The standard quark-lepton modelEdexcel International A Level Physics: Subtopic test

1. 1
   A detector team at a particle accelerator is sorting the particles recorded in a set of collision events into the groups of the standard quark-lepton model. The particles identified were a proton, a neutron, a positive pion, an electron, an electron neutrino and a gamma-ray photon.

   (a)

   Which of the particles recorded is a meson?

   [1 mark]

   • AProton
   • BNeutron
   • CPositive pion
   • DElectron neutrino

   (b)

   Which row gives a lepton followed by a baryon from the recorded particles?

   [1 mark]

   • AElectron, neutron
   • BProton, electron
   • CElectron neutrino, positive pion
   • DGamma-ray photon, neutron

   (c)

   A positive pion is made of one up quark and one anti-down quark. Explain why the positive pion is classified as a meson and not as a baryon.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A research group studying antimatter produces antiprotons in a collision experiment and stores them in a magnetic trap. A separate source supplies positrons, which are the antiparticles of electrons. The group compares the properties of each antiparticle with those of its particle.

   (a)

   Which statement about an antiproton is correct?

   [1 mark]

   • AIt has charge +e and the same mass as a proton
   • BIt has charge −e and the same mass as a proton
   • CIt has charge −e and a smaller mass than a proton
   • DIt has no charge and the same mass as a proton

   (b)

   Which pair consists of a particle and its antiparticle?

   [1 mark]

   • AProton and neutron
   • BElectron and electron neutrino
   • CPositive pion and proton
   • DPositive pion and negative pion

   (c)

   Use the properties of the electron to deduce the charge and the lepton number of a positron.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   By 1977 experiments had identified five quarks (up, down, strange, charm and bottom) and six leptons: the electron, the muon and the tau, each paired with its own neutrino. Physicists noticed that the quarks did not fit the same pattern as the leptons. In 1995 a sixth quark, the top quark, was discovered, with a mass far greater than that of any other known quark.

   (a)

   Explain how the symmetry of the standard quark-lepton model led physicists to predict the existence of a sixth quark.

   [3 marks]

   (b)

   The top quark was discovered in 1995. Suggest why this discovery was strong evidence for the model, and why it was found so much later than the other quarks.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A museum exhibit sorts particles into four drawers labelled baryons, mesons, leptons and photons. Visitors are given cards for a neutron, a negative pion, an electron antineutrino and a gamma-ray photon, and must decide which drawer each card belongs in. A student helping to write the exhibit labels writes: 'An antiparticle has exactly the same properties as its particle, so the only difference is the name.' The student's notes list the proton and antiproton, the electron and positron, the neutron and antineutron, and the electron neutrino and electron antineutrino as examples.

   (a)

   Explain which drawer each of the four particles belongs in, justifying each choice from the structure or nature of the particle.

   [6 marks]

   (b)

   Evaluate the student's statement, using the four pairs listed.

   [6 marks]

   Total for question 4: 12 marks

End of questions