Extracts from the Internet

Testing the Standard Model

Recently, several experiments have been performed to test the predictions of the Standard Model of elementary particles (for background see Physics Uspekhi 42 1193 (1999)). Experiments at the Stanford Linear Accelerator Center (SLAC) and the KEK laboratory in Japan have measured the violation of charge parity (CP) in a system of B mesons. CP violation manifests itself as a slight difference in decay time between the B meson and its antiparticle. The results of both experiments are in precise agreement with the predictions of the Standard Model.

The g-2 collaboration at Brookhaven National Laboratory has now performed a new measurement of the anomalous magnetic moment of the muon. The accuracy of the experiment is two times better than in previous experiments. The measured anomalous moment is different in value from that calculated within the Standard Model. This discrepancy may suggest that there are new particles or new interactions beyond the Standard Model - for example, theoretically predicted `superstrings.' The measurements of the muon's anomalous magnetic moment and neutrino oscillation data present the only deviations from the Standard Model currently known. Source: Physics News Update, Number 600, Phys. Rev. Lett. 89 101804 (2002)

Superconductivity in magnesium diboride

S Louie and M Cohen and their colleagues at the University of California and the Berkeley National Laboratory have put forward a new theoretical explanation of the superconductivity of magnesium diboride. According to their calculations, MgB₂ has not one energy gap as usual low-temperature superconductors, but two gaps, with a different critical temperature for either. The combination of these temperatures gives a transition temperature for magnesium diboride of 39 K. The two gaps appear due to the strong interaction of electrons and photons along the planes of the crystal lattice formed by boron atoms. The experimentally observed temperature dependence of the heat conductivity of MgB₂ is also successfully explained by the two-gap model. Source: Nature 418 758 (2002)

Quantum properties of plasmons

It is known that photons can pass through holes in a metal foil whose diameter is smaller than the photon wavelength. The passage occurs due to the conversion of photons into surface plasmons (electron excitations) and the subsequent re-emission of photons by plasmons on the other side of the foil. A team at the University of Leiden in the Netherlands studied the passage of pairs of photons in entangled quantum states through a set of holes in a gold foil. The photon wavelength in this experiment was three times the hole diameter. As it turned out, most of the photon pairs remained in entangled states after passage through the foil. This result is unexpected in that the quantum correlation of a photon pair persists in spite of the fact that plasmons consist of 10¹⁰ electrons and are therefore macroscopic systems. This experiment appears to be the first to observe the quantum properties of plasmons. A theoretical description of the discovered effect has not yet been given. Source: Nature 418 304 (2002)

The merger of supermassive black holes

There exist many galaxies which are known to interact and to merge. Once two galaxies have merged, supermassive black holes residing at their cores are supposed to gradually descend to the center of the new galaxy and to form a binary system. It has been unclear, however, whether there is enough time for the black holes to merge under the action of dynamic friction and gravitational radiation. According to many calculations, the time required for the merger to occur exceeds the current age of the Universe. A study by D Merritt (USA) and R D Ekers (Australia) provides a serious argument in favour of the merger scenario. The structure of radio galaxies is observed to contain jets directed along the axes of rotation of the central black holes. Importantly, jets are found more often in more massive galaxies (i. e., those with larger-mass black holes) than in less massive ones. Calculations by D Merritt and R D Ekers showed that immediately before a merger, the jet ejected by the more massive black hole undergoes a sharp tilt at an average angle of 50 degrees. The tilt is due to the addition of the black holes' intrinsic and orbital angular moments. Observations show that about 7% of radio galaxies indeed have X-like jets with brightness in one direction much lower than in others. This jet configuration is evidence for a sharp tilt the jets undergo - most likely when the merger takes place. A slow precession of the axis of rotation would lead to S-like jets observed in other radio galaxies. The result of the study adds optimism to the search for the gravity wave bursts which accompany the merger of black holes. Source: http://www.arXiv.org/abs/astro-ph/0208001