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...

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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...

We report on a new measurement of the CP-violating permanent Electric Dipole Moment (EDM) of the neutral 129 Xe atom. Our experimental approach is based on the detection of the free precession of co-located nuclear spin-polarized 3 He and 129 Xe samples. The EDM measurement sensitivity benefits strongly from long spin coherence times of several hours achieved in diluted gases and homogeneous weak magnetic fields of about 400 nT. A finite EDM is indicated by a change in the precession frequency, as an electric field is periodically reversed with respect to the magnetic guiding field. Our result, (−4.7 ± 6.4) · 10 −28 ecm, is consistent with zero and is used to place a new upper limit on the 129 Xe EDM: |dXe| < 1.5 · 10 −27 ecm (95% C.L.). We also discuss the implications of this result for various CP-violating observables as they relate to theories of physics beyond the standard model.

The detection of the free precession of co-located 3 He/ 129 Xe nuclear spins (clock comparison) is used as ultra-sensitive probe for non-magnetic spin interactions, since the magnetic dipole interaction (Zeeman-term) drops out in the weighted frequency difference, i.e., ω = ω He - Features of frequency standards and clocksSince Galileo Galilei and Christiaan Huygens invented the pendulum clock, time and frequency have been the quantities that we can measure with the highest precision. Since 1967 the Cs atomic clock defines our unit of time, the second, as the period during which a cesium-133 atom oscillates 9,192,631,770 number of cycles on the hyperfine clock transition |F = 4, m F = 0 → |F = 3, m F = 0 in the 6 2 S 1/2 atomic ground state. Cesium atomic clocks have been gradually improved to the point where modern cesium-fountain clocks realize the definition of the second with a relative uncertainty of about 4 × 10 −16 [1]. In the near future, the cesium clock defining the fundamental timing reference will be replaced with an optical clock, since suppression of systematic effects shifting the frequency of a standard is greatly facilitated by the use of higher frequencies.Thanks to the incredible high relative accuracy of frequency determination, atomic clocks may touch the μHz range on an absolute scale, but will essentially not go far below.To address fundamental questions in physics often associated with the experimental search for violation of fundamental symmetries in nature, much smaller frequencies or frequency shifts as a result of tiny changes in the clock transition must be tracked. From that point of view it is more appropriate to develop a "clock" that oscillates at low frequencies (∼10 Hz), but shows the same relative accuracy as a Cs atomic clock. Thus, frequency shifts in the pHz range caused by hypothetical interaction potentials might be accessible."Spin clocks" which are based on nuclear spin precession are the most promising approach to reach such sensitivity limits. The ,,spin clock" described here is based on the detection of free spin-precession of gaseous, nuclear spin-polarized 3 He or 129 Xe samples [2]. Like in a free induction decay (FID) measurement, the decay of the transverse magnetization is monitored and the Larmor frequency ω of the precessing sample magnetization is related to the magnetic field B 0 through ω = γ · B 0 , where γ is the gyromagnetic ratio of the corresponding nucleus. Since this type of clock will preferably operate at low magnetic fields and thus at low frequencies, using a SQUID as magnetic field detector is appropriate due to its high sensitivity in that spectral range. The 3 He/ 129 Xe nuclear spins are polarized by means of optical pumping. Thus, the nuclear polarization obtained exceeds the Boltzmann polarization as used in typical NMR experiments by four to five orders of magnitude.