Gravity Prob ┬: results
1 June 2011
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.
Neutrino oscillations in the MINOS experiment
1 June 2011
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
1 June 2011
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  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
1 June 2011
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
1 June 2011
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)
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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