Exploring Lepton Annihilation and Hadron Formation in the Standard Model

[Bob S]

When a positron approaches an electron, it first gets captured into atomic states, just like the states that bind the electron to the proton in hydrogen. The positron then gets captured by the electron and annihilated in several nanoseconds. If the mass of the neutron were slightly less than the mass of the proton, then the electron in the hydrogen atom would eventually get absorbed by the proton to produce a neutron and a neutrino. However, the neutron is heavier than the proton, and the neutron decays into a proton, an electron, and a (nearly) massless neutrino. In nuclei, beta (electron)decay is an example of neutron decay. Beta (positron) decay in nuclei is an example of proton decay in nuclei.
There is one additional effect called K capture, in which a proton in a nucleus "captures" an electron in the atomic K shell and becomes a neutron with the emission of a neutrino. This is also called inverse beta decay.