Constraints on monopole-dipole potential from tests of gravity

Abstract

An ultralight axion like particle (ALP) can mediate a macroscopic force with long-range monopole-dipole interactions between the Earth and the Sun, if the Earth is treated as a polarized source. Because of the geomagnetic field, there exists an estimated 10 ratio 42 polarized electrons within the Earth. These electrons, in a polarized state, can interact with the unpolarized nucleons in the Sun, giving rise to a monopole-dipole potential between the Sun-Earth system. This phenomenon ultimately influences the trajectories of light and celestial bodies, resulting in observable effects such as gravitational light bending, Shapiro time delay, and perihelion precession of planets. We investigate two scenarios for constraining the monopole-dipole coupling strength. In the first scenario, we establish a constraint on the monopole-dipole strength based on a single astrophysical observation for the first time, treating the Earth as a source of polarized electrons. The perihelion precession of Earth sets an upper limit on the monopole-dipole coupling strength as gSgP ≲ 1.75×10−16 for the ALP of mass ma ≲ 1.35 × 10−18 eV. This bound surpasses the limits obtained from gravitational light bending and Shapiro time delay. In the second scenario, constraints on monopole-monopole coupling strength gS (≲ 3.51 × 10−25) arise from the perihelion precession of the planet Mars, while the limit on dipole-dipole coupling strength gP (≲ 1.6 × 10 −13) is taken from the measurement of the tip of the red giant branch in ω Centauri using Gaia DR2 data. Together, they yield a hybrid constraint on the monopole-dipole coupling strength as gSgP ≲ 5.61 × 10−38. Our hybrid bound is 3 orders of magnitude more stringent than the Eöt-Wash experiment and 1 order of magnitude stronger than the current hybrid (Lab)NS × (Astro)eP limit.