While it is true that we cannot directly observe individual quarks or atoms with our eyes or conventional optical microscopes, the existence of quarks and their role in the structure of subatomic particles has been inferred through various experimental and theoretical investigations. The evidence for the existence of quarks comes from a combination of experiments in particle physics, scattering experiments, and the development of the theory of quantum chromodynamics (QCD).
Here are some key points on how we know about quarks:
Scattering experiments: Particle accelerators are used to collide particles together at high energies. By studying the interactions and scattering patterns of these particles, scientists can infer the underlying structure of matter. Experiments such as deep inelastic scattering, where high-energy electrons are scattered off protons and neutrons, provided evidence for the existence of point-like constituents within these particles.
The Eightfold Way: In the 1960s, Murray Gell-Mann and George Zweig independently proposed a classification scheme known as the Eightfold Way, which explained the patterns observed in the properties of various particles. The scheme introduced the concept of quarks as fundamental building blocks of matter. This classification scheme later formed the basis for the development of the quark model.
Quark model: The quark model provides an explanation for the observed properties of hadrons (particles made of quarks), such as protons, neutrons, and mesons. It suggests that these particles are composed of combinations of quarks. Quarks have fractional electric charges (1/3 or 2/3 of the elementary charge) and carry color charges (a property related to the strong nuclear force). The model successfully explains the behavior of particles in terms of their quark content.
Deep inelastic scattering and parton distribution functions: Deep inelastic scattering experiments, which involve scattering high-energy electrons off protons and neutrons, revealed that these particles have substructure. By studying the momentum transfer and energy loss in these experiments, scientists discovered that the nucleons (protons and neutrons) contain point-like constituents known as partons. These partons were identified as quarks and gluons, which are the carriers of the strong nuclear force in QCD.
QCD and the strong force: Quantum chromodynamics (QCD) is the theory that describes the strong nuclear force and the interactions of quarks and gluons. QCD provides a mathematical framework that explains the behavior of quarks and how they bind together to form composite particles. The theory has been extensively tested and verified through a wide range of experimental data, including scattering experiments, high-energy collisions, and other observables.
While we cannot directly observe individual quarks or atoms, the cumulative evidence from these experiments and theoretical frameworks provides a strong basis for our understanding of quarks and their role as fundamental constituents of matter.