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| Issue 12, 2024 |
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Extracts from the Internet | ||
Mass of the Ω-b-baryon1 August 2009The CDF collaboration at the Tevatron accelerator has measured the mass of the Ω-b-baryon consisting of a b-quark and two s-quarks. The existence of a doubly strange particle Ω-b is predicted by the Standard model of elementary particles; it has first been observed by the D0 collaboration and announced in August 2008. These particles was created in the CDF experiment in p-anti-p collisions at the center-of-mass energy of 1.96 TeV and were identified at the confidence level of 5.5σ through the chain of decays to lighter particles: Ω-b→J/ΨΩ-, J/Ψ→μ+μ-, Ω-→ΛK^-, Λ→pπ-. On the whole, approximately 5×1011 p-anti-p collisions were studied and 16+6-4 production events of the Ω-b-baryon were recorded. The resulting mass of 6054.4±6.8(stat.)±0.9(syst.) MeV differs from the value 6165±10(stat.)±13(syst.) MeV reported by the D0. The results of the D0 and CDF are statistically incompatible and neither experiment found any special features near the mass measured in the other experiment. Such features could be an evidence that in fact the two experiments observed different particles. Furthermore, the rate of creation of Ω-b-baryons in the CDF experiment was found to be lower than in the D0. The causes of these discrepancies have not yet been identified. Russian scientists from JINR and ITEP take part in the CDF Collaboration. Source: arXiv:0905.3123v1 [hep-ex] Novel magnetic effect1 August 2009Researchers at the National Institute of Standards and Technologies (NIST, USA) and the Institute of Solid State Physics (Chernogolovka, Russia) have discovered that the magnetic ordering in ferromagnets longer range under certain conditions than was earlier expected. They studied heterostructures consisting of a thin ferromagnetic film coated with a grid of antiferromagnetic compound FeMn. They used the technique of magnetooptical visualization which makes it possible to follow in real time the formation, growth and disappearance of magnetic domains. The antiferromagnetic grid creates the pinning effect — the ferromagnetic magnetization gets pinned in certain directions. It was assumed that the magnetic interaction of this type should penetrate into ferromagnetic films to a depth of at most several tens of nanometers beneath the FeMn film. In fact the structure of domain walls (domain boundaries) in the ferromagnetic film was sensitive to this influence even at a distance of 50 µm from the nearest FeMn strip, which is greater by three orders of magnitude than the expected penetration range. One possible explanation of this effect is the topological stability of the domains in the upper grid of the antiferromagnetic; however, further investigation is required to fully clarify the phenomenon discovered. The results of the experiment are important for designing magnetic storage devices with high density of data recording. Source: Phys. Rev. B 79 144435 (2009) Quantum walks1 August 2009Researchers at the Bonn University were able to implement the algorithm of quantum random walks suggested by R. Feynman, for neutral cesium atoms in the potential field of two overlapping optical lattices. Standing waves formed by two laser beams created a one-dimensional periodic potential barrier of height kB×80 µK where kB is the Boltzmann constant. Cesium atoms possessed thermal energy kB×10 µK and could exist in two states of hyperfine splitting of levels each of which was quantum-correlated with one of the directions of atomic displacement to the left or to the right; the permeability of the barrier depended on the internal state of the atom owing to a certain polarization of laser beams. The “step” of an atom in the lattice was initiated by switching the polarization, after which the position of the atom could be determined using its fluorescent emission. In contrast to the classical random walk that takes place, for example, in diffusion, in the quantum case momentum imparted to the atom transferred it to a state of quantum superposition of two possible directions of motion. The next pulse created a new configuration of the atomic wave function which included a superposition of the states of the preceding steps. The experiment implemented up to N=24 steps. The observation of the final positions of trapped atoms showed that their displacements could indeed be regrarded as quantum walking in which the summary displacement proportional to N. In the case of decoherence at each step of quantum walking the characteristics of walking tended to the classical law N½. The quantum walk algorithm has already been implemented earlier in a number of systems but the experiment of Ì. Karski and his coworkers outlined above is the closest to the original one suggested by R. Feynman. Source: Science 325 174 (2009) ; arXiv:0907.1565v1 [quant-ph] A single-molecule optical transistor1 August 2009J. Hwang and his colleagues at the Swiss Federal Institute of Technology (ETH Zurich) designed a transistor using a single molecule of dibenzanthanthrene dye impregnated into a crystalline matrix of an organic compound tetradecane. When the molecule was cooled by liquid helium to a temperature of 1.4 K, its effective cross section of interaction with a photon increased to the value of the cross section of focused laser beams used in the experiment. One of the beams served as the “gate” of the transistor, controlling the quantum state of the molecule. Depending on what energy level the molecule was on, it scattered the second, more powerful beam differently and this allowed the control over the passage of the beam through the molecule; this is an analogy to how current is controlled in a conventional electron transistor. In the future such photonic devices may become a viable alternative to electronic systems owing to high speed and low heat production, for instance for building optical computers. Source: Nature 460 76 (2009) Generation of gamma radiation very close to a black hole1 August 2009Joint observations using gamma and radio telescopes allowed astronomers to establish that the high-energy gamma radiation of the M87 galaxy is generated in the immediate vicinity of a supermassive black hole. The giant elliptic galaxy M87 is the central galaxy of the Virgo galaxy cluster and is located at a distance of 50 million light years from the Earth. A black hole with a mass of about 3×109 solar masses is found at the center of the M87 galaxy. Matter falls onto the black hole from the accretion disk and produces high-power electromagnetic flares in various frequency ranges and ejecting relativistic plasma jets thrown to distances of thousands of light years. Currently gamma telescopes have low resolving power and are incapable of pinpointing the exact area within the galaxy from which the highest energy radiation is emitted. It proved possible to overcome this difficulty since gamma flares are accompanied by simultaneous radio flares. When a gamma flare ends, charged particles continue to move along the jet and therefore the radio emission intensity continues to rise for considerably longer but gamma and radio flares start practically at the same time and therefore must be produced in the same source which was identifiable in view of very high resolution of the radiotelescopes. Gamma flares from the nucleus of the M87 galaxy were recorded for two years using the atmospheric Cerenkov detectors VERITAS, H.E.S.S. and MAGIC, and radio flares were observed in parallel, using an array of ten VEBA radiotelescopes of the National Radio Observatory (NRAO). It was established by radio observation that flares are generated at a distance from the central black hole not bigger than 50 times the radius of its event horizon. These observational data can help improve the theoretical models of jet formation and clarify the mechanisms of generation of emission. Source: Science 325 444 (2009) |
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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