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| Issue 12, 2024 |
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Extracts from the Internet | ||
A new meson discovered1 June 2003A new elementary particle, called Ds+(2317), has been discovered in the BaBAr experiment at the Stanford Linear Accelerator Center (SLAC). It is discovered as a resonance in the system of Ds+-pi mesons. The new particle is probably a p-wave excitation of the Ds+ meson (the ground state of the system of c and anti-s quarks). Its existence has been predicted theoretically, but its measured mass, 2.317 GeV, turned out to be much lower than predicted. Nor is the particles' decay pattern consistent with the theoretical model of the meson (no radiation transitions to the ground state occur). Alternative hypotheses are suggested, according to which the newly discovered particle is either a four-quark system or a hadron molecule. The particle Ds+(2317) was produced in electron-positron collisions at the PEP-II asymmetric storage ring. The international BaBar collaboration consists of more than 500 specialists, including Russian scientists from the G I Budker Institute of Nuclear Physics in Novosibirsk. Source: http://www-public.slac.stanford.edu/babar/ Atomic levels of fermium1 June 2003The electronic energy levels of the element 255Fm have been studied for the first time at the University of Kassel in Germany. This makes Fm the heaviest element whose spectrum has been studied. Fermium atoms, with a half-time of 20.1 h., were produced in the reactor at the Oak Ridge laboratory in the US and then delivered to Germany. The absorption lines of the atomic transitions involved were detected using the method of resonance ionization spectroscopy. The ionization of atoms was produced in two stages: the first laser pulse promoted electrons to an excited level, the second pulse knocked electrons out of an atom. After that, the number of produced ions was counted. In this way two energy levels were found whose characteristics were identical to those calculated by the Dirac-Fock method. In calculating electronic states in heavy atoms it is necessary to include relativistic effects, which somewhat changes the usual classification rules for stationary orbitals. Source: Phys. Rev. Lett. 90 163002 (2003) Long alpha-decay1 June 2003P de Marcillac and his colleagues, in France, have measured the longest alpha-decay half-life known. The natural bismuth isotope 209Bi, according to calculations, should decay into 205Tl with a half-life of 4.6x1019 years. That long time is due to the small difference between the binding energies of the nuclei and the fact that 209Bi neighbours doubly magic nuclei 208Pb. The low energy (3 keV) of the alpha-particles emitted in the decay process has prevented their detection in previous experiments. In particular, attempts to detect them by using nuclear emulsions have been unsuccessful. The new experiment used a scintillating bolometer enclosed in a reflecting cavity and cooled to a temperature of 20 mK. The bolometer consisted of two detectors, one of which, a crystal of Bi4Ge3O12, converted a temperature change into an electrical signal, and the other, a germanium photocell, registered photons. As the alpha particles were absorbed by the germanium target, light was emitted and heat released. Over 5 days of experiment, 128 alpha particles were detected, with a 3.14 keV decay line of 209Bi in their spectrum. The half-life of (1.9 + - 0.2)x1019 year obtained based on detection rate measurements is close to the theoretically calculated value. The experimental facility employed is a prototype for the more massive detectors of the dark matter search experiment ROSEBUD now planned. Source: Nature 422 876 (2003) Superconductivity of magnesium diboride1 June 2003Evidence has been found in a number of experiments that the intermetallic superconductor MgB2 has at least two gaps in the electronic energy spectrum (see Physics Uspekhi 44(3) 330 (2001) and Physics Uspekhi 45(9) 998 (2002)). The experiments failed to characterize the gaps, however, because only the averaged electron momentum distribution was measured. T Takahashi and his colleagues used angle-resolved photoemission spectroscopy to obtain direct data on the two gaps. The energy gaps are due to the sigma and pi-bonds of electrons in boron atoms and have values of (6-7)x10-3eV and (1-2)x10-3eV, respectively. It is the former which is mainly responsible for the superconducting properties of MgB2. Source: Nature 423 65 (2003) Young stars in the Andromeda halo1 June 2003Among the stars in the spherical halo of the galaxy M31 (Andromeda nebula), only bright giant ones have previously been observed. Now, using the ASC camera onboard the Hubble Space Telescope, it has proved possible to resolve about 300,000 of normal- and low-brightness stars. It turned out that about a third of these stars have an unexpectedly short age of 6 to 8 billion years, much less than stars in the halo of our Galaxy, about 11-13 billion years. In addition, the discovered young stars are richer in heavy elements compared to those in the halo of the Galaxy. The astronomers believe that the composition difference between the halos of the two galaxies is due to their different formation histories. The Andromeda nebula may have undergone a merger with another galaxy. Disk stars of one of the merged galaxies dispersed into the halo or, alternatively, the merger initiated the birth of new stars in the halo itself. Source: http://hubblesite.org/news/2003/15 |
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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