The Standard Model Particle Physics

The Standard Model (SM) describes the Universe in terms of six Quarks, six Leptons and interacting force carriers called Bosons. The Photon, Gluon and Z and W Bosons act as the force carriers for the Electromagnetic, Strong and Weak forces respectively. The model does not include a Gauge description of Gravity but must be able to explain how particles acquire mass. For this reason the Higgs Boson has been introduced but has remained elusive to discovery at colliders.

The three generations of particles are shown in the diagram I have created above. The Quarks are highlighted pink and include the Up, Charm, Top, Down, Strange and Bottom Quarks. The Leptons are highlighted green and include the Electron Neutrino, Muon Neutrino, Tau Neutrino and the Electron, Muon and Tau. The force carriers known as Bosons are highlighted in blue and include the Photon, Gluon, Z and W Bosons. A particle in a later generation will have a larger mass than particles in an earlier generation from which it can be concluded that the first generation are stable. The heaviest SM particle is the Top Quark with an experimentally determined mass of 171.4GeV. All other Quark masses are two orders of magnitude smaller than this.

I will be referring in this article to a diagram of the Standard Model found here, produced by the D0 Collaboration.

Excluding the bosons the twelve elementary particles are known as fermions because of their charactistic spin quantum number being one-half. Every fermion listed also has a corresponding anti-particle with opposite charge. The classification of fermions is based on how they interact with pairs being grouped together to form a generation. The Quarks themselves are different to Leptons because they partake in the strong interaction. They carry an additional property called colour charge which allows the strong interacting gluon to couple to them. Quarks like the Electron, Muon and Tau particles also carry electric charge allowing them to interact through the Electromagnetic force. The Neutrinos are neutral, light particles and so only interact through the Weak force via the W and Z Bosons. These particles are extremely light and so are often not detected in experiments, having their existence inferred from conservation of momentum.

The Higgs particle is classified as a Boson and has no intrinsic spin. It is the only fundamental particle not yet discovered at particle colliders but is expected to surface at the Large Hadron Collider. The Higgs plays an important role in the Standard Model; explaining why other particles are massive, like the W and Z Bosons.