The nucleon is not a hard sphere: screening fifth forces with Skyrmions

Abstract

Laboratory searches for chameleon and symmetron fifth forces increasingly rely on individual nucleons as sources or probes, but model them as uniform spheres. Motivated by the Skyrme picture, we instead consider a nucleon profile with a constant-density core and an $r^{-6}$ pion tail, together with its exponential truncation at the physical pion mass. We derive analytic screening charges for the uniform sphere and the $r^{-6}$ profile, and extend the comparison to the exponential truncation at the physical pion mass using numerical solutions. Because the outer tail is too dilute to be screened at any coupling strength, the $r^{-6}$ tail produces a power-law enhancement over the sphere charge in strong screening, and the exponential tail has a weaker, logarithmic enhancement. Propagating this difference through the published analyses, nucleon-sourced exclusion regions extend systematically further into the strongly coupled regime. For example, symmetron constraints from neutron interferometry at $μ=1\,\mathrm{keV}$ reach maximal self-couplings about $1.4$ orders of magnitude higher than for a uniform-sphere neutron ($3.0$ orders of magnitude in the chiral limit),and at the dark-energy scale the hydrogen $1s$--$2s$ chameleon bound reaches couplings $1.8$ orders of magnitude weaker than for a uniform-sphere proton ($2.4$ in the chiral limit), overtaking the muonium bound.