Questions on classification of particles

Question 1

State the two groups into which particles are classified according to whether they experience the strong interaction, and identify which group does.

Accepted Answer

Hadrons and leptons (1). Hadrons feel the strong interaction; leptons do not (1). Hadrons are further split into baryons and mesons.

Question 2

Distinguish between a baryon and a meson.

Accepted Answer

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).

Question 3

Give two examples of leptons.

Accepted Answer

Any two of the following, one mark each (2): the electron; the muon; the electron neutrino; the muon neutrino; or any of their antiparticles.

Question 4

State the number of quarks in a baryon and in a meson.

Accepted Answer

A baryon is made of three quarks; a meson is a quark and an antiquark (1).

Question 5

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.

Accepted Answer

The pion (1). It is a meson (1).

Question 6

State what a kaon decays into, and name the interaction responsible for the decay.

Accepted Answer

A kaon decays into pions (also accept muons and neutrinos) (1). The decay proceeds by the weak interaction (1).

Question 7

State the baryon number of a baryon, an antibaryon and a meson, and explain how baryon number is used.

Accepted Answer

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).

Question 8

State the lepton number of an electron, a positron and an electron antineutrino.

Accepted Answer

Electron: L_e = +1 (1). Positron: L_e = −1 (1). Electron antineutrino: L_e = −1 (1).

Question 9

A free neutron decays: n → p + e⁻ + anti-ν_e. Show that this decay conserves both baryon number and lepton number.

Accepted Answer

Baryon number before: 1 (1)
Baryon number after: 1 + 0 + 0 = 1, so it is conserved (1)
Lepton number before: 0 (1)
Lepton number after: 0 + (+1) + (−1) = 0, the electron having L_e = +1 and the antineutrino L_e = −1 (1)

Question 10

Describe how strange particles are produced and how they decay, and state what this implies about strangeness.

Accepted Answer

Strange particles are produced in pairs through the strong interaction (1), but decay through the weak interaction (1). Strangeness is conserved in strong interactions but can change by 0 or ±1 in weak interactions (1).

Question 11

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.

Accepted Answer

Muon lepton number: 1 → 0 + 0 + 1 = 1 ✓ (1). Electron lepton number: 0 → (+1) + (−1) + 0 = 0 ✓ (1). Each family balances separately, as required, so the decay is allowed (1).

Question 12

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.

Accepted Answer

Particles are classified by the interactions they experience, not by their mass (1). The muon does not feel the strong interaction (1) and it is fundamental, not made of quarks, so it belongs with the leptons (1).

Question 13

Explain why the proton is the only stable free baryon.

Accepted Answer

Baryon number is conserved in every interaction (1), so any decaying baryon must leave a baryon among its products (1). The proton is the lightest baryon, so there is nothing lighter for it to decay into, while every other free baryon eventually decays, in steps, to a proton (1).

Question 14

State two differences between a kaon and a pion.

Accepted Answer

A kaon has strangeness +1 or −1, whereas a pion has strangeness 0 (1). Kaons decay into pions, by the weak interaction, whereas the pion is the exchange particle of the strong force between nucleons (1).

Question 15

A proposed interaction is p + e⁻ → n + ν_e. By checking charge, baryon number and lepton number, state whether it is allowed.

Accepted Answer

Charge: (+1) + (−1) = 0 → 0 + 0 = 0, conserved (1)
Baryon number: 1 + 0 = 1 → 1 + 0 = 1, conserved (1)
Lepton number: 0 + (+1) = +1 → 0 + (+1) = +1, conserved (1)
All three are conserved, so the interaction is allowed (1)

Question 16

Classify each of these particles as a baryon, a meson or a lepton: proton, pion, muon, neutron, electron neutrino.

Accepted Answer

Proton: baryon. Pion: meson (1). Muon: lepton (1). Neutron: baryon. Electron neutrino: lepton (1).

Question 17

State two properties that distinguish a lepton from a hadron.

Accepted Answer

Leptons are fundamental (not made of quarks) (1) and do not feel the strong interaction (1); hadrons are made of quarks and do feel the strong interaction (1).

Question 18

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.

Accepted Answer

Creation in pairs by the strong interaction is the behaviour of strange particles (1): the strong interaction conserves strangeness, so it can only make strangeness +1 and −1 together (1). The comparatively slow decay shows the decay is by the weak interaction, the only interaction that can change strangeness (1). So X is a strange particle (for example a kaon) and it decays by the weak interaction (1).

Question 19

Explain why the interaction ν_μ + n → e⁻ + p is never observed, even though it conserves both charge and baryon number.

Accepted Answer

Lepton number must be conserved separately for each family (1). Muon lepton number: 1 → 0, so it is not conserved (1). Electron lepton number: 0 → +1, also not conserved; either failure on its own forbids the interaction (1).

Question 20

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.

Accepted Answer

It feels the strong interaction, so it is a hadron (1). Its baryon number is 0, so it must be a meson (1). A meson with charge −1 and zero strangeness is the π⁻ (1).