E05-017: Proton structure and the distortion caused by two-photon effects
About four decades ago, it was discovered that the proton is made up of quarks bound tightly together by the strong force, but even today we do not have a complete picture of how the properties of the proton are built up from the properties of its quarks. For example, if the charge and magnetism of the proton are simply the sum of the quarks' intrinsic charges and magnetism, then these distributions would be identical inside the proton. A new generation of experiments at Jefferson Lab found significant differences between the distributions, while earlier measurements had indicated that the distributions were identical. The recent measurements, using a new experimental technique, suggest a more complicated system. Understanding the difference between these two sets of results is critical to having a clear picture of the structure of the proton.
To measure these distributions, physicists use very energetic electrons to probe the proton by exchanging a single photon. This is a simple interaction that provides a clear picture of the proton structure, but this picture can be distorted by the small chance that two photons are exchanged. This effect is quite small, but was still enough to distort our view of the proton. The newer measurements, using a technique which is less sensitive to these distortions, provided a clearer picture of the proton, yielding both new insight into the proton structure and the importance of understanding the distortion caused by these two-photon effects.
The goal of this experiment is to make improved measurements using the earlier "Rosenbluth" technique to carefully map out the distortion caused by these two-photon effects in order to enhance our understanding of these corrections and the details of the quark structure of the proton. Because single-photon exchange is a commonly used tool, what we learn about these corrections will also provide more accurate pictures of other details of the structure of matter.