Conservation laws in particle interactions

Not every reaction that conserves energy and momentum is actually allowed to happen. Particle physics imposes a further set of conservation laws that any proposed reaction must satisfy simultaneously, and a reaction violating even one of them is strictly forbidden, regardless of how much energy is available to drive it — these laws function as a checklist that a proposed process must pass in full before it can be taken seriously as physically possible.

Some of these conservation laws are familiar from earlier physics: total energy and total momentum must both be conserved in every particle interaction, exactly as in classical mechanics, now applied relativistically. Electric charge conservation is similarly strict — the total electric charge before a reaction must exactly equal the total charge after, with no exceptions observed in any interaction ever recorded.

Beyond these familiar quantities, particle physics introduces additional conserved quantum numbers specific to the particle world. Baryon number counts baryons (particles like protons and neutrons, built from three quarks) as +1 each and antibaryons as −1, and this total must be conserved in every known interaction. This law has a strikingly direct experimental consequence: since a proton is the lightest baryon, and baryon number conservation forbids it from decaying into any combination of lighter, non-baryonic particles without violating the baryon count, the proton is observed to be extraordinarily stable — experiments have searched for proton decay and found none, placing an enormous lower bound on the proton's lifetime, consistent with baryon number being an exactly conserved quantity in the Standard Model. Lepton number plays an analogous role for leptons (electrons, muons, taus and their neutrinos), similarly forbidding reactions that would change the total lepton count.

Together, these conservation laws are what make it possible to predict, before ever running an experiment, which particle reactions can occur and which cannot — a proposed decay or collision product is ruled out instantly if it fails to conserve even one of energy, momentum, charge, baryon number, or lepton number, regardless of how energetically favourable it might otherwise appear.