 |
RSS feeds |
|
|||||
| Issue 12, 2024 |
|
||||||
|
|
|
||||||
Extracts from the Internet | ||
Multi-muon processes1 December 2008New results have been obtained at the E. Fermi Tevatron Laboratory which, in case their validity is confirmed, may point to detection of new particles or interactions beyond the Standard Model of elementary particles. Researchers studied proton-antiproton collisions with energy 1.96 TeV in a vacuum beam pipe 1.5 cm in diameter. Collisions generated mesons which decayed into, among other things, muons which were recorded by detectors. Previous experiments revealed a discrepancy between the number (and the distribution) of muons and the predictions of the Standard Model. The aim of the new experiment was to identify the causes of discrepancies. By installing additional detectors it was possible to establish where exactly muons were created. Complete agreement with theoretical calculations has been found for those muons whose tracks originated from within the colliding beams. However, it was unexpectedly found that a significant fraction of muons were created outside the beams and even outside the vacuum pipe at a distance of several centimeters from the location of pp collisions. Also, multiple cascade creation processes (creating up to eight particles) were also observed in which both μ- and μ+ muons in various combinations were created with equal probability and the trajectories of their flight were remote from the axis of the pp beam; several muons formed a “muon jet”. One possible explanation of the observed exotic processes could be the existence of unknown particles which were created in collisions within the beam, flew out far from the beam edge and there decayed into other particles and muons. According to the computations of P. Giromini and his colleagues, cascades need at least three new particles for explanation, with masses of 3.6 GeV, 7.3 GeV, and 15 GeV, and they are absent from the Standard Model. Further checking and clarification of these results requires new independent experiments. Sources: http://arxiv.org/abs/0810.5357, http://arxiv.org/abs/0810.5730 Nanoclaster transports electrons1 December 2008S. Gordeev (Bath University, UK) and his co-workers implemented experimentally the transfer of electrons between two electrodes via metallic nanoclusters. Such hypothetical “nanoshuttle” was theoretically suggested 10 years ago by L.Yu. Gorelik et al, and attempts to implement the “nanoshuttle” were undertaken using semiconductors and fullerenes, with ambiguous results. In the successful experiment of S. Gordeev and his colleagues a gold nanocluster 20 nm in diameter was fixed to two electrodes by a monolayer of elastic organic molecules. Electric field forced the nanocluster to rapidly vibrate between the two electrodes, transferring four electrons per cycle. The current flowing between the electrodes depends on the frequency of oscillations of the nanocluster (it reached 1011 Hz in the experiment), which in turn depends on the nanocluster mass and the elasticity of organic molecules. The current-voltage characteristics of the device agree well with those calculated in the theory of L.Yu. Gorelik et al. The method allows development of new nanoelectromechanical devices with unique properties; for instance, a similar device with three electrodes may become a nanoelectromechanical analog of the transistor. Source: http://arxiv.org/abs/0810.2430 Nanorotor1 December 2008Researchers at the Liverpool University (UK) and colleagues at the Academy of Sciences of China designed a molecular nanorotor characterized by a number of useful differences from other molecular rotors developed at the moment. Complex molecules of tetra-tert-butyl zinc phthalocyanine were evaporated onto gold substrate and their nitrogen atoms bonded the molecules to single atoms of gold. The point of attachment — the axis of rotation — did not coincide with the center of the molecule so that the molecule rotated as a whole around a point on its boundary. Rotation was driven by the energy of thermal degrees of freedom of the molecule. An important achievement was that the rotation axis was fixed in space. The investigation of nanorotors was carried out using a scanning tunneling microscope. The researchers also created an array of such rotating molecules on an area of several square micron. If an atom of a magnetic metal is placed at the center of the molecule, it becomes possible to control the angular velocity of rotation by means of an electric current passing through the molecule and creating a magnetic field. Source: Phys. Rev. Lett. 101 197209 (2008) Study of inner orbitals in molecules1 December 2008A technique called high harmonic generation, or HHG, has been developed at a SLAC laboratory which makes it possible to study inner electron levels of molecules. A beam of nitrogen molecules was irradiated with laser light. Molecules were thus raised to an excited state and on reverse transitions emitted photons whose spectrum permitted reconstruction of the electron structure of molecules. In contrast to earlier similar experiments, it became possible for the first time to observe radiative transitions not only for electrons of the outer electron orbital but also of a deeper orbital corresponding to the next energy level. The data from this study is important for understanding the mechanism of chemical reactions in which electron bonds in molecules are restructured. Source: http://www.physorg.com/news145214579.html Nearby source of cosmic rays1 December 2008The ATIC balloon experiment measured the intensity of cosmic rays and gamma photons in upper layers of the atmosphere. The data obtained revealed a curious feature on the electron spectrum: excessive abundance in the energy range 300-800 GeV and sharp decline towards higher energies. The excess is observed relative to the background level that corresponds to a very carefully developed model of generation of electrons in supernova remnants and the model of electron propagation in galactic magnetic fields. Since electrons are absorbed quite easily by interstellar matter, this excess can be produced only by a source in relative proximity to the Earth. It was found, however, that spectra of electron emission from ordinary sources such as neutron stars or microquasars cannot explain the observed energy spectrum. At the same time, good agreement between calculations and observations is achieved in the model with annihilation of dark matter particles of mass 620 GeV in Kaluza – Klein-type field-theory models with compactified extra dimensions. It is not impossible therefore that the ATIC experiment detected a dense clump of dark matter in which annihilation occurred. Another hypothesis assumes that particles may annihilate in a high-density peak near an intermediate-mass black hole. Source: Nature 456 362 (2008) |
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. | |
|
© 1918–2024 Uspekhi Fizicheskikh Nauk Email: ufn@ufn.ru Editorial office contacts About the journal Terms and conditions |