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.
With the discovery of the Higgs boson, the last major fundamental prediction of the Standard Model of particle physics has been confirmed. Nonetheless, there remain several fundamental mysteries in modern physics that defy explanation within the Standard Model, such as the origin of the predominance of matter over antimatter in the universe and the nature of dark matter and dark energy. To date no prediction of a new particle theory extending beyond the Standard Model has been confirmed experimentally, and thus, in some sense, fundamental physics has entered an ``era of speculation'' where it is likely that many possibilities must be explored before solutions to these fundamental mysteries will be found. One such possibility is that the new physics required to explain the matter-antimatter asymmetry, dark matter, or dark energy will be in the form of new fundamental forces, and it is possible that these new forces will interact with the intrinsic spins of elementary particles. Our research program at California State University - East Bay has focused on the search for such new spin-dependent forces. The two major scientific goals of this project are (1) to search for a long-range spin-mass (spin-gravity) coupling of the proton using a dual-isotope rubidium comagnetometer and (2) to search for short-range anomalous spin-spin interactions. During the period of this grant we have completed our search for anomalous short-range spin-spin interactions using two different methods (a) studying spin-exchange collisions between helium-3 and potassium, which established the first constraints on anomalous spin couplings of the neutron at the atomic scale, and (b) studying molecular energy shifts (scalar J-coupling) in deuterated molecular hydrogen, which improved constraints on proton spin couplings at the atomic scale by over two orders of magnitude and neutron spin couplings by over eight orders of magnitude. The development and construction of the experimental apparatus for our search for a long-range spin-mass (spin-gravity) coupling has been completed (see figure) and we are now in the data collection and analysis phase. We anticipate this experiment to be completed soon as we are presently taking data. We have carried out and published a comprehensive review of systematic effects, and have developed experimental schemes to control all known systematic errors at our proposed level of sensitivity. This experiment should enable detection of long-range spin-mass (spin-gravity) interactions with the proton at levels more than an order-of-magnitude weaker than have previously been searched for. One of the most important results of our project is the broader impact of the work on our students and community. Prior to this project, there had never been an externally funded physics research project at California State University - East Bay. Since the beginning of this project, more than forty undergraduate students have worked in our research laboratory. These students have won a number of awards and gone on to excellent jobs in the high-tech industry (at national labs, Hewlett-packard, and Google) and several are now enrolled in top Ph.D. programs (University of California at Berkeley, Pennsylvania State University, University of New Mexico, University of California at Davis). California State University - East Bay is one of the most diverse undergraduate institutions in the nation and our program has involved a significant number of women and underrepresented minorities in cutting-edge physics research. Another important outcome has been the recent publication of a text that the PI coedited with Dmitry Budker from the University of California at Berkeley entitled "Optical Magnetometry" (Cambridge University Press, 2013), describing experimental techniques we have employed in this project, disseminating our work to a wide audience and facilitating new applications of the technology and techniques developed during this research.
