If quarks had internal structure (contradicting current beliefs), what is the lowest upper bound on their "radius" based on current experimental results?
If possible, I'd prefer to only consider experiments which probe protons and neutrons (not other shorter-lived particles since their interpretations get biased more by the standard model).
My only understanding is that this radius must be less than roughly 0.2 fm since spacings are found to be 2 fm in high-energy proton scattering experiments. I imagine "higher-energy scattering experiments" and "excited angular momentum experiments" have probed this further, but am not familiar with any other results. Or, is there some other reason why this radius must be zero? Honestly, with the surprise of quarks 10,000x smaller than the electron cloud, it wouldn't be surprising if we found some internal structure after another 10,000x zoom.