The fractional electric charges of quarks, such as +2/3 and -1/3, were initially proposed as part of the quark model, which was developed independently by Murray Gell-Mann and George Zweig in the 1960s. The quark model was proposed to explain the observed patterns and symmetries in the behavior of hadrons (particles made up of quarks) and the strong nuclear force.
The fractional electric charges of quarks were inferred indirectly based on experimental data and theoretical considerations. Here's a brief overview of the process:
Symmetry patterns: The observation of certain patterns and symmetries in the behavior of hadrons led physicists to propose a fundamental building block model for these particles. The quark model was developed to explain these patterns and symmetries.
Fractional electric charges: In order to explain the observed charges of hadrons, Gell-Mann and Zweig assigned fractional electric charges to the quarks. They assigned +2/3 to what we now call "up" quarks and -1/3 to what we now call "down" quarks. These values were chosen to match the observed charges of known hadrons, such as protons and neutrons.
Conservation of electric charge: The fractional charges of quarks were consistent with the conservation of electric charge. For example, a proton, composed of two up quarks (+2/3 each) and one down quark (-1/3), has a total charge of +1, which is consistent with the observed charge of a proton.
Experimental evidence: Over time, experiments were conducted to study the properties and interactions of particles, including deep inelastic scattering experiments and particle accelerators. These experiments provided evidence for the existence of quarks and their fractional electric charges.
It's important to note that quarks themselves have never been observed in isolation due to a property known as color confinement. They are always found bound together in composite particles, such as protons and neutrons, or in particle collisions where they produce a shower of other particles. However, the evidence from experimental data and the success of the quark model in explaining various phenomena strongly support the existence of quarks and their fractional electric charges.