RUI: Search for Anomalous Proton Spin Interactions with A Dual-Isotope Rubidium Magnetometer

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This award supports continuation of a research program using a dual-isotope rubidium (Rb) magnetometer to search for a hypothetical long-range coupling between Rb nuclear spins and the mass of the earth. It will also support research to explore use of the dual-isotope Rb magnetometer and related experimental techniques to search for anomalous intermediate-range (over distance scales of about one meter) and short-range (over distance scales of about one nanometer) spin-spin couplings. The dual-isotope Rb magnetometer is particularly sensitive to non-magnetic, spin-dependent interactions of the proton so that anomalous interactions of the proton spin produce a differential shift between the Rb spin-precession frequencies, whereas most sources of systematic error produce common-mode shifts of the spin-precession frequencies which can be controlled through auxiliary measurements. The majority of recent searches for similar effects limit anomalous couplings of either the neutron or electron spin, so the proposed experiment searches a parameter space to some degree, depending on the theoretical model, orthogonal to that constrained by previous experiments. The optimized dual-isotope Rb magnetometer has sufficient shot-noise-projected sensitivity to improve experimental limits on long-range spin-mass couplings by an order of magnitude in general and by two orders of magnitude for the proton spin in particular, and improve limits on anomalous spin-spin couplings by over an order of magnitude.There is renewed interest in scalar-tensor theories of gravity with massless scalar/pseudoscalar fields since they can explain the recent observation of "dark energy" over cosmological distances. The pseudoscalar component of such a field generates a long-range interaction between spins and the gravitational field of the Earth. This interaction in the simplest scalar-tensor theory causes a shift of the spin precession frequencies smaller than present limits but just within the range of this experiment. Detection of this type of interaction would be evidence that gravity violated parity and time-reversal symmetries, as well as the equivalence principle which underlies the theory of general relativity. A result consistent with general relativity would still provide new tests of numerous alternative theories, such as torsion gravity, that hypothesize anomalous spin-mass and spin-spin couplings. As the only externally funded physics experiment in the history of California State University / East Bay, this research program has led to tripling the number of physics majors in the past three years and sixteen students, including four women and nine underrepresented minority students, have made meaningful contributions to the project.