Protons at Very High Energy
In a high-energy collision we take a very peculiar look at the protons (or nuclei). The collision process acts as microscopic study with enormous spatial resolution. One has found that, when looking with increasing resolution, the apparent number of quarks and gluons inside a proton increases dramatically creating an extremely dense system, which however, unlike the quark-gluon plasma, is rather cold. At some point the density should be so high that the system is completely filled with colored particles, in particular with gluons – it saturates (see figure). In fact this saturated state can be described as a classical color field, in a similar way as the classical electromagnetic field is a description of a system with a very large number of photons. (Photons are the quanta of the electromagnetic interaction, as gluons are the quanta of the strong interaction.) In fact it could be the only realization of the classical field limit of a subatomic interaction, and is thus of fundamental interest. This state is often called the color-glass condensate: a condensate, because it has high density, color, because that is the important characteristic of gluons, and glass, because it behaves like a solid, i.e. it doesn’t change, on short time scales, even though it may be moving on very long time scales, much like ordinary glass. The knowledge of the internal state of protons (and nuclei) under these conditions is also crucial for the interpretation of many of the other measurements performed in such collisions. The color-glass condensate would provide a universal description of all such strongly interacting systems.