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| Issue 12, 2024 |
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Extracts from the Internet | ||
Neutrino-nucleon interaction1 January 2002At Fermilab (near Chicago), many years of data on the collisions of high-energy neutrinos with nucleons have revealed a unexpected departure from the Standard Model of elementary particles. The quantity measured in the experiments was the so-called mixing angle θ, a characteristic of the contribution which charged and neutral weak currents give to the cross section for the interaction involved. Charged currents are carried by W bosons and cause the muon neutrino to become a muon. The exchange of Z bosons does not involve the creation of muons, thus enabling a distinction to be made between events with and without the participation of charged and neutral weak currents. The neutrino was one of the decay products of the pions and kaons produced by collisions of accelerated protons with the beryllium oxide target at the Tevatron accelerator. To record various types of events caused by the interaction of neutrinos with nuclei, a 700-ton detector complex was used. The measured value of sin2θ differs by about three standard deviations from its theoretical value. One reason for the departure from the Standard Model prediction might be the existence of new interactions or possibly new particles - leptoquarks, for example. Up to now, the Standard Model has served as a highly accurate description for all experimental data except for those on neutrino oscillations. Recent reports of an anomalously large magnetic moment of the muon (see Phys.-Usp. 44 330 (2001)) proved to be due to an error in theoretical calculations, whose elimination restored agreement between theory and experiment. Source: http://xxx.lanl.gov New substances1 January 2002
Oxygen O4. A team of researchers in Italy (F Cacace, G de Petris,
and A Troiani) has for the first time succeeded in creating
molecules of O4 which were predicted to exist back in the 1920s.
Positive O+4
ions formed in a mixture of neutral O2 molecules with O2+
ions and were separated out by a mass spectrometer. The
irradiation of O4+ with electrons produced neutral O4 molecules.
Upon repeated ionization the mass spectrograph again revealed O4+,
indicative of the stability of the molecules.
Source: www.nature.com Superfast insulator-metal transition1 January 2002A Cavalleri and his colleagues in the US and Canada have been exploring the dynamics of vanadium oxide VO2 making a transition to the conducting state. This phase transition is due to the reorganization of the crystal structure of the oxide. Previously, only slow transitions occurring on heating VO2 to a temperature of 340 K have been observed. A Cavalleri and his colleagues irradiated the oxide with a powerful, 50 fs laser pulses which caused a phase transition to occur in as short time as 100 fs. The state of the crystal was monitored by an x-ray diffraction technique as well as optically. The team also established that the phase transition did not result from the growth of islands of the new phase, as is the case usually, but occurred over the entire sample volume at once. Source: Phys. Rev. Lett. 87 237401 (2001) Microlensing identification1 January 2002Ten years of observations have revealed about 20 cases of stars in the Large Magellanic Cloud being gravitationally microlensed by compact dark objects in the halo of the Milky Way galaxy (see Phys.-Usp. 40 869 (1997) for a review). Until recently, however, the nature of these objects was not understood. Now an international collaboration of astronomers has for the first time identified and studied one of these gravitational lenses. Using the Hubble Space Telescope, it is found that the lens is a brown dwarf - a faint star of class M, 5-10% the mass of the Sun, located about 600 light years from Earth. The fact that the star belongs to class M is confirmed by the analysis of it spectrum using the Very Large Telescope (VLT) at the European Southern Observatory (ESO). The statistics of microlensing events reveal that `microlenses' might constitute up to 50% of all dark matter in the Galaxy. It is still unclear whether brown dwarfs were also involved in the remaining 20 or so minus one microlensing events observed. Source: http://physicsweb.org/article/news/5/12/3 |
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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