SM: Leptons


Leptons in the standard model are defined as those fundamental particles which do not interact via the strong force. The strong force is one of the three fundamental forces in the standard model – the other two being electromagnetism and the weak force.

Leptons come in three different generations…


the electron, the muon and the tau and their associated neutrinos.

You may not have heard of neutrinos before but they are in fact all around is. They are created in the sun by a decay involving the weak force.


If you were to hold your thumb out then about 100 billion of neutrinos pass through it every second. You don’t feel it because neutrinos are very weakly interacting meaning they have to travel through a lot of stuff before having a sizeable probability of interacting. There is even an experiment in using part of the ice sheet of Antarctica to try and detect them – this experiment is affectionately called ICE CUBE.

Electrons are also worth a mention, since they pretty much determine the whole of chemistry. Most of the matter around us is made up of atoms…


… at the centre of which is a nucleus.  This nucleus is surrounded by electrons, held there by the electromagnetic force, and which in part determine the chemical properties of that atom.

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SM: Fundamental Forces

The standard model of particle physics contains 3 of the 4 fundamental forces in the universe: Electromagnetism, the weak force and the strong force.

The 4th fundamental force is called gravity (which I am guessing you are all familiar with). Gravity is the weakest of these forces, and its effect only comes into play at very large energies – much larger than the ‘standard model of particle physics’ is there to predict.

The other forces in the standard model are mediated by a types of particles called ‘Gauge bosons’…


The electromagnetic force is mediated by a gauge boson called the photon. Photons are all around, forming what is commonly termed as light. Although it is photons that we use to see they also allow electrically charged objects to attract or repel. For example when you touch a table a stream of photons moves from your hand to the table (or vice versa) causing a repulsive force.

The weak force is mediated by, not one, but three different types of Gauge boson. The weak force is responsible for certain types of decays. In particular without the weak force we would not have stars, since the mechanics they undergo to produce light and energy requires decays mediated by the weak force.

The strong force is mediated by a gauge boson called a gluon. As the name suggests this glues together matter. As you sit there reading this blog post, (nearly) everything around you is made up of atoms. At the centre of these atoms is a nucleus which is held (or should I say ‘glued’) together by the strong force.

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