A topic in the Open Knowledge Graph — a free, open map of 15,495 topics and the order to learn them in.
Particle reactions and decays obey conservation of energy, momentum, angular momentum, and electric charge, plus additional additive quantum numbers. Baryon number (+1 for protons, neutrons, and other baryons, −1 for antibaryons, 0 for mesons and leptons) has never been seen to change. Lepton numbers, counted separately for the electron, muon, and tau families, are conserved in all observed reactions except neutrino oscillations. Strangeness is conserved by the strong and electromagnetic interactions but can change by one unit in weak decays, which is why strange particles live far longer than strong-interaction time scales.
Build a table of particles with columns for charge, baryon number, each lepton number, and strangeness. Then test proposed reactions column by column, marking which law each forbidden reaction breaks. Use observed lifetimes to show how strangeness-changing decays are slow. Finish with the stability of the proton as the most striking consequence of baryon number conservation.
Energy, momentum, angular momentum, and charge are conserved in every reaction ever observed. They do not explain everything, though. A proton could decay into a positron and a neutral pion while satisfying all four, yet no such decay has been seen. Physicists therefore introduced further conserved quantities, each an integer that adds up over all particles in a reaction.
Baryon number B is +1 for each baryon (proton, neutron, Λ, and so on), −1 for each antibaryon, and 0 for mesons, leptons, and photons. In the quark model each quark carries B = 1/3. No experiment has ever seen B change. The proton, as the lightest baryon, has nowhere to go: any decay would lower B. Large detectors such as Super-Kamiokande have watched enormous numbers of protons and place the lifetime above about 10³⁴ years, vastly longer than the age of the universe.
Lepton number comes in three kinds, Lₑ, L_μ, and L_τ. The electron and electron neutrino have Lₑ = +1, and their antiparticles have Lₑ = −1, with the same pattern for the muon and tau families. In neutron decay, n → p + e⁻ + ν̄ₑ, the electron and antineutrino carry Lₑ = +1 and −1, so the total stays zero. This is why the neutral particle emitted in beta-minus decay is an antineutrino. The separate family numbers are broken only by neutrino oscillations, in which a neutrino created as one flavor is later detected as another.
Strangeness S tracks the strange quark, with S = −1 for each s quark and +1 for each anti-s. In the 1950s, particles such as kaons and the Λ⁰ puzzled physicists. They were produced copiously in strong collisions but decayed slowly, with lifetimes near 10⁻¹⁰ s instead of the 10⁻²³ s typical of strong decays. Murray Gell-Mann and Kazuhiko Nishijima explained this with strangeness. It is conserved by the strong interaction, so strange particles are made in pairs, as in π⁻ + p → K⁰ + Λ⁰, where S goes from 0 to +1 − 1 = 0. The lightest strange particles can decay only by changing S, which only the weak interaction does, so their decays are slow.
These rules turn particle physics into bookkeeping that can be checked. For any proposed reaction, add up charge, B, each L, and S on both sides. A strong or electromagnetic process that changes S is forbidden. A process that changes B or the total lepton number has never been observed, and experiments continue to search for one, because some theories beyond the Standard Model predict very rare violations.