# The Little Neutral One

The neutrino is one of the most interesting particles that has ever been discovered. Its fascinating history started with the observation of beta decays, i.e., the emission of electrons in nuclear decays such as $$\begin{equation*} {}^{14}_ {6} \mathrm C \to {}^{14} _ {7}\mathrm N + \mathrm e^{-} + \bar\nu_{\mathrm e}. \end{equation*}$$ The fact that the electron energy spectrum in the beta decay process is continuous indicates the existence of a third product other than ${}^{14}_{7}\mathrm N$ and $\mathrm e^-$. In 1930, Pauli wrote a letter 1 to a workshop in Tubingen explaining to the “Radioactive Ladies and Gentlemen” about his so called “neutron” as the missing particle in the beta decay at that time. It was later called neutrino since the name “neutron” had been used to name one of the nucleons. The missing particles in beta decays were proven to be anti-neutrinos. In a beta decay process, the charged current weak interaction converts a down quark in the neutron to an up quark while releasing an electron and an anti-electron neutrino: $$$$n\to p + e^- + \bar \nu_e .$$$$

Reaction TypeProcessMediator(s)
Electron emission${}^A_Z X \to {}^A_{Z+1}X' + e^- +\bar \nu_e$$W^{\pm} Positron emission{}^A_Z X \to {}^A_{Z-1}X' + e^+ + \nu_e$$W^{\pm}$
Electron capture${}^A_Z X + e^- \to {}^A_{Z-1}X' + \nu_e$$W^{\pm} Positron capture{}^A_Z X + e^+ \to {}^A_{Z+1}X' + \bar\nu_e$$W^{\pm}$
$e^{\pm}$ annihilation$e^- + e^+ \to \nu + \bar\nu $$W^{\pm}, Z BremsstrahlungX+X' \to X + X' + \nu + \bar\nu$$Z$
$\nu_{\mathrm e} (\bar\nu_{\mathrm e})$ capture${}^A_{Z}X + \overset{(-)}{\nu_e} \to {}^A_{Z\mp 1}X' + e^\pm $$W^{\pm} e^\pm\nu scatteringe^- + \overset{(-)}{\nu} \to e^- + \overset{(-)}{\nu}$$W^{\pm}$, $Z$
Nucleon scattering${}^A_Z X + \overset{(-)}{\nu} \to {}^A_Z X + \overset{(-)}{\nu}$Z

Neutrino Reactions: Neutrino related nuclear and leptonic reactions.

More generally, the positron/electron emission and capture processes are all neutrino-related nuclear reactions which are listed in Table Neutrino Reactions. There are three different flavors of neutrinos, namely the electron flavor, the muon flavor, and the tau flavor as shown in Table Neutrino Properties. The first direct detection of neutrinos was done by Clyde Cowan and Frederick Reines in 1956 2 who used nuclear reactor neutrinos as the source of the experiment.

Electric Charge0
Spin$1/2$
Mass$<2~\mathrm{eV}$
InteractionsWeak, Gravitation
Flavors$\nu_e$, $\nu_\mu$, $\nu_\tau$
ChiralityLeft
Hypercharge$-1$

Neutrino Properties: The physical properties of the neutrino 3.

1. The original letter can be found at https://www.bibnum.education.fr/physique/physique-nucleaire/chers-mesdames-et-messieurs-radioactifs ↩︎

2. C L Cowan, "Detection of the free neutrino: a confirmation.", Science 124, 103-4 (1956) . ↩︎

3. C. Patrignani, "Review of particle physics", Chinese Physics C 40, 100001 (2016) . ↩︎