Quarks are elementary particles that are fundamental building blocks of matter. They are considered to be point-like particles with no known substructure. The fractional charge of quarks is a property that arises from the way they interact with the strong nuclear force.
Quarks are affected by a fundamental interaction called the strong nuclear force, which is mediated by particles called gluons. The strong force is responsible for holding quarks together inside atomic nuclei and is also involved in the processes of particle decay and interaction. The strong force is characterized by a property known as color charge, which comes in three types: red, green, and blue (these are just labels and don't correspond to the colors we see).
Quarks carry a fractional electric charge, such as +2/3e or -1/3e, where "e" represents the elementary charge. This fractional charge is a consequence of the way quarks interact with the strong force. In the theory of quantum chromodynamics (QCD), which describes the strong interactions, quarks are considered to be confined within particles called hadrons (such as protons and neutrons). The confinement of quarks means that they cannot exist as isolated free particles, but rather they are always bound together in combinations that have integer electric charges.
The fractional charges of quarks can be understood through a phenomenon called color confinement. According to QCD, the strong force binds quarks together by exchanging gluons between them. These gluons carry color charge, and as quarks move apart, the exchange of gluons increases, leading to an increase in the strong force between them. As a result, the energy required to separate quarks becomes large enough to create new quark-antiquark pairs from the vacuum. These new pairs combine with the original quarks, forming color-neutral combinations that have integer electric charges. This phenomenon ensures that quarks are always confined within hadrons and cannot be observed as isolated particles.
It's worth noting that the fractional charge of quarks is not directly related to the electric charge we typically associate with particles. The electric charge of a quark is canceled out by the presence of other quarks in a hadron, resulting in an overall integer electric charge for the hadron as a whole.