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.

We report on the search for a CPT and Lorentz invariance violating coupling of the 3 He and 129 Xe nuclear spins (each largely determined by a valence neutron) to background tensor fields which permeate the universe. Our experimental approach is to measure the free precession of nuclear spin polarized 3 He and 129 Xe atoms in a homogeneous magnetic guiding field of about 400 nT using LTC SQUIDs as low-noise magnetic flux detectors. As the laboratory reference frame rotates with respect to distant stars, we look for a sidereal modulation of the Larmor frequencies of the co-located spin samples. As a result we obtain an upper limit on the equatorial component of the background field interacting with the spin of the bound neutronb n ⊥ < 6.7 · 10 −34 GeV (68% C.L.). Our result improves our previous limit (data measured in 2009) by a factor of 30 and the world's best limit by a factor of 5. [4,5] test the isotropy of the interactions of matter itself. Searches for an anomalous spin coupling to a relic background field which permeates the universe have been performed with electron and nuclear spins with increasing sensitivity [6][7][8][9][10][11][12][13][14][15][16][17][18]. The theoretical framework presented by A. Kostelecký and colleagues parametrizes the general treatment of CPT and Lorentz invariance violating (LV) effects in a Standard Model Extension (SME) [19][20][21]. The SME was conceived to facilitate experimental investigations of Lorentz and CPT symmetry, given the theoretical motivation for violation of these symmetries. Although Lorentz-breaking interactions are motivated by models such as string theory [21,22], loop quantum gravity [23][24][25][26], etc., the low-energy effective action appearing in the SME is independent of the underlying theory. The SME contains a number of possible terms that couple to the spins of fundamental Standard Model particles like the electron, or composite particles like the proton and (bound) neutron. These terms are small due to Planckscale suppression (M p ), and in principle are measurable in experiments by detecting tiny energy shifts of order ∆E (n) ∼ ( mw Mp ) n · m w , where the low energy scale is set by the mass m w of the particle. Since n = 1 is largely ruled out by present experiments [27], tuning the measurement sensitivity to second order effects (n = 2) in Planck scale suppression is the current challenge 1 . To de- * Corresponding author: allmendinger@physi.uni-heidelberg.de 1 For the neutron (mn = 939 MeV) this is ∆E (2) ≈ 10 −38 GeV.termine the leading-order effects of a LV potential V , it suffices to use a non-relativistic description for the particles involved given bywhich can be interpreted as a coupling of the electron, proton or neutron spin σ w J to a hypothetical background fieldb w J . The most sensitive tests so far were performed on the bound neutron using a 3 He-129 Xe Zeeman maser [12, 13], a 3 He-129 Xe co-magnetometer [28] based on free spin precession, and a K-3 He co-magnetometer [7]. The latter one so far gave the highest energy resol...

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couple to the spins of standard model particles like the electron, proton, and nucleon (mostly the bound neutron) [23]. These terms have set the most stringent limits on CPT and Lorentz violations. To determine the leading-order effects of a Lorentz violating potential V, it suffices to use a non-relativistic description for the particles involved given by [23](with J = X, Y, Z ; w = e, p, n) .(1)