MICROSCOPE limits for new long-range forces and implications for unified theories

Abstract

Many theories beyond the Standard Model involve an extra U(1) gauge group. The resulting gauge boson U, in general mixed with the Z and the photon, may be massless or very light, and very weakly coupled. It may be viewed as a generalized dark photon interacting with matter through a linear combination (\epsilon_Q Q + \epsilon_B B+\epsilon_L L) e, involving B-L in a grand-unified theory, presumably through B-L-.61 Q, inducing effectively a very small repulsive force between neutrons. This new force, if long-ranged, may manifest through apparent violations of the Equivalence Principle. They are approximately proportional to \epsilon_B+\epsilon_L/2, times a combination involving mostly \epsilon_L. New forces coupled to B-L or L should lead to nearly opposite values of the E\"otv\"os parameter \delta, and to almost the same limits for \epsilon_{B-L} or \epsilon_L, as long as no indication for \delta \neq 0 is found. We derive new limits from the first results of the MICROSCOPE experiment testing the Equivalence Principle in space. A long-range force coupled to (\epsilon_Q Q + \epsilon_{B-L} (B-L)) e or (\epsilon_Q Q + \epsilon_L L) e should verify |\epsilon_{B-L}| or |\epsilon_L| < .8 10^{-24}, and a force coupled to (\epsilon_Q Q + \epsilon_B B) e, |\epsilon_B| < 5 10^{-24}. We also discuss, within supersymmetric theories, how such extremely small gauge couplings g", typically \simle 10^{-24}, may be related to a correspondingly large \xi"D" term associated with a huge initial vacuum energy density, \propto 1/g"^2. The corresponding hierarchy between energy scales, by a factor \propto 1/\sqrt g" \simge 10^{12}, involves a very large scale ~ 10^{16} GeV, that may be associated with inflation, or supersymmetry breaking with a very heavy gravitino, leading to possible values of {\delta} within the experimentally accessible range.