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In nuclear physics, '''beta decay''' is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted. In the case of electron emission, it is referred to as "beta minus" (β⁻), while in the case of a positron emission as "beta plus" (β⁺).

In β⁻ decay, the weak interaction converts a neutron (''n''⁰) into a proton (''p''⁺) while emitting an electron (''e''⁻) and an anti-neutrino

 

At the fundamental level, this is due to the conversion of a down quark to an up quark by emission of a W^- boson; the W^- boson subsequently decays into an electron and an anti-neutrino.

In β⁺ decay, energy is used to convert a proton into a neutron, a positron (''e''⁺ ) and a neutrino

 

Fundamentally, an up quark is converted into a down quark, emitting a W⁺ boson which then decays into a positron and a neutrino.

So, unlike beta minus decay, beta plus decay cannot occur in isolation, because it requires energy, the mass of the neutron being greater than the mass of the proton. Beta plus decay can only happen inside nuclei when the absolute value of the binding energy of the daughter nucleus is higher than that of the mother nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron and a neutrino and into the kinetic energy of these particles.

In all the cases where β⁺ decay is allowed energetically (and the proton is a part of a nucleus with electron shells), it is accompanied by the electron capture process, when an atomic electron is captured by a nucleus with the emission of a neutrino:

 

 

But if the energy difference between initial and final states is low (less than 2m_ec²), then β⁺ decay is not energetically possible, and electron capture is the sole decay mode.

If the proton and neutron are part of an atomic nucleus, these decay processes transmute one chemical element into another. For example:

(beta minus),

(beta plus),

(electron capture).

Beta decay does not change the number of nucleons ''A'' in the nucleus but changes only its charge ''Z''. Thus the set of all nuclides with the same ''A'' can be introduced; these ''isobaric'' nuclides may turn into each other via beta decay. Among them, several nuclides (at least one) are beta stable, because they present local minima of the mass excess: if such a nucleus has (''A'', ''Z'') numbers, the neighbour nuclei (''A'', ''Z''−1) and (''A'', ''Z''+1) have higher mass excess and can beta decay into (''A'', ''Z''), but not vice versa. It should be noted, that a beta-stable nucleus may undergo other kinds of radioactive decay (alpha decay, for example). In nature, most isotopes are beta stable, but a few exceptions exist with half-lives so long that they have not had enough time to decay since the moment of their nucleosynthesis. One example is ⁴⁰K, which undergoes all three types of beta decay (beta minus, beta plus and electron capture) with half life of 1.277×10⁹ years.

Some nuclei can undergo double beta decay (ββ decay) where the charge of the nucleus changes by two units. In most practically interesting cases, single beta decay is energetically forbidden for such nuclei, because when β and ββ decays are both allowed, the probability of β decay is (usually) much higher, preventing investigations of very rare ββ decays. Thus, ββ decay is usually studied only for beta stable nuclei. Like single beta decay, double beta decay does not change ''A''; thus, at least one of the nuclides with some given ''A'' has to be stable with regard to both single and double beta decay.