E.3.9—Neutrinos and antineutrinos

Identify the neutral leptons

A neutrino $\nu_e$ and an antineutrino $\bar{\nu}_e$ are neutral, extremely low-mass leptons. They interact very weakly with matter, so they are difficult to detect directly.

Choose the correct particle

Beta-minus decay emits an electron and an electron antineutrino: $n\rightarrow p+e^-+\bar{\nu}_e$. Beta-plus decay emits a positron and an electron neutrino: $p\rightarrow n+e^++\nu_e$ (inside a nucleus).

Check lepton number

An electron has lepton number $+1$, so the accompanying antineutrino has $-1$. A positron has $-1$, so the accompanying neutrino has $+1$. Each beta reaction therefore keeps the initial total lepton number at zero.

Common trap

Do not swap the beta partners: $\beta^-$ pairs with $\bar{\nu}_e$, while $\beta^+$ pairs with $\nu_e$. The continuous beta spectrum is treated separately in the HL objective E.3.18.