In December of 1930 Wolfgang Pauli postulated the existence of the neutrino to explain the non-conservation of energy in beta-decays ($\beta$). In a $\beta$ decay an atomic nucleus ***X*** spontaneously decays into a different nucleus ***X'*** by emitting either an electron or a positron and becomes a different element with the same mass number (**A**) but with a different atomic number (**Z**). In 1930 this type of decay scientists believed that this reaction was as follows: \begin{eqnarray} ^A_Z X \rightarrow ^A_{Z+1}X' + e \end{eqnarray} In order to determine the total energy released in a given nuclear decay we can use the mass-energy equivalence, and use the **Q-value**. The Q value is defined as the total energy released in a given nuclear decay, considering the energy conservation the general definition of Q based on mass-energy equivalence, where K is kinetic energy and m is mass (for equation (1)): \begin{eqnarray*} Q&=&K_f-K_i = (m_i-m_f)c^2\\ &=&\left[m\left({}_{Z}^{A}\mathrm {X} \right)-m\left({}_{Z+1}^{A}\mathrm {X'} \right)-m_{e}\right]c^{2} \end{eqnarray*} Beta particles can therefore be emitted with any kinetic energy ranging from 0 to Q. Studying the experimental Q-values obtained for $\beta$ decays Pauli noticed that the energy of these reactions was not conserved, there was always an energy deficit. In 1927 Ellis and Wooster performed an experiment in which they measured the total energy released in the disintegration of a ${}^{210}Bi \rightarrow {}^{210}Po$ . The calorimeter was thick enough to stop all the emitted electrons and they expected to measure a total energy of $1.05$ MeV. In fact they observed a total energy $E = 344 \pm 34$ keV. The experiment was repeated in Berlin with an improved calorimeter by Meitner and Orthman in 1930 and the result was $337 \pm 20$ keV. Pauli was intrigued by these results and thought there could be two possible explanations for the energy deficit: 1. The conservation laws were not valid when applied to regions of subatomic dimensions. OR 2. There must be a new invisible fundamental particle that accounts for the loss of energy from the nucleus. The second explanation was preferred because it maintained the integrity of the conservation laws. This led Pauli to postulate the existence of a new particle, the **neutron, that later became known as neutrino $\nu$**. The neutrino would be emitted by the nucleus simultaneously with the electron, and would carry the missing energy and momentum, but would not be detected. Taking this into account one can then re-write reaction (1) as: \begin{eqnarray} ^A_Z X \rightarrow ^A_{Z+1}X' + e\, + \bar{\nu_e} \end{eqnarray} And then the new Q-value becomes: \begin{eqnarray*} Q=\left[m\left({}_{Z}^{A}\mathrm {X} \right)-m\left({}_{Z+1}^{A}\mathrm {X'} \right)-m_{e}-m_{{\overline {\nu }}_{e}}\right]c^{2} \end{eqnarray*} It would take physicists 26 years to experimentally detect the first neutrino. Figure 1 (below) is a schema of a $\beta^-$ decay, where a neutron decays into a proton and emits an electron:  Figure 1: $\beta^-$ decay, a nuclueus emits an elctron. A neutron decays into a proton and emits an electron and an anti-neutrino. This is a copy of the original letter (in German) by W. Pauli in 1930. Pauli calls this particle a neutron, today we know it as a neutrino. The neutron as we know it today was only discovered two years later, in 1932, by James Chadwick, see [Discovery of the neutron.](https://en.wikipedia.org/wiki/Discovery_of_the_neutron) In 1933 Enrico Fermi changes the name of Pauli's postulated particle to **neutrino**. The neutrino particle played a crucial role in the first theory of nuclear beta decay formulated by Enrico Fermi in 1933 and which later became known as the weak force. Fermi was Italian and **neutrino was the obvious choice because it means the little neutral one.** Neutrinos have a very low cross-section, they are very penetrating particles, extremely hard to detect. It would take physicist 26 years to detect the first neutrino in 1956. Today we know that there are three different neutrino flavours, one for each lepton: $\nu_e$ electron neutrino, $\nu_{\mu}$ muon neutrino and $\nu_{\tau}$ tau neutrino. (and the corresponding anti-neutrinos) **$\nu_e$ electron neutrino:** It was postulated by W. Pauli in 1930 and it was only discovered in 1956 by a team led by Clyde Cowan and Frederick Reines. You can learn more here: [Cowan–Reines Neutrino Experiment](https://en.wikipedia.org/wiki/Cowan–Reines_neutrino_experiment) **$\nu_{\mu}$ muon neutrino:** The muon neutrino was first hypothesized in the early 1940s, and was discovered in 1962 by Leon Lederman, Melvin Schwartz and Jack Steinberger. They were awarded the 1988 Nobel Prize in Physics for their discovery. You can learn more here: [Discovery of the Muon Neutrino](https://www.bnl.gov/bnlweb/history/nobel/nobel_88.asp) **$\nu_{\tau}$ tau neutrino:** The existence of the tau neutrino was immediately implied after the tau particle was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC–LBL group. The tau neutrino was first observed in July 2000 by the DONUT collaboration. You can learn more here: [Direct Observation of the Nu Tau](https://en.wikipedia.org/wiki/DONUT) This and other letters from W. Pauli can be found in the CERN archives [here](https://cds.cern.ch/record/83282?ln=en). Also check out [this short portrait page](https://library.ethz.ch/en/locations-and-media/platforms/short-portraits/pauli-wolfgang.html) and very interesting [virtual exhibition page](https://library.ethz.ch/en/locations-and-media/platforms/virtual-exhibitions/wolfgang-pauli-and-modern-physics.html) at the ETH Zürich library.