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| Issue 12, 2024 |
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Extracts from the Internet | ||
Neutrino oscillations in the Daya Bay experiment1 April 2012The international Daya Bay Reactor Neutrino Experiment in China recorded oscillations of reactor-generated electron antineutrinos as found from their shortage in the beam; the researchers also determined the mixing angle sin2( 2θ13 ) = 0.092 ± 0.016(stat.) ± 0.005(syst.), which effectively eliminated the version θ13 = 0 at the 5.2 σ level. A similar result was obtained earlier in the T2K experiment at a lower confidence level. Six atomic reactors served as the source of anti-νe with energies of several MeV. Antineutrinos at the baseline point in the beams were detected using three detectors at distances of 470-576 m from the sources while three remote detectors were placed underground at a distance of 1648 m. As the near and remote detectors are identical, errors due to differences in their design were reduced to a minimum. The effect of inverse β decay anti-νe + p → e+ + n was used for the observation of anti-νe. A characteristic feature of anti-νe was found in correlated bursts of light caused by positrons and by interactions of neutrons with nuclei. Each detector uses 20 tons of gadolinium-doped liquid scintillator as target. During 55 days of the experiment the remote detectors recorded 10416 electron antineutrino candidates, which is 6 % less than their expected number deduced from the data of near detectors. This deficit is due to oscillations (conversions) of anti-νe into other sorts of antineutrino. The team of Daya Bay researchers includes some Russian scientists from the JINR (Dubna). Source: arXiv:1203.1669v1 [hep-ex] Dineutron decay of 16Be nuclei1 April 2012A. Spyrou (University of Michigan, USA) and her colleagues at the National Superconducting Cyclotron Laboratory recorded the emission of dineutrons — short-lived weakly bound states of two neutrons — from 16Be nuclei. In the past only indirect evidence of the formation of dineutrons inside neutron-rich nuclei was reported. 16Be nuclei in ground state were produced in the cyclotron in nuclear collisions of 17B with beryllium target. The decay of 16Be with emission of single neutrons is unfavored but these nuclei can emit two neutrons. Very soon after leaving the nucleus, a dineutron decayed into two single neutrons; the method of detecting them was based on searching for pair coincidences. Reconstruction of decay events showed that the dineutron scenario was the most likely explanation of the experimental data among scenarios with emission of a single neutron, a dineutron, or two independent or three neutrons. The scenario of emission of a dineutron from a 16Be nucleus agrees with the computational model in which these nuclei contain a halo of paired neutrons around the more compact central core. Source: Phys. Rev. Lett. 108 102501 (2012) Detection of electric currents based on second-harmonic generation1 April 2012Â.À. Ruzicka (University of Kansas, USA) et al. studied electrical currents in crystals via generation of second harmonics in transmitted light. Generation of harmonics produced by electric field has already been observed earlier. In the experiment reported here electric fields are responsible for only a small contribution while the effect is mostly caused by the electric current. A GaAs crystal was illuminated by focused pulses from IR laser linearly polarized in the direction of current flow, and experimenters studied the spectrum of the transmitted light. Two versions of the experiment were run: the electric current was either produced by field applied to electrodes, or it was produced for a short time in response to a laser pulse. Electric current immediately generated second harmonics in the optical spectrum; the harmonics intensity was proportional to current density. As in the case of electric field, the generation of harmonics due to electric current results from asymmetric charge distribution in momentum space. This asymmetry makes it possible to emit photons at double frequency; in the case of symmetry the photons would remain virtual. This effect was predicted theoretically in 1995 by J.B. Khurgin. Source: Phys. Rev. Lett. 108 077403 (2012) The transfer of single photons between molecules1 April 2012Y.L.A. Rezus (Swiss Higher Technical School of Zurich) and his colleagues have carried out an experiment in which photons were emitted by a specific single molecule and absorbed by a second molecule, identical to the first, at a distance of several meters. Interaction with single photons is usually effective only for molecules in a resonant cavity but Y.L.A. Rezus et al. succeeded in having the molecule absorb a single photon flying in free space without special cavities or waveguides. The absorption cross section was large enough since the photon's resonant frequency was that of the molecular transition. The organic dye molecules were embedded into the tetradecane crystal cooled to 1.5 K. After laser excitation, the source molecule emitted photons which were focused onto the second molecule. Correlation measurements using splitters confirmed that the photons propagating in the beam were indeed single photons. Microelectrodes placed close to the target molecule produced the electric field, and when frequency was driven away from the resonance by the Stark frequency shift, the photons were more efficiently reflected by the molecule than absorbed by it; this confirmed the fundamental role of the resonance transition. Source: Phys. Rev. Lett. 108 093601 (2012) Rapid flow of gas from black hole1 April 2012Investigation by the Chandra Space Observatory of a stellar-mass black hole (BH) in the system IGR J17091-3624 revealed high speed of matter outflow (approximately 3 % of the speed of light) in the plane of accretion disk around the BH. Observations conducted two months earlier found no such fast matter flows, which points to variability of the effect. The binary system IGR J17091-3624 consists of a Sun-like star and a BH in the central region of the Galaxy. The gas flow velocity, 9300+500-400 km s-1, was calculated from the measurement of the shift of Fe spectral lines. This velocity is roughly an order of magnitude greater than what one finds near other stellar-mass BH. Observations with EVLA radio telescopes showed that jets along the axis of the disk were absent when the equatorial gas flow was apparent but they were observed earlier when there were no equatorial winds. This anticorrelation, also observed in other systems, is probably caused by a changed topology of magnetic field in the disk region. The loss of mass produced by the outgoing gas jet may exceed the rate of matter accretion onto the BH. Radiation pressure in the system IGR J17091-3624 is incapable of creating such an intense flow of matter. It probably stems from other thermal or magnetic processes. Sources: arXiv:1112.3648v2 [astro-ph.HE], www.nasa.gov |
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources. | |
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