Quarks, leptons and hadronsEdexcel A-Level Physics: Subtopic test

1. 1
   Particle physicists classify the particles detected in high-energy collisions using the standard quark–lepton model. The up quark has charge +2/3 e, and the down and strange quarks have charge −1/3 e. Every quark has baryon number +1/3, and the proton is made of two up quarks and one down quark (uud).

   (a)

   A baryon called the sigma-plus has quark composition uus. What is its charge?

   [1 mark]

   • A+5/3 e
   • B+1/3 e
   • C+1 e
   • D−1/3 e

   (b)

   Which of the following is a meson?

   [1 mark]

   • AA pion, made of an up quark and a down antiquark (u d̄)
   • BA proton, made of three quarks (uud)
   • CAn antiproton, made of three antiquarks (ū ū d̄)
   • DA muon

   (c)

   Show that the quark composition of the proton gives it a charge of +1 e and a baryon number of +1.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The omega-minus (Ω⁻) was first detected in a bubble chamber experiment in 1964. It is a baryon made of three strange quarks (sss). A strange quark has charge −1/3 e and baryon number +1/3. Every particle has an antiparticle.

   (a)

   What is the quark composition of the antiparticle of the Ω⁻?

   [1 mark]

   • AThree strange quarks (sss)
   • BThree up antiquarks (ū ū ū)
   • COne strange quark and one strange antiquark (s s̄)
   • DThree strange antiquarks (s̄ s̄ s̄)

   (b)

   The Ω⁻ is described as a baryon. What is a baryon?

   [1 mark]

   • AA lepton made of three quarks
   • BA hadron made of three quarks
   • CA hadron made of a quark and an antiquark
   • DA particle that does not experience the strong interaction

   (c)

   Deduce the charge and the baryon number of the antiparticle of the Ω⁻.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   By 1977 physicists had identified five quarks (up, down, strange, charm and bottom) and six leptons (the electron, muon and tau, and their three neutrinos). The sixth quark, the top quark, was not discovered until 1995 at Fermilab. Its charge is +2/3 e.

   (a)

   Explain how symmetry in the standard model led physicists to predict the existence of the top quark before it was discovered.

   [3 marks]

   (b)

   A K⁺ meson has quark composition u s̄, where the strange antiquark has charge +1/3 e. Determine the charge and baryon number of the K⁺, and the quark composition and charge of the K⁻, its antiparticle.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hadrons are particles that experience the strong interaction; leptons do not. Protons, neutrons and pions are hadrons, while electrons and neutrinos are leptons. The neutron has quark composition udd. An antiquark has the opposite charge and baryon number to its quark.

   (a)

   Compare the properties of baryons, mesons and leptons, using the proton, the π⁺ and the electron as examples.

   [6 marks]

   (b)

   The antineutron is neutral, like the neutron. Explain why it is nevertheless a different particle from the neutron, and deduce its quark composition, charge and baryon number.

   [6 marks]

   Total for question 4: 12 marks

End of questions