RUI: Continuation of Support for Short-range Tests of Gravity at Humboldt State University

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Humboldt State University (HSU) is a federally-recognized Hispanic Serving Institution in a predominantly rural area of Northern California, and the Gravitational Physics Laboratory (GPL) is the only dedicated hands-on physics research laboratory in the Department of Physics and Astronomy. This award ensures that HSU undergraduates, many of whom are first-generation college students and members of underrepresented groups in STEM fields, gain valuable research skills necessary for the pursuit of graduate studies or careers in industry. Undergraduates will also be sent to perform research at the University of Washington through a collaboration formed with a gravity research group there. This collaboration gives the students experience working in a world-class research environment. Alumni of the laboratory have secured prestigious graduate fellowships and industrial positions. This award greatly contributes to the expansion of research at HSU in the STEM fields. Within the major program, software and hardware developed for this project will be transferable to educational opportunities in the upper division classroom. This award also allows undergraduates to participate in valuable dissemination efforts through presentations at national and regional conferences, as well as publication of research results. Finally, tests of gravity are of great interest to research in diverse fields ranging from astrophysics and cosmology to particle physics and precision measurement science. This award supports high-precision experiments that will test the Weak Equivalence Principle (WEP) of General Relativity and the gravitational Inverse-Square Law (ISL) at unprecedented levels. The WEP will be tested at the millimeter scale and the ISL will be probed over sub-millimeter distances with unmatched sensitivity down to approximately 25 microns. Both tests use a novel parallel-plate torsion pendulum and attractor mass design. In recent years the study of the gravitational interaction at short (sub-millimeter) distance scales has gained much attention due in a large part to unification scenarios arising in string or M theory that require more than three spatial dimensions, some of which could be macroscopic. Short-range tests of gravity and the WEP also investigate possible proposed mechanisms that attempt to explain the accelerated expansion of the universe, generally attributed to Dark Energy. Finally, such tests present the possibility to search for hypothetical new interactions due to exotic particles or other phenomena. Many scenarios concerning these effects predict a violation of the WEP or ISL at short distances. Since 1999, experimental limits have improved dramatically so that the ISL has proven valid over distances down to approximately 55 microns. A dedicated test of the WEP, however, has not been performed below the millimeter scale. The work described in this proposal will probe the WEP for test mass separations at the sub-millimeter scale. This measurement will be accomplished with a parallel-plate torsion pendulum design that will provide an essentially null experiment and use multiple composition dipole combinations. Similarly, an unparalleled test of the gravitational ISL will be pursued by decreasing the pendulum/attractor test mass separation to 100 microns.