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| Issue 12, 2024 |
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Extracts from the Internet | ||
Quark-gluon plasma1 July 2003In recent years, lead- and gold-ion collision experiments at CERN (Switzerland) and Brookhaven National Laboratory (US) have provided evidence for the existence of quark-gluon plasma, a mixture of free quarks and free gluons with hundred times the density of nuclear matter. Now Brookhaven physicists have provided further confirmation of this. What they did was to discover the jet quenching phenomenon that had been predicted theoretically. In high energy nuclear collisions the ejection of a pair of quarks usually gives rise to two particle jets. In the Brookhaven experiments, only one jet was occasionally observed in head-on collisions of gold nuclei. A likely explanation is that the second jet is absorbed by quark-gluon plasma. The absorption occurs when a quark-antiquark pair emerges near the surface of a nucleus and one of the quarks flies through the central collision region. As a control experiment, collisions between a beam of gold nuclei and that of deuterons were studied. In this case quark-gluon plasma is not created, and the particle jets come in pairs. Source: http://www.bnl.gov/bnlweb/pubaf/pr/2003/bnlpr061103.htm Orthopositronium lifetime1 July 2003Positronium, a system of an electron and a positron bound together, lives for about 142 ns before annihilating. For parapositronium (a positronium variety with the electron and positron spins oppositely directed) the measured lifetime is in excellent agreement with theory. For orthopositronium, however, the experimental and QED-calculated values have differed by about 0.1%, giving rise to a number of exotic explanations beyond the Standard Model of elementary particles. Now P Vallery and his colleagues have used a new technique to make the most accurate measurement yet of the orthopositronium lifetime. Orthopositronium `atoms' were created by letting a beam of positrons pass through a thin quartz film. To register the gamma radiation produced by annihilation reactions, a scintillation detector was used. The measured lifetime was found to be within 0.014% of the predicted value. In the previous experiments, the authors believe, there was a systematic error due to the fact that many of the orthopositronium `atoms' annihilate in the detector walls. Source: Phys. Rev. Lett. 90 203402 (2003) Magnetic anisotropy energy1 July 2003A group of researchers from Switzerland, Italy, France, and Germany have measured the highest magnetic anisotropy energy known. The measurement was made on a layer of cobalt atoms deposited on a platinum substrate. The layer was grown by the molecular-beam epitaxy technique. The researchers placed the material in a magnetic field of 7 T and measured its magnetization along and perpendicular to the field. The magnetic anisotropy energy was found to be 9.3x10-3eV per cobalt atom, 200 times larger than for cobalt atoms in a bulk crystal and several times larger than maximum values for other materials. Source: Science 300 1130 (2003) Spin current1 July 2003Two groups of researchers, one at the University of Iowa (USA) and one at the University of Marburg (Germany) have independently created a directed electron spin current with no accompanying electrical charge transfer. In both experiments a semiconductor material was illuminated by two differently polarized laser beams, with photons in one of them twice as energetic as in the other. The radiation promoted electrons into the conduction band and produced two counterpropagating electron flows with oppositely directed spins. Due to the interference phenomenon, the charge flows exactly cancelled one another whereas the spin flows added. The latter is due to the fact that, as far as spin transfer is concerned, the motion of a specific spin state in one direction is equivalent to the motion of an opposite spin state in the opposite direction. Sources: Phys. Rev. Lett. 90 136603 (2003), Phys. Rev. Lett. 90 216601 (2003) Magnetic field of alone neutron star1 July 2003A group of astronomers from Italy and France have measured the magnetic field of a solitary neutron star, one which is not a member of a multiple star system. Solitary neutron stars are amenable to observation in visible and X-ray wavelengths. Among the neutron stars known, only the object 1E1207.4-5209 reveals spectral absorption features in addition to the thermal X-ray continuum. The most precise spectroscopic observations of the neutron star 1E1207.4-5209 were made using the XMM-Newton telescope. Absorption features were observed at energies of 0.7, 1.4, and 2.1 keV, and some evidence for spectral features at 2.8 keV was found. The most likely interpretation of these features is the electron cyclotron resonance scattering of X-ray radiation. The corresponding magnetic field should be about 8x1010 Gauss, which is 50 to 100 times less that indirectly estimated from the neutron star's slowdown rate. The cyclotron interpretation is supported by periodic variations in the features, presumably due to the rotation of the star and because the cyclotron resonance scattering cross section depends on the angle between the magnetic field and the photon momentum. Source: http://arXiv.org/abs/astro-ph/0306189 |
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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