Does quark have internal structures?

Abstract

Quark is one of the fundamental particles so far known by human being as a building block of our matter world. It is the only one which participates all known fundamental interactions in the framework of the Standard Model (SM). As part of the SM, the gauge theory that describes the strong interactions between quarks and gluons is called Quantum Chromo-Dynamics (QCD). Since quarks and gluons always carry “color charges”, any single quark or gluon cannot be isolated from hadrons to act as a free particle. This phenomenon, known as “color confinement”, implies that hadrons instead of quarks or gluons are actually the so-far smallest inseparable particles. The other feature with QCD is that when the larger energy or momentum transfers occur between quarks, the weaker their interactions become. This paves a way for probing the structure of quarks by high energy hadron-hadron scatterings or electron-hadron scatterings which can better resolve the structure of such tiny colored objects. Evidence for smaller fundamental particles than the quarks or possible proof for internal refined structures of the quarks would request new physics beyond the SM and they may dramatically change our view of the matter world. These studies are actually the high energy frontier of modern high energy physics in both experiment and theory.