The knowledge about the quark structure of protons and other hadrons (particles composed of quarks) is derived from a combination of experimental evidence and theoretical models, particularly the success of the quark model in explaining a wide range of experimental data.
While it is true that quarks have never been observed in isolation (a phenomenon known as quark confinement), there is compelling evidence for their existence and the quark model. Here are some key pieces of evidence:
Deep inelastic scattering: Experiments involving high-energy electron scattering off protons and neutrons have provided detailed information about their internal structure. These experiments revealed that the scattering patterns were consistent with the presence of point-like constituents within the protons and neutrons, which were later identified as quarks.
Particle collision experiments: Particle colliders, such as the Large Hadron Collider (LHC), accelerate particles to high energies and collide them. These collisions produce a variety of particles, including those containing quarks. The analysis of these collision products supports the existence of quarks and their interactions.
Quark counting rules: The observed pattern of particle production in high-energy collisions follows specific "quark counting rules," where the number of produced particles is related to the number of quarks involved. These counting rules align with the predictions of the quark model.
Hadron spectroscopy: The study of the properties of hadrons, including their masses, spin, and decay patterns, has provided substantial evidence for the quark model. The observed properties can be explained by combining the properties of the constituent quarks within the hadrons.
Furthermore, theoretical developments, such as quantum chromodynamics (QCD), provide a framework for understanding the strong interaction that binds quarks together within hadrons. QCD is a fundamental theory describing the strong nuclear force and predicts the existence of quarks and their behavior within hadrons, despite their confinement.
While quarks cannot be isolated individually due to confinement, their collective properties and their impact on the behavior of hadrons can be inferred from experimental observations and theoretical models. The consistency between experimental results, theoretical predictions, and the success of the quark model in explaining a wide range of phenomena strongly supports the conclusion that protons and other hadrons are indeed composed of three quarks.