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###### Particles of the Standard Model

Physics (Year 12) - The Standard Model

Dev Lohar

# What is the Standard Model?

Throughout your entire academic journey, you wouldâ€™ve learnt that atoms are the smallest â€˜thingsâ€™ to exist in the universe and they are the building blocks of everything around us. The latter is true, however there are particles smaller than atoms. These are referred to as subatomic/fundamental/elementary particles.

There is a well-accepted theory called the Standard Model which describes the properties and interactions of these fundamental particles. The Standard model describes and explains three of the four known fundamental forces (electromagnetic, strong, and weak) and how these forces interact with matter in the universe. It also classifies all known fundamental particles.

# Subatomic particles

*Standard Model diagram*

Subatomic particles are divided into 2 types; fermions and bosons. Fermions are particles which follow the Fermi-Dirac statistics and the Pauli exclusion principle and are further divided into quarks and leptons. Bosons are particles which carry force and mediate electromagnetic, strong, and weak interactions.

All subatomic particles have common properties such as mass, charge, and spin. Quarks and leptons have a special property called quantum number. Quarks possess baryon number and leptons possess lepton number.

Every charged subatomic particle, has an associated anti-particle which has the same properties as the particle, but with the opposite sign of their charge and quantum number. For example, the anti-particle of an electron is an anti-electron which has a charge of +1 (ie. opposite to the charge of an electron) and a lepton number of -1 (ie. opposite to the lepton number of an electron).

# Fermions

As stated earlier, Fermions are divided into quarks and leptons. There are 6 quarks and they experience the strong nuclear force and there are 6 leptons which experience the electromagnetic force (if the lepton has a charge).

Hadrons are composite particles which are made up of 2 or more quarks and these quarks are held together by the strong nuclear force. There are 2 types of hadrons; baryons, and mesons. Baryons are made up of 3 quarks and make up most of ordinary matter since protons and neutrons are types of baryons. Protons are made up of 2 ups and 1 down quark, whilst neutrons are made up of 2 down and 1 up quark. Mesons are made of 2 quarks where one is a normal quark, whilst the other is an anti-quark.

# Bosons

As stated earlier, bosons are particles which carry force and mediate electromagnetic, strong, and weak interactions. When particles interact and they experience a force between them, in reality what is happening is that the particles are exchanging bosons which allow them to experience different forces. For example, we know what happens when 2 electrons approach one another; they repel. But why do they repel? Weâ€™ve learnt from the standard model that when the electrons approach each other, both of them emit photons that is absorbed by the other. This exchange of photons is what causes the electrons to repel because photons are responsible for all electromagnetic force interactions.

Every fundamental force has an associated boson. Weâ€™ve learnt from the example above that photons are responsible for electromagnetic force which includes any form of electric/magnetic attraction/repulsion. Gluons are responsible for the strong nuclear force which is a force that acts between quarks and holds atoms of the nucleus together. W+, W-, and Z bosons are responsible for the weak nuclear force which is part of radioactive decay. There is a fourth, theoretical boson called the graviton which may be responsible for the attraction between any 2 masses.

It is also important to note the relative strengths of the fundamental forces from weakest to strongest; gravity, weak nuclear, electromagnetic, strong nuclear. This explains why protons are able to stay together in a nucleus despite the fact that they have like charges and should repel. Strong nuclear force has a higher strength than electromagnetic force and so the force keeping the protons together is much larger than the force repelling them.

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