Yu.P. Braginetz

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.

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.

The crystal acceleration effect for cold neutrons

A new mechanism of neutron acceleration is discussed and studied experimentally in detail for cold neutrons passing through the accelerated perfect crystal with the energies close to the Bragg one. The effect arises due to the following reason. The crystal refraction index (neutron-crystal interaction potential) for neutron in the vicinity of the Bragg resonance sharply depends on the parameter of deviation from the exact Bragg condition, i.e. on the crystal-neutron relative velocity. Therefore the neutrons enter into accelerated crystal with one neutron-crystal interaction potential and exit with the other. Neutron kinetic energy cannot vary inside the crystal due to its homogeneity. So after passage through such a crystal neutrons will be accelerated or decelerated because of the different energy change at the entrance and exit crystal boundaries.

Bragg resonance behavior of the neutron refractive index and crystal acceleration effect

The energy dependence of neutron refraction index in a perfect crystal for neutron energy, close to the Bragg ones, was studied. The resonance shape of this dependence with approximately the Darwin width was found. As a result, the value of deviation from the exact Bragg condition can change during the neutron time of flight through the accelerated crystal and so the refraction index and the velocity of outgoing neutron can change as well. Such new mechanism of neutron acceleration in the accelerating perfect crystal was proposed and found experimentally. This mechanism is march more effective then known one concerning with the neutron acceleration in the accelerating usual media.

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.

Anomalous behavior of neutron refraction index in a perfect crystal near the Bragg reflex

Anomalous behavior of neutron refraction index in a perfect crystal near Bragg resonance was studied. This phenomenon is connected with the resonance behavior of potential of neutron interaction with crystal near the Bragg reflex. The amplitude of this resonance is equal to magnitude of g-harmonic of neutron interaction potential Vg and width is about the Bragg width of reflex. Recently, it was shown that for the case of noncentrosymmetric crystal this effect result in a large electric field acting on a neutron (value of the field can reach about 108V/cm) . This effect is planed using to search for the electric dipole moment of a neutron. If the degree of crystal imperfect is less than the Bragg reflection width (case of perfect crystal) the width of the reflex is determined by the own width of crystal reflex that is about 10−5 of the neutron energy. The value of g-harmonics of interaction of neutron with crystal Vg and optical potential of the interaction of neutron with crystal V0 are usually about the same. Therefore the variation of neutron energy on a 10−5 of its value will change significantly a potential of neutron interaction with crystal.