The Standard Model
Introduction
This is kind of a filler article until I manage to finish the final dark matter article. The Standard Model is an extremely useful tool for all Particle Physicists, in fact I’d say it’s their version of the Periodic Table. Well their version of the Periodic Table would be the Periodic Table… you get what I mean.
“The top quark was discovered in 1995, and since then, the Higgs has become our obsession because the standard model was incomplete without it.”
- Fabiola Gianotti
The name “Standard Model” was first introduced by two Physicists: Abraham Pais and Sam Treiman. It essentially showcases what Physicists like to call “The fundamental particles”.
Before we attempt to understand the standard model, we must first understand how particles are classified.
Particles are classified like so:
So before we dive into this, we need to understand the exact definition for a particle.
A particle is a small object which can have several properties such as mass, charge, density and volume.
Particles can be classified into two groups: Fermions and Bosons.
Fermions
A fermion is any particle with half integer spin. You don’t need to know what spin is, it’s a bit hard to explain. It is essentially a type of angular momentum.
Fermions can be broken down into two further groups: Hadrons and Leptons
Hadrons
Hadrons are particles that experience the strong force (check out my post on the four fundamental forces). Hadrons are particles that are made up of smaller particles called quarks. A quark is basically a subatomic particle that are constituents of larger particles. We once thought atoms were the smallest thing in the Universe but we now know they are built of smaller particles called protons, neutrons and electrons. Similarly quarks are kind of like subatomic particles, except instead of building atoms, they build other particles.
Hadrons can be classified into two further groups: Baryons and Mesons. Hadrons are classified further based on how quarks combine.
Quark (interlude)
There are six types of quarks: Up (u), Down (d), Charm [c], Strange (s), Top (t), Bottom (b). The up, charm and top quarks have a charge of +(2/3)e. The down, strange and bottom quarks have a charge of -(1/3)e. What is e? e is known as the elementary charge:
You’ve probably been taught that protons and electrons have a charge of +1 and -1 respectively. That’s wrong, they have a charge of +1e and -1e respectively. There are three types of each quark as well (three up quarks, three strange quarks etc.), these are based on colour but that’s a very complicated topic.
Baryons
Baryons are types of hadrons. Baryons are particles which are composed of three quarks. You probably already know two baryons: The proton and neutron. The proton is made up of the up, up and down quark. It can be represented like so: [uud]. If we add up the charges for the three quarks (2/3e + 2/3e -1/3e), we get +1e. This is the charge of a proton. The Neutron is made up of the up, down, down quark. [udd]. If we add up the charges, we get 0.
Mesons
Mesons are types of hadrons. Mesons are particles which are composed of two quarks. However, they must be a quark — antiquark pair. What is an antiquark? An antiquark is a version of a quark with the same mass but with opposite charges. An anti-up quark would have a charge of -2/3e while an anti-strange quark would have a charge of +1/3e. However, quarks cannot combine with its own antiquark to form a meson. This is because a process known as annihilation occurs where the particles kind of cancel each other out and emit photons. An example of a meson is the pion. The pion is made of the up and anti-down quark.
All baryons are hadrons.
Not all hadrons and baryons.
All mesons are hadrons.
Not all hadrons are mesons.
Leptons
On the same level as hadrons, we have the lepton. Leptons are known as fundamental particles; they cannot be broken down further. Hadrons are not fundamental as they can be broken down to get quarks. There are six types of leptons and you’re probably already familiar with one: The electron. The other five leptons are the Tauon, Muon, Electron neutrino, Tauon neutrino, Muon neutrino.
Bosons
Bosons are particles with integer spin. Gauge bosons are bosons that carry the four fundamental forces.
There are different types of Bosons: Photons, Gluons, W+, W-, Z⁰ and Gravitons. The photons mediate the electromagnetic force. W+, W- and Z⁰ bosons mediate the weak force. Gluons mediate the strong force while gravitons mediate gravity.
The Standard Model
There is another boson we haven’t talked about which is the Higgs Boson. That is another can of worms for a separate article.
I hope you now have a basic understanding of the Standard Model.
I hope you enjoyed!
