Hadrons and leptonsAQA A-Level Physics: Subtopic test

1. 1
   In a high-energy collision experiment, protons are fired at protons in a liquid hydrogen target. The physicists check each proposed reaction against the conservation laws before searching for it in the data.

   (a)

   Which of these particles is a baryon?

   [1 mark]

   • APion
   • BKaon
   • CMuon
   • DNeutron

   (b)

   What is the baryon number of an antiproton?

   [1 mark]

   • A−1
   • B0
   • C+1
   • D+2

   (c)

   Explain why the reaction p + p → p + π⁺ cannot occur.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Cosmic rays striking the upper atmosphere produce muons, some of which reach ground-level detectors, where they are observed to decay. A negative muon decays into an electron and two other leptons.

   (a)

   The decay is μ⁻ → e⁻ + X + Y. Which pair of particles X and Y conserves both electron lepton number and muon lepton number?

   [1 mark]

   • A$\bar{\nu}_e$ and $\bar{\nu}_\mu$
   • B$\nu_e$ and $\nu_\mu$
   • C$\bar{\nu}_e$ and $\nu_\mu$
   • D$\nu_e$ and $\bar{\nu}_\mu$

   (b)

   What are the electron lepton number Le and the muon lepton number Lμ of a positron?

   [1 mark]

   • ALe = +1, Lμ = 0
   • BLe = −1, Lμ = 0
   • CLe = 0, Lμ = −1
   • DLe = −1, Lμ = −1

   (c)

   The decay μ⁻ → e⁻ + γ is never observed. Use lepton numbers to explain why.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Physicists study the decays of mesons and baryons produced when high-energy protons strike a target. They record the kaon decay K⁺ → π⁺ + π⁰ and the decay of free neutrons.

   (a)

   Show that charge and baryon number are both conserved in the decay K⁺ → π⁺ + π⁰.

   [3 marks]

   (b)

   Explain why a free neutron decays but the proton is the only stable baryon.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A large international particle physics laboratory fires beams of protons into targets and records the particles produced. Its detectors identify hadrons such as protons, pions and kaons, and leptons such as electrons and muons.

   (a)

   Compare hadrons and leptons, using the proton, pion, electron and muon as examples. Include the interactions that they experience, how hadrons are classified and the quantities that are conserved.

   [6 marks]

   (b)

   Discuss why discoveries of new particles at this laboratory depend on collaboration between large teams of scientists and engineers, and explain how new knowledge is validated before it is accepted.

   [6 marks]

   Total for question 4: 12 marks

End of questions