What are hadrons and how are they classified?

Hadrons are subatomic particles composed of quarks, classified into two groups: baryons and mesons.

Hadrons are a type of composite subatomic particles that are made up of quarks, which are the fundamental constituents of matter. The term 'hadron' comes from the Greek word 'hadros', meaning 'stout' or 'thick'. They are held together by the strong nuclear force, one of the four fundamental forces of nature. This force is so strong that it overcomes the repulsion between the quarks, which are all positively charged.

Hadrons are classified into two main categories: baryons and mesons. Baryons, such as protons and neutrons, are made up of three quarks. They are fermions, meaning they have half-integer spin and follow the Pauli Exclusion Principle, which states that no two fermions can occupy the same quantum state simultaneously. This is why, for example, in an atom, electrons (which are also fermions) fill up different energy levels.

Mesons, on the other hand, are composed of a quark and an antiquark. They are bosons, meaning they have integer spin and do not follow the Pauli Exclusion Principle. This allows them to occupy the same quantum state. Mesons often act as the exchange particles for the strong nuclear force between baryons.

The classification of hadrons is based on their quark content, which determines their properties such as charge, spin, and mass. The study of hadrons and their interactions is a key part of quantum chromodynamics, the theory that describes the strong nuclear force. Understanding hadrons and their behaviour helps us to understand the fundamental structure of matter and the forces that hold it together.