In the Standard Model of Particle Physics, neutrinos are elementary particles in the family of leptons (same family as the electron), thus not sensitive to strong interaction, and with no electric charge, thus not sensitive to electromagnetic interaction.
The study of neutrino properties, from the measurement of its helicity to the long-awaited discovery of oscillations, had an important impact on particle theory. The early experimental data (or its theoretical interpretation) seemed to suggest that neutrinos, unlike other fermions, were massless. Indeed, the Goldhaber experiment showed in 1958 that the neutrino helicity is negative [Gol58] ; because of this, the founders of the Standard Model (S.L. Glashow, S. Weinberg and A. Salam) did not include right-handed neutrinos in their theory [Gla61,Wei67,Sal68]. Neutrinos are therefore massless in the Standard Model.
This did not prevent theorists like Pontecorvo, to consider the possibility of neutrino flavour oscillations [Pon67]. Oscillations, which require neutrinos to be massive, seem to be in contradiction with the Standard Model. But the Standard Model itself could be the low-energy limit of a more fundamental theory, such as one of the Grand Unified Theories developed in the 1970s by Georgi, Glashow and other theorists. In this case, the Standard Model would be supplemented by additional interaction terms (so-called higher-dimensional operators) suppressed by powers of a scale Λ characteristic of the high-energy theory. Weinberg noticed that the interaction term llHH/Λ (known as the Weinberg operator) [Wei79], which involves two lepton and two Higgs doublets and violates lepton number, generates a mass mν~M2W/Λ for the Standard Model neutrinos, which are then promoted to Majorana neutrinos. For mν~0.05 eV, this gives Λ~1014 GeV, close to the Grand Unification scale MGUT ~1016 GeV.
Roughly at the same time, several theorists realized that Grand Unified Theories based on the SO(10) gauge group contain all the ingredients needed to generate light neutrino masses [Gel79,Gla79,Moh80]. These theories predict the existence of heavy Majorana neutrinos, whose coupling to the lepton and Higgs fields induce the Weinberg operator via the so-called “seesaw mechanism” [Min77,Yan79]. The masses of these heavy neutrinos lie typically a few orders of magnitude below MGUT, yielding masses in the eV or sub-eV range for the Standard Model neutrinos. The existence of light Majorana neutrinos can therefore be viewed as a prediction of the seesaw mechanism and of Grand Unified Theories, even though alternative mechanisms could be at the origin of neutrino masses. It is, unfortunately, very hard to test this appealing idea. A first step – necessary but not sufficient – would be to prove, through the observation of neutrinoless double beta decay, that neutrinos are Majorana fermions. If, in addition, proton decay were observed, this would strongly support the Grand Unification hypothesis and give some credit to the seesaw mechanism.
 It is also possible that neutrino masses arise from some low-energy physics that would complement the Standard Model. The simplest option is to add right-handed neutrinos to the Standard Model and to impose lepton number conservation, in which case neutrinos are Dirac fermions, but there is no natural explanation for the smallness of their masses.
During the conference on the History of the Neutrino (Sept. 5-7, 2018 in Paris) the subject of Neutrinos and Particle Physics Models was reviewed by Pierre Ramond (University of Florida, USA) : here the slides and the video of his talk.
|Gel79||M. Gell-Mann, P. Ramond and R. Slansky||Complex Spinors and Unified Theories||Talk at the 19th Sanibel Symposium, February 1979, retro-print arXiv:hep-ph/9809459 & Supergravity, ed. by D. Freedman and P. van Nieuwenhuizen, North-Holland (1979) p. 315, retro-print arXiv:1306.4669|
|Gla61||S.L. Glashow||Partial symmetries of weak interactions||Nucl. Phys. 22 (1961) 579|
|Gla79||S.L. Glashow||The future of elementary particle physics||Cargèse lectures, July 1979, in Quarks and Leptons, Plenum, 1980|
|Gol58||M. Goldhaber, L. Grodzins, A. Sunyar||Helicity of neutrinos||Phys. Rev. 109 (1958) 1015|
|Min77||P. Minkowski||m → e g at a rate of one out of 1 billion muon decays||Phys. Lett. B67 (1977) 421|
|Moh80||R.N. Mohapatra and G. Senjanovic||Neutrino mass and spontaneous parity nonconservation||Phys. Rev. Lett. 44 (1980) 912|
|Pon67||B. Pontecorvo||Neutrino experiments and the question of leptonic-charge conservation||ZETF 53 (1967) 1717 – Soviet Physics JETP 26 (1968) 984|
|Sal68||A. Salam||Weak and electromagnetic interactions||Proceedings of the 8th Nobel Symposium, Conf. Proc.C 680519 (1968) 367|
|Wei67||S. Weinberg||A model of leptons||Phys. Rev. Lett. 19 (1967) 1264|
|Wei79||S. Weinberg||Baryon and lepton non conserving processes||Phys. Rev. Lett. 43 (1979) 1566|