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Particle Physics: Question 7

Syllabus 11.2

Structured AS 7 marks

A physics teacher lists five particles on the board for a classification exercise: a proton, an electron, a neutron, a π+\pi^+ meson (quark composition udˉu\bar{d}, i.e. an up quark bound to a down antiquark), and a muon-neutrino.

(a) State which of these five particles are leptons and which are hadrons. Give a reason for your classification, based on whether each type of particle has an internal quark composition. [2]

(b) Among the hadrons identified in (a), state which are baryons and which are mesons. Give a reason for your classification, based on the number and type of quarks each contains. [3]

(c) State one similarity and one difference between the electron and the proton, in terms of their internal structure. [2]

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Worked solution

Part (a): Leptons versus hadrons

Leptons are fundamental particles, they have no internal quark composition. Of the five particles listed, the electron and the muon-neutrino are leptons.

Hadrons are particles built from quarks. Of the five particles listed, the proton, the neutron and the π+\pi^+ meson are hadrons, since each is composed of quarks (or a quark and an antiquark).

Part (b): Baryons versus mesons

Among the three hadrons identified above:

  • The proton (uuduud) and the neutron (uddudd) are baryons, because each is built from exactly three quarks.
  • The π+\pi^+ meson (udˉu\bar{d}) is a meson, because it is built from a quark-antiquark pair (one quark and one antiquark), not three quarks.

Part (c): Electron versus proton. Internal structure

Similarity: both the electron and the proton are charged particles, with charges of equal magnitude but opposite sign.

Difference: the electron is a fundamental lepton. It has no smaller constituents and no internal quark composition. The proton, by contrast, is a composite hadron (specifically a baryon), built from three quarks (uuduud) bound together.

Final answers

  • (a) Leptons: electron, muon-neutrino (no quark composition). Hadrons: proton, neutron, π+\pi^+ (built from quarks).
  • (b) Baryons: proton (uuduud), neutron (uddudd), three quarks each. Meson: π+\pi^+ (udˉu\bar{d}), a quark-antiquark pair.
  • (c) Similarity: both carry equal-magnitude charge. Difference: the electron has no internal quark structure; the proton is composed of three quarks.