Studies of the Structure of the Nucleon

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Advances in the physics goal of understanding the universe at its most fundamental level have shown that matter is composed of a set of fundamental particles that combine to form other particles that group together to generate the structures that we recognize around us. For example, the nucleus of an atom is formed from a mixture of protons and neutrons. These protons and neutrons are made from constituents, called quarks and gluons, which give substructure to these particles. The goal of this project is to determine aspects of the structure of the proton and neutron, and how this structure is affected when placed in the nucleus. The experiments focus on how the quarks are distributed in space and in momentum. An on-going puzzle is whether the proton size is not the same when measured with atomic hydrogen and its counterpart, muonic-hydrogen, where the electron in the hydrogen atom has been replaced by its heavier cousin, the muon. The project includes measurement of electron and muon scattering from protons to provide an independent measurement of the proton radius. In addition to the direct scientific goals of the project, the experiments provide students and young scientists experience and training in working in the international collaborations of modern scientific experiments, with state of the art technology.The primary goal of this proposal is study the structure of the nucleon. The ongoing Fermilab SEAQUEST experiment uses Drell-Yan cross sections to determine the ratio of anti-d to anti-u quarks in the nucleon as a function of quark momentum, to study transverse momentum distributions, to investigate the EMC effect, to study quark energy loss in cold nuclear matter, and to investigate the possible existence of dark photons in a parameter region of higher mass but lower coupling constant to ordinary matter. The Fermilab MINERvA experiment provides a complementary view of nucleon and nuclear structure by scattering few GeV neutrinos from protons and from nuclei. At Jefferson Lab, experiments on tritium targets will be run in 2017. The MARATHON experiment, through a comparison of deep inelastic scattering of electrons on 3H to 3He, provides information on the ratio of the proton and neutron F2 structure functions at high x, testing decades old perturbative quantum chromodynamics model calculations. The preliminary phases of creating a new experiment to elastically scatter muons and electrons from the proton, the MUSE experiment at the Paul Scherrer Institute in Switzerland, are in progress, with the goal of measuring the distribution of charge in the proton.