The standard quark-lepton model (Topic 8B)
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According to the standard quark-lepton model, all particles can be classified as either hadrons or leptons:
- Hadrons experience the strong interaction (strong nuclear force) and are composed of smaller particles called quarks.
- Leptons do not experience the strong interaction and are fundamental (this means they cannot be broken down into anything smaller).
Other examples of hadrons include neutrons, pions and kaons.
Other examples of leptons include muons, electron neutrinos and muon neutrinos.
Hadrons are particles composed of fundamental particles called quarks.
Fundamental particles have no internal structure: they cannot be divided into smaller constituents.
Quarks do not exist in isolation, but only in pairs or in groups of three (or higher for exotic particles) to form hadrons:
- Hadrons experience the strong nuclear force.
- Hadrons decay by the weak nuclear force and also experience the electromagnetic force if charged.
- There are two classes of hadrons: baryons and mesons.
It is useful to know that exotic hadrons consisting of four or more quarks have been observed in particle colliders, but they are extremely unstable and short-lived.
Hadrons are divided into two groups: baryons and mesons.
- Baryons are made of three quarks.
- Mesons are made of one quark and one antiquark.
Protons are the only stable baryons. All other baryons eventually decay into lighter particles, making protons unique among the baryons studied at A-level.
It is useful to note that some scientists hypothesise that protons will eventually decay, but this has never actually been observed, and current experiments establish the lower limit on the proton’s lifetime to be greater than years. Moreover, exotic hadrons consisting of four or more quarks have been observed in particle colliders, but they are extremely unstable and short-lived.
Mesons are hadrons made from one quark and one antiquark. They can have positive, negative or zero charge, depending on their quark composition. Mesons have a baryon number of 0.
The two main mesons considered are pions and kaons. Both pions and kaons are unstable particles that decay into other particles. They are short-lived particles that often appear when high-energy collisions briefly create new matter.
It is important to note that kaons are strange particles. They are produced by the strong interaction but decay via the weak interaction, making them important examples in the study of conservation laws.
Leptons are fundamental particles that are not subject to the strong interaction.
All leptons experience gravity and the weak interaction. Charged leptons, such as electrons and muons, also experience the electromagnetic interaction.
Every lepton has a corresponding antilepton with the same mass but opposite charge and lepton number.
The muon has the same charge as the electron but is approximately 200 times more massive. Unlike the electron, it is unstable and eventually decays into an electron and neutrinos.
Electron neutrinos and muon neutrinos and their corresponding antiparticles have no charge and a very small mass.
Examiners want the top quark presented as a prediction, not just a fact. The standard quark-lepton model rests on a symmetry between quarks and leptons: both fall into three generations of two, as in the table below:
| Generation | Up-type quark | Down-type quark | Charged lepton | Neutrino |
|---|---|---|---|---|
| 1st | (up) | (down) | (electron) | |
| 2nd | (charm) | (strange) | (muon) | |
| 3rd | (top) [predicted] | (bottom) | (tau) |
Six leptons were known, so by symmetry there should be six quarks. When only five had been found in 1977 (up, down, strange, charm, bottom), the bottom quark had no partner. The symmetry therefore predicted a sixth quark: the top, to complete the third generation, and its later discovery in 1995 confirmed the model.
The Universe is composed of matter particles such as protons, neutrons, and electrons.
All matter particles have corresponding antimatter particles, which are identical in most properties but have the opposite charge:
- If a matter particle is positively charged, its antimatter counterpart is negatively charged, and vice versa.
- If a matter particle has zero charge, its antiparticle also has zero charge.
Common matter–antimatter particle pairs are shown in the figure below.
For particles other than electrons, their antiparticle counterpart has the same name, named with the prefix “anti-” and is symbolised by the same letter with a bar above it (e.g. for antiproton).
Be careful not to confuse neutral particles, like the neutron and neutrino, with their antiparticles.
Antiparticles are written with a bar over the symbol, so do not rely solely on the charge to identify a particle or antiparticle.
Antimatter particles have the same mass as their corresponding matter particles.
Charged antimatter particles have an equal and opposite charge to their corresponding matter particles.
The electron and positron have the same mass but an opposite charge:
The proton and antiproton also have the same mass but an opposite charge.
The rest mass–energy of a particle is the energy equivalent to the mass of the particle when it is at rest.
It can be calculated using Einstein’s famous equation:
Where:
- is the rest mass–energy,
- is the particle’s rest mass, and
- is the speed of light
Rest mass–energy is important for calculations related to particle interactions, in which mass can be converted to energy.
Rest mass–energy has SI units of joules (J) but is often expressed in for convenience.
The table below provides the mass in and the rest mass–energy in for a proton, neutron, electron and neutrino.
The rest mass–energy for the neutrino is which is on the scale of magnitudes smaller than the other particles in the above table.
The following conservation laws apply to particle decays:
- Charge conservation: the total charge before and after a decay must be the same.
- Lepton number conservation: the total lepton number is conserved (within a family of leptons).
- Baryon number conservation: the total baryon number remains unchanged during a decay.
Baryon number is a quantum number used to describe particle interactions. Quantum numbers are quantities that must be conserved in particle reactions.
Charge is an example of a conserved quantity that we have already encountered.
Baryon number is defined as:
- baryons: +1
- antibaryons: −1
- all other particles: 0
The total baryon number before and after an interaction must remain the same. If it does not, the interaction is not possible.
Lepton number is a quantum number that must be conserved in particle interactions. In particle physics, lepton number is split into two independent quantities:
Electron lepton number ():
- Electron and electron neutrino:
- Positron and antielectron neutrino:
Muon lepton number ():
- Muon and muon neutrino:
- Antimuon and antimuon neutrino:
All non-leptons have lepton number 0. Electron and muon lepton numbers must be conserved separately in any interaction.
Lepton number conservation is a key rule used to determine whether particle interactions are possible.
Muon decay is an important example because it always produces an electron and neutrinos to conserve both the electron and muon lepton numbers.