Quarks are fundamental building blocks of matter. They are most commonly found inside protons and neutrons, the particles that make up the core of each atom in the universe. Based on current experimental evidence, quarks seem to be truly fundamental particles; thus cannot be further subdivided.
In the present standard model, there are six different species of quarks called flavors, each of which has a unique mass. Physicists usually refer to them in terms of three pairs: up/down, charm/strange, and top/bottom. (Also, for each of these quarks, there is a corresponding anti-quark.)
Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively. Quarks also carry another type of charge called color charge.
The quark forces are attractive and observable only in “colorless” combinations of three quarks (baryons), quark-anti-quark pairs (mesons) and possibly larger combinations such as the penta-quark that could also meet the colorless condition.
The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks. For reasons still unknown, nature also designed two copies each of the up and down quarks, identical except for having larger masses. The heavier copies of the up quark are called charm and top quarks; the copies of the down quark are named strange and bottom quarks. Converting energy into mass, accelerators produce these heavier, short-lived quarks through particle collisions.
