Precision Measurements of the Proton Structure

The proton is quite a fantastic particle. If free, it doesn’t decay on any timescale people have been able to explore. It has an immense ability to heal itself, demonstrated in the high rate of diffraction even for interactions with large momentum transfer. What the author really knows, is actually quite limited. The charge was determined to be “+1”, the mass was measured to be 1.6 × 10−27 kg and the spin is 1/2. Spin will not be discussed in this contribution; there are others who will write about it. If the proton is probed with enough energy, three valance quarks are revealed. If it is probed with even more energy, the QCD affliction of the proton, i.e. the glue and the sea become visible. QCD is always used when the results of one measurement are used to make predictions for another. And one part of this ansatz are parton distributions functions, PDFs, of the proton. They are a very successful tool. However, their shape is entirely heuristic; QCD cannot predict them from first principle. Protons are a vital part of nuclei. Together with neutrons, they provide the rich world of elements that we so dearly love. However, in this environment QCD is generally not the theory of choice to predict what happens. Inside a nucleus, a proton can decay because the energy to become a neutron comes from the nucleus. Nuclei are not spheres; the proton itself is often depicted as one, but that is also too simplistic.