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| Issue 12, 2024 |
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Extracts from the Internet | ||
Unusual elementary particle1 May 2009The CDF collaboration detected an unusual particle in an experiment on the Tevatron accelerator; so far this particle could not be classified in terms of the accepted quark recipe of building mesons and baryons. The researchers selected B+ → J/ψφK+ decays. It was found that the intermediate state of the decays are new particles Y(4140) with a mass of about 4140 MeV. The experimenters recorded 14±5 such events at statistical significance of 3.8 standard deviations. The assumption that Y(4140) is one of the states of the system of c-anti-c quarks met with serious difficulties in describing the observed characteristics of the decays; the structure of the new particle is still unclear. There is a possibility that Y(4140) is a hadronic molecule, a hybrid particle that includes gluons as its components, or that Y(4140) is a new four-quark state. The Y(4140) particle is another in a series of exotic particles discovered in recent years (see Phys. Usp. 50 1289 (2007)). Source: arXiv:0903.2229v1 [hep-ex] The Efimov effect for four particles1 May 2009In 2006 H.-C. Nagerl and coworkers at the University of Insbruck in Austria observed for the first time bound quantum states of three cesium atoms (see Phys. Usp. 49 438 (2006)). The object of study was the Bose – Einstein condensate in which the intensity of interaction between atoms was controlled by the magnetic field and the bound states were identified by determining recombination losses. The effect of formation of trimers, predicted by V.I. Efimov in a theoretical paper in 1970, can arise even in the absence of bound paired states of atoms. After this discovery, in 2007-2008 two groups of researchers made a theoretical prediction that a similar effect is possible for four particles too; furthermore, some observational evidence of formation of bound quartets was found among the data of the first experiment. H.-C. Nagerl and his colleagues carried out a new experiment using a technique completely reproducing that of the earlier experiment. The presence of bound quantum states of four particles was demonstrated unambiguously. The range of applicability of the Efimov effect was thus extended to quartets of atoms. In contrast to trimers, bound quartets form only a pair of universal states and not an infinite sequence. The data obtained in this work will be used to test principally important aspects of quantum mechanics. Source: Phys. Rev. Lett. 102 140401 (2009) Doubly magic nucleus 24Î1 May 2009Nuclei of the unstable oxygen isotope 24Î having a large excess of neutrons were studied at the GSI laboratory (Darmschtadt, Germany). It was found that these nuclei are doubly magic, i.e. that their proton and neutron shells are completely filled, despite a strong distorsion of level structure at the nucleus stability boundary. A beam of 24Î nuclei was generated in the accelerator by sending 48Ca nuclei into a target. This process created three 24Î nuclei per second which then collided with a carbon target, losing one neutron each. The form of the momentum distribution of the resulting 23Î nuclei made it possible to determine the structure of nucleon shells in 24Î nuclei. It was confirmed that 24Î are indeed doubly magic nuclei with spherically symmetric shells. The study of 24Î isotopes is important for nuclear astrophysics because they may be born in supernova explosions; these nuclei may also be present in the crust of neutron stars. Source: Phys. Rev. Lett. 102 152501 (2009) Optical Maxwell's demon1 May 2009J.J. Thorn and his colleagues at the Oregon University created an optical barrier that lets through atoms of 87Rb in one direction only. Two almost parallel laser beams separating a dipole atomic trap into two parts were focused near its center. The frequency of one of the beams differed from the resonance frequency of transition between sublevels of hyperfine splitting in 87Rb. This beam created a potential barrier whose permeability for an atom depends on its state: atoms on the lower subsevel pass through the beam unobstracted while the potential created by the barrier for the excited atoms is repulsive. The frequency of the second laser equals the frequency of transition between the sublevels. If an atom in the ground state crosses the second beam and passes from one side of the trap to the other, it absorbs a photon from the second beam and after this is unable to pass through the barrier in the opposite direction. As a result, atoms accumulate on only one side of the barrier. This system of beams which operates as an atomic diode is an analog of Maxwell's demon. By analogy to Maxwell's thought experiment, it is possible to experimentally check how entropy is transferred between the parts of the system in the case of the optical barrier. The reduction in entropy caused by the displacement of atoms to one side of the trap is compensated for by the production of entropy when photons are scattered. Furthermore, this technique is of interest for developing new ways of laser cooling of atoms. Source: arXiv:0903.3635v1 [physics.atom-ph] Superconductivity in SrFe2As21 May 2009Scientists at the Tokyo Institute of Technology discovered that after the compound SrFe2As2 was treated with steam, it become a superconductor with the temperature of superconducting transition Tc ≈ 25 Ê. The compound SrFe2As2 belongs to a class of new layered iron-based superconductors. Its intense study began in 2007 (see Phys. Usp. 51 1201 (2008)). However, superconductivity arises in these compounds only when they are doped with certain impurities or under high pressures. H. Hosono and his coworkers exposed a film of pure (undoped) SrFe2As2 to moist atmospheric air which resulted in the samples becoming superconducting. Checking demonstrated that exposure to individual components of air (nitrogen, oxygen or carbon dioxide) without H2O did not produce such changes. So far the exact mechanism of the effect of H2O remains unclear. It is suggested that either oxygen atoms of water molecules penetrate the structure of the crystal SrFe2As2, or that strontium atoms bind to the OH group and form atomic vacancies. Superconductivity arises in response to application of water in some other (not iron-based) layered compounds too but neutron diffraction studies established that interlayer spacings in SrFe2As2 do not grow in response to water treatment but are reduced. It was also found that the superconductivity of SrFe2As2 caused by water is strongly anisoropic in magnetic field (Tc depends on the field orientation) — in contrast to the almost isotropic superconductivity of SrFe2As2 doped with cobalt atoms. Source: arXiv:0903.3710v1 [cond-mat.supr-con] |
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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