PHYS08200604017 Manimala Mitra - Homi Bhabha National Institute

Chapter 2
Neutrino Mass and Mixing
From many decades, the existence of neutrino is very well-known. In 1930 Pauli [1]
postulated the existence of a very light neutral particle neutrino to rescue the principle
of energy-momentum conservation in nuclear β-decay. After the discovery of the neutron
by James Chadwick, it was speculated that the particle which Pauli postulated could be
neutron. However, soon it was realized that the particle which Pauli proposed should
be much lighter than neutron. In 1956, Clyde Cowan and Frederick Reines [2] observed
antineutrinos, emitted by a nuclear reactor. The observed neutrino was later determined
as a partner of the electron. In 1962, muon neutrinos were observed [3]. Finally, tau
neutrinos were discovered in 2000 by the experiment DONUT at Fermilab [4], and so the
tau neutrino became the last observed particle of the standard model. Hence, with the
discovery of the tau-neutrino we have three flavors of neutrinos electron, muon and tau
neutrinos ν e , ν µ and ν τ in the standard model.
In the standard model the three flavors of neutrinos belong to three different doublet
representations of the gauge group SU(2) L
and they have hypercharge Y = −1. There is
no right handed neutrino in the standard model and hence theoretically with the particle
contents of the standard model it is not possible to generate the neutrino masses. Given
the experimental possibilities in the 60’s, when the SM was being built, no evidence of
neutrino masses were observed. Therefore the standard model with only left-handed neutrinos
was compatible with data. However, the standard model, from 60 onwards faced
two major problems, the solar and atmospheric neutrino anomalies. In 1968, Ray Davis
detected the solar neutrinos coming from the Sun with a chlorine based detector in the
Homestake mine, USA [5]. The flux measured in this experiment was reported to be
only 1/3 of the expected one. The discrepancy originated the long-lasting ”solar neutrino
problem”. Similar discrepancy has also been observed in the flux of the muon neutrinos
coming from the Earth’s atmosphere [6]. The discrepancies in the solar as well as in the
atmospheric neutrino fluxes are possible to explain in terms of the phenomenon referred
to “neutrino oscillation” [7], the transformation of one flavor of neutrino into another
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