Particle interactions and equationsEdexcel International A Level Physics: Flashcards

Exam questions on Particle interactions and equations

1. Free neutrons are released inside a research reactor. A free neutron is unstable and decays into a proton, an electron and one other particle, which is almost impossible to detect. Physicists use the conservation laws to work out what that third particle must be. The electron has lepton number +1 and the electron neutrino has lepton number +1; their antiparticles have lepton number −1.
   Show that charge and baryon number are both conserved in the decay n → p + e⁻ + ν̄ₑ.2 marks
2. A detector next to a nuclear reactor identifies reactor antineutrinos by the reaction they cause with protons in a large water tank. The team also considers other reactions that might, in principle, be used. In these questions the proton and neutron each have baryon number +1, the electron and the electron neutrino each have lepton number +1, and the positron and the electron antineutrino each have lepton number −1.
   Deduce whether the reaction p + e⁻ → n + νₑ is possible, showing your working for each quantity.2 marks
3. A cosmic-ray laboratory records two decays. In the first, a proton in a neutron-deficient nucleus changes into a neutron and emits a positron. In the second, a positive pion (a meson) decays into an antimuon, μ⁺, and a muon neutrino, νμ. Muons and muon neutrinos have lepton number +1; antimuons have lepton number −1. The electron has lepton number +1 and the positron −1.
   Write the particle equation for the first decay, and use lepton number to justify the type of neutrino that must be emitted with the positron.3 marks