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Issue 12, 2024
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Extracts from the Internet


A new particle is discovered

Until now, only hadrons consisting of two or three quarks have been observed. The existence of the five-quark particle uudds was predicted theoretically by D. Dyakonov, V. Petrov, and M. Polyakov in 1997. Now N. Nakano and his colleagues in Japan have for the first time detected this particle in an experiment on collisions between gamma photons and neutrons. The gamma radiation was produced by scattering laser light from a beam of accelerated electrons. The photons flew through a plastic scintillator and interacted with neutrons in carbon nuclei. Five-quark particle Theta+ with mass of 1.54 GeV were created in the reaction gamma n -> K-Theta+-> K-K+n and were identified through a resonance in the energy spectrum of K+-mesons. Subsequent experiments at the Institute of Theoretical and Experimental Physics (ITEF) in Russia and T. Jefferson Lab in US also detected the five-quark state, thus confirming the results of the Japanese scientists. In the ITEF experiment, which involved collisions of K+-mesons with liquid xenon, K+Xe->K0pXe, a resonance corresponding to a five-quark particle was discovered in the energy spectrum of the reaction products. There remains the probability, however, that the five-quark state detected in the above experiments is in fact not a hadron resonance but a molecular meson-barion one. Sources: Phys. Rev. Lett. 91 012002 (2003), hep-ex/0304040

A Superconductor with a large second critical field

H.J. Niu and D.P. Hampshire (both of Durham University, UK) have developed a method for fabricating superconducting materials with a large value of the second critical magnetic field Bc2. This is the external field at which the superconductivity of a material disappears completely due to the entire volume of the superconductor being filled with magnetic vortices. According to Ginzburg-Landau theory, Bc2 grows with decreasing either the coherence length or the electron mean free path - which is due to the decrease in the vortex size. H.J. Niu and D.P. Hampshire reduced the mean free path considerably in the superconductor PbMo6S8 by milling it into a very fine powder and sintering it at high temperature and pressure. The resulting sample has a granular structure with crystalline grains about 20 nm in size. Electrons in the superconductor undergo strong scattering from the disordered grain boundaries, the coherence length near 2nm turning out to be much less than the characteristic grain size. The value of Bc2 in a granulated sample is about 100 T, twice the value for a single crystal PbMo6S8 . It is as yet technologically impossible to make the windings of superconducting magnets from a brittle granulated material. Source: Phys. Rev. Lett. 91 027002 (2003)

Formation of molecules in a degenerate Fermi gas

D. Jin and her colleagues at the National Institute of Standards and Technology (NIST) and the University of Colorado have for the first time obtained ultracold molecules from atoms of a degenerate Fermi gas. Weakly bound molecular states of the atoms of potassium-40 appeared due to the Feshbach resonance at a magnetic field of about 224 Gauss and temperature less then 150nK. The number of separate atoms in the optical trap was controlled by spectroscopic methods. Using the Zeeman effect the energy of atoms near the resonance could be changed smoothly by varying the magnetic field. On reaching the resonance, the number of separate atoms decreased almost by half in a jump, indicating that some of the atoms joined together to form molecules. About 250,000 40K2 molecules were obtained in the experiment. Away from the resonance molecules broke, and the number of atoms returned to its original value. The binding energy of the molecules as measured from the way they absorb radio waves, turned out to be in exact agreement with the theoretical value. Further experiments are planned to see if superfluidity is possible in the molecular gas obtained. Source: Nature 424 47 (2003)

Experimental study of the Landau-Pomeranchuk effect

L.D. Landau and I Ya. Pomeranchuk, in 1953, showed theoretically that at high energies the radiative energy loss of electrons in material decreases and their penetrating power increases. This phenomenon, described as an increase in the radiation length formally, is due to the fact that high-energy electrons interact with many atoms simultaneously. Earlier, however, the Landau- Pomeranchuk effects has been studied experimentally only for electrons with energies below 25 GeV, and the dependence of radiation length on electron energy has not been revealed. Now H.D. Hansen and his colleagues at CERN has conducted a new experiment for electron energies of 149, 207, and 287 GeV and have for the first time seen an increase in radiation length with increasing electron energy. A beam of electrons from an accelerator was directed into an iridium target, and the spectrum of Bremsstrahlung radiation photons was measured. The results of the experiments are in good agreement with A.B. Migdal's 1956 calculations of the effect. The Landau-Pomeranchuk effect may have a major role in the development of cosmic-ray-produced air showers near the Greisen-Zatsepin-Kuzmin threshold. Source: Phys. Rev. Lett. 91 014801 (2003)

Evolution of galaxies and of supermassive black holes

Experimental observations of recent years have shown that almost all elliptical and spiral galaxies harbour black holes 106-109 the mass of the Sun at their cores. However, the formation mechanism of the supermassive black holes is not yet reliably established. The black holes might have formed in the `dark ages' of the universe (and then increased their mass through the accretion process) or at a later time, from the material of destroyed stars in the central regions of galaxies. Observations made by astronomers at J. Hopkins University in Baltimore made it possible to establish that the mass growth of distant galaxies and the formation of star systems took place synchronously. Data on 120,000 galaxies from the Sloan Digital Sky Survey were employed. Statistical analysis showed that star formation processes and the activity of cores are time correlated and possibly interrelated. Source: www.nature.com

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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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