What Are Neutrinos and Antineutrinos, and Why Were They Proposed?

Neutrinos and antineutrinos are electrically neutral, extremely low-mass particles that interact through the weak interaction. They were proposed to explain the continuous energy spectrum of beta particles while preserving conservation of energy, momentum, and angular momentum.

The Reasoning

In beta-minus decay, a neutron in an unstable nucleus changes into a proton, producing an electron and an electron antineutrino:

$$\text{n} \rightarrow \text{p} + \text{e}^{-} + \bar{\nu}_{\text{e}}$$

In beta-plus decay, a proton in a suitable unstable nucleus changes into a neutron, producing a positron and an electron neutrino:

$$\text{p} \rightarrow \text{n} + \text{e}^{+} + \nu_{\text{e}}$$

If beta-minus decay produced only a daughter nucleus and an electron, the electron should have an almost fixed kinetic energy for a particular nuclear transition. Experiments instead showed a continuous beta-energy spectrum: electrons emerged with a range of energies.

In 1930, Wolfgang Pauli proposed an undetected neutral particle that could carry away the apparently missing energy and momentum. Because beta decay is a three-body process, the available energy can be divided differently among the daughter nucleus, electron or positron, and neutrino or antineutrino. The particle’s spin also allows angular momentum to be conserved.

A common misconception is that beta decay violates energy conservation. It does not: the total energy remains fixed, but it is shared between multiple products. Neutrinos were initially undetected because they interact extremely weakly with matter.

Exam Technique

For SL and HL, know the particles included in beta-minus and beta-plus equations. The continuous beta spectrum as evidence for the neutrino is an AHL extension in E.3 Radioactive decay. In an Explain response, link the observed range of beta-particle energies to variable energy sharing and conservation laws.