Particle reactions are constrained by conserved quantities associated with symmetries and interaction structure. Energy, momentum, angular momentum and electric charge must be conserved in every isolated process.

Additional bookkeeping quantum numbers are useful. Baryon number assigns approximately $+1$ to baryons, $-1$ to antibaryons and $0$ to leptons and mesons. Lepton number distinguishes leptons from antileptons, with flavor-specific versions useful in many low-energy processes. In the Standard Model, some of these classical conservation rules are approximate or emerge in subtler combinations, but they remain powerful for checking ordinary reactions.

Consider neutron beta decay: $$n\rightarrow p+e^-+\bar\nu_e.$$ Electric charge is conserved: $$0=(+1)+(-1)+0.$$ Baryon number remains $1=1$. Electron-family lepton number is initially zero and finally $$0=(+1)+(-1).$$

A proposed decay such as $$p\rightarrow e^++\gamma$$ conserves electric charge but would change baryon number. It is therefore forbidden in the Standard Model and would signal new physics if observed.

Conservation laws do more than balance arithmetic after a reaction is known. They eliminate impossible processes, constrain allowed final states and reveal which symmetries or interactions must be involved. Particle physics relies heavily on this logic because short-lived states are often inferred from their decay products rather than observed directly.