Conservation of Charge, Baryon Number and Lepton Number

In every particle interaction and decay, three quantities are always conserved: charge Q, baryon number B and lepton number L. The total of each before must equal the total after. If any one is not conserved, the reaction cannot happen.Values to learn:• quarks B = +1/3, antiquarks B = −1/3
• baryons (p, n) B = +1, antibaryons B = −1, mesons B = 0
• leptons (e⁻, ν_e) L = +1; antileptons (e⁺, ν_e) L = −1
• hadrons have L = 0; leptons have B = 0Worked example 1: check β⁻ decay, n → p + e⁻ + ν_e.Q: 0 → +1 − 1 + 0 = 0   conserved
B: +1 → +1 + 0 + 0 = +1   conserved
L: 0 → 0 + 1 − 1 = 0   conservedThe decay is allowed. This is why the particle emitted with the electron must be an antineutrino: an ordinary neutrino would make L = 2 after the decay.Worked example 2: could a proton decay by p → π⁰ + e⁺? Charge: +1 → 0 + 1, conserved. Baryon number: +1 → 0 + 0, not conserved. So the decay is forbidden, which fits with the proton being stable.Always check all three quantities in turn, and write the totals for each side.