This proposal will support continuing leadership roles for the Indiana University experimental nuclear physics group in three critical areas of basic research: (1) understanding the contributions from the internal "sea" of gluons and quark-antiquark pairs to the overall spin and magnetism of the proton and neutron; (2) elucidating the properties of nuclear matter at extreme temperatures, such as those believed to pertain during the earliest instants of the universe's evolution following its Big Bang origin; (3) testing the ability of the Standard Model of particle physics to account precisely for the properties and interactions of low-energy neutrons and of neutrinos. Measurements in category (1) will be made via two complementary techniques: studies of spin-polarized proton beam collisions at high energy exploiting the Relativistic Heavy Ion Collider (RHIC) facility and a major new addition to the STAR detector at that facility just completed by the Indiana group; comparative studies of neutrino-proton and neutrino-neutron scattering processes. Experiments relevant to category (2) will also utilize RHIC, with focus on the production in high-energy nucleus-nucleus collisions of mesons composed from a heavy quark-antiquark pair, and on differences between the interactions of quarks vs. gluons in the unique, hot matter produced in those collisions. Studies of fundamental particle interactions will include: completion of an ongoing experiment to confirm or refute a controversial signal for the oscillation of neutrinos from one type to another, a signal that cannot be easily accommodated in the emerging picture of neutrino states in nature; dramatic advances in the state-of-the-art for tests of nature's nearly perfect time-reversal symmetry, by exploiting innovative concepts to search for forbidden electric interactions of the spin of an electron; improved characterization of the normally masked weak interaction among neutrons and protons via highly sensitive exposure of deviations from perfect mirror symmetry in neutron-nucleus interactions and in nuclear structure. The proposed research will also involve the development of innovative new technology for tracking products from neutrino interactions and for producing beams of "ultra-cold" neutrons for fundamental interaction experiments
