. Rates comparable to the experimental limit for decays into final states involving additional neutrinos (massive or massless) cannot be ruled out.