Quarks and antiquarksAQA A-Level Physics: Subtopic test

1. 1
   A student is learning how hadrons are built from quarks. The only quarks she needs are the up (u), down (d) and strange (s) quarks and their antiquarks.

   (a)

   Which combination of quarks makes a neutron?

   [1 mark]

   • Au u d
   • Bu d d
   • Cu u s
   • Du $\bar{d}$

   (b)

   What are the baryon number and the strangeness of a strange antiquark $\bar{s}$?

   [1 mark]

   • AB = +1/3, S = −1
   • BB = −1/3, S = −1
   • CB = +1/3, S = +1
   • DB = −1/3, S = +1

   (c)

   The π⁺ meson is made of an up quark and an anti-down quark. Show that its charge is +1 and that its baryon number is 0.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   At an accelerator, beams of antiprotons and kaons are produced, and the physicists need to identify the quark structure of the particles they detect.

   (a)

   Which combination of quarks and antiquarks makes an antiproton?

   [1 mark]

   • A$\bar{u}$ $\bar{u}$ $\bar{d}$
   • Bu u d
   • C$\bar{u}$ $\bar{u}$ d
   • D$\bar{d}$ $\bar{d}$ $\bar{u}$

   (b)

   The K⁺ meson is made of an up quark and a strange antiquark. What are its charge and strangeness?

   [1 mark]

   • ACharge +1, strangeness −1
   • BCharge 0, strangeness +1
   • CCharge +1, strangeness +1
   • DCharge −1, strangeness +1

   (c)

   The K⁻ is the antiparticle of the K⁺, which is made of an up quark and a strange antiquark. Deduce the quark composition of the K⁻ and show that its strangeness is −1.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A free neutron decays through the weak interaction: n → p + e⁻ + $\bar{\nu}_e$. At the quark level, one quark in the neutron changes into a different quark.

   (a)

   Identify the change that happens to a quark in the neutron, name the interaction responsible, and show that charge is conserved at the quark level.

   [3 marks]

   (b)

   Use the quark compositions of the neutron and the proton to show that charge and baryon number are both conserved in the decay of the neutron.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the quark model, baryons and mesons are made of up, down and strange quarks and antiquarks. Physicists use the properties of the quarks to decide which combinations correspond to which particles.

   (a)

   Describe the quark structure of pions and kaons, and explain how this accounts for their charge, baryon number and strangeness.

   [6 marks]

   (b)

   A student proposes a particle made of two up quarks and one strange quark (u u s). Determine its charge, baryon number and strangeness, and the same properties of its antiparticle, and classify both particles.

   [6 marks]

   Total for question 4: 12 marks

End of questions