To search for new laws of nature beyond our current understanding researchers can use very high energy accelerators, such as the Large Hadron Collider at CERN, or alternatively they can make precise experimental measurements of basic particle properties and search for deviations from the known physical laws. This award supports research that centers around experiments making precise measurements of physical properties of the free neutron. In the UCNA (Ultra-Cold Neutron Asymmetry) experiment, carried out at the Los Alamos National Laboratory, extremely slow neutrons are used, which have such low energy that they can be trapped and studied. This allows the scientists to make a precise measurement of the radioactive decay of the neutron into a proton, an electron and a particle called an anti-neutrino. This provides needed information about the theory of radioactivity (also called the weak nuclear force) as well as looking for small deviations from the known physical laws as a signature of new physics. In addition, this group is leading the development of equipment for a very sensitive experiment to search for the electric dipole moment (EDM) of the neutron. The existence of this EDM would be a signal of new physics beyond our current knowledge and could help to explain why the observed universe is dominated by matter. Our present Standard Model of particle physics predicts that there should be nearly equal amounts of matter and antimatter in today's universe, which is not observed. This remains one of the great puzzles in our basic understanding of the evolution of our universe. Participation by postdoctoral scholars, graduate students, and undergraduates is an essential part of the research program, affording young researchers exceptional opportunities to advance their training and education in this frontier area of nuclear physics.

This project is primarily focused on experimental research addressing fundamental properties of the free neutron. In particular the group will continue analyzing data from the UCNA experiment, in particular searching for new scalar and tensor forces not present in our present understanding of particle interactions. The group is also constructing experimental equipment for a new, highly sensitive experiment to search for the neutron's EDM. New results from the UCNA experiment will provide a sensitive search for new scalar or tensor components in the weak force, the so-called Fierz interference, by comparing the energy dependence of both the beta-decay asymmetry and the electron energy spectra simultaneously to further improve the sensitivity. The PI and his group are also collecting data for an improved measurement of the neutron lifetime using UCN at the Los Alamos UCN facility and anticipate results from both of these projects during the period of this grant. The PI plays a leading role in the development and construction of a new experiment searching for the electric dipole moment of the free neutron. The goal of the new neutron Electric Dipole Moment experiment - nEDM - at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is to improve the sensitivity of the measurement by two orders of magnitude compared to previous work. The observation of a neutron electric dipole moment signals violation of time-reversal symmetry, and could provide important information related to the origin of the matter-antimatter asymmetry of the universe and the new physical phenomena that is responsible for it. The critical cryogenic magnets and magnetic shielding of the experiment will be commissioned at Caltech and should receive the first cold-neutron beam at SNS for neutron polarization studies during the period of this proposed grant.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

National Science Foundation (NSF)
Division of Physics (PHY)
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Allena K. Opper
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California Institute of Technology
United States
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