# 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 = Charge/e =
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
proton uud

Mesons consist of quark-antiquark pairs. For example:

Meson quark composition charge/e
pion
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.

#### Particle Numbers

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.