1 July 2007
Until recently only one baryon Λb (quark structure udb) incorporating a b quark was observed experimentally. A D0 experiment on the Tevatron accelerator of the Enrico Fermi National Accelerator Laboratory discovered a new baryon Ξb- (also known as "cascade b") and its antiparticle Ξb+ that consist of three quarks, one from each quark family, d, s and b. Indirect evidence of the existence of the baryon Ξb- was reported at CERN. In the D0 experiment, the Ξb- baryon was created in proton-antiproton collisions with center-of-mass energy 1.96GeV and was identified at the confidence level 5.5σ in decay chains Ξb-→J/ψΞ-,
Ξ-→Λπ-→pπ-π-. Experimental data gathered in the period from 2002 to 2006 were used. The rate at which Ξb- baryons were created was approximately 28% of the creation rate of Λb in the same collisions. The measured Ξb- baryon mass was found to be 5.774±0.011±0.015GeV (the first error is statistical, the second is systematic). This value is in good agreement with the theoretical value 5.806±0.008GeV. The team of the international experiment D0 includes Russian physicists from JINR (Dubna), ITEP (Moscow), Moscow University and the
B.P.Konstantinov INP (Sankt-Peterburg). Sources: http://arxiv.org/abs/0706.1690
Superfluidity in solid helium?
1 July 2007
In 1969 A.F.Andreev and E.M.Lifshits gave a theoretical prediction that at low temperatures microscopic defects in solids (atomic vacancies) should undergo Bose-Einstein condensation. The experimental discovery of this effect was announced by ┼. Kim and M. H. W. Chan in 2004. In their experiment on measuring the momentum of inertia of torsion pendulums filled with 4He, the phase transition set in at a temperature Tc≈200m╩ and at pressure of 25-135atm, and the maximum mass fraction of the superfluid in the the sample (about 1.5%) was observed at a pressure of 50atm (see Physics Uspekhi 47 215 (2004)). Experiments conducted immediately after this by other groups failed to produce an unequivocal result (for details see Physics Uspekhi 49 1307 (2006)). S.O.Diallo and his colleagues in the USA and UK carried out a new independent study of solid 4He specimens at a pressure of 41 atm by the neutron scattering technique in the temperature range 80 to 500m╩. Particle concentration n(k) was measured in the zero quantum state k=0 which corresponds to the Bose-Einstein condensate. No changes were observed in the state of specimens on crossing the Tc point and to within the measurement error, n(k)=0 for k=0 was obtained. To summarize, the new experiment did not confirm the existence of the Bose-Einstein condensation, so that the superfluid state observed earlier may have been of a different origin.
Sources: Phys. Rev. Lett. 98 205301 (2007)
The Casimir effect in liquids
1 July 2007
The Casimir effect arises as a result of absence of long-wavelength modes of zero quantum oscillations between conducting bodies, leading to attractive forces between bodies - the Casimir force. If space is surrounded by dielectric bodies, the resulting force is known as the Casimir-Lifshitz force. Numerous studies of the Casimir effect were carried out previously in vacuum and in air, and the Casimir-Lifshitz force was also measured at short distances in liquids in the Van der Waals mode. J.N.Munday and F.Capasso (Harvard University) first measured the Casimir-Lifshitz force between metal surfaces separated with a sufficiently thick layer of liquid - the case of significant Casimir effect and at the same time negligible Van der Waals forces. Munday and Capasso measured the attractive force in ethanol between a macroscopic gold-coated sphere and a metal plate. To calibrate the measuring device, the sphere was put in motion and the force of hydrodynamic friction was measured; at the same time it was possible to calculate it from the Stokes formula. The force of attraction in the liquid was found to be lower by 20% than in vacuum for the same configuration of bounding surfaces. The results of measurements are in good agreement with the results of calculations using the theory of E.M.Lifshitz, I.E.Dzyaloshinsky and L.P.Pitaevsky.
Cavitation in microchannels
1 July 2007
Researchers at the University of Twente (The Netherlands) developed a new technique for controlling the flow of viscous liquids in microscopic capillaries. Pulsed laser light focused into the liquid brings it to boiling which generates a gas bubble. The collapse of this bubble is accompanied with cavitation and vortical motion of the liquid. The type of motion depends on how close the capillary is and the shape of the wall. In some cases it is possible to use this technique to mix the liquid and push it faster along the microchannel. This method may prove useful in ôon-the-chipö devices where it is necessary to transport the liquid and control the rate of chemical reactions.
The structure of the stellar halo of the Galaxy
1 July 2007
E.F.Bell and his co-workers used the data of the galactic Sloan Digital Sky Survey Data Release 5 to study the distribution of stars in the halo of our Galaxy. The survey covers about 1/4 of the celestial sphere; from it, about four million old main sequence stars were selected. On average, the halo was found to be essentially non-spherical. The ratio of semiaxes of ellipsoidal surfaces of constant density is approximately 0.6. The total stellar mass in the distance range 1-40 kpc from the Galactic center is 4x108 solar masses, and the density profile is a power-law curve of the type r-3. The main result of this study was the discovery of strong inhomogeneity in the distribution of stars. The stars concentrate into several large elongated structures and into numerous small clumps. The inhomogeneity of the halo, as well as its nonsphericity, increases with distance away from the center of the Galaxy. Strong inhomogeneity is an indication that most stars in the halo have not been born in it but were incorporated into the Galaxy when it cannibalized its satellite galaxies. After these satellites were broken by gravitational tidal forces, part of their stars scattered through the Galaxy, leaving behind strong inhomogeneities such as, for example, tidal streams. This behavior corresponds to the hierarchical model of galaxy formation, in which large galaxies result from merger of smaller ones.
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.