Conservation laws and particle equationsEdexcel A-Level Physics: Mind map

Exam questions on Conservation laws and particle equations

1. In β⁻ decay a neutron in an unstable nucleus changes into a proton, and an electron and a third particle X are emitted. The neutron and proton are baryons with baryon number +1, and the neutron, proton and electron have lepton numbers 0, 0 and +1 respectively.
   Write the particle equation for the decay and show that charge and lepton number are conserved.2 marks
2. A positive pion (π⁺) decays into a positive muon and a muon neutrino, π⁺ → μ⁺ + νμ. The pion is a meson with baryon number 0 and lepton number 0. Muons and neutrinos are leptons with lepton number +1, and their antiparticles have lepton number −1.
   The positive muon decays as μ⁺ → e⁺ + νe + ν̄μ. Show that charge and lepton number are conserved in this decay.2 marks
3. Particle physicists analyse the products of high-energy collisions between protons. Pions are mesons with baryon number 0 and lepton number 0; the π⁺, π⁻ and π⁰ have charges +1 e, −1 e and 0. An antiproton has charge −1 e and baryon number −1.
   In one collision, p + p → p + n + X, where X is a single particle. Use conservation laws to identify X.3 marks