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Neutrino oscillations in the Daya Bay experiment
1 April 2012
The 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 nuclei
1 April 2012
A. 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 generation
1 April 2012
Â.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 molecules
1 April 2012
Y.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 hole
1 April 2012
Investigation 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
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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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