Particle interactions and equationsEdexcel International A Level Physics: Subtopic test

1. 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.

   (a)

   Which particle completes the equation n → p + e⁻ + ?

   [1 mark]

   • AElectron antineutrino
   • BElectron neutrino
   • CPositron
   • DPhoton

   (b)

   Which row gives the total charge, baryon number and lepton number of the products of n → p + e⁻ + ν̄ₑ?

   [1 mark]

   • A0, 1, 1
   • B+1, 1, 0
   • C0, 1, 0
   • D0, 0, 0

   (c)

   Show that charge and baryon number are both conserved in the decay n → p + e⁻ + ν̄ₑ.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which reaction is possible and could be used to detect an electron antineutrino?

   [1 mark]

   • Ap → n + e⁻ + ν̄ₑ
   • Bn → p + e⁻ + νₑ
   • Cp + e⁻ → n + ν̄ₑ
   • Dp + ν̄ₑ → n + e⁺

   (b)

   The team considers the reaction n + ν̄ₑ → p + e⁻. Which quantity is not conserved?

   [1 mark]

   • ACharge only
   • BLepton number only
   • CBaryon number only
   • DCharge and baryon number

   (c)

   Deduce whether the reaction p + e⁻ → n + νₑ is possible, showing your working for each quantity.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   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]

   (b)

   Show, using the conservation laws, that the decay π⁺ → μ⁺ + νμ is possible, and state which total is zero on both sides of the equation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   At a particle physics masterclass, students test proposed reactions against the conservation laws of charge, baryon number and lepton number. The proton and neutron each have baryon number +1 and all other particles named here have baryon number 0. The electron and electron neutrino have lepton number +1; the positron and electron antineutrino have lepton number −1. Photons have no charge, baryon number or lepton number.

   (a)

   For each reaction, determine whether it is possible. If it is not, state a quantity that is not conserved. (i) p → e⁺ + γ (ii) p + p → p + n + e⁺ (iii) n + νₑ → p + e⁻

   [6 marks]

   (b)

   In the reactions below, X and Y are single particles. Deduce the identity of X and of Y, and write each complete equation. (i) νₑ + n → p + X (ii) p + p → p + n + Y + νₑ

   [6 marks]

   Total for question 4: 12 marks

End of questions