Progress of the nEDM experiment at the Paul Scherrer Institute

Advances in experimental searches for a neutron Electric Dipole Moment (nEDM, d _n ) are motivated by the potential discovery of a new source of CP violation beyond the Standard Model of particle physics. The nEDM experiment at the Paul Scherrer Institute (PSI), which with accumulated sensitivity of 1.09 × 10^?26 e?cm (September 2016) is currently the most sensitive nEDM experiment worldwide, uses the Ramsey technique of separated oscillatory fields applied to stored ultracold neutrons. The nEDM measurements depend upon precise information about the magnetic field, which is monitored by a ¹⁹⁹Hg co-magnetometer and an array of ¹³³Cs magnetometers. The principle of the magnetic field measurement is based on the optical detection of the Larmor precession frequency of atoms polarized by optical pumping. In this article we present the recent progress of the nEDM experiment as well as details of a magnetic field measurements with special focus on the laser-operated array of high-sensitivity ¹³³Cs magnetometers.     

Present status of the 129Xe comagnetometer development for neutron EDM measurement

A ¹²⁹Xe comagnetometer designed for the measurement of neutron electric dipole moment (nEDM) as precisely as 1 × 10^?27e cm is presented. Highly nuclear spin polarized ¹²⁹Xe are introduced into an EDM cell where the ¹²⁹Xe spin precession is detected by means of the two-photon transition. The geometric phase effect (GPE) which generates the false nEDM was quantitatively discussed and the systematic error of nEDM from the GPE was estimated considering the buffer-gas suppression due to Xe atomic collisions. Research and development are in progress to construct the ¹²⁹Xe comagnetometer with a field sensitivity of 0.3 fT. At present, about 70 % nuclear spin polarized ¹²⁹Xe atoms have been obtained in a spin exchange opitial pumping cell, that are in the process of being transferred into the EDM cell via a cold trap.     

Detection of 3He spins with ultra-low field nuclear magnetic resonance employing SQUIDs for application to a neutron electric dipole moment experiment

The precession of ³He spins is detected with ultra-low field NMR. The absolute strength of the NMR signal is accurately measured and agrees closely with theoretical calculations. The sensitivity is analyzed for applications to a neutron electric dipole moment (nEDM) fundamental symmetry experiment under development.     

CP violation in gauge theories and the electric dipole moment of the neutron

A nonvanishing contribution to the neutron electric dipole moment in CP-violating gauge theories of the weak interactions, arising from interaction of the photon with two-quark subsystems of the three-bound-quark neutron system, is calculated. In the Kobayashi-Maskawa model the resulting value of the moment is estimated as O(10^?32) e cm; however, strong interaction corrections (gluonic radiative corrections) give quark moment contributions which may be numerically larger (possibly 10^?30±1 e cm). Either case clearly distinguishes gauge-sector CP violation from Higgs-sector CP violation which typically gives a neutron moment of order 10^?24 e cm.     

Rotation of neutron spin in passing through a noncentrosymmetric single crystal

Rotation of the spin of cold neutrons passing through a noncentrosymmetric single crystal is observed. This effect is caused by the Schwinger interaction of the magnetic moment of a moving neutron with the crystalline electric field in a noncentrosymmetric single crystal and depends both on the direction of neutron trajectory in the crystal and on its energy. It is shown that the characteristic magnitude of the effect for α-quartz is ?(1–2)×10^?4 rad/cm over a wide wavelength range (from 2.8 to 5.5 Å) and is determined by the degree of beam monochromaticity [Δλ/λ=(2–5)×10^?2 in our experiment]. This magnitude corresponds to an electric field of ?(0.5–1)×10⁵ V/cm acting on a neutron. The measured value agrees well with the theoretical calculation.     

The electric dipole moment of the neutron in the Kobayashi-Maskawa model

A possible mechanism for producing a neutron electric dipole moment to order G_F² in the Kobayashi-Maskawa model is proposed; the consequent order of magnitude of the dipole moment would be 10^?30 cm in units of the electric charge.     

New measurements of the neutron electric dipole moment

We report a new measurement of the neutron electric dipole moment with the PNPI EDM spectrometer using the ultracold neutron source PF2 at the research reactor of the ILL. Its first results can be interpreted as a limit on the neutron electric dipole moment of |d _n | < 5.5 × 10^?26 e cm (90% confidence level).     

Program of Fundamental-Interaction Research for the Ultracold-Neutron Source at the the WWR-M Reactor

The use of ultracold neutrons opens unique possibilities for studying fundamental interactions in particles physics. Searches for the neutron electric dipole moment are aimed at testing models of CP violation. A precise measurement of the neutron lifetime is of paramount importance for cosmology and astrophysics. Considerable advances in these realms can be made with the aid of a new ultracold-neutron (UCN) supersource presently under construction at Petersburg Nuclear Physics Institute. With this source, it would be possible to obtain an UCN density approximately 100 times as high as that at currently the best UCN source at the high-flux reactor of the Institute Laue–Langevin (ILL, Grenoble, France). To date, the design and basic elements of the source have been prepared, tests of a full-scale source model have been performed, and the research program has been developed. It is planned to improve accuracy in measuring the neutron electric dipole moment by one order of magnitude to a level of 10^?27 to 10^?28e cm. This is of crucial importance for particle physics. The accuracy in measuring the neutron lifetime can also be improved by one order of magnitude. Finally, experiments that would seek neutron–antineutron oscillations by employing ultracold neutrons will become possible upon reaching an UCN density of 10³ to 10⁴ cm^?3. The current status of the source and the proposed research program are discussed.     

Ion emission in humid air

An electric discharge with a current of about 1 pA that appears at an air humidity >60% in an electric field >10 kV/cm is described. The experimental data demonstrate the presence of a discharge induced by ion emission from a wet surface rather than of a corona discharge. Emission decreases the potential of the insulated electrode according to the law U(t) ～ e ^?k√t . The static electric field is measured with a fixed collector and a vibrating screening grid.     

PNPI differential EDM spectrometer and latest results of measurements of the neutron electric dipole moment

In this work, the double chamber magnetic resonance spectrometer of the Petersburg Nuclear Physics Institute (PNPI) designed to measure the neutron electric dipole moment (EDM) is briefly described. A method for long storage of polarized ultracold neutrons in a resonance space with a superposed electric field collinear to the leading magnetic field is used. The results of the measurements carried out on the ILL reactor (Grenoble, France) are interpreted as the upper limit of the value of neutron EDM |d_n| < 5.5 × 10^–26 e cm at the 90% confidence level.     

Search for P-ODD asymmetry in the radiative cross-section of the interaction of neutrons with lead nuclei

The P-odd effect in the radiation cross section of capture of longitudinally polarized neutrons in a sample of natural lead is measured. The experiment was performed at PF1B facility at the Institut Max von Laue-Paul Langevin. The neutron polarization P _n was 92%, the total flux of polarized neutrons was ～3 × 10¹⁰ n/s, and the mean neutron wavelength was λ = 4.7 Å. Taking into account “0-test” we estimated the asymmetry: a _γ(^natPb) = (2.3 ± 3.5) × 10^?7, i.e., α _γ ≤ 8.1 × 10^?7 at 90% confidence level.     

In the wonderland of ultra-parallel neutron beams

Bragg reflections from single crystals yield angular widths of a few arcsec for thermal neutron beams. The Bonse-Hart proposal to attain a sharp, nearly rectangular profile by Bragg reflecting neutrons multiply from a channel-cut single crystal, was realized in its totality three and a half decades later by achieving the corresponding Darwin reflection curves for 5.23 Å neutrons. This facilitated SUSANS (Super USANS) measurements in the Q ～ 10^?5 Å^?1 range. The polarized neutron option was introduced into the SUSANS set-up by separating the up- and down-spin neutron beams by ～10 arcsec with a magnetic (air) prism. The neutron angular width has recently been reduced further by an order of magnitude to ～0.6 arcsec by diffracting 5.3 Å neutrons from a judiciously optimized Bragg prism. This constitutes the most parallel monochromatic neutron beam produced to date. I present the first SUSANS spectra probing the Q ～ 10^?6 Å^?1 domain, recorded with this beam.     

Neutron optics of noncentrosymmetric crystals: New possibility of searches for the neutron electric dipole moment and CP-violating forces

New neutron-optics effects, which were predicted and discovered quite recently in noncentrosymmetric crystals and which can be used to study the fundamental properties of the neutron, are discussed. In particular, strong electric fields of strength up to 10⁸ or 10⁹ V/cm may act in such crystals on the neutron, and this provides new possibilities for measuring the neutron electric dipole moment (EDM) by the crystal-diffraction method. It also becomes possible to perform searches for pseudomagnetic forces acting on neutrons and violating CP invariance. For the range of such forces that satisfies the condition λ _A < 10^?5 cm, the best constraints on the product of the scalar and pseudoscalar coupling constants have already been obtained for the interaction induced by the exchange of a light pseudoscalar (axion-like) particle. The present-day status and prospects of neutron-optics crystal-diffraction experiments aimed at searches for the neutron EDM and pseudomagnetic forces are considered.     

Quantum properties of strong accumulation layers on ZnO surfaces

Extremely strong accumulation layers with surface electron densities ΔN approaching 10¹⁴ cm^?1 have been achieved on ZnO surfaces in contact with an electrolyte. Quantization effects, which are very pronounced in such narrow (?10 Å) layers, are studied by measurements of ΔN versus surface barrier height V_s. Comparison of the results with self-consistent calculations shows very good agreement up to ΔN = 2 × 10¹³ cm^?2. Deviations observed at higher ΔN are probably associated with the huge electric fields (～10⁷ V/cm) experienced by the surface electrons.     

The neutron electric dipole moment as a test of the superweak interaction theory

Theoretical calculations of the neutron electric dipole moment D_n are reviewed for various theories of CP violation. It is shown that for the superweak interaction theory D_n is less than 10^?29e · cm in contrast to values of 10^?23 to 10^?24 predicted by many but not all milliweak theories. It is concluded that prospective measurements of D_n may provide decisive evidence against or significant evidence in favour of the superweak theory.     

Magnetic-field-induced slowing-down of molecular rotation in C<Subscript>60</Subscript> crystals

The molecular dynamics of C₆₀ crystals was studied by inelastic neutron scattering at T=290 K, i.e., above the first-order phase transition temperature (T_C≈260 K), in the region of free C₆₀-spheroid rotation in the lattice. The energy broadening of the original neutron spectrum 2Γ₀≈0.1 meV for a momentum transfer q=2 Å^?1 is in agreement with NMR data on the rotational relaxation time of the molecule τ～10^?11 s～ ?Γ₀. This effect was observed to decrease in magnetic fields H=2.5–4.5 kOe applied along the scattering vector: Γ_H=0.7Γ₀. The slowing-down of the molecular rotation is discussed in connection with the interaction of a magnetic field with the molecular currents, which fluctuate when the C₆₀ cage rotates.     

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 λ between 2 × 10^?5 m and 0.02 m is interesting because only little experimental information is available there. Axionlike 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 _s g _p of scalar and pseudo-scalar dimensionless constants, as a function of the force range λ, g _s g _p λ² ≤ 2 × 10^?21 [cm²] (C.L.95%) for 10^?4 cm < λ < 1 cm. For 0.1 cm < λ < 1 cm previous limits are improved by 4 to 5 orders of magnitude.     

High-spin states in 166Lu

It is shown that the discrepancy between the results obtained for different neutron energy ranges, when neutron polarizability is derived from the neutron scattering data, can be removed if one assumes that at the fast neutron scattering a strong-interaction long-range potential of Van der Waals (～ r ^?6 ) or Casimir-Polder (～ r ^?7 ) is observed. This strong-interaction long-range potential has possibly some experimental confirmation in the elastic p-p scattering.     

Creation of electron-positron plasma with superstrong laser field

We present a short review of recent progress in studying QED effects within the interaction of ultra-relativistic laser pulses with vacuum and e ^? e ⁺ plasma. Current development in laser technologies promises very rapid growth of laser intensities in the near future. Two exawatt class facilities (ELI and XCELS, Russia) in Europe are already in the planning stage. Realization of these projects will make available a laser intensity of ～ 10²⁶?W/cm² or even higher. Therefore, discussion of nonlinear optical effects in vacuum are becoming compelling for experimentalists and are currently gaining much attention. We show that, in spite of the fact that the expected field strength is still essentially less than E _S = m ² c ³/e? = 1.32 · 10¹⁶?V/cm, the nonlinear vacuum effects will be accessible for observation at the ELI and XCELS facilities. The most promissory effect for observation is pair creation by a laser pulse in vacuum. It is shown, that at intensities ～ 5 · 10²⁵?W/cm², creation even of a single pair is accompanied by the development of an avalanche QED cascade. There exists a distinctive feature of the laser-induced cascades, as compared with the air showers arising due primarily to cosmic rays entering the atmosphere. In our case the laser field plays not only the role of a target (similar to a nucleus in the case of air showers) but is also responsible for the acceleration of slow particles. It is shown that the effect of pair creation imposes a natural limit for the attainable laser intensity and, apparently, the field strength E ～ E _S is not accessible for a pair-creating electromagnetic field at all.     

Specific features of the magnetoresistance in multilayer systems of magnetic nanoislands in weak magnetic fields

New magnetic structures such as multilayer systems of magnetic nanoislands being alternating layers of nanoislands of various magnets have been proposed. The electric, magnetic, and magnetooptical properties of the systems have been studied. The magnetoresistance of ～2% related to the anisotropic effect has been revealed. In multilayer structures of magnetic nanoislands, a unidirectional axis of predominant magnetization has been found, which changes its orientation depending on the structure parameters. The magnetic field required to reorient the axis in the opposite direction has been estimated to be 2 kOe < H _A < 20 kOe. The periodic multilayer structures of magnetic nanoislands are very sensitive to hyperweak magnetic fields (to 10^?6 Oe).