Extracts from the Internet


A new limit on neutrinoless double-beta-decay

Following reports of the neutrinoless double-beta-decay of 76Ge nuclei (see Phys. Usp. 45 345 (2002)), and given the subsequent discussion questioning the discovery, the independent experimental study of such decays in 76Ge and other nuclei has come on the agenda. Because neutrinoless double-beta-decay does not conserve lepton number, its observation would yield insight into new physics beyond the Standard Model of particle physics. Of interest in this context are new results from a search for the neutrinoless double-beta-decay of 130Te nuclei in the CUORICINO experiment at the Gran Sasso laboratory in Italy. The CUORICINO decay detector is an array of 62 bolometers made of crystalline TeO2 and totaling a mass of 40.7kg (the mass of tellurium is 11.0kg). The heat capacity of TeO2 at the detector working temperature of 8mK is so low that nuclear decay products generate enough heat for rare decay events to be detected. The experiment is located in an underground laboratory and carefully protected from external radioactive sources. CUORICINO did not detect any evidence for neutrinoless double-beta-decays, implying, more specifically, that the half-life of the double-beta-decays (if it really exists) should be in excess of 1.8x1024years years. This result, in addition to providing a new stringent limit on the decay probability, also puts a limit on the neutrino mass, <0.2eV, provided the neutrino is a Majorana particle. CUORICINO is the first step toward a larger-scale experiment, an array of 19 detectors of this kind, which will greatly improve the accuracy of the measurements. Source: Phys. Rev. Lett. 95 142501 (2005)

Bose-Einstein Condensation of Magnons

The term `magnons' refers to quasiparticles (elementary excitations) in systems of interacting particles. Magnons obey Bose-Einstein statistics, and accordingly a number of theoretical studies have been made to see whether Bose-Einstein condensation can occur in a gas of magnons. Now a team of researchers from Germany, Russia (Joint Institute for Nuclear Research, Dubna), the UK and Poland has observed such a condensation experimentally in a crystal of antiferromagnetic Cs2CuCl4. Measuring the heat capacity of the crystal in an external magnetic field at low temperature, the researchers observed a phase transition at a magnetic field Bc=8.51T, with the critical temperature varying with the magnetic field as (Bc-B)1/p, where p=1.5. This is exactly the dependence predicted theoretically for the Bose-Einstein condensation of magnons. At B>Bc, magnetic moments perpendicular to the field are no longer arranged in a correlated fashion. At this temperature a gap opens up in the energy spectrum of the magnons, which has previously been shown to exist in neutron scattering experiments. Although there is some experimental evidence, obtained back several years ago, for the Bose-Einstein condensation of magnons in TeCuCl3, the critical exponent p in those experiments was different from 1.5, and hence Bose-Einstein condensate was not actually obtained there in its pure form. In the new experiment by T.Radu and colleagues, the existence of a condensate in Cs2CuCl4 is established beyond reasonable doubt. Source: Phys. Rev. Lett. 95 127202 (2005)

Hall effects for phonons

Researchers in France have experimentally discovered an analogue of the Hall effect for the flow of phonons in a magnetic field (phonons are excitation quasiparticles, or lattice vibration quanta). C.Strohm, G.Rikken, and P.Wyder established a temperature difference of about 1K between the ends of a small dielectric rod made of the compound Tb3Ga5O12 and applied a magnetic field perpendicular to the flow of phonons that arose (and carried heat) due to the temperature difference. The magnetic field was varied from zero to 4T, and as it was increased, a temperature difference arose between the rod's side surfaces, perpendicular to both the photon flow and the magnetic field, which was linear with the field and ranged between 1.0 and 3.0mK. Directing the magnetic field parallel to the photon flow did not lead to temperature difference between the rod faces, suggesting that a transverse magnetic effect was observed. A transverse effect for heat flow through metals, discovered back in the 19th century, eight years after the discovery of the Hall effect, is due to the electron contribution to thermal conductivity. The effect of a magnetic field on electrically neutral phonons has a different nature and is related to the anisotropic scattering electrons undergo as they move diffusively in a magnetic field. An accurate theory of this effect has not yet been developed, though. Source: Phys. Rev. Lett. 95 155901 (2005)

Cosmic gamma-ray bursts

Short-burst afterglow Cosmic gamma-ray bursts divide into two classes: long ones, lasting more than 2s and having a relatively soft spectrum, and short, spectrally hard ones. A number of long bursts have been seen to have an afterglow in the optical, X-ray, and radio bands, allowing their association with supernova explosions in distant galaxies (see Phys. Usp. 46 557 (2002)). The origin of the short burst has, on the contrary, been unclear. Calculations show that a supernova explosion cannot produce a short burst. Now, the sources of two gamma-ray bursts have been identified for the first time from their afterglow by pooling data from ground- and space-based telescopes. Observations with the Swift satellite X-ray telescope showed that the source of the gamma-ray burst GRB050509B is located in a bright elliptical galaxy with a redshift z=0.225. Because star formation in that galaxy has long been over and because massive stars are short-lived, it is highly unlikely that the explosion of a massive star gave rise to this burst. The spectral and time characteristics of the GRB050509B suggest that its cause is the merger of a neutron star with another neutron star or with a black hole. For the short gamma-ray burst GRB 050709, the X-ray and optical afterglow have been observed for the first time using an array of telescopes including the Hubble Space Telescope. GRB050709 was originally detected by gamma-ray-detectors onboard the HETE satellite, and shortly afterwards other telescopes were used to investigate the burst's localization region. The burst is at a distance of 3.8kpc from the center of an irregular dwarf galaxy at z=0.16. The burst's luminosity curve together with the absence of characteristic supernova explosion features in the emission spectrum indicate that GRB050709 occurred when a pair of neutron stars or, alternatively, a neutron star and a black hole merged together. Such mergers of compact objects produce bursts of gravitational waves, so the identification of short gamma-ray bursts increases the hope that the LIGO detector will be able to register such waves some time soon. Source: Nature 437 pp.845, 851, 855, 859
The most distance burst The afterglow of the cosmic gamma-ray burst GRB050904 in the optic and the short-wavelength IR range has been detected by the 8.2-m VLT telescope. From the spectral characteristics of the afterglow, the redshift of the burst is found to be z=6.3, making it the most distant of the afterglow-producing bursts. Furthermore, the source of the burst, together with several quasars and young galaxies, are the most distant z>6 objects known in the Universe. GRB050904 belongs to the class of long bursts, and its characteristics do not in principle differ from those of other, closer long bursts. While all the indications are that its source was a supernova explosion, it is not yet firmly established explosions of exactly which high- redshift stars produced gamma-ray bursts. Source: astro-ph/0509766

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