The Yukawa potential of a non-homogeneous sphere, with new limits on an ultralight boson

Abstract

Extremely weak long-range forces may lead to apparent violations of the Equivalence Principle. The final MICROSCOPE result, leading at 95 % c.l. to $|\delta| < 4.5 \times 10^{-15}$ or $6.5 \times 10^{-15}$ for a positive or negative E\"otv\"os parameter $\delta$, requires taking into account the spin of the mediator, and the sign of $\Delta (Q/A_r)_{\rm{Ti-Pt}}$ ($Q$ denoting the new charge involved). A coupling to $B-L$ or $B$ should verify $|g_{B-L}|<1.1 \times 10^{-25}$ or $|g_{B}| < 8 \times 10^{-25}$, for a spin-1 mediator of mass $m < 10^{-14}$ eV$/c^2$, with slightly different limits of $1.3 \times 10^{-25}$ or $\,6.6 \times 10^{-25}$ in the spin-0 case. The limits increase with $m$, in a way which depends on the density distribution within the Earth. This involves an hyperbolic form factor, expressed through a bilateral Laplace transform as $\Phi(x=mR)= \langle\,\sinh mr/mr \,\rangle$, related by analytic continuation to the Earth form factor $\Phi(ix)= \langle \,\sin mr/mr \,\rangle $. It may be expressed as $\Phi(x) = \frac{3}{x^2}\, (\cosh x - \frac{\sinh x}{x}) \times\, \bar\rho(x)/\rho_0\,$, where $\bar\rho(x)$ is an effective density, decreasing from the average $\rho_0$ at $m=0$ down to the density at the periphery. We give general integral or multishell expressions of $\Phi(x)$, evaluating it, and $\bar\rho(x)$, in a simplified 5-shell model. $\Phi(x)$ may be expanded as $\, \sum \frac{x^{2n}}{(2n+1)!} \frac{\langle \,r^{2n}\,\rangle}{R^{2n}} \simeq 1 + .0827\ x^2 + .00271 \ x^4 + 4.78 \times 10^{-5}\,x^6 + 5.26\times 10^{-7}\, x^8 +\ ... \ $, absolutely convergent for all $x$ and potentially useful up to $x\approx 5$. The coupling limits increase at large $x$ like $mR \ e^{mz/2}/\sqrt{1+mr}$ ($z=r-R$ being the satellite altitude), getting multiplied by $\simeq 1.9,\ 34$, or $1.2\times 10^9$, for $m = 10^{-13},\ 10^{-12}$ or $10^{-11}$ eV$/c^2$, respectively.