The Standard Model (Edexcel International A Level Physics)

Baryons & Mesons

• All particles of matter are made up of either quarks and/or leptons

  • The standard model of particle physics categorises quarks and leptons by charge and mass

Quarks and leptons form the standard model of particle physics. The first generation of particles make up all ordinary matter

• Hadrons are made up of quarks and interact with the strong nuclear force

• Baryons and mesons are types of hadron

  • Baryons consist of 3 quarks

  • Mesons consist of a quark-antiquark pair

• The most common baryons are protons and neutrons

• The most common mesons are pions and kaons

Hadrons may be either a baryon or a meson. Both baryons and mesons interact with the strong nuclear force

Anti-hadrons may be either an anti-baryon or an anti-meson

• Quarks have never been discovered on their own, always in pairs or groups of three

• Note that all baryons or mesons have integer (whole number) charges eg. +1e, -2e etc.

• This means quarks in a baryon are either all quarks or all anti–quarks. Combination of quarks and anti–quarks don’t exist in a baryon

  • e.g.

    

• The anti–particle of a meson is still a quark and anti–quark pair. The difference being the quark becomes the anti–quark and vice versa

Leptons & Photons

Leptons

• Leptons are a group of fundamental (elementary) particles

  • This means they are not made up of any other particles (no quarks)

• Leptons interact with other particles via the weak, electromagnetic or gravitational interactions

  • Unlike hadrons (baryons & mesons), leptons do not interact via the strong nuclear force

• The most common leptons are:

  • The electron, e^–

  • The electron neutrino, v_e

  • The muon, μ^–

  • The muon neutrino, v_μ

The most common leptons are the electron and muon, along with their associated neutrinos

• The muon is similar to the electron but is 200 times more massive

• Electrons and muons both have a charge of -1e

• Electrons have a mass of 0.0005 u

• Muons have a mass of 0.11 u

• Neutrinos are the most abundant leptons in the universe and have no charge and negligible mass (almost 0)

• Although quarks are fundamental particles too, they are not classed as leptons

Photons

• Photons are a group of particles which mediate the electromagnetic interaction

  • They are uncharged

  • They have zero mass

• They are sometimes called "exchange bosons" because they mediate one of the fundamental forces (electromagnetism)

  • For example, the electrostatic repulsion between two electrons is understood in terms of exchanging photons 

Symmetry of the Standard Model

• The first four quarks discovered were: 

  • The up quark

  • The down quark

  • The strange quark

  • The charm quark

• The symmetry of the standard model predicted a third generation of particles, namely the top and bottom quark

  • Experiments were carried out to discover these, and eventually they were found as predicted

• Therefore, the three generations of quarks are and their respectively charges are:

The three generations of quarks. e is the charge of an electron.

• They each have their own anti-quark, which has the opposite charge

Three generations of anti-quarks. These have the same properties as the quarks except opposite charges.