The Weak Force
According to the Standard Model of particle physics, there are twelve known fundamental particles of matter in the Universe. Six of these are types of quarks: up, down, charm, strange, top, and bottom. The other six are called leptons, and include electrons, muons, tau, and three kinds of neutrinos. Among other things, the weak force allows quarks to turn into other types of quarks, and leptons to turn into other leptons. The most well-known consequence of such a change is radioactive decay, which occurs when an atomic nucleus is unstable.
In alpha radioactive decay, the nucleus ejects a cluster of four nucleons - two protons and two neutrons, called an alpha particle (identical to a helium atom). This reduces the total number of nucleons in the nucleus.
In beta radioactive decay, the number of protons in the nucleus changes, which changes the chemical element, but the total number of nucleons stays the same. Either a neutron turns into a proton, or a proton turns into a neutron. More precisely, a down quark becomes an up quark, or an up quark becomes a down quark. When this happens, the neutron emits a W- boson, which then becomes an electron (also called a beta particle) and an electron antineutrino, or the proton emits a W+ boson which becomes a positron (an anti-electron) and an electron neutrino.