The major goal of subatomic physics is understanding the structure of the nucleon. The structure can determined by scattering proton, electron, or neutrino probes from nucleons. Each probe is sensitive to different aspects of the nucleon structure. By varying the energy of the probe, the structure can be determined at different length scales. The objectives of this project are to measure the structure of the nucleon at distances from about 10% of the nucleon size up to the size of the nucleon. In addition, the characteristics of neutrino interactions with various nuclei will be determined with the MINERvA experiment. The results of these measurements are an important ingredient for other experiments which aim to understand the properties of neutrinos.
Besides the scientific impact, this work will serve to train both undergraduate and graduate students, as well as post-doctoral students, in the methods of scientific research. This training in fundamental research provides a strong basis for careers in and outside of physics. Our students and post-docs have successfully moved to varied careers in medical physics, homeland security, and finance, as well as continuing in fundamental research.
Nuclear physics seeks to understand the structure of the nucleon, the modification of the structure of the nucleon in the nucleus, nuclear structure, and nuclear reactions. This project focuses on the first two objectives. The structure of the nucleon and its modifications in the nuclear environment are explained, in principle, by the theory of Quantum Chromo-dynamics (QCD). Although the calculations are difficult and still cannot accurately predict all observed properties, considerable progress has been made. Precise experiments are needed to test theory where predictions can be made, as well as guide phenomenological models which describe nuclear reactions with various probes. Measurements of reactions of neutrinos with the nucleon and nuclei also aid interpretation of experiments studying neutrino oscillations, another forefront topic in subatomic physics. The charge distribution of the nucleon has been a focus of our work for many years. A recent area of interest to the nuclear physics community is the proton radius puzzle, the difference between the radius when measured with electrons vs. muon probes. Our recent JLab experiment measured polarization transfer at modest momentum transfer, confirming the proton radius puzzle, and polarized beam - polarized target data at much lower momentum transfer are being analyzed. We now have an approved experiment at the Paul Scherrer Institute πM1 beam line to measure ep and μp elastic scattering simultaneously, to further test lepton universality, two-photon exchange corrections, and the proton radius puzzle. Much of our work this past year has been developing this novel experiment. Related to this, the dissertation data of our most recent Jefferson Lab student indicated that two-photon exchange in elastic electron-nucleon scattering is not sufficiently understood. A second focus has been the study of neutrino-nucleus interactions. The MINERvA experiment is studying neutrino interactions on several nuclei. The results obtained in this last year, part of which provided the thesis data for a Rutgers student, demonstrated the inadequacy of current models for neutrino-nucleus interactions and provided data that will improve the interpretation of data obtained in neutrino oscillation experiments. The most significant broader impact outcome of this research is the training of students and post-docs in the methods of basic research. Learning how to approach problems where the method of solution is unclear and which require a complex variety of techniques is one that is best approached through basic research. The students and post-docs trained by this grant and its predecessors have gone on to a wide variety of careers reflecting the strength of this training. Beyond academic careers, they have gone on to careers in medical physics, national security, and the financial industry. Their skills in solving problems with new and innovative techniques are developed and honed through their training in basic research. The project has also involved undergraduates. The opportunity to engage in basic research both trains and inspires younger students, encouraging them to pursue careers in science. Several of undergraduates working on this grant have gone on to graduate school at leading institutions, including Duke, Johns Hopkins, and Yale.
