The Quark model of Hadrons

The Quark model of Hadrons

Hadrons are generally more massive than leptons. They are sub-divided into baryons (the most massive) such as protons and neutrons, and mesons (somewhat less massive) such as the pion and kaon.

Unlike the leptons, which do not experience the strong nuclear force, hadrons experience all four fundamental forces.

Experiments in which electrons are scattered from nucleons have revealed that neutrons and protons are not fundamental particles but are composed of yet smaller particles, quarks.

There are three quarks: up, down and strange and each has a corresponding antiquark of opposite charge.

Charge/e = Copyright S-cool Charge/e = Copyright S-cool
up u down d
charm c strange s
top t bottom b

These are all unstable

An odd property of quarks is that they have charges that are fractions of the electron charge. The up quark has a charge of +2/3e and the strange and down quarks -1/3e.

Baryons consist of 3 quarks. For example:

Baryon quark composition charge/e
neutron udd Copyright S-cool
proton uud Copyright S-cool

Mesons consist of quark-antiquark pairs. For example:

Meson quark composition charge/e
pion Copyright S-cool Copyright S-cool Copyright S-cool
Copyright S-cool Copyright S-cool Copyright S-cool
Copyright S-cool Copyright S-cool Copyright S-cool
Notice that in all cases, the charges of the particles add to a whole number multiple of the electron charge e, or 0. Any of the baryons or mesons can be made from combinations of up and down quarks. The up and down properties of quarks are called the quark's flavour. Hence, we say it is possible to make any baryon or meson from combinations of the quarks of flavour up and down.

In order to understand the interactions of these particles better they are assigned numbers as described below:

Charge (Q) is conserved in all interactions.

Baryons are assigned a Baryon number (B). B = 1 for baryons and B = -1 for antibaryons. Baryon number is conserved in all interactions.

Strange quarks possess a property called Strangeness (S). S = -1 for strange quarks and S = 1 for antistrange quarks. Strangeness is conserved in all but the weak interaction (this is because the weak interaction involves one type of quark changing into another).

These numbers help particle physicists to make predictions about particle interactions and track patterns in behaviour.

The Quark model of Hadrons

All of the Hadrons are unstable. For many years protons were thought to be stable but recent theories have shown that protons have a half-life of around 1032 years - a very long time! It is thought that protons eventually decay into leptons but due to the very long half-life this will be difficult to show.

Neutrons are relatively stable inside of the nucleus but have a half-life of only around 15 minutes outside of it. The decay is shown below.

In this process a down quark in the neutron is changed into an up quark in the proton.