S.Yu. Semenikhin

Measurement of the neutron electric dipole moment via spin rotation in a non-centrosymmetric crystal

AbstractWe have measured the neutron electric dipole moment using spin rotation ina non-centrosymmetric crystal. Our result is d n = (2.5 ± 6.5 stat ± 5.5 syst ) ·10 −24 ecm. The dominating contribution to the systematic uncertainty is sta-tistical in nature and will reduce with improved statistics. The statistical sensi-tivity can be increased to 2·10 −26 ecm in 100 days data taking with an improvedsetup. We state technical requirements for a systematic uncertainty at the samelevel. Keywords: electric dipole moment, CP violation, perfect crystal, neutron,diffraction, three-dimensional polarisation analysis PACS: 14.20.Dh, 61.05.fm, 04.80.Cc1. IntroductionElectric dipole moments (EDMs) of elementary particles belong to the mostsensitive probes for CP violation beyond the Standard Model of Particle Physics[1]. Constraining or detecting EDMs of different systems allows to gather ex-perimental information about models for new physics that is complementary tohigh energy physics data.For the neutron EDM (nEDM), the most sensitive results [2, 3] were ob-tained using ultracold neutrons and Ramsey’s resonance method. See [4] fora recent review of measurements using free neutrons. Measurements using theinteractionof neutrons with the atomic electric field in absorbingmatter werepi-oneered by Shull and Nathans [5]. Abov and colleagues [6] first discussed a spin-dependent term in the scatteringamplitude for neutronsin non-centrosymmetricnon-absorptive crystals. This term is caused by the interference of nuclear and

Measurement of the neutron electric dipole moment by crystal diffraction

An experiment using a prototype setup to search for the neutron electric dipole moment by measuring spin rotation in a non-centrosymmetric crystal (quartz) was carried out to investigate statistical sensitivity and systematic effects of the method. It has been demonstrated that the concept of the method works. The preliminary result of the experiment is dn=(2.5±6.5)×10-24ecm. The experiment showed that an accuracy of ~2.5×10-26ecm can be obtained in 100 days data taking, using available quartz crystals and neutron beams.

Anomalous behavior of the dispersion of a neutron transmitting through a crystal at nearly Bragg energies

The features of the propagation of a neutron through a crystal at nearly Bragg energies has been studied within the framework of the preparation of an experiment on the search for the electric dipole moment of a neutron by the crystal diffraction method. The time of passage of the neutron through the crystal has been studied as a function of the deviation from the Bragg condition. The anomalous behavior of the dispersion of the neutron, i.e., the energy dependence of its average velocity, has been observed. It has been shown that the derivative dv/dE for the diffracting neutron at nearly Bragg energies can be three or four orders of magnitude larger than this derivative for a free neutron. This opens new possibilities in precision neutron spectroscopy.

Observation of small changes in the energy of a neutron in an alternating magnetic field

A method for measuring small changes in the energy of a neutron has been proposed on the basis of the anomalous behavior of the dispersion of the neutron in the crystal near Bragg “resonance.” A high sensitivity of the method allows the observation of the acceleration of the neutron in the alternating magnetic field. It has been found that the small difference between the energies of two spin states of the neutron (parallel and antiparallel to the magnetic field) leads to significant spatial splitting of wave packets and, correspondingly, to the depolarization of the neutron beam.

Dispersion of the refractive index of a neutron in a crystal

The dispersion (resonance) behavior of the refractive index of a neutron moving in a crystal with energies close to Bragg values has been studied. It has been shown that a small change in the energy of the neutron by about the Bragg width in this case (ΔE/E ∼ 10−5) results in a significant (several tens of percent) change in the potential of the interaction of the neutron with the crystal. A new phenomenon—the acceleration of the neutron passing through a perfect crystal moving at a variable velocity near a Bragg resonance—has been observed. The effect appears because the parameter of deviation from the Bragg condition and, therefore, the neutron-crystal interaction potential change during the time of flight of the neuron through the accelerated crystal. As a result, the kinetic energy of the neutron leaving the crystal changes.

Analysis of spatial resolution of an experiment on verification of the equivalence principle for a neutron by the diffraction method

Investigations of the spatial resolution of a setup for verification of the equivalence of inertial and gravitational masses of a neutron by the method of diffraction in a perfect crystal have been performed. Tests were conducted at Bragg angles of 74°–82°. A decrease in spatial resolution at Bragg angles >78° is detected.

Two-crystal focusing effect

The effect of two-crystal focusing of neutrons at Laue diffraction from large perfect silicon crystals has been studied. It has been shown that the focusing effect makes it possible to reach an angular resolution better than 0.03″, which is about 0.01 of the width of a Bragg reflection. This circumstance allows a new ultraprecision neutron-spectrometry method based on the method of spin-echo small-angle neutron scattering in combination with Laue diffraction.