Our knowledge of the inner structure of baryons is based on deep inelastic scattering of leptons (electrons, neutrinos or muons) from protons. Such experiments have been done repeatedly, at higher and higher incident energies, since around 1970. Even the earliest experiments revealed that baryons are made of pointlike objects, both real and virtual, and that the number of such objects within the proton is essentially limitless. These objects turn out to be "valence" quarks, and virtual quark-antiquark pairs ("sea" quarks) as well as the quanta of the strong interaction, gluons.

As I hope you learned in 373, the scattering amplitude as a function of center-of-momentum scattering angle, or of momentum transfer, carries information about the size and shape of what the beam scatters from. As long as the probability wavelength of the beam particles (preferably pointlike themselves) is comparable to the size of the scatterer, you will see basically a Fresnel diffraction pattern in the scattered particles, strongly forward peaked, with diffraction fringes. If the scatterer is diffuse, not a compact shape, there is still a characteristic shape to the scattering amplitude. But what if the scatterers are point particles, or have a size much, much, much smaller than the wavelength of the incoming beam particles? Then the scattering amplitude is isotropic, with no dependence on scattering angle or momentum transfer. If the beam energy is increased more and more and more, and the amplitude remains isotropic, then for all practical purposes the particles that scatter the beam are physical points. According to the Standard Model, all fundamental particles are physical points, so we would expect and do see an isotropic scattering amplitude. But what if we select kinematically the situations where the particle that does the scattering carries an almost totally negligible fraction of the total momentum of the system from which we scatter? Observations show an inexplicable result, namely that the scattering amplitude has a small positive slope. We are working in the muddy "bottom of the barrel" residue of baryon constituents, and it is no surprise that this result might have no explanation within the Standard Model.  However, it is generally assumed to measure an increase in the density of gluons corresponding to tiny x.