New search for the neutron electric dipole moment with ultracold neutrons at ILL

The search for an electric dipole moment (EDM) of the neutron is a crucial test for theoretical particle physics models with violation of time and spatial invariance. A new experiment recently has been carried out at the High-Flux Reactor at Institut Laue-Langevin, using the upgraded double-chamber magnetic resonance spectrometer developed at Petersburg Nuclear Physics Institute. The result is interpreted as an upper limit on the value of the neutron EDM, $|d_n|<5.5\times {10}^{-26}{e}_{}\mathrm{cm}$ (90% C.L.). This article provides a detailed description of the setup and experimental procedures, along with a discussion of possibilities for further improvement of the experimental accuracy.

Figure 1

Sensitivity of present and future EDM measurements to the mass parameters of an exemplary supersymmetric model relevant for discussion of the electroweak baryon genesis (from Ref. [14]).

Figure 2

Chronology of experimental upper limits on the neutron EDM and a perspective for an increase of precision.

Figure 3

Central parts of the Ramsey resonance curves measured using the detectors D1 and D2 connected to the two neutron chambers (see Fig. 4) and for phase shifts of 90° and 270° between the rf-pulses. The visibility of the resonance was about 0.7 for T = 95 s.

Figure 4

Layout of the EDM spectrometer (vertical cut through the center).

Figure 5

Photograph of the PNPI spectrometer implemented on its platform in the experimental hall of ILL.

Figure 6

Measurements of UCN storage in the traps.

Figure 7

Block scheme of the power module.

Figure 8

Layout of the cascade voltage multiplier.

Figure 9

External view on the cascade multiplier.

Figure 10

The magnetometer channels of the EDM spectrometer and a dismounted pair of magnetometers. The black cables contain the light guides.

Figure 11

Block diagram of the noise-immune cesium magnetometer: (1) platform made of Plexiglas; (2) working cell; (3) rf coil; (4) lens; (5) prism; (6) circular polarizer and D1 filter; (7 and 8) light guides; (9) spectral lamp unit; (10) feedback amplifier; (PD) photodiode, (WBA) wideband amplifier, 0.01–2 kHz; (AGC) automatic gain control unit; (PS) phase shifter; (LED) light-emitting diode, part of the optoelectronic device; (OEC) optoelectronic couple; (11) frequency meter; (12) part of the optoelectronic device placed on the magnetometer platform (PD is a photodiode of nonmagnetic design, and C is a blocking capacitor).

Figure 12

Block scheme for averaging of frequencies of a quartet of magnetometers.

Figure 13

System for producing the rf field for neutron magnetic resonance. М1–М8: the $\mathrm{Cs}$ self-generating magnetometers; В1 and В2: devices for averaging the output frequencies from the $\mathrm{Cs}$ magnetometers of the top and bottom neutron chambers, respectively; В3: device for averaging the mean frequencies from B1 and B2; D: digital synthesizer producing the signal for the neutron rf coil.

Figure 14

Two examples of magnetic field fluctuations in the spectrometer. Shown are averaged values from the $\mathrm{Cs}$ magnetometer close to the upper and the lower neutron chambers, respectively, and the mean value of them. (a) Quite magnetic situation; (b) Noisy magnetic environment caused by bridge-crane movements. Correlated changes of the magnetic field with amplitudes up to 1 nT are observed.

Figure 15

Mean magnetic field fluctuations in the UCN storage volume area measured by means of $\mathrm{Cs}$ magnetometers (see text). The $\mathrm{Cs}$ frequency values are quoted after division by the factor 120, i.e., corresponding to the neutron resonance frequency.

Figure 16

Distribution of the “measurement” results for random configurations of the leakage currents. The simulated histograms for two independent chambers and for the summary result are given.

Figure 17

Exemplary series of measurements. $\mathrm{ED}{\mathrm{M}}_{\mathrm{top}}$ and $\mathrm{ED}{\mathrm{M}}_{\mathrm{bott}}$ are values measured for the top and bottom chambers separately (see text). EDM is the quantity defined in Eq. (3) providing the neutron EDM using measurements with both chambers and all four UCN detectors.

Figure 18

Results of measurements of the various quantities given in Eq. (3) for different ranges of leakage current. For explanation of the lowest plot see text.

Figure 19

Distribution of measured values for the quantities defined in Eq. (3). For each quantity is shown the ratio of the width of the measured values distribution to its uncertainty defined by counting statistics, as well as ${\chi }^2$ obtained from fitting a normal distribution.