Particle interactions (collisions, decays and annihilations) must obey a set of conservation laws. In every interaction the following are conserved:• charge
• baryon number
• lepton number
• mass-energy
• momentumTo test whether an interaction is possible, add up each quantity on the left and on the right. An interaction is only possible if all of the conservation laws are obeyed. If even one fails, it cannot happen.Mass-energy: rest mass is not conserved on its own. Kinetic energy can be converted into the rest mass of new particles (using ΔE = c2Δm), and rest mass can be converted into kinetic energy. A particle can only decay spontaneously if the total rest mass of the products is less than the rest mass of the parent; the difference appears as kinetic energy.Momentum: when a stationary electron and positron annihilate, the total momentum is zero, so two photons are produced travelling in opposite directions. A single photon could not have zero momentum.Worked example: check charge in β− decay, n → p + e− + ν̄e.left: 0 right: (+1) + (−1) + 0 = 0 Charge is conserved. Baryon number and lepton number are checked in exactly the same way, and the values used for each particle are set out in the next lesson.
• baryon number
• lepton number
• mass-energy
• momentumTo test whether an interaction is possible, add up each quantity on the left and on the right. An interaction is only possible if all of the conservation laws are obeyed. If even one fails, it cannot happen.Mass-energy: rest mass is not conserved on its own. Kinetic energy can be converted into the rest mass of new particles (using ΔE = c2Δm), and rest mass can be converted into kinetic energy. A particle can only decay spontaneously if the total rest mass of the products is less than the rest mass of the parent; the difference appears as kinetic energy.Momentum: when a stationary electron and positron annihilate, the total momentum is zero, so two photons are produced travelling in opposite directions. A single photon could not have zero momentum.Worked example: check charge in β− decay, n → p + e− + ν̄e.