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Laboratory of Researches of Weak Interactions

Head of laboratory Lobashev V.M., Corresponding member of RAS


    The main field of scientific activity of laboratory is a research of properties of symmetry of Fundamental interactions. Experimental study of parity non-conserving effects was started by V.M. Lobashev and V.A.Nazarenko with coworkers in the beginning of the sixties. In these works the unique technique, combining integral method of registration of -quanta and resonant filtering of a signal with the help of a high Q-factor filter (pendulum of astronomical clock), was used. It has allowed to measure extremely small effects of circular polarization of -quanta - P( ) = 10-5 - 10-7. The effects of parity non-conservation were found out in a lot of transitions of radioactive nuclei, that together with works of ITEPh group was first reliable confirmation of hypothesis about universality of weak interaction. These works were awarded by the Lenin prize (V.M.Lobashev, V.A.Nazarenko - PNPI; Yu.G.Abov, P.A.Krupchitsky - ITEPh).

   During carried out researches the properties of polarimeter used in measurements were studied in detail. As a result the new phenomenon - rotation of plane of linear polarization of -quanta passing through magnetized ferromagnetic - was discovered. It is similar to Faraday's effect in optics (V.M.Lobashev, A.P.Serebrov, L.M.Smotritsky et al.).

   The experiment for measurement of circular polarization of -quanta in reaction n+p d+ was of great importance. As a proton target a light-water neutron trap in the reactor core was used. Such a source of -quanta had a volume 3 liters and its intensity was 1016 -quanta per second. In this experiment the upper limit for -odd circular polarization of -quanta was established: P( ) < 510-7 (V.M.Lobashev, V.A.Nazarenko, E.A.Kolomensky, A.N.Pirozhkov, A.I.Egorov et al.) This value has the large meaning for the theory of weak nucleon-nucleon interactions.

   Within several years in laboratory the effects of parity violation were investigated in nuclear reactions with the polarized neutrons. A number of the new phenomena is found out:

  • the left-right asymmetry in fission of heavy nuclei by slow polarized neutrons;

  • caused by parity non-conservation circular polarization for integral spectrum of -quanta from the ( n,)-reaction on intermediate nuclei;

  • dependence of the total and radiative cross-sections on helicity of neutrons; it was shown that the parity non-conserving part of the total cross-section of interaction of longitudinal polarized neutrons is wholly caused by dependence of radiative capture cross-section on helicity of neutrons, i.e. it is connected to processes proceeding through a compound-nucleus;

  • the experimental confirmation for the phenomenon of resonant enhancement of PNC effects for nuclei having p-wave compound-resonance near to a thermal point was obtained.

   The experiment for search for the electric dipole moment of the neutron (EDM) became most significant. It was started at reactor WWR-M by V.M.Lobashev with a group of physicists, who came to work in PNPI in the seventies. An use of ultra cold neutrons (UCN) was a major feature of the experiment The preparation for this experiments was started in 1968. The technique of work with UCN was advanced, as a matter of fact from zero, beginning from their production (sources of UCN), up to creation of magnetic resonance spectrometer with resolving power of 10-17eV. This experiment is one of key experiments for elementary particle physics. It is connected to search for effect of or T-parity violation, underlying formation of the Universe. A new upper limit for EDM of a neutron |dn| < 9,710-28 em has allowed practically completely to close Weinberg's model of P-violation, and it gives the indication for the benefit of other models of CP-non-conservation: Supersymmetric, left-right and model by Kobayashi-Mascawa.

   The obtained limit dn gives rise to so called the "strong CP problem" for the QCD sector, where CP violation is produced via the -mechanism of gluon exchange. The problem remains open, and it gives an impetus to search for different factors that could suppress the -therm to the level of < 10-9.

   The UCN method in experiment for search for EDM of a neutron has still not exhausted their opportunities. Now in the laboratory the works are conducted for modernization of experimental installation with the purpose to increase its sensitivity.

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