Quarks, leptons and hadronsEdexcel A-Level Physics: Revision notes
Section 1
The standard quark–lepton model
The standard model groups the fundamental particles into quarks, leptons and the exchange particles such as the photon.
There are six quarks in three pairs: up and down, charm and strange, top and bottom. There are six leptons in three pairs: the electron and its neutrino, the muon and its neutrino, the tau and its neutrino.
Quarks have fractional charges and baryon number +1/3. Leptons have lepton number +1 and baryon number 0. The photon carries the electromagnetic interaction and has zero charge, baryon number and lepton number.
Section 2
Hadrons: baryons and mesons
Hadrons are the particles made of quarks. They experience the strong interaction.
- A baryon is made of three quarks (qqq), e.g. the proton (uud) and neutron (udd). Baryon number +1.
- A meson is made of a quark and an antiquark (q q̄), e.g. the pion π⁺ (u d̄). Baryon number 0.
Leptons are not made of quarks and do not feel the strong interaction.
Quark charges: up, charm, top +2/3 e; down, strange, bottom −1/3 e. Antiquarks have the opposite charge.
Adding quark charges as whole numbers. Use fractions: uud is 2/3 + 2/3 − 1/3 = +1, not +3 − 1.
Section 3
Deducing properties from quark composition
Add the charges and baryon numbers of the quarks.
Worked example: the Σ⁺ is uus. Charge = 2/3 + 2/3 − 1/3 = +1 e. Baryon number = 3 × 1/3 = +1, so it is a baryon.
The K⁺ is u s̄. Charge = +2/3 + 1/3 = +1 e. Baryon number = 1/3 − 1/3 = 0, so it is a meson.
To identify a hadron: three quarks means a baryon, three antiquarks an antibaryon, one quark and one antiquark a meson.
Section 4
Antiparticles
Every particle has an antiparticle with the same rest mass but opposite charge, baryon number and lepton number. Its quark composition is the matching antiquarks.
- Proton uud: antiproton ū ū d̄, charge −1, baryon number −1
- Neutron udd: antineutron ū d̄ d̄, charge 0, baryon number −1
- Electron: positron, charge +1, lepton number −1
- Electron neutrino: electron antineutrino, lepton number −1
The antineutron is neutral but is not the neutron. The photon is its own antiparticle.
To find an antiparticle's properties, reverse the sign of every charge and quantum number. Mass stays the same.
Section 5
Symmetry and the top quark
The six leptons fall into three pairs (generations). Physicists expected the quarks to follow the same pattern of three pairs. By 1977 five quarks had been found (up, down, charm, strange and bottom), so the bottom quark's partner was missing.
This symmetry predicted a sixth quark, the top quark (charge +2/3 e), which was found in 1995. Predictions based on patterns and symmetry are a major strength of the model.
Must know
- Six quarks and six leptons in three pairs, plus the photon
- Baryon = qqq (baryon number +1); meson = q q̄ (baryon number 0); both are hadrons
- Quark charges are ±2/3 e and ±1/3 e; add them as fractions
- Antiparticle: same mass, opposite charge, baryon and lepton number
- Symmetry predicted the top quark
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Quarks, leptons and hadrons
- 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).Show that the quark composition of the proton gives it a charge of +1 e and a baryon number of +1.2 marks
- 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.Deduce the charge and the baryon number of the antiparticle of the Ω⁻.2 marks
- 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.Explain how symmetry in the standard model led physicists to predict the existence of the top quark before it was discovered.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).