particle physics - In which experiment did protons seem to consist of infinite amount of quarks?

In this video Richard Feynman is telling that in some experiment it seems that the proton should consist of infinite amount of quarks.

What is this case he's mentioning? Is it solved now?

Answer

Thanks for finding this amazing historical video.

He's talking about the deep inelastic scattering electron proton experiment at SLAC. This showed evidence that high energy electrons scattered off pointlike charged particles within the proton, which Feynman named 'partons'. It took some time to establish that these partons are the same as quarks, which had been postulated to make sense of the patterns of mesons and baryons. We now understand that they are the same, but that the proton consists of three 'valence' quarks (up up down) plus a 'sea' of quarks and antiquarks which the electrons will scatter off (as well as gluons). So in a sense there are three quarks in a proton and in a sense there are an infinite number.

The SLAC measurements were confirmed by later experiments, particularly the HERA electron proton ring at DESY, with much more detail. In particular the early evidence for 'scaling': that scattering depended only on $x$, the fraction of the proton momentum embodied in the stuck Parton, and not on $Q^2$, the mass of the exchanged virtual photon, turned out to be wrong. The experiment just happened to look at a region where it was approximately true, and maybe that misled us for a while. But apart from that the results hold, and we now understand that the contradiction that was puzzling Feynman in the video is not a contradiction after all.