We present for the first time a detailed and comprehensive analysis of the experimental results that set the current world sensitivity limit on the magnitude of the electric dipole moment (EDM) of the neutron. We have extended and enhanced our earlier analysis to include recent developments in the understanding of the effects of gravity in depolarizing ultracold neutrons; an improved calculation of the spectrum of the neutrons; and conservative estimates of other possible systematic errors, which are also shown to be consistent with more recent measurements undertaken with the apparatus. We obtain a net result of d n ¼ −0.21 AE 1.82 × 10 −26 e cm, which may be interpreted as a slightly revised upper limit on the magnitude of the EDM of 3.0 × 10 −26 e cm (90% C.L.) or 3.6 × 10 −26 e cm (95% C.L.).

We present the result of an experiment to measure the electric dipole moment (EDM) of the neutron at the Paul Scherrer Institute using Ramsey's method of separated oscillating magnetic fields with ultracold neutrons. Our measurement stands in the long history of EDM experiments probing physics violating timereversal invariance. The salient features of this experiment were the use of a 199 Hg comagnetometer and an array of optically pumped cesium vapor magnetometers to cancel and correct for magnetic-field changes. The statistical analysis was performed on blinded datasets by two separate groups, while the estimation of systematic effects profited from an unprecedented knowledge of the magnetic field. The measured value of the neutron EDM is d n ¼ ð0.0 AE 1.1 stat AE 0.2 sys Þ × 10 −26 e:cm.

We search for a spin-dependent P -and T -violating nucleon-nucleon interaction mediated by light pseudoscalar bosons such as axions or axion-like particles. We employed an ultra-sensitive low-field magnetometer based on the detection of free precession of co-located 3 He and 129 Xe nuclear spins using SQUIDs as low-noise magnetic flux detectors. The precession frequency shift in the presence of an unpolarized mass was measured to determine the coupling of pseudoscalar particles to the spin of the bound neutron. For boson masses between 2 µeV and 500 µeV (force ranges between 3·10 −4 m -10 −1 m) we improved the laboratory upper bounds by up to 4 orders of magnitude. origin into a photon in the presence of a static magnetic field. However, any axion or axion-like particle that couples with both scalar and pseudoscalar vertices to fundamental fermions would also mediate a parity and time-reversal symmetryviolating force between a fermion f and the spin of another fermion f σ , which is parameterized by a Yukawatype potential with range λ and a monopole-dipole coupling given by [8]:σ is the spin vector and λ is the range of the Yukawa-force with λ= /(m a c). Thus, the entire axion window can be probed by searching for spin-dependent short-range forces in the range between 20 µm and 0.2 m. g f s and g fσ p are dimensionless scalar and pseudoscalar coupling constants which in our case correspond to the scalar coupling of an axion-like particle to a nucleon (g . Accordingly, we have m fσ = m n .r is the unit distance vector from the bound neutron to the nucleon. The potential given by Eq. 1 effectively acts near the surface of a massive unpolarized sample as a pseudomagnetic field and gives rise to a shift ∆ν sp = 2 · V Σ /h, e.g., in the precession frequency of nuclear spin-polarized gases ( 3 He and 129 Xe), which according to the Schmidt model [9] can be regarded as an effective probe of spinpolarized bound neutrons. The potential V Σ is obtained by integration of V sp (r) from Eq. 1 over the volume of the massive unpolarized sample averaged over the volume of the polarized spin-sample, each having a cylindrical shape. Based on the analytical derivation of V Σ,∞ for disc-shaped spin-and matter samples with respective thicknesses D and d [10], we obtainη(λ) takes account for the finite size in transverse direction of our cylindrical samples and ∆x represents the finite gap between them. Furthermore, κ = 2 g N s g n p /(8π · m n ) and N is the nucleon number density of the unpolarized matter sample. η(λ) 1 is determined numerically for our cylindrically shaped spin-and matter samples at "close"-position (see Fig. 1). Our experimental approach to search for non-magnetic, spin-dependent interactions is to use an ultra-sensitive low-field comagnetometer based on detection of free spin precession of gaseous, nuclear polarized samples [11]. The Larmor frequencies of 3 He and 129 Xe in a guiding magnetic field B are given by ω L,He(Xe) = γ He(Xe) · B, with γ He(Xe) being the gyromagnetic ratios of the respective gas species...

We discuss the design and performance of a very sensitive low-field magnetometer based on the detection of free spin precession of gaseous, nuclear polarized 3 He or 129 Xe samples with a SQUID as magnetic flux detector. The device will be employed to control fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron as well as in a new type of 3 He/ 129 Xe clock comparison experiment which should be sensitive to a sidereal variation of the relative spin precession frequency. Characteristic spin precession times after one day. Even in that sensitivity range, the magnetometer performance is statistically limited, and noise sources inherent to the magnetometer are not limiting. The reason is that free precessing 3 He ( 129 Xe) nuclear spins are almost completely decoupled from the environment. That makes this type of magnetometer in particular attractive for precision field measurements where a long-term stability is required.

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