Search for macroscopic CP violating forces using a neutron EDM spectrometer

The search for CP violating forces between nucleons in the so-called axion window of force ranges $\lambda$ between $2\cdot10^{-5}\,{\rm m}$ and $0.02\,{\rm m}$ is interesting because only little experimental information is available there. Axion-like particles would induce a pseudo-magnetic field for neutrons close to bulk matter. A laboratory search investigates neutron spin precession close to a heavy mirror using ultracold neutrons in a magnetic resonance spectrometer. From the absence of a shift of the magnetic resonance we established new constraints on the coupling strength of axion-like particles in terms of the product $g_sg_p$ of scalar and pseudo-scalar dimensionless constants, as a function of the force range $\lambda$, $g_sg_p\lambda^2\leq2\cdot10^{-21}\,[{\rm cm}^2]$ (C.L.95%) for $10^{-4}\,{\rm cm}<\lambda<1\,{\rm cm}$. For $0.1\,{\rm cm}<\lambda<1\,{\rm cm}$ previous limits are improved by 4 to 5 orders of magnitude.