This award supports a nuclear physics experimental project at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. The goal of the experiment is a better understanding of the internal quark and gluon structure of the proton and neutron constituents of nuclear matter. The proposed research will provide fundamental information on the momentum distributions of the quarks inside the nucleons of the helium-3 and tritium atoms, using a new method of comparing directly measurements of inelastic electron scattering from helium-3 and tritium nuclei. This method is complementary to experiments done in the past at the Stanford Linear Accelerator Center, which showed the existence of point-like "partons" inside the nucleons, by scattering electrons from hydrogen and deuterium nuclei. The new method will eliminate inherent theoretical uncertainties in the Stanford method. The results will test long-standing predictions of the Quark Model of the nucleon, and of Quantum Chromodynamics, the theory describing the interactions between quarks and gluons. In addition, a new proposal will be prepared for a measurement of elastic electron scattering from the tritium nucleus, which will provide critical information for the development of a "standard model" describing the structure and dynamics of the simplest, lightest nuclei in nature. The project is expected to make significant contributions to the advancement of basic nuclear physics science, and to graduate education and training by offering doctoral dissertation topics on related science and instrumentation, which provide experience needed for careers in education, medical physics, homeland security, high technology, and national laboratories research.
The project will make use of a tritium, helium-3, and deuterium target system under development at Jefferson Lab, and magnetic spectrometers in the Hall A facility of the Lab. Unpolarized electrons with energy up to 11 gigaelectronvolt will be scattered from the above nuclear targets. For the inelastic measurements, scattered electrons will be detected with the electron High Resolution Spectrometer and a refurbished BigBite Spectrometer. The ratio of the measured cross sections for tritium and helium-3 will determine reliably the ratio of the neutron to proton structure functions, and subsequently the ratio of the probability distributions for an up quark compared to a down quark in the nucleon. The ratio of the tritium and helium-3 cross sections to the deuterium cross section will provide additional information about the effect of the nuclear environment on quarks in the nucleon. A comparison of the effect for these two mirror nuclei is considered to be essential for a full explanation of this effect. For the elastic measurements, the hadron High Resolution Spectrometer will be also used for detection of tritium recoiling nuclei. The two tritium form factors will be measured up to large momentum transfers by means of a Rosenbluth separation. This group will play a major role in all aspects of the project, namely the i) refurbishment, checkout, and calibration of the BigBite spectrometer, ii) preparation of all experimental apparatuses to be used, iii) acquisition of the experimental data, iv) data analysis, and v) publication of the results. The graduate students involved will be exposed to and gain expertise with both hardware and software development.
