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| Issue 11, 2024 |
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Extracts from the Internet | ||
Pion production by neutrinos on argon1 February 2015ArgoNeuT Collaboration at the National Fermi laboratory (USA) performed the first measurements of charged-current coherent pion production cross section upon muon neutrino and antineutrino scattering by argon nuclei. The coherent scattering is due to the entire nucleus and not to separate nucleons in its composition. Studied were νμ+Ar → μ-+π++Ar and anti-νμ+Ar → μ++π-+Ar reactions with low energy transfer to Ar nuclei where after π± production the states of the nuclei remained unchanged. Neutrinos from a beam passed through a reservoir filled with liquid argon, and the charged particles born in the interaction were accelerated by the electric field and were registered by a wire detector. Among a lot of events, several of the above-mentioned reactions were selected according to a number of selection criteria. The pion production cross section was found to be 2.6+1.2-1.0(stat.)+0.3-0.4(syst.) × 10-38 cm2 for neutrinos with energy of 9.6 GeV and 5.5+2,6-2,1(stat.)+0,6-0.7(syst.) × 10-39 cm2 for antineutrinos with 3.6 GeV. The measured cross section for anti-νμ agrees well with the calculations of D. Rein and L.M. Sehgal, while the result for νμ differs from the theoretical value by approximately 1.2 σ. Source: Phys. Rev. Lett. 113 261801 (2014) Measurement of gravitational field curvature1 February 2015G. Rosi (University of Florence and the National Institute of Nuclear Physics, Italy) with colleagues measured the gravitational field curvature using simultaneously three atomic interferometers. The same team earlier conducted an analogous experiment with two interferometers where the field gradient was measured. The field measurement in several places at a time makes it possible to subtract the general vibration noise and to heighten the precision of the result. The idea of such experiments was suggested by J.M. Mc Guirk with colleagues in 2001. In the new experiment, three small clouds of ultracold 87Rb gas were jolted in a vertical tube to different heights. The upper part of the tube was surrounded by bobs of tungsten alloy with the total mass of 156 kg inducing additional inhomogeneity of the gravitational field. Near the upper points of the trajectories the gas clots were exposed to a series of laser pulses going up and down the tube. Part of the atoms absorbed and immediately emitted photons undergoing two-photon Raman transitions. The additional pulse received by the atoms led to a displacement of the upper point of the trajectory and, accordingly, to a phase shift relative to the atoms that did not absorb photons. The interference pattern observed using fluorescent radiation of atoms allowed measurement of the gravitational field at three heights and determination of the field curvature. The results obtained agree well with the theoretical calculations of the structure of the field generated by the Earth and by the additional mass. A similar method can be employed to specify the value of the gravitational constant and can be applied in seeking minerals by variations of the gravitational field of the Earth above the fields. Source: Phys. Rev. Lett. 114 013001 (2015) Conductance quantization in a neutral matter flux1 February 2015Electric conductance quantization determining a charged particle flux has already been observed in experiments. T. Esslinger (Federal School of Technology, Zurich, Switzerland) with colleagues observed for the first time the conductance quantization (transmission capacity) of a thin channel for a neutral atom flux. First, an elongated cloud of ultracold (T = 42 nK) degenerate Fermi gas consisting of ≈ 105 6Li atoms was created in a magneto-optical trap. With the help of laser beams passing through a special mask and a microscope the cloud fell into two parts joined by a channel 1.5 ± 0.3 µm wide. This width was smaller than the Fermi wavelength (2 µm), for which reason the regime of single transverse conductance modes was reached and the conductance quantization could be observed. Another laser changed the channel conductance by creating an additional repulsive potential. The two parts of the cloud contained a different number of atoms, and the chemical potential difference δμ induced an atomic flux through the channel I = Gδμ, where G is conductance. The conductance was determined by the rate of variation of the number of atoms in the two parts of the cloud. With increasing additional potential, the conductance varied in steps. In the region of the first plateau it was equal to the inverse value of the Planck constant 1/h as was predicted by the theory of R. Landauer. The graph of conductance demonstrated up to three plateaus altogether. Source: Nature 517 64 (2015) Iron opacity at high temperatures1 February 2015The character of energy transfer inside the Sun depends on plasma composition and on the scattering properties of each type of ions. At the present time divergence exists between the spectral observations of photosphere and the observations of acoustic oscillations in the Sun by the helioseismological method. Comparison of these data shows that the solar matter opacity is higher by approximately 15 % than that predicted by the theory. J.E. Bailey (Sandia National Laboratories, USA) with colleagues carried out the first direct measurements of iron opacity at temperatures Te = 1.9-2.3 mln K and concentrations ne = (0.7-4.0) × 1022 cm-3 of electrons, i.e., in conditions close to those in the Sun at the boundaries of convection zones and radiative transfer. In the “Z machine” the iron and magnesium foil was heated and evaporated by high-power X ray pulses. In the 22 experiments carried out within three years, 450 spectra of the radiation emitted from plasma were detected. The measured iron opacity appeared to be by 30-400 % (depending on the radiation wavelength) higher than that predicted by the calculations. Integrally this makes up about half of the increase of the general opacity necessary for consistency with the helioseismological data. To clarify the nature of the remaining part of the difference, similar measurements should be taken for the other elements. Source: Nature 517 56 (2015) Gamma rays from the center of galaxy IÑ 3101 February 2015Observation of the fast variability of gamma-ray emission from the center of galaxy IÑ 310 has shown that the size of the emitting region must make up less than 20 % of the radius of supermassive black hole horizon in the galactic center. Such small scales cannot now be investigated in direct observations, e.g., using radio telescopes. Galaxy IC 310 lies in Perseus constellation at a distance of 260 mln light years from the Earth. Its black hole mass 3+4-2×108M☉ was determined by the known black hole mass correlation with the value of central velocity dispersion. The observations were carried out using MAGIC telescopes placed on the La Palma island. They detect Cherenkov radiation from cascades of charged particles produced by gamma-ray photons in atmosphere. The characteristic time of flux doubling in gamma-ray range was 4.8 minutes. The radiation-generating processes cannot propagate faster than light, which implies the above-mentioned restriction from above on the emitting region size. Particle acceleration by the electric field in the magnetospheric gap at the base of the relativistic jet is regarded as a probable mechanism of rapidly varying gamma-ray emission. This mechanism is well known in the case of pulsars. Source: Science 346 1080 (2014) |
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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