The standard quark-lepton modelEdexcel International A Level Physics: Flashcards

Exam questions on The standard quark-lepton model

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 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
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.
   Use the properties of the electron to deduce the charge and the lepton number of a positron.2 marks
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.
   Explain how the symmetry of the standard quark-lepton model led physicists to predict the existence of a sixth quark.3 marks