The Elementary Particles

The current model of the universe’s fundamental particles is called the Standard Model. We know it’s not 100% accurate and needs a bit of tweaking but overall it does a fantastically good job of describing the universe to an incredible degree of accuracy.

In the Standard Model the elementary particles are split into two groups: fermions (named after the Italian physicist Enrico Fermi) and bosons (named after the Bengali physicist Satyendra Nath Bose).

Fermions are spin 1/2 particles and there are two types of fermion in the Standard Model: leptons and quarks. For each type of fermion, leptons and quarks, there are three generations of particles.

Leptons are either charged or neutral. The three generations of charged leptons are, from lightest to heaviest, the electron, the muon and the tau. The neutral leptons are called neutrinos and there is one neutrino for each type of charged electron, i.e. the electron neutrino, the muon neutrino and the tau neutrino.

In the Standard Model neutrinos are massless, although recent experiments have shown that, in fact, neutrinos do have mass, albeit an incredibly tiny one.

Quarks either have a charge of +2/3 e or -1/3 e, where e is the magnitude of the charge on an electron (or muon or tau). The three generations of +2/3 e quarks are, from lightest to heaviest, the up quark, the charm quark, and the top quark. The three generations of -1/3 e quarks are, from lightest to heaviest, the down quark, the strange quark and the bottom quark (which is also known as the beauty quark).

In ordinary everyday matter only electrons and up and down quarks are present. The other particles, being heavier, are unstable and would simply decay into the lighter particles.

The heavier particles existed right at the beginning of the universe in the first few seconds after the Big Bang. They can also be produced in particle accelerators where particles, such as electrons, positrons (the anti-particle of the electron) and protons, are made to collide at very high energies in order to produce conditions similar to those present at the time of the Big Bang.

Quarks do not exist on their own. They group together to form mesons (two quarks) or baryons (three quarks) which are either neutral or have a charge equal to integer multiple of e (the charge on an electron).

The proton, for example, is a baryon which contains two up quarks and one down quark, giving it an overall charge of +1e.

The neutron is also a baryon. It contains two down quarks and one up quark, giving it no overall charge, i.e. it is neutral.

Each of the particles mentioned above also has an anti-matter equivalent, which has opposite charge but all other properties, including mass, are the same. The anti-electron is called the positron. Other anti-particles are just referred to by putting ‘anti’ in front of the particle’s name.

The bosons have integer spin and are referred to as the force carriers. The bosons of the Standard Model are: the photon, the W boson, the Z boson and the gluons. There is also the Higgs boson.

The bosons are known as force carriers because each of the fundamental forces of nature is mediated by a boson.

The photon is the mediator of the electromagnetic force and is massless. The electromagnetic force is the force that exists between particles with electrical charge. Whenever two particles undergo an electromagnetic interaction they exchange a photon.

The W and Z bosons are the mediators of the weak interaction and are relatively heavy. The W bosons have charge +e or -e and the Z boson is neutral. The weak interaction is the force responsible for nuclear interactions such as radioactive decay.

There are eight gluons and they are the mediators of the strong interaction. Unlike the other force carriers, gluons can interact with other gluons. The strong force is the force that binds the quarks together.

The Higgs boson has not yet been observed but it has been postulated that such a particle exists in order to provide the other fundamental particles with mass.