In the framework of the Standard Model of particle physics, quarks are considered to be fundamental particles that exist in three "generations" or "flavors": up, down, charm, strange, top, and bottom. These quarks are the building blocks of composite particles, such as protons and neutrons, which are made up of three quarks each.
The interactions between quarks are mediated by gluons, which are the force-carrying particles for the strong nuclear force. Gluons bind quarks together inside nucleons and other hadrons, forming a state known as a color singlet. This color singlet state ensures that the strong force confines quarks within these composite particles.
While the Standard Model describes quarks as point-like particles, some theoretical extensions and exotic theories propose the existence of additional types of particles or structures beyond the three generations of quarks we currently know. For instance, certain theories, such as technicolor models or composite Higgs models, suggest the existence of new strongly interacting particles that can form bound states with different geometries or topologies.
However, it is important to note that these speculative theories are still highly theoretical and have not yet been confirmed by experimental evidence. The study of such exotic particles and structures is an active area of research, and scientists are conducting experiments at particle colliders, such as the Large Hadron Collider (LHC), to search for evidence of new physics beyond the Standard Model.
In summary, while the current understanding within the Standard Model describes quarks as point-like particles forming nucleons through the exchange of gluons, there are theoretical possibilities for more exotic structures and particles beyond our current knowledge. Exploring and testing these possibilities is an ongoing endeavor in particle physics research.