Who predicted the existence of the muon neutrino?

Question

The Wikipedia article on the muon neutrino says:

The muon neutrino is a lepton, an elementary subatomic particle which has the symbol $\nu_\mu$ and no net electric charge. Together with the muon it forms the second generation of leptons, hence the name muon neutrino. It was first hypothesized in the early 1940s by several people, and was discovered in 1962 by Leon Lederman, Melvin Schwartz and Jack Steinberger.

But it doesn't say who hypothesized it, and I haven't been able to find that out.

So: who predicted the existence of the muon neutrino, and in what papers did they do it?

Answer 1

This seems to be a rather complicated issue. The earliest source I've been able to find proposed the existence of a muon counterpart to the electron neutrino is Sakata & Inoue's On the Correlations between Mesons and Yukawa Particles, published in English in 1946 but formulated several years prior. They postulated the existence of a charged meson $m^{\pm}$ and a neutral meson $n$ which could interact with the then-called "Yukawa meson" $Y^{\pm}$ (the charged pion) by $$m^{\pm}\leftrightarrow n+Y^{\pm},\quad n\leftrightarrow m^{\pm}+Y^{\mp}$$ In particular, they described $n$ as a

neutral meson which is assumed in the following discussions to have a negligible mass, and consequently may be regarded as equivalent with the neutrino

Decades later (I can't determine the precise data), Masami Nakagama wrote in Neutrinos and Sakata: A Personal View that it was later assumed by many "from the convenience and economy principles" that the beta decay neutrinos (electron neutrinos) and the "neutral mesons" of Sakata and Inoue were the same, something Sakata apparently resisted. In conjunction with Sakata's objections, Ogama & Kamefuchi's On the µ-Meson Decay explored some problematic consequences of assuming that the two particles were identical, meaning that the debate was going on as of 1950. The upshot? It seems that the community may have shifted from the idea that there was a sibling to the [electron] neutrino to the idea that this particle was the same as the neutrino; then shifted back in the aftermath of the 1962 experiments at Brookhaven (interestingly enough, the Danby et al. paper reporting the experiments mentions none of the abovementioned theories).

An additional reason I say that this is complicated is that proponents of the distinct-particle theory might not have still classified both particles under the umbrella of "neutrino" - in other words, it's not clear to me that Sakata & Inoue intended for their "neutral meson" to be thought of as a true sibling to the neutrino, or just an analogous counterpart in a pair of sort-of-analogous interactions. But that may not be an objection that others think is substantial.

Answer 2

I may have found it. I'm quoting Wiki's article on Schoichi Sakata:

Sakata and Inoue proposed their two-meson theory in 1942.[3] At the time, a charged particle discovered in the hard component cosmic rays was misidentified as the Yukawa’s meson ($\pi^\pm$, nuclear force career particle). The misinterpretation led to puzzles in the discovered cosmic ray particle. Sakata and Inoue solved these puzzles by identifying the cosmic ray particle as a daughter charged fermion produced in the $\pi^\pm$ decay. A new neutral fermion was also introduced to allow $\pi^\pm$ decay into fermions.

We now know that these charged and neutral fermions correspond to the second generation leptons $\mu$ and $\nu_\mu$ in the modern language. They then discussed the decay of the Yukawa particle, \begin{equation} \pi^+\rightarrow \mu^+ + \nu_\mu \end{equation} Sakata and Inoue predicted correct spin assignment for the muon, and they also introduced the second neutrino. They treated it as a distinct particle from the beta decay neutrino, and anticipated correctly the three body decay of the muon. The English printing of Sakata-Inoue’s two-meson theory paper was delayed until 1946,[4] one year before the experimental discovery of $\pi\rightarrow\mu\nu$ decay.

And there's the link to said paper: https://academic.oup.com/ptp/article/1/4/143/1846220

I might be wrong but it's my best shot.