Extracts from the Internet


Gravity Prob ┬: results

The summary of the final results has been presented of the space experiment Gravity Prob ┬ which was measuring two relativistic effects of gravitation: geodetic precession and frame dragging by a rotating mass (the Lense – Thirring effect). The satellite was in polar orbit, and the common axis of rotation was fixed in the direction of the star IM Pegasi. The measurement system was built around four gyroscopes Ś quartz balls with superconducting niobium coating. The rotating balls created magnetic field measured by a SQUID magnetometer. This technique allowed conducting high-accuracy measurements of the angular displacement of gyroscope axes for almost a year. The results of the measurements are in agreement with the calculations within the framework of general relativity. The predictions have been confirmed at the level of the achieved accuracy: 0.28% for the geodetic precession and 19% for the Lense – Thirring effect. These effects have already been measured at a comparable accuracy in laser ranging experiments. In this way, Gravity Prob ┬ presented a reliable independent confirmation of earlier results. Numerous innovative solutions have been found and technologies have been developed (which found important practical applications) in the process of designing and implementing the Gravity Prob ┬ project created for studying fundamental scientific problems. Sources: www.nasa.gov, arXiv:1105.3456v1 [gr-qc]

Neutrino oscillations in the MINOS experiment

In the MINOS experiment the muon neutrinos νμ are recorded near the base of the neutrino beam produced by the accelerator and at a distance of 735 km along the beam. The deficiency (disappearance from the beam) of νμ in the second detector leads to a conclusion that νμ transformed (oscillated) into other types of neutrino which are not directly recorded in this experiment. According to recent MINOS data, the difference between squared masses of different neutrino mass states is |Δ m²| = (2.32-0.08+0.12) × 10-3 eV², and the mixing angle is sin²(2θ) > 0.90 at 90% confidence level. Also, the νμ decay hypothesis and the quantum decoherence hypothesis were excluded at the 7 σ and 9 σ levels, respectively. The MINOS experiment also provided new data on the oscillations of νμ antineutrinos. The experiments with anti-νμ and νμ give consistent results with accuracy of 2.0% if the oscillation parameters in the two cases are identical. Furthermore, a constraint < 22% was obtained for the fraction of νμ which could oscillate into the hypothetical sterile neutrino. Sources: arXiv:1103.0340v1 [hep-ex], arXiv:1104.0344v3 [hep-ex], arXiv:1104.3922v2 [hep-ex]

Periodicity in the stripe phase of the quantum Hall effect

I.V. Kukushkin (Max-Planck-Institute For Solid State Physics Research, Germany and the Institute of Solid State Physics, Chernogolovka, Russia) and his colleagues from Germany and Israel have investigated the state known as the quantum Hall stripe phase, in quasi-two-dimensional heterostructure GaAs/AlGaAs with a filling factor of Landau levels ν = 9/2. This phase looks like an array of linear domains with alternating full and zero filling at the upper Landau level thus forming charge density wave. The direction of the stripes corresponds to the [110] of GaAs crystal lattice. The stripe phase was observed in a number of other systems, including high-temperature superconductors. To measure the momentum of magnetophonon quasiparticles as a function of energy, the system was subjected to combined action of surface acoustic waves, microwave radiation and laser light which caused photoluminescence. When the impacts of the first two factors are in resonance with the dispersion relation, two-dimensional electron gas heats up and thereby affects the photoluminescence spectrum. Consequently, measuring this spectrum is a means of evaluation of dispersion properties. If the wave vector of the surface acoustic wave kSAW is parallel to stripes, the dispersion characteristics have the theoretically predicted form, for instance, the curve reveals a roton minimum. The observation that in the transverse direction the dispersion curve is not flat but slowly increases as kSAW increases was an unexpected result. The exact mechanism of this behavior has not yet been identified. Also, as kSAW was increased, the authors observed a periodical (in wavelength) variation of the absorption of surface acoustic waves with period 3.6 Rc where Rc is the cyclotron radius. The observed geometric resonance makes it possible to measure the wavelength of charge density wave in the quantum Hall stripe phase. According to calculations, the period would have to be 2.7 Rc; it has not been established yet what caused the discrepancy. Source: Phys. Rev. Lett. 106 206804 (2011)

Theoretical calculation of Hoyle energy level

In 1954 F. Hoyle predicted the existence of the energy level of the nucleus 12C near the threshold of the fusion reaction 8Be + α-particle. This level, required for the occurrence of the nuclear cycle in stars, joins three α-particles and results in formation of carbon. Soon after Hoyle's prediction, this energy level was discovered experimentally. However, it proved impossible until recently to calculate the Hoyle level by ôfirst principlesö calculations, i.e. starting with the fundamental principles of quantum chromodynamics. ┼. Epelbaum (Ruhr University, Bochum, Germany) and his colleagues were able to accomplish this for the first time in lattice computations in the framework of the effective field theory which is based on expansion of quantities in series in powers of the characteristic particle momentum Q; it is important that ┼. Epelbaum et al. took into account all terms of third order O(Q³) and lower. The calculations faithfully reproduced the energy of the ground and excited states with spin 2 and in addition revealed a resonance at -85 ± 3 MeV whose properties are fully consistent with the Hoyle level. The Hoyle level is an example of fine-tuning of parameters: the synthesis of elements making organic life possible would have been impossible at the slightest shift of its energy so that the Hoyle level is often discussed in the context of anthropic principle. Source: Phys. Rev. Lett. 106 192501 (2011)

Rotational Doppler broadening in the photoelectron spectra of molecules

T.D. Thomas (University of Oregon, USA) and his colleagues carried out the first experimental investigation of the effect of rotation of dimers on the spectrum of photoelectrons they emit. In the case of off-center emission an electron may gain additional energy, and this explains the rotational Doppler effect. The contribution of this effect to the broadening of spectral lines may be comparable in magnitude with the contribution of the conventional Doppler effect caused by the movement of centers of mass of molecules. High-resolution electron spectroscopy was used to record photoelectrons emitted by a gas mixture of N2 and Kr irradiated by x-rays. Monatomic krypton gas was used for calibration: monitoring of Kr spectral lines made it possible to eliminate the contribution of the conventional Doppler effect. The measured dependence of line broadening on gas temperature and photoelectron energy is in good agreement with the theoretical model of Y.-P. Sun et al. which takes quantum effects into account but is also described rather well in terms of the simple classical model. Source: Phys. Rev. Lett. 106 193009 (2011)

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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.

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