Following the Reines-Cowan experiment, physicists in the late 1950s were faced with two further problems concerning the neutrino. The first—the question of whether the neutrino and antineutrino were identical or distinct particles—was thought, wrongly, to have been answered. It was believed that they were distinct. As we shall see below, this answer was incorrect. The second question was whether there was only one kind of neutrino. Was there one neutrino associated with the electrons in β decay https://www.w3.org/1998/Math/MathML"> ν e / ν ¯ e https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429199783/d9d7f6b3-b84f-4140-a573-7684ed8d06ff/content/TNF-CH007_eqn_0001.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> and another with the muon https://www.w3.org/1998/Math/MathML"> ν μ / ν ¯ μ ? https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429199783/d9d7f6b3-b84f-4140-a573-7684ed8d06ff/content/TNF-CH007_eqn_0002.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> As Reines remarked, “Having detected a neutrino associated with nuclear beta decay we puzzled as to whether the neutral particle from (π, μ) decay, was the same as the neutrino from nuclear beta decay. We wrote in a 1956 article in Nature

The question arises as to the identity of these neutrino-like particles with the neutrino of nucleón decay. It is to be noted that in nuclear beta decay the initial and final nuclei both quite obviously interact strongly with nuclei. This is not the case in (π, μ) decay, where the emission of a ‘neutrino’ converts the interaction from strong to weak. Furthermore, despite the apparent equality of nuclear beta-decay matrix elements with (μ,e) decay, both the initial and final products of the latter interact weakly with nuclei.