What are up and down quarks?

Up and down quarks are the lightest of all quarks, fundamental particles that combine to form protons and neutrons.

Quarks are elementary particles and a fundamental constituent of matter. They are the smallest things that we know of and are part of the family of particles that physicists call 'fermions'. Fermions are particles that obey the Pauli Exclusion Principle, which states that no two identical fermions can occupy the same quantum state simultaneously.

The up and down quarks are the lightest and most stable of the six types of quarks, which also include charm, strange, top, and bottom quarks. The up quark carries a charge of +2/3, while the down quark carries a charge of -1/3. This is in contrast to the electron, which carries a full negative charge of -1.

In the standard model of particle physics, protons and neutrons, which make up the atomic nucleus, are composed of up and down quarks. A proton is made up of two up quarks and one down quark, giving it a total charge of +1. A neutron, on the other hand, is made up of one up quark and two down quarks, giving it a total charge of 0.

Quarks are never found alone in nature. They are always found in combinations of two or three, forming particles known as mesons and baryons, respectively. This is due to a property of quarks known as 'colour charge', which is completely unrelated to visual colour, but is a useful way to categorise quarks. The 'colour' of a quark can be red, green, or blue, and the 'colour' of an anti-quark can be anti-red, anti-green, or anti-blue. A stable particle must be 'colourless', which means the combination of quarks must result in a neutral 'colour' charge.

The discovery and study of quarks have greatly expanded our understanding of the fundamental structure of matter. It's a fascinating field that continues to evolve as physicists probe deeper into the subatomic world.