Discoveries of the Neutrino

In a gist, when Beta decay was discovered, the electron was expected to have a fixed energy due to energy conservation. But a spectrum of energies of the decay product (electron) was observed instead.

So, the existence of a new, electrically neutral particle in beta decay was proposed to explain the anomaly of a continuous spectrum.

$(A,Z)\to (A,Z+1)+e^{-}+\nu$

It was Fermi who proposed that this particle (dubbed as a “neutrino”) should be light, spin-1/2 and electrically neutral.

It was later showed that the interaction cross-section of a neutrino must be very small ⇒ it can pass through the earth without deviation.

The famous Cowan-Reines experiment (indirectly) proved the existence of an electron neutrino. Gaillard, Lederman, Schwartz and Steinberger built a Spark Chamber to detect muon neutrinos coming from pion decays.

It was found that neutrinos couple to weak gauge bosons and will modify their decay width. The width measurement of Z bosons by the Large Electron Positron collider (LEP) confirmed 3 generations of neutrinos.

$N_{\nu }=\frac{{\Gamma }_{inv}}{{\Gamma }_{\nu }}=\frac{{\Gamma }_{total}-{\Gamma }_{vis}}{{\Gamma }_{\nu }}=\frac{{\Gamma }_{total}-{\Gamma }_{hadrons}-3{\Gamma }_{leptons}}{{\Gamma }_{\nu }}$

The Tau lepton was discovered: $e^{+}+e^{-}\to {\tau }^{+}+{\tau }^{-}\to e^{\pm }+{\mu }^{\pm }+4\nu$

It was confirmed by the DONUT experiment that the products had 4 neutrinos and the OPERA experiment observed about 10 tau neutrinos!