Neutrinos and Neutrettos

Until I improved it, the Wikipedia article on the muon neutrino said:

The muon neutrino is a lepton, an elementary subatomic particle which has the symbol ν_μ 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 didn’t say who hypothesized it! So I got curious: who predicted the existence of the muon neutrino, and in what papers did they do it?

A sort of obvious theory is this: when people realized the muon was a lot like an electron, they began to suspect that just as electron can turn into an electron neutrino in some reactions, a muon could turn into a muon neutrino.

This is approximately right, but the history is not so simple. To understand it you need a bit of background. In 1935, Hideki Yukawa predicted that the attractive force between nucleons was carried by a particle lighter than these particles but heavier than the electron. This middleweight particle was dubbed a ‘mesotron’, or later ‘meson’.

A particle in this mass range was discovered in 1937, and people thought at first it was Yukawa’s meson. But later they realized it wasn’t: it didn’t interact much with nucleons!

In 1947, Yukawa’s meson was found. They named it the ‘pi meson’, or ‘pion’ for short. The original impostor was renamed the ‘mu meson’, or ‘muon’. For a while it, too, was considered a meson of sorts. But by now we consider it a wholly different sort of beast, so the term ‘mu meson’ is no longer used.

A negative pion usually decays into a muon and a muon antineutrino. A muon, in turn typically decays into an electron, a muon neutrino and an electron neutrino. Note that ‘electron-ness’ and ‘muon-ness’ are separately conserved in these processes.

Studying these processes made people suspect the existence of the muon neutrino and its antiparticle. But people first observed them in the midst of sorting out the confusion between muons and pions. So everything was a mess for while.

In fact, at one point what we now call the muon neutrino was called the ‘neutretto’!

I asked around about these issues, and I got three very interesting answers.

Over on the History of Science and Mathematics StackExchange I asked:

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

and someone named Conifold answered:

Nobody in particular, it was what is called “folklore”. The idea came up naturally when muon decay was observed by several groups in 1948, see Anicin’s The neutrino – its past, present and future:

“When in 1948 the electron spectrum from muon decay was found to be continuous it became obvious that not one but two neutrinos are emitted along with the electron. Pontecorvo witnesses that at that time everybody felt that the two neutrinos should be different. They were even named differently, the “neutrino” and the “neutretto”, but with time the idea seem to have been forgotten and it was only in 1962, when the difference between the two neutrinos has been clearly demonstrated in the first of a long series of important accelerator neutrino experiments, that the electron and the muon neutrino were finally given life.”

The Pontecorvo reference is to his The infancy and youth of neutrino physics: some recollections, where we read:

“Several groups, among which J. Steinberger, E. Hincks and I, and others were investigating the (cosmic) muon decay. The result of the investigations was that the decaying muon emits 3 particles: one electron (this we found by measuring the electron Bremsstrahlung) and two neutral particles, which were called by various people in different ways: two neutrinos, neutrino and neutretto, ν and ν’, etc. I am saying this to make clear that for people working on muons in the old times, the question about different types of neutrinos has always been present. True, later on many theoreticians forgot all about it, and some of them “invented” again the two neutrinos (for example M. Markov), but for people like Bernardini, Steinberger, Hincks and me … the two neutrino question was never forgotten… How to perform the decisive experiment I was able to formulate /40/ clearly enough (the use of muon neutrino beams). At the time the idea of the experiment was not obvious, although the statement may be strange today: one must search for electrons and muons produced in matter by muon neutrinos.”

/40/ is the reference to Pontecorvo’s 1959 paper in JETP, The Universal Fermi interaction and astrophysics.

Over on Physics StackExchange someone named HDE 226868 wrote:

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 and a neutral meson which could interact with the then-called “Yukawa meson” (the charged pion) by

In particular, they described 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.

Also on Physics StackExchange, Karim Chahine wrote:

I may have found it. I’m quoting Wikipedia’s article on Schoichi Sakata:

“Sakata and Inoue proposed their two-meson theory in 1942. At the time, a charged particle discovered in the hard component cosmic rays was misidentified as the Yukawa’s meson (π^±) 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 π^± decay. A new neutral fermion was also introduced to allow π^± decay into fermions.”

We now know that these charged and neutral fermions correspond to the second generation leptons μ and ν_μ in the modern language. They then discussed the decay of the Yukawa particle,

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, one year before the experimental discovery of decay.

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