The first experimental evidence for the existence of quarks came from deep inelastic scattering experiments performed in the 1960s and 1970s. These experiments involved shooting high-energy electrons or neutrinos at protons and other target particles and studying the scattering patterns of the resulting particles.
One of the key experiments in this regard was the SLAC-MIT experiment conducted at the Stanford Linear Accelerator Center (SLAC) in the late 1960s. In this experiment, electrons were accelerated to high energies and collided with protons. By measuring the scattering angles and energies of the scattered electrons, researchers were able to deduce the internal structure of protons and observed that they had substructure, later identified as quarks.
The experiments provided evidence for the presence of charged constituents (quarks) within protons and other hadrons. The observed scattering patterns were consistent with the idea that protons contained three point-like objects, which were later identified as up (u) and down (d) quarks.
Determining the spin of quarks is a complex process that requires a combination of experimental techniques and theoretical interpretations. One of the key experiments that helped determine the spin properties of quarks was the European Muon Collaboration (EMC) experiment in the 1980s. The EMC experiment used muons to probe the spin structure of nucleons (protons and neutrons).
The experiment observed that the contribution of quark spins to the overall spin of the nucleon was much smaller than expected. This result, known as the "spin crisis," indicated that the quark spins accounted for only a fraction of the nucleon's total spin. This unexpected finding sparked further research and led to the realization that the gluons and the orbital angular momentum of quarks and gluons also play significant roles in the nucleon's spin.
The spin properties of quarks have been further studied through various high-energy scattering experiments, such as deep inelastic scattering and polarized scattering experiments. These experiments, combined with theoretical frameworks such as Quantum Chromodynamics (QCD), have provided insights into the spin structure and properties of quarks within hadrons.