A positive pion decays into a positive muon and a neutrino.

$${\mathrm{\pi <!--\; \pi \; -->}}^{+}\to <!--\; \to \; -->{\mathrm{\mu <!--\; \mu \; -->}}^{+}+v_{\mathrm{\mu <!--\; \mu \; -->}}$$

The momentum of the muon is measured to be 29.8 MeV c^–1 in a laboratory reference frame in which the pion is at rest. The rest mass of the muon is 105.7 MeV c^–2 and the mass of the neutrino can be assumed to be zero.

For the laboratory reference frame