Quarks and leptons – fundamental particles

About century ago the proton (first thought of by Rutherford in 1911) the neutron (the properties of neutrons and protons were defined by James Chadwick in 1932) and electron (discovered by J J Thompson in 1897) were thought to be the fundamental particles. As research progressed over the last half of last century it became clear that some of these particles were themselves made of smaller bits. So far it has not been possible to split up an electron - that is still thought of a one of the fundamental particles, part of the group called leptons.

We currently think that there are two types of fundamental particles - quarks and leptons.

Quarks

Family of quarks

The quark seems to be one of the basic constituents of matter. Six types have been identified but two are particularly common. The six types are quaintly known as known as flavours: up, down, charm, strange, top, and bottom. (I suppose they had to be called something). The up and down varieties survive in large quantities forming protons and neutrons, these are the only two stable quarks in our universe.

The other four have brief life spans as do the particles they form. In addition every quark has an antiparticle - a sort of mirror image, with the same mass but an opposite charge. In total there are therefore 12 quarks.

Leptons

Lepton family

There are three types or “flavours” of leptons (or six, if you count the corresponding neutrinos separately or twelve if you count antiparticles!) have been identified:

• the electron,

• the muon,

• the tau lepton or tau

Each flavour consists of a pair of particles called a “weak doublet”. One of this pair is a relatively massive and charged particle that has the same name as its flavour (such as the electron). The other is a neutral particle of very tiny mass called a neutrino (for example the electron neutrino). And of course there are corresponding antimatter leptons and neutrinos.

The antimatter electron has a special name - the positron. The electron and the positron are stable (unless they meet of course) as are all the neutrinos.

Note- if a particle meets its’ antimatter mirror image then the two will mutually destruct,  producing energy.

Hadrons, baryons and mesons

Hadrons are all made of of quarks, again split into two into two groups:

• The baryons are the family of subatomic particles all of which are made of three quarks. The family notably includes protons and neutrons, which make up the atomic nucleus, but many other unstable baryons exist as well.

Mesons, made up of a quark and an antiquark pair.

 

You can download a pdf copy of these notes here:Introducing quarks and leptons