The standard model of hadrons and leptons is a united set of quantum mechanical theories encompassing electromagnetism; the weak force, which is responsible for beta decay; and the strong force, which...The standard model of hadrons and leptons is a united set of quantum mechanical theories encompassing electromagnetism; the weak force, which is responsible for beta decay; and the strong force, which holds atomic nuclei together. The lepton number is the number of leptons minus the number of antileptons, with a similar definition for baryon number.