The standard quark-lepton modelEdexcel International A Level Physics: Revision notes
Section 1
Classifying particles
In the standard quark-lepton model every particle is placed in one of four classes. The classification depends on whether a particle is made of quarks and, if so, how many.
- Baryons: made of three quarks, e.g. the proton and the neutron.
- Mesons: made of a quark and an antiquark, e.g. pions.
- Leptons: fundamental particles, not made of anything smaller, e.g. the electron and the neutrino.
- Photons: the quanta of electromagnetic radiation.
Baryons and mesons are built from quarks. Leptons and photons are not.
A particle can be classified from its structure alone: three quarks means baryon, quark plus antiquark means meson, no quark structure means lepton or photon.
Section 2
Baryons and mesons
A baryon contains three quarks, so protons and neutrons are baryons. A meson contains one quark and one antiquark. The positive pion, for example, is made of an up quark and an anti-down quark.
Because baryons and mesons are made of quarks, they are not fundamental particles.
Do not call a pion a baryon. It has only two constituents, a quark and an antiquark, so it is a meson.
Section 3
Leptons and photons
Leptons are fundamental, so they have no internal quark structure. The electron and the neutrino are the examples named in the specification. The electron, muon and tau each form a pair with their own neutrino, giving six leptons in all.
Photons are the quanta of electromagnetic radiation and are fundamental particles in their own class: they are neither quarks nor leptons.
Section 4
Symmetry and the top quark
The model has six leptons in three pairs. Its symmetry suggests that there should be six quarks, also in three pairs.
When only five quarks were known (up, down, strange, charm and bottom), the bottom quark had no partner. This predicted a sixth quark, the top quark. It has a very large mass, so very high-energy collisions were needed to make it, and it was discovered in 1995.
A model that makes a correct prediction before the experiment is supported by evidence. It is not proved.
Section 5
Antiparticles
Every particle has a corresponding antiparticle. An antiparticle has the same mass as its particle, but its charge, baryon number and lepton number are all opposite.
- Electron: charge −e, lepton number +1. Positron: charge +e, lepton number −1.
- Proton: charge +e, baryon number +1. Antiproton: charge −e, baryon number −1.
- Neutral particles still differ: the neutron and antineutron have baryon numbers +1 and −1, and the neutrino and antineutrino have lepton numbers +1 and −1.
To deduce an antiparticle's properties, keep the mass and reverse the sign of every other quantity. The same method works backwards from an antiparticle to its particle.
Do not say a neutral particle has no antiparticle. The neutron and antineutron differ in baryon number even though both have zero charge.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The standard quark-lepton model
- A detector team at a particle accelerator is sorting the particles recorded in a set of collision events into the groups of the standard quark-lepton model. The particles identified were a proton, a neutron, a positive pion, an electron, an electron neutrino and a gamma-ray photon.A positive pion is made of one up quark and one anti-down quark. Explain why the positive pion is classified as a meson and not as a baryon.2 marks
- A research group studying antimatter produces antiprotons in a collision experiment and stores them in a magnetic trap. A separate source supplies positrons, which are the antiparticles of electrons. The group compares the properties of each antiparticle with those of its particle.Use the properties of the electron to deduce the charge and the lepton number of a positron.2 marks
- By 1977 experiments had identified five quarks (up, down, strange, charm and bottom) and six leptons: the electron, the muon and the tau, each paired with its own neutrino. Physicists noticed that the quarks did not fit the same pattern as the leptons. In 1995 a sixth quark, the top quark, was discovered, with a mass far greater than that of any other known quark.Explain how the symmetry of the standard quark-lepton model led physicists to predict the existence of a sixth quark.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).