INSTITUTE OF PHYSICS PUBLISHING EUROPEAN JOURNAL OF PHYSICS Eur. J. Phys. 27 (2006) 1257–1264 doi:10.1088/0143-0807/27/5/025 The uses of neutrinos: a historical perspective G Mateos 1 and J Navarro 2 1 Centro de Estudios Interdisciplinarios en Ciencias Y Humanidades, Universidad Autonoma Nacional de Mexico, Mexico 2 Department of History and Philosophy of Science, University of Cambridge, Free School Lane, Cambridge, CB2 3RH, UK Received 30 May 2006, in final form 30 July 2006 Published 21 August 2006 Online at stacks.iop.org/EJP/27/1257 Abstract When Pauli postulated the existence of a new elementary particle, the ‘neutron’, in the late 1930s, he was introducing an entity to fill the gaps in the physics of the day. There was no direct evidence for such a particle; only the need to account for the missing energy in beta decay processes and to explain the statistics of some nuclei. Thus, Pauli’s neutron was called into existence to solve problems in the areas of radioactivity and nuclear physics; but, in time, it would prove useful in many other areas of physics. The aim of this paper is to describe the many uses the neutrino has had in physics since its appearance, uses that have determined the ontological status of this elusive particle. Interestingly, the experimental detection of neutrinos 50 years ago was a minor episode in understanding the nature of such particles. As we shall argue, other moments in the biography of the particle proved more significant in determining the uses of neutrinos and their ontology. 1. Nuclear physics and radioactivity As is well known [1], Pauli introduced the concept of ‘neutron’ as a desperate way out to solve serious problems in physics [2]. At the time, atoms and atomic nuclei were supposed to consist of protons and electrons in different arrangements. Beta radioactivity was obviously seen, in this context, as the emission of electrons from the nuclei. However, both the composition of nuclei and the energy of the electrons emitted were long standing anomalies in physics. On the one hand, the statistics of some nuclei was different to that expected considering the number (even or uneven) of particles composing them. On the other hand, the spectra of the electrons emitted by nuclei were always continuous, a fact that contradicted the principle of conservation of energy, since the state of the nuclei before and after the emission was fixed [3]. The magnetic neutron came to solve both problems. With a mass between those of the proton and the electron, Pauli’s neutron would be a nuclear component (thus solving the odd statistics of some nuclei) that could be emitted together with the electron, carrying with it the missing energy in beta radioactivity. 0143-0807/06/051257+08$30.00 c  2006 IOP Publishing Ltd Printed in the UK 1257