The understanding that a proton consists of three quarks comes from a combination of experimental evidence and theoretical understanding. The concept of quarks was initially proposed in the 1960s by physicists Murray Gell-Mann and George Zweig as a way to explain the properties and behavior of particles observed in high-energy scattering experiments.
Experimental evidence supporting the existence of quarks and their arrangement in nucleons (such as protons and neutrons) has been gathered through various methods, including deep inelastic scattering experiments and electron-proton scattering. These experiments involved bombarding protons with high-energy particles and studying the resulting scattering patterns.
One significant piece of evidence came from deep inelastic scattering experiments conducted at the Stanford Linear Accelerator Center (SLAC) in the 1960s and 1970s. These experiments involved firing high-energy electrons at protons and measuring the scattering angles and energies of the scattered electrons. The observed data indicated that the proton's charge is concentrated in three distinct regions within the particle, suggesting a composite structure.
Furthermore, theoretical advancements, particularly the development of quantum chromodynamics (QCD), provided a framework for understanding the strong nuclear force and the behavior of quarks. QCD describes how quarks interact through the exchange of gluons, the force-carrying particles of the strong nuclear force.
Through a combination of experimental results and theoretical understanding, the quark model emerged as the most successful explanation for the internal structure of nucleons. According to this model, a proton is composed of two up quarks and one down quark, forming a combination of electric charges that results in an overall positive charge.
It's worth noting that while the quark model provides an effective description of the nucleon's structure, the strong nuclear force makes the study of quarks challenging due to its property of confinement. This means that quarks are always bound within composite particles and cannot be observed as free particles in isolation. Nonetheless, the evidence from experimental observations and the successful predictions made by the quark model have led to a broad scientific consensus regarding the tri-quark structure of the proton.