The Main Results of Fundamental & Applied Researches for 1998.
A new mechanism of leptogenesis is proposed in which the asymmetries in lepton numbers are produced through the CP-violating oscillations of "sterile" (electroweak singlet) neutrinos. The asymmetry is communicated from singlet neutrinos to ordinary leptons through their Yukawa couplings. The lepton asymmetry is then reprocessed into baryon asymmetry by electroweak sphalerons
Sufficient conditions are established for the existence of stable non-topological solitons, Q-balls in electroweak models with low-energy supersymmetry breaking. These objects may have been created in the early Universe. It is found that the relic Q-balls can accumulate inside a neutron star and gradually absorb the baryons into the scalar condensate. This causes a slow reduction in the mass of the star. When the mass reaches a critical value, the neutron star becomes unstable and explodes. The cataclysmic destruction of the distant neutron stars may be the origin of the gamma-ray bursts.
The first three-loop QCD analysis of the data for xF_3 structure function of deep-inelastic neutrino-nucleon scattering, obtained by CCFR collaboration at Fermilab Tevatron, is performed. The values of alpha_s(M_Z) and twist-four power corrections are extracted.
A consistent analysis of Upsilon sum rules and B-meson semileptonic width in the next-to-next-to-leading order in the strong coupling constant was performed. It is shown that new light gauge boson which might be produced in the decays of pseudoscalar mesons could be effectively searched for in neutrino experiments via the Primakoff effect. An estimate of the branching ratio of new gauge boson in pi^0 decay for the NOMAD neutrino detector at CERN is given.
On the "Troitsk nu-mass" installation in INR RAN a new limit to an electron antineutrino rest mass 2.5 eV/c^2 has been obtained at a 95% C.L. An existence of a narrow bump in the beta-spectrum has been also confirmed. The bump is placed 5-15 eV below the end point with a relative intensity about 10^{-10}. The most interesting feature of this bump is a periodical shift of its position with respect to the end point. The measurements which were carried out at the end of 1997 and in 1998 together with the previous data give a period of the shift variation 0.504 +- 0.003 year. The coincidence of this value with a half of the period of Earth rotation around the Sun allows one to discuss on a new basis the possibility of the explanation of the observed effect by the capture of low energy degenerate relic neutrinos which are concentrated around the Sun in a cloud with a density of about 0.5*10^15 neutrinos/cm^3. In spite of the very speculative nature of this hypothesis it stimulates new measurements and development of new spectrometers with higher sensitivity.
When studying an anomalous suppression of production of charmonium J/psi and psi` states in collisions of Pb nuclei with the energy of 158 GeV/nucleon we found a threshold effect in the dependence of J/psi yield on a transversal energy (INR as a member of the NA50 collaboration). For peripheral interactions the data agree well with a model of J/psi nuclear absorption. As far as central interactions are concerned a J/psi yield drastically decreases which can testify quark-gluon plasma origin for collisions with a maximal density of energy.
A method has been worked out to search for WIMP with a sensitivity of 10^{-5} events per kilogram of substance per day which is 4 order of magnitude better than that for the up-to-date methods. In INR an experiment is under way on search for WIMP by means of xenon two-phase ionization chamber. A calculated value of the background in the experiment is 10^{-5} events per kilogram of substance per day which is 4 order of magnitude less than that in the experiments fulfilled.
It is shown for the first time that in the subthreshold production of kaons by protons on light nuclei a two-step mechanism of kaon production through intermediate pions is not sure to dominate as it has been considered before. A dependence of a relative strength of a direct and two-step mechanisms of kaons production in the subthreshold mode on kinematics of the reaction analyzed has been demonstrated.
A project has been completed and a manufacture of a Superluminous Lead Neutron Slowing-Down Spectrometer (which is by a factor of 10^3-10^4 greater than the best world setups) is now under way. The proton source here is a spallation reaction under action of the Moscow Meson Factory (MMF) proton beam.
Manufacture and a successful trial of the superconducting magnet system are the ground for making the experiments on the polarized neutrons beam of JINR with a polarized proton target. In the framework of experiment DELTA-SIGMA (collaboration JINR - INR RAS - PINP - KhPTI - CEA/Sacley) we completely manufactured and conducted trials of proton polarized target. A test run has been carried out on the polarized neutrons beam of JINR for adjustment and calibration of the detection equipment.
Precise experimental data on photon asymmetry of photoproduction of positive pions on proton have been obtained. The most part of the data have been obtained for the first time. In the framework of collaboration "GRAAL" new precise experimental data on photon asymmetry of photoproduction of positive pions on proton have been obtained in the angle range 40-160 degrees for energies 550-1100 MeV. The most part of the data have been obtained for the first time at angles more than 120 degrees. This result testifies that the F15(1680) resonance contributes much less than it has been considered before.
Unique avalanche MRS-detectors for scintillation counters have been developed which allow one to detect single photons at a room temperature with a high efficiency. In 1995-1998 INR RAS together with JINR, Dubna developed a new approach to the construction of the solidstate analogs of PM - single photon counters. The new method is based on using an avalanche process into the multilayer MRS-structures (metal-resistivity layer-semiconductor). Tests of a sample of the avalanche MRS-photodiode with a sensitive area (0.5*0.5) mm^2 and with a gain coefficient greater than 10^4 showed that single photons can be registered at a room temperature with a probability 23 per cent (within 100 nsec).
Theoretical description of the s-, p- and d-wave neutron strength function and potential scattering lengths of s- and p-neutrons is obtained for even-even spherical nuclei with 56<=A<=206 in the framework of the unified approach proposed earlier for the description of low-energy neutron interactions with such nuclei. The d-wave strength functions and the potential scattering lengths of p-neutrons are reproduced for the first time. It is shown that this approach allows one to describe the totality of low-energy neutron data and to calculate neutron cross sections and strength functions of the nuclei for which direct measurements are extremely difficult.
The MMF experiments. As a result of study of reactions p + d --> p + d + (gamma) and p + d --> p + p + (gamma+n) at 305 MeV two narrow structures in missing mass spectra at 1905 and 1924 MeV with widths equal to the experimental resolution 3 MeV were observed. The comparison of the experimental yields with the Monte Carlo simulations showed that the found peaks can be explained as the supernarrow dibaryons the decay of which into two nucleons is suppressed by Pauli principle. The shape of an angular dependence of the charged particles emitted from the decay of the dibaryon with mass 1905 MeV agrees well with the calculated curve for dibaryon D(T=1,JP=1+,S=1) or D(1,1-,0).
For the first time emission of light charged particles (LCP) from fragments of uranium fission by 1700 MeV pions was observed. The isotropic angular distribution of LCP makes it possible to conclude that they are emitted from non-accelerated fragments. (Earlier at lower energies of incident particles the emission of LCP from accelerated fragments only was observed.) Such a process is possible when the fission of a strongly excited proton-rich nucleus (formed at the cascade-evaporation stage) proceeds rather rapidly.
An INR neutron complex has been physically put into operation. It comprises a high-intensity pulsed neutron source, a trap of neutron and proton beams and a superpower light gathering lead neutron slowing-down spectrometer. The INR neutron complex was started on 8.12.98-15.12.98. It contains the first Russian high-intensity pulsed neutron source on the base of a high-current proton linac and a water-cooled tungsten target, the only Russian trap of neutron and proton beams and a superpower light-gathering lead neutron slowing-down spectrometer. The main unit of the source is its water-cooled tungsten target with a light water moderator. Thus, Russia has got a new up-to-date high-intensity neutron source providing neutrons through interactions of average energy protons with the substance of the stretched target of heavy nuclei. During the start all life-support systems of the source were tested and a 209 MeV proton beam was guided from the high-current linac at a pulse repetition rate of 1 Hz, duration of 60 mcs and average current of about 1 mcA. A contribution of fast neutrons to the spectra was estimated and background conditions were examined. The source opens up new wide prospects for studying fundamental problems of condensed media physics and nuclear physics. The physical start of the only Russian trap of neutron and proton beams made for studies in the field of radiation physics showed that it can be successfully used for investigations.
The results of research of solar neutrino fluxes at the underground Gallium Germanium neutrino telescope of the Baksan Neutrino Observatory, RAN (Russian-American experiment SAGE) are an evidence for existence of neutrino oscillations, and therefore, observation of physical phenomena beyond the Standard Model of elementary particles. The Gallium Germanium neutrino telescope is placed in the underground complex of the Baksan Neutrino Observatory, INR RAS (vil. Neutrino, Kabardino-Balkaria, the Caucasus) at a depth of 4800 mwe. Over 50 tons of metallic Gallium are used as substance for the neutrino obsorber target. The large-scale observations (since 1990) of the Sun neutrino irradiation accompanying the inertial fusion processes (Russian-American experiment SAGE) revealed a rather great deficit of low-energies flux at a level of about 52% of the amount predicted by Standard Solar Model. The results of the SAGE experiment combined with those obtained in other neutrino experiments (Chlorine - in the USA, water-Cherenkov - in Japan, Gallium - in Italy) are a good evidence for conversion of neutrinos with different lepton numbers to each other (neutrino oscillations), and therefore, observation of the phenomena beyond the so-called Standard model. Thus, we are on the threshold of one of the most important discoveries in modern physics which is likely to result in revising all our conceptions about elementary particles and a role of neutrinos in evolution of celestial matter and early Universe.
Baikal deep underwater neutrino telescope HT-200 which is one of the largest high energy neutrino detectors in the world has been mounted and put into operation in the mode of permanent data taking (the winter expedition of 1998). It is the first successful experience on developing a stationary large-scale deep underwater Cherenkov detector of elementary particles which opens up new prospects for further investigations in the field of neutrino astrophysics. Since April 1998 the detector has been operating in the designed configuration where 192 optical units are assembled into 8 vertical clusters 72 m long and installed at a depth of about 1100-1200 meters. The detector makes it possible to search for new particles and rare processes at the sensitivity level which cannot be achieved by any other methods and becomes a powerful tool for monitoring the South Baikal region ecology. (INR - Moscow State Univ. - Irkutsk State Univ. - Nizhni Novgorod Polytechnical Univ. - St. Petersburg State Marine Technical Univ. - N.N.Andreev Acoustic Inst., Institute of Ecological Toxicology - State Scientific Center `Kurchatov Institute` - DESY - IfH (Berlin/Zeuthen) - JINR (Dubna).
By means of multiwire wall-less high-pressure proportional counters for the first time in the world a limit has been obtained on the Kr-78 semi-decay period relative to the two-neutrino double K-acceptance: T1/2>1.8*10^20 years (68% C.L.). A stage has been completed on search for the two-neutrino double K-acceptance of Kr-78 isotope by means of multiwire wall-less high-pressure proportional counters. The entire data-taking time period is 1817 hours. The measurements with the Crypton sample enriched in Kr-78 up to 94% were the first in the world to give a limit on the semi-decay period relative to this process: T1/2>1.8*10^20 years (68% C.L.). With background spectrum peculiarities (revealed on the base of big statistics) taken into account a sensitivity of the setup for a year of measurements with this sample appeared to be 4.0*10^20 years (68% C.L.)
For the first time a spectrum of the Extensive Atmosphere Showers (EAS) has been obtained on the total number of muons within the multiplicity interval 75-3500 muons corresponding to the energy region of primary cosmic rays 10^15 - 10^17 eV. The data provided by the Baksan Underground Stintillation Telescope (BUST) testify that the knee in the energy spectrum (~ 3*10^15 eV) occurs at an equal energy per nucleus and that a chemical composition of primary cosmic rays after the knee of the spectrum is close to that observed at lower energies (~ 10^14 eV). A new method has been elaborated for conversion of multiplicity spectrum of muons to spectrum EAS on the total number of muons which allows a direct comparison of data obtained in different experiments with muon bundles. A direct comparison of data at n_{mu}>1800 and n_{mu}=75-650 has been done for the first time. Muons scattering in the ground above the setup and an exact view of muons survival probability p(E,H) has been taken into account. For the first time there has been obtained a spectrum EAS, F(n_{mu}) in the interval n_{mu}=75-3500 which corresponds to the Primary Cosmic Rays (PCR) energy region 2*10^15 - 2*10^17 eV for protons and 8*10^14 - 10^16 eV for Fe nuclei. The combined data obtained on BUST at n_{mu}>1800 and n_{mu}=75-650 show that the knee in the PCR energy spectrum (at E>3*10^15 eV) occurs at an equal energy per nucleus and that the PCR chemical composition in the region 10^15 - 10^17 eV is close to that observed at 10^14 eV.
The second tower of the mutual Russian-Italian setup LVP situated in the Gran Sasso laboratory has been put into operation. The two LVD towers contain about 700 tons of scintillator and are the largest scintillation track installation in the world providing investigations in the field of neutrino astrophysics and physics of neutrino and cosmic rays.
In 1998 the high-intensity linear accelerator of the Moscow meson factory provided acceleration of a proton beam within 2000 hours during four runs. On reason of economy its energy was limited by 210 MeV.
Strontium-82 isotope has been produced in the amount of 6.8 Curie for medical purposes.
To increase a beam average current a system of injection to the MMF linac on the base of the booster section with a RF focusing has been reconstructed. For the first time in the World an accelerator with a RF focusing has been put into operation in the energy range 400-750 KeV. An accelerated beam has been obtained with a pulse current of 12 mA at a pulse duration of 140 mcs, which corresponds to an average current of 85 mcA.
On the polarized hydrogen ion source with an atomic beam accumulation of polarized hydrogen atoms in the ionizer accumulation cell has been investigated. A polarized protons beam has been obtained with a current in pulse I=11 mA, polarization P=0.8 with a record quality factor IP^2=7 mA and normalized emittance 1.0 pi.mm.mrad.
In agreement with the TRIUMF laboratory (Canada) we developed, manufactured and tested at a nominal level of average power of 10 kW a three-gap beam buncher at a frequency of 105 MHz for operating with heavy ions.
A hypothesis for the impact of natural alpha-radiation of uranium and thorium decay on the enhanced diffusion of gas-forming elements into mantle-originated crystals of age exceeding 400 million years is proposed and confirmed experimentally. Numerical simulations on olivine crystals using the nuclear reaction 12C(d,p)13C have shown (at deuteron doses equivalent in defect formation to alpha-particles) enhancing the carbon diffusion into the crystal depth even at T=370K. A confirmation of the found data for other gas-forming elements can lead to correction of the conventional concepts about gas conditions of the Earth and its degasation mechanisms.