In 1935 Yukawa proposed that nucleon interaction is mediated by a new particle, which he termed "meson" because its mass was predicted to be intermediate between electron and nucleon. In 1937 Neddermeyer and Anderson discovered a candidate for such a particle in cosmic rays, which was called "$\mu$ meson". However, after the war new experiments showed in 1947 that "$\mu$ meson" neither interacted strongly with matter, nor decayed as predicted. So it couldn't have been Yukawa's particle. Rabi even quipped "who ordered that?" about it, and the name "$\mu$ meson" was shortened to the modern "muon".

Experiments soon revealed the manner of muon's decay, into an electron and a pair of neutrinos, and the shape of the electron spectrum was first calculated by Louis Michel in 1950, who introduced a parameter $\rho$ to describe it. This is where "Michel electron" comes from. After the non-conservation of parity in weak interactions proposal of Lee and Yang three more parameters ($\eta$, $\xi$ and $\delta$) had to be added, and now all four are referred to collectively as "Michel parameters".

Depommier's paper gives more historical details, and a reference to the Michel's 1950 paper Interaction between Four Half-Spin Particles and the Decay of the $\mu$-Meson.

In 1935 Yukawa proposed that nucleon interaction is mediated by a new particle, which he termed "meson" because its mass was predicted to be intermediate between electron and nucleon. In 1937 Neddermeyer and Anderson discovered a candidate for such a particle in cosmic rays, which was called "$\mu$ meson". However, after the war new experiments showed in 1947 that "$\mu$ meson" neither interacted strongly with matter, nor decayed as predicted. So it couldn't have been Yukawa's particle. Rabi even quipped "who ordered that?" about it, and the name "$\mu$ meson" was shortened to the modern "muon".

Experiments soon revealed the manner of muon's decay, into an electron and a pair of neutrinos, and the shape of the electron spectrum was first calculated by Louis Michel in 1950. This is where "Michel electron" comes from. Michel introduced a single parameter $\rho$ to describe the shape. However, after the non-conservation of parity in weak interactions proposal of Lee and Yang three more parameters ($\eta$, $\xi$ and $\delta$) had to be added, and now all four are referred to collectively as "Michel parameters".

Depommier's paper gives more historical details, and a reference to the Michel's 1950 paper Interaction between Four Half-Spin Particles and the Decay of the $\mu$-Meson.