The indivisibility of quarks is a fundamental concept in particle physics that is based on a combination of experimental evidence and theoretical considerations. Quarks are elementary particles, meaning they are not composed of smaller constituents according to our current understanding of particle physics.
Here are a few lines of evidence supporting the notion that quarks are indivisible:
Deep Inelastic Scattering: In the 1960s and 1970s, experiments studying the scattering of high-energy electrons and neutrinos off protons and neutrons revealed a surprising result. The scattering patterns indicated that protons and neutrons are made up of smaller entities with point-like structure, which were later identified as quarks. The deep inelastic scattering experiments provided indirect evidence for the existence of quarks and their point-like nature.
Asymptotic Freedom: The theory of Quantum Chromodynamics (QCD) describes the interactions of quarks and the strong nuclear force. One of the remarkable features of QCD is asymptotic freedom, which was discovered in the 1970s by physicists David Gross, David Politzer, and Frank Wilczek. Asymptotic freedom implies that at very high energies or short distances, the strong nuclear force weakens, and quarks become nearly free and behave as if they are point-like. This behavior is consistent with the notion of indivisible quarks.
Hadron Spectrum: The properties and behavior of hadrons (particles composed of quarks, such as protons and neutrons) can be studied in experiments and calculations. The observed spectrum of hadrons, along with theoretical models and calculations based on the quark model, provide evidence for the existence of quarks as fundamental particles. The quark model successfully explains various properties of hadrons, such as their masses, quantum numbers, and the patterns observed in their decays.
It's important to note that quarks have never been observed in isolation due to a property known as confinement. The strong nuclear force binds quarks together tightly, and they are always found in composite particles called hadrons. This confinement is another piece of evidence supporting the indivisibility of quarks.
While the evidence strongly suggests that quarks are indivisible, our understanding of fundamental particles and their properties continues to evolve. Future experiments and theoretical advancements may shed further light on the nature of quarks and the fundamental constituents of matter.