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| Issue 11, 2024 |
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Extracts from the Internet | ||
Mixing of D0-mesons1 April 2007Transformation of D0-mesons to their antiparticles was observed at the Stanford Center of Linear Accelerators (SLAC) using the ÂàÂàã detector. The mesons were created when electron and positron beams collided in the storage ring and were counted in decay products. The effect of mixing D0- and anti-D0-mesons which is responsible for the transformation of particles into antiparticles is predicted by the Standard Model of elementary particles, while mixing-free models are excluded by this experiment at the level of 3.9 standard deviations. The Standard Model also predicts an asymmetry (a small difference in decay rates) in the decays of D0- and B0-mesons due to the violation of the CP invariance; however, the experiment is so far insufficiently sensitive to measure this asymmetry. A similar effect was earlier observed with B- and K-mesons (see e.g. Phys. Uspekhi 49 549 (2006)). The group of experimenters included some Russian scientists from the G.I.Budker Institute of Nuclear Physics (Novosibirsk). Sources: hep-ex/0703020 Quantum critical point in an antiferromagnet1 April 2007P. Gegenwart and co-workers in Germany (Max Planck Institute of Chemical Physics of Solids in Dresden) and their colleagues in the USA studied low-temperature phase transitions in an antiferromagnetic compound YbRh2Si2 in magnetic field. The measured characteristics were the isothermal magnetostriction - an expansion of a specimen as a function of magnetic field at various fixed temperatures - and also electric properties of specimens. The growth of bubbles of a new phase at the initial stage is characterized by fluctuations, such that at sufficiently low temperatures the zero quantum fluctuations dominate thermal ones. A transition was found at temperatures below 0.8Ê, which does not sit well with the conventional scenario of “universal classes” of phase transitions that works nicely in the case of thermal fluctuations. At the moment no detailed theoretical description exists of the mechanism of the new phase transition but it was suggested that the arising entangled quantum states of conduction electrons and magnetic moments of valence electrons play the key role. This produces quasiparticles whose properties resemble those of heavy electrons. According to this model, the appearance of a “liquid of heavy electrons” is the factor that is responsible for the observed phase transition. Sources: http://physicsweb.org/articles/news/11/2/18/1 Observation of photons in a resonator1 April 2007S. Gleyzes and co-workers in France carried out an experiment in which for the first time single photons were observed in an electromagnetic resonator by a non-demolition method. The tool that served to carry out the measurements was a beam of rubidium atoms in Rydberg states, sent across the resonator. The resonator consisted of two superconducting niobium mirrors placed in a protection shield that cancelled the effects of thermal radiation and external electromagnetic fields. Before crossing the resonator, Rb atoms were prepared as a superposition of quantum states with the principal quantum numbers N=50 and N=51. The resonator frequency was somewhat different from the frequency of transitions between atomic levels, so a photon was not absorbed in the interaction with an atom and only a phase shift occurred in the dipole oscillations of the atom. Hence, the observation of the photon was nondestructive. The phase shift increased the probability of the subsequent transition of to a level with the principal quantum number N=51. Atoms emerging from the resonator were detected using an atomic interferometer. The presence of a photon in the resonator was established by knowing the relative number of atoms in the state N=51 emerging from the resonator. Time and again a photon would be spontaneously created in the resonator. Hundreds of atoms had time to cross the resonator during the life of the photon. The atomic beam thus made it possible to monitor the entire “lifetime” of a photon in the resonator from its creation until its disappearance. Sources: Nature 446 297 (2007) Vortices in the Bose-Einstein condensate1 April 2007Vortex formation was observed at the Arizona University when independent clouds of Bose-Einstein condensate of 87Rb atoms were allowed to merge together. Laser beams created potential barriers inside a cylindrical atomic trap which partitioned the trap into three identical circular segments. An independent Bose-Einstein condensate cloud formed in each segment in the course of evaporative cooling. When the potential barriers were turned off, the condensate clouds merged together and one or several vortices were seen to form. The number of vortices depends on the rate of merging of the clouds, which is controlled by the rate at which the barrier is removed. This phenomenon is a result of phase differences between the wave functions of the independent condensate clouds and therefore, of the nonzero total angular momentum possessed by the condensate. Source: Phys. Rev. Lett. 98 110402 (2007) The Greisen—Zatsepin-Kuzmin effect (the GZK Cutoff)1 April 2007In 1966 Ê.Greisen, G.T.Zatsepin and V.A.Kuzmin gave a theoretical prediction that owing to the interaction between particles of cosmic rays and microwave background photons the spectrum of cosmic rays should have a cutoff energy of about 6x1019eV. A number of experiments were carried out since that time to observe “broad atmospheric showers”, that is, cascades of particles that are created in the interaction between cosmic rays and atoms in the atmosphere. In several cases particles were encountered with energies above the cutoff threshold but these results were ambiguous and carried large errors. Cosmic ray particles with superhigh energies are probably a product of acceleration at shockwave fronts in other galaxies or galaxy clusters. Reports on detection of particles with energies above the cutoff threshold led to creation of alternative “top-down” models of the origin of cosmic rays in decays of hypothetical supermassive particles in our Galaxy. The situation may get clearer through using new detectors that already started measurements. At the moment one of the latest experiments, the HiRes, that collected data from 1997 to 2006, built the best statistics of observations of superhigh-energy cosmic rays. The HiRes consists of two telescopes that measure the UV radiation of nitrogen molecules in the Earth’s atmosphere excited by broad atmospheric showers. According to the data of this experiment, the spectrum does indeed manifest a cutoff at the confidence level of about 5 sigma at an energy 5.6x1019eV. One can thus conclude with a sufficient measure of confidence that the Greisen—Zatsepin-Kuzmin effect has been observed in the HiRes experiment. HiRes also confirmed a feature in the cosmic-ray spectrum known as the “ankle” which was predicted by V.S.Berezinsky and S.I.Grigoreva in 1988. Sourcess: astro-ph/0703099 |
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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