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Classification of particles questions

The many known particles sort into a short hierarchy. Hadrons feel the strong interaction and divide into baryons and mesons; leptons are fundamental and do not feel it. Baryon number, lepton number and strangeness are the quantum numbers that keep the classification consistent.

20 original questions · 55 marks · the classification of particles notes · Particles

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2 marks · state · warm-up
State the two groups into which particles are classified according to whether they experience the strong interaction, and identify which group does.
Mark scheme
Hadrons and leptons (1). Hadrons feel the strong interaction; leptons do not (1). Hadrons are further split into baryons and mesons.
2 marks · warm-up
Distinguish between a baryon and a meson.
Mark scheme
A baryon is a hadron made of three quarks, baryon number +1, such as the proton or neutron (1). A meson is a hadron made of a quark and an antiquark, baryon number 0, such as a pion or kaon (1).
2 marks · give · warm-up
Give two examples of leptons.
Mark scheme
Any two of the following, one mark each (2): the electron; the muon; the electron neutrino; the muon neutrino; or any of their antiparticles.
1 mark · state · warm-up
State the number of quarks in a baryon and in a meson.
Mark scheme
A baryon is made of three quarks; a meson is a quark and an antiquark (1).
2 marks · warm-up
The strong nuclear force between nucleons is carried by an exchange particle. Name this particle and state the class of hadron to which it belongs.
Mark scheme
The pion (1). It is a meson (1).
2 marks · state · warm-up
State what a kaon decays into, and name the interaction responsible for the decay.
Mark scheme
A kaon decays into pions (also accept muons and neutrinos) (1). The decay proceeds by the weak interaction (1).
3 marks · state
State the baryon number of a baryon, an antibaryon and a meson, and explain how baryon number is used.
Mark scheme
Baryon +1, antibaryon −1 (1), meson 0 (1). Baryon number is a conserved quantity: in any allowed interaction the total baryon number before equals the total afterwards (1).
3 marks · state
State the lepton number of an electron, a positron and an electron antineutrino.
Mark scheme
Electron: L_e = +1 (1). Positron: L_e = −1 (1). Electron antineutrino: L_e = −1 (1).
4 marks
A free neutron decays: n → p + e⁻ + anti-ν_e. Show that this decay conserves both baryon number and lepton number.
3 marks · describe
Describe how strange particles are produced and how they decay, and state what this implies about strangeness.
3 marks
A muon decays by μ⁻ → e⁻ + anti-ν_e + ν_μ. Show that both electron lepton number and muon lepton number are conserved in this decay.
3 marks
A student claims that the muon must be a meson because its mass lies between the mass of the electron and the mass of the proton. Explain why the muon is in fact a lepton.
3 marks · explain
Explain why the proton is the only stable free baryon.
2 marks · state
State two differences between a kaon and a pion.
4 marks · stretch
A proposed interaction is p + e⁻ → n + ν_e. By checking charge, baryon number and lepton number, state whether it is allowed.
3 marks · stretch
Classify each of these particles as a baryon, a meson or a lepton: proton, pion, muon, neutron, electron neutrino.
3 marks · state · stretch
State two properties that distinguish a lepton from a hadron.
4 marks · stretch
In a cosmic-ray experiment a new particle X is observed. X is only ever created together with a second new particle, in collisions that occur on strong-interaction timescales, but X decays into pions comparatively slowly, with a lifetime of about 10⁻¹⁰ s. Deduce, with reasons, whether X is a strange particle, and name the interaction by which it decays.
3 marks · explain · stretch
Explain why the interaction ν_μ + n → e⁻ + p is never observed, even though it conserves both charge and baryon number.
3 marks · stretch
A particle has baryon number 0, charge −1 and strangeness 0, and it experiences the strong interaction. Deduce its classification, and name a particle it could be.

The same practice on paper: the printable workbook for this topic, questions and a worked answer book.

Practise classification of particles one question at a time

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