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| Issue 11, 2024 |
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Extracts from the Internet | ||
ÅÌÑ effect in light nuclei1 December 2009In 1983 the European Muon Collaboration (ÅÌÑ) discovered an effect which reflected how the maximum momentum of quarks in a nucleon depends on the characteristics of the nucleus to which the nucleon belongs. J. Seely et al. at the Thomas Jefferson Laboratory carried out an experiment which revealed new features of the ÅÌÑ effect. A conclusion was made in a number of theoretical papers attempting to interpret the ÅÌÑ effect that it is linked to the mean density or mass of the nucleus. The new experiment demonstrated, however, that these parameters give an ambiguous description of the effect and may be of only secondary importance. This experiment studied the scattering of 5.8 GeV electrons by targets of 2H, 3Íå, 4Íå, 9Âå and 12Ñ nuclei. It was found that the magnitude of the ÅÌÑ effect of the nucleus 9Âå is close to that of 12Ñ even though the mean density of the nucleus 9Âå is considerably lower. The conclusion is that the mean nuclear density is not a decisive factor for the ÅÌÑ effect. It was also established that the light nuclei 3Íå and 4Íå differ greatly in the magnitude of the ÅÌÑ effect. The key to the explanation of the obtained results may lie in the cluster structure of nuclei. E. g. the nucleus of beryllium may be regarded as two bound nuclei of 4Íå plus an additional neutron revolving around them, the average density of the 9Âå nucleus being considerably lower than the density of each of the 4Íå nuclei. In view of this, the ÅÌÑ effect in such nuclei may be a function of not average but local density so that the ÅÌÑ effect of the nucleus 9Âå is analogous to the effect of individual nuclei 4Íå. In other words, the properties of nucleons in nuclei are determined not by the mass or mean density of the nucleus as a whole but by the local environment of the nucleon, such as the characteristics of the clusters contained in this nucleon. Source: Phys. Rev. Lett. 103 202301 (2009) Emission cone of Vavilov – Cherenkov radiation in “left-handed” matter1 December 2009Researchers at the Zhejiang University, Hangzhou (China) and Massachusetts Institute of Technology (USA) observed for the first time the reversed Vavilov – Cherenkov radiation generated in a “left-handed” medium (a medium whose dielectric permittivity and magnetic permeability are simultaneously negative). As V.G. Veselago predicted in 1967 (see Phys. Usp. 10 509 (1968)), the cone of emission of the Cherenkov radiation and the energy flow in “left-handed” materials are directed backward relative to the motion of the particle. The experiment described here used microwave radiation propagating through a metamaterial, that is, an array of conductors. A charged particle was imitated by a sequence of dipoles with the phase changing in a certain manner; the dipoles were excited in a waveguide consisting of 14 gaps. The speed at which this “particle” was moving was v = 1,9c/n where n is the refractive index of the metamaterial. This set of dipoles is completely equivalent from the standpoint of emission of radio waves to a real charged particle; however, the imitation made it possible to achieve considerably higher (and measurable) intensity of the Vavilov – Cherenkov radiation in the frequency range 8.1-9.5 GHz. The Vavilov – Cherenkov radiation could not propagate in metamaterials studied earlier (owing to the nature of their anisotropy) so for observing the Vavilov – Cherenkov radiation a metamaterial with a special configuration of unit cells was fabricated. The reversed Vavilov – Cherenkov radiation may prove useful in fast particle detectors, e.g. in accelerator experiments. For details on media with negative refractive index see papers by V.G. Veselago in Phys. Usp. 46 764 (2003), Phys. Usp. 52 649 (2009) . Source: Phys. Rev. Lett. 103 194801 (2009) Bose – Einstein condensate of strontium atoms1 December 2009Two independent groups of researchers from the Rice University in US and the Institute for Quantum Optics and Quantum Information (IQOQI) in Austria prepared the Bose – Einstein condensate of atoms of strontium isotope 84Sr whose natural abundance is only 0,56%. Even though the abundances of the isotopes 86Sr and 88Sr are much higher, they cannot be cooled evaporatively owing to the excessively large (in the case of 86Sr) or too small (88Sr) atomic scattering length. Contrary to these two, the rare isotope 84Sr has the scattering length of 123 Bohr radii which suits cooling ideally; in the experiments of both groups, evaporative cooling was the concluding stage after laser cooling in magnetooptic trap. The transition to condensate state was identified by monitoring the optical profile of the cloud of gas and from the value of the chemical potential calculated from the dynamics of expansion of the cloud. It is suggested that the condensate of 84Sr atoms be used in ultraprecise experiments, in new systems for quantum computation and as a buffer gas in cooling other isotopes, e. g. the fermion isotope 87Sr, to degenerate state. Sources: Phys. Rev. Lett. 103 200402 (2009) ; Phys. Rev. Lett. 103 200401 (2009) Laser acceleration of neutral atoms1 December 2009U. Eichmann and his colleagues at the Institute for Optics and Atomic Physics (Berlin) and the Max Planck Institute discovered the effect of acceleration of neutral atoms by pondermotive force in the field of nonuniform laser radiation. Typically, one considers the effect of the pondermotive force on charged particles but in fact a similar force may arise in the case of neutral atoms in view of the dynamical polarization of atoms after they were excited to Rydberg states. The electron on a distant orbit may then be accelerated as a free particle by the pondermotive force (the acceleration of the nucleus is much weaker because of its large mass). If the electron after acceleration remains bonded to the atomic nucleus, the momentum of the accelerated electron is transferred to the atom as a whole. In the experiment of the German scientists a beam of neutral helium atoms was illuminated by short focused pulses of laser light, and roughly one per cent of atoms felt acceleration. In some cases the acceleration of an atom was greater than the acceleration of free fall g by a factor of 1014 — record-high for accelerations of neutral atoms in external fields ever observed. Source: Nature 461 1261 (2009) Polarization of microwave background and cosmological parameters1 December 2009The observation of polarization of cosmic microwave background is one of the most efficient methods of studying physical processes in the early Universe and of improving the cosmological parameters. Measurements of polarization of microwave background became technically possible in 2002 and have been conducted since then with gradually better precision by a number of instruments. From 2005 to 2007 observations were conducted at the South Pole with a 2.6 m QUaD radio telescope equipped with 31 pairs of orthogonal bolometers sensitive to the polarization of electromagnetic waves and functioning at two frequencies: 100 and 150 GHz. By now the data gathered during this period has been processed and more accurate values of cosmological parameters were computed. The accuracy of results is the highest if datasets of several detectors are used simultaneously (WMAP, ACBAR, QUaD etc.). For example, according to the latest data, the most probable value of the Hubble constant is H0 = 70.6 km s-1Mpc-1, the index of the density perturbation spectrum ns = 0.960, and a scenario is possible in which the index depends on scale (on the running index). It was also possible to improve the constraint on the tensor mode of perturbations (gravitational waves) in comparison with scalar perturbations (density perturbations); now the ratio of these components is estimated as r<0,33 at the confidence level 95%. Source: Astrophysical Journal 705 978 (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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