Electron-positron annihilation into Dirac magnetic monopole and antimonopole: the string ambiguity and the discrete symmetries

We address the problem of string arbitrariness in the quantum field theory of Dirac magnetic monopoles. Different prescriptions are shown to yield different physical results. The constraints due to the discrete symmetries (C and P) are derived for the process of electron- positron annihilation into the monopole-antimonopole pair. In the case of the annihilation through the one-photon channel, the production of spin 0 monopoles is absolutely forbidden; spin 1/2 monopole and antimonopole should have the same helicities (or, equivalently, the monopole-antimonopole state should be p-wave $^1P_1$).Comment: 14 pages, revtex, 3 figure

Impurity-induced modulation of terahertz waves in optically excited GaAs

The effect of the photoinduced absorption of terahertz (THz) radiation in a semi-insulating GaAs crystal is studied by the pulsed THz transmission spectroscopy. We found that a broad-band modulation of THz radiation may be induced by a low-power optical excitation in the spectral range of the impurity absorption band in GaAs. The measured modulation achieves 80\%. The amplitude and frequency characteristics of the resulting THz modulator are critically dependent on the carrier density and relaxation dynamics in the conduction band of GaAs. In semi-insulating GaAs crystals, the carrier density created by the impurity excitation is controlled by the rate of their relaxation to the impurity centers. The relaxation rate and, consequently, the frequency characteristics of the modulator can be optimized by an appropriate choice of the impurities and their concentrations. The modulation parameters can be also controlled by the crystal temperature and by the power and photon energy of the optical excitation. These experiments pave the way to the low-power fast optically-controlled THz modulation, imaging, and beam steering.Comment: 5 pages, 3 figure

Search for Millicharged Particles at SLAC

Particles with electric charge q < 10^(-3)e and masses in the range 1--100 MeV/c^2 are not excluded by present experiments. An experiment uniquely suited to the production and detection of such "millicharged" particles has been carried out at SLAC. This experiment is sensitive to the infrequent excitation and ionization of matter expected from the passage of such a particle. Analysis of the data rules out a region of mass and charge, establishing, for example, a 95%-confidence upper limit on electric charge of 4.1X10^(-5)e for millicharged particles of mass 1 MeV/c^2 and 5.8X10^(-4)e for mass 100 MeV/c^2.Comment: 4 pages, REVTeX, multicol, 3 figures. Minor typo corrected. Submitted to Physical Review Letter

Use of ultrasound densitometry for the assess of structural and functional disorders of bone tissue and prediction of fractures risk

To determine the dependence of lumbar spine compression degree on BMD state and the patients’ age. The relationship of BMD state, the patients’ age and the degree of compression of the vertebrae were studied. Correlation and regression analysis of the relationship between SOS index, score of compression and patients' age have been examined. Results. The data obtained allowed us to develop a mathematical model for predicting BMD reduction and the severity of vertebral compression fractures. Conclusions. Ultrasonic indexes of bone mineral density may be used not only for its screening assessment in general population of different ages, but also in assessing the degree of structural and functional changes of BT, predicting the severity of low-energy osteoporosis fractures. It will help to assign treatment in preclinical stage, and carry out prevention of compression fractures

Off-limb EUV observations of the solar corona and transients with the CORONAS-F/SPIRIT telescope-coronagraph

The SPIRIT telescope aboard the CORONAS-F satellite (in orbit from 26 July 2001 to 5 December 2005), observed the off-limb solar corona in the 175 Å (Fe IX, X and XI lines) and 304 Å (He II and Si XI lines) bands. In the coronagraphic mode the mirror was tilted to image the corona at the distance of 1.1...5 &lt;I&gt;R&lt;/I&gt;&lt;sub&gt;sun&lt;/sub&gt; from the solar center, the outer occulter blocked the disk radiation and the detector sensitivity was enhanced. This intermediate region between the fields of view of ordinary extreme-ultraviolet (EUV) telescopes and most of the white-light (WL) coronagraphs is responsible for forming the streamer belt, acceleration of ejected matter and emergence of slow and fast solar wind. We present here the results of continuous coronagraphic EUV observations of the solar corona carried out during two weeks in June and December 2002. The images showed a &quot;diffuse&quot; (unresolved) component of the corona seen in both bands, and non-radial, ray-like structures seen only in the 175 Å band, which can be associated with a streamer base. The correlations between latitudinal distributions of the EUV brightness in the corona and at the limb were found to be high in 304 Å at all distances and in 175 Å only below 1.5 &lt;I&gt;R&lt;/I&gt;&lt;sub&gt;sun&lt;/sub&gt;. The temporal correlation of the coronal brightness along the west radial line, with the brightness at the underlying limb region was significant in both bands, independent of the distance. On 2 February 2003 SPIRIT observed an expansion of a transient associated with a prominence eruption seen only in the 304 Å band. The SPIRIT data have been compared with the corresponding data of the SOHO LASCO, EIT and UVCS instruments

Spin flip from dark to bright states in InP quantum dots

We report measurements of the time for spin flip from dark (non-light emitting) exciton states in quantum dots to bright (light emitting) exciton states in InP quantum dots. Dark excitons are created by two-photon excitation by an ultrafast laser. The time for spin flip between dark and bright states is found to be approximately 200 ps, independent of density and temperature below 70 K. This is much shorter than observed in other quantum dot systems. The rate of decay of the luminescence intensity, approximately 300 ps, is not simply equal to the radiative decay rate from the bright states, because the rate of decay is limited by the rate of conversion from dark excitons into bright excitons. The dependence of the luminescence decay time on the spin flip time is a general effect that applies to many experiments.Comment: 3 figure