The standard model of particle physics is a theory that describes three of the four known fundamental forces (electromagnetic, weak and strong interactions, not including gravitational force) in the Universe, as well as the classification of all known elementary particles.

The fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and electromagnetic interactions, which produce significant long-range forces whose effects can be seen directly in everyday life, and the strong and weak interactions, which produce forces at minuscule, subatomic distances and govern nuclear interactions.

Interaction |
Mediators |
Range |
Particles |

weak | W and Z bozons | 10^{−18} |
neutrinos, leptons, quarks |

electromagnetic | photons | ∞ | leptons, quarks |

strong | gluons | 10^{−15} |
quarks |

All particles will decay to lighter particles unless prevented from doing so by some conservation law. The timescales for these such decays are

- 10
^{-23}s for the strong interaction - 10
^{-16}s for the electromagnetic interaction - 10
^{-13}s – 15 minutes for the weak interaction

The **Electric charge**, Q, the total charge of the particles before and after the interaction takes place is conserved in all interactions.

The **Baryon number** is B = (n_{q} − n_{¯q})/3 where n_{q} is the total number of quarks and n_{¯q} the total number of antiquarks. The Baryon number is conserved in strong, electromagnetic and weak interactions as the total number of quarks and anti-quarks is constant. The proton is the lightest baryon so it cannot decay into any other baryon.

Quarks are assigned B = 1/3, antquarks B = −1/3. Baryons have 3 quarks and so B=+1, antibaryons have B=−1, mesons have B=0, leptons have B=0.

The **Lepton number** is given by L = n_{l} − n_{l¯} where n_{l} is the number of leptons and n_{l¯} the number of anti-leptons. Since a lepton is not made of other leptons we see L = +1 for leptons, L = −1 for antileptons and L = 0 for other particles. L is conserved in strong, electromagnetic and weak interactions.

Leptons can be grouped into families each with a conserved lepton quantum number:

- L
_{e}=+1 for e^{−}, ν_{e}and L_{e}=−1 for e^{+}, ¯ν_{e} - L
_{µ}=+1 for µ^{−}, ν_{µ}and L_{µ}=−1 for µ^{+}, ¯ν_{µ} - L
_{τ}=+1 for τ^{−}, ν_{τ}and L_{τ}=−1 for τ^{+}, ¯ν_{τ}