There are 6 quarks. However, up and down are enough to make every atom in the universe.
Particles made of quarks are called hadrons. They are held together by the strong interaction.
There are two families of hadrons, baryons and mesons. Baryons are qqq and mesons are \(q\bar{q}\).
Nucleons are baryons made of up and down quarks.
The neutron is ddu and the proton is uud.
The proton is the only stable free hadron. All other free hadrons decay.
Table of quark properties
$$\begin{array}{|c|c|c|} \hline \text{name} & \text{charge (e)} & \text{rest mass (MeV)} \\ \hline \text{Up} & + \frac{2}{3} & 2.3 \\ \hline \text{Down} & - \frac{1}{3} & 4.8 \\ \hline \text{Strange} & - \frac{1}{3} & 95 \\ \hline \text{Charm} & + \frac{2}{3} & 1275 \\ \hline \text{Bottom} & - \frac{1}{3} & 4180 \\ \hline \text{Top} & + \frac{2}{3} & 173210 \\ \hline \end{array}$$
We will concentrate on the first three quarks, up, down and strange.
$$\begin{array}{|c|c|} \hline \text{name} & \text{charge (e)} \\ \hline \text{Up} & + \frac{2}{3} \\ \hline \text{Down} & - \frac{1}{3} \\ \hline \text{Strange} & - \frac{1}{3} \\ \hline \end{array}$$
The proton is composed of three quarks up up down or uud.
This is the most stable hadron.
All baryons decay into the proton eventually.
A free proton has a life time of ...
They were originally formed soon after the Big Bang as the universe cooled down and quarks were no longer free.
The neutron is composed of three quarks up down down or udd.
The free neutron is unstable and will decay into a proton via the weak interaction.
$$ n \rightarrow p + e^- + \bar{\nu}_e $$
This is actually the change of flavour of a down quark into an up quark - the decay of a down quark.
$$ udd \rightarrow uud + e^- + \bar{\nu}_e $$
$$ d \rightarrow u + e^- + \bar{\nu}_e $$
When a quark changes flavour this is a weak interaction.
The free neutron has a life time of ...
The nucleons are a family as they are very similar and composed of up and down quarks.
They can be thought of as two states of baryons with up and down quarks.
It is possible to think that states uuu and ddd exist logically. They are possible but have an added issue of spin to consider. Only two quarks of the same type can occupy the same state. Adds instability to uuu and ddd. These are known as delta particles.
$$ \Delta^- \text{ and } \Delta^{++} $$
Delta states are heavier and so unstable.
A baryon is a hadron made of three quarks.
This is simply decribed as qqq.
Baryons must have integer charge.
We describe a baryon as having a Baryon number of 1. So each quark must have a Baryon number of \( \frac{1}{3} \).
Anti-baryons have opposite charge and opposite sign baryon number.
The strange quark is given strangeness -1.
Strange baryons can have 1, 2 or 3 strange quarks in them.
Strange particles are very unstable.
The strange quark decays by weak interaction into an up quark via the emission of a W^-.
If one strange quark is present then there are 3 possible arrangements of quarks (plus one spin 3/2 state).
uus \( \Sigma^+ \), uds \( \Sigma^0 \) and dds \( \Sigma^- \).
With two strange particles the states are ssu \( \Xi^0 \) and ssd \( \Xi^- \).
Finally the state with three strange quarks is sss \( \Omega^- \).
Mesons are the other major family of hardons. Mesons are quark anti-quark pairs - \( q \bar{q} \).
These always decay and often produce leptons as a result of weak interaction decays.
The can produce photons if the is a quark anti-quark pair of the same flavour. This is basically the quark anti-quark pair annihilating.
Mesons have Baryon number of 0.
The pions are the mesons made from up and down quarks.
They are \( \pi^+ \; u\bar{d}\), \( \pi^- \; d\bar{u}\) and \( \pi^0 \; u\bar{u} \text{ and } d\bar{d} \).
The \( \pi^0 \) decays by annihilation producing two gamma photons.
$$ \pi^0 \rightarrow 2\gamma $$
The \( \pi^+ \) will decay by the weak interaction.
$$ \pi^+ \rightarrow \mu^+ + \nu_\mu $$
The \( \pi^- \) will decay by the weak interaction.
$$ \pi^- \rightarrow \mu^- + \bar{\nu}_\mu $$
The kayons are the mesons made from one strange quark / anti-quark and either up or down quarks or anti-quarks.
The three kayons are \( K^+ \; u\bar{s} \), \( K^- \; s\bar{u} \) and \( K^0 \; d\bar{s} \) or \( K^0 \; s\bar{d} \).
The kaons decay into pions and leptons in various combinations by the weak interaction.