First axion dark matter search with toroidal geometry

We firstly report an axion haloscope search with toroidal geometry. In this pioneering search, we exclude the axion-photon coupling $g_{a\gamma\gamma}$ down to about $5\times10^{-8}$ GeV$^{-1}$ over the axion mass range from 24.7 to 29.1 $\mu$eV at a 95\% confidence level. The prospects for axion dark matter searches with larger scale toroidal geometry are also considered.Comment: 5 pages, 5 figures, 1 table and to appear in PRD-R

MRI On the Fly: Accelerating MRI Imaging Using LDA Classification with LDB Feature Extraction

To improve MRI acquisition time, we explored the uses of linear discriminant analysis (LDA), and local discriminant bases (LDB) for the task of classifying MRI images using a minimal set of signal acquisitions. Our algorithm has both off-line and on-line components. The off-line component uses the k-basis algorithm to partition a set of training images (all from a particular region of a patient) into classes. For each class, we find a basis by applying the best basis algorithm on the images in that class. We keep these bases to be used by the on-line process. We then apply LDB to the training set with the class assignments, determining the best discriminant basis for the set. We rank the basis coordinates according to discriminating power, and retain the top M coordinates for the on-line algorithm. We keep the top M coordinates, which index the basis functions with the most discriminating capability, for on-line purposes. Finally, we train LDA on these transformed coordinates, producing a classifier for the images. With the off-line requirements complete, we can take advantage of the simplicity and speed of the on-line mechanism to acquire an image in a similar region of the patient. We need acquire only the M important coordinates of the image in the discriminant basis to create a ``scout image.\u27\u27 This image, which can be acquired quickly since M is much much smaller than the number of measurements needed to fill in the values of the 256 by 256 pixels, is then sent through the map furnished by LDA which in turn assigns a class to the image. Returning to the list of bases that we kept from the k-bases algorithm, we find the optimal basis for the particular class at hand. We then acquire the image using that optimal basis, omitting the coefficients with the least truncation error. The complete image can then be quickly reconstructed using the inverse wavelet packet transform. The power of our algorithm is that the on-line task is fast and simple, while the computational complexity lies mostly in the off-line task that needs to be done only once for images in a certain region. In addition, our algorithm only makes use of the flexibility of MRI hardware, so no modifications in hardware design are needed

Far Ultraviolet Observations of the Dwarf Nova VW Hyi in Quiescence

We present a 904-1183 A spectrum of the dwarf nova VW Hydri taken with the Far Ultraviolet Spectroscopic Explorer during quiescence, eleven days after a normal outburst, when the underlying white dwarf accreter is clearly exposed in the far ultraviolet. However, model fitting show that a uniform temperature white dwarf does not reproduce the overall spectrum, especially at the shortest wavelengths. A better approximation to the spectrum is obtained with a model consisting of a white dwarf and a rapidly rotating ``accretion belt''. The white dwarf component accounts for 83% of the total flux, has a temperature of 23,000K, a v sin i = 400 km/s, and a low carbon abundance. The best-fit accretion belt component accounts for 17% of the total flux, has a temperature of about 48,000-50,000K, and a rotation rate Vrot sin i around 3,000-4,000 km/s. The requirement of two components in the modeling of the spectrum of VW Hyi in quiescence helps to resolve some of the differences in interpretation of ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of a second component (and its exact nature) in VW Hyi itself is still relatively uncertain, given the lack of better models for spectra of the inner disk in a quiescent dwarf nova.Comment: 6 figures, 10 printed page in the journal, to appear in APJ, 1 Sept. 2004 issue, vol. 61

The locally covariant Dirac field

We describe the free Dirac field in a four dimensional spacetime as a locally covariant quantum field theory in the sense of Brunetti, Fredenhagen and Verch, using a representation independent construction. The freedom in the geometric constructions involved can be encoded in terms of the cohomology of the category of spin spacetimes. If we restrict ourselves to the observable algebra the cohomological obstructions vanish and the theory is unique. We establish some basic properties of the theory and discuss the class of Hadamard states, filling some technical gaps in the literature. Finally we show that the relative Cauchy evolution yields commutators with the stress-energy-momentum tensor, as in the scalar field case.Comment: 36 pages; v2 minor changes, typos corrected, updated references and acknowledgement

Photochemistry of ozone over the western Pacific from winter to spring

Aircraft measurements of ozone (O3) and its precursors, including NO, CO, H2O, and nonmethane hydrocarbons (NMHCs), were made over the western Pacific in the 20° - 45°N latitude range in January and April-May 2002 during the Pacific Exploration of Asian Continental Emission (PEACE)-A and B campaigns. These measurements have provided data sets that, in combination with Transport and Chemical Evolution over the Pacific (TRACE-P) data taken in March 2001, enable studies of O3 photochemistry from winter to late spring. A photochemical box model is used to calculate ozone formation (F(O3)) and destruction (D(O3)) rates constrained by the observed species concentrations. The values of F(O3) and D(O3) are controlled directly by NO, J(O1D) (O3 photolysis frequency), H2O, OH, and HO2. Changes in HO2 concentration cause corresponding changes in both F(O3) and D(O3) leading to their coupling. Concentrations of these species, which are strongly influenced by photochemistry and transport from the Asian continent, underwent large seasonal variations. In the boundary layer (0-3 km), NO was much higher in January than in April-May, because of stronger winds, lower convective activities, and lower oxidation rates by OH in winter. The net O3 formation rate, given by P(O3) = F(O3) - D(O3), was largely positive in the boundary layer at 30°-45°N (1.5-4 ppbv d-1) in January, mainly because of high NO and low H2O values. Net O3 formation continued from January to the end of March, demonstrating that the western Pacific is an important O3 source region during this season. Net O3 formation nearly ceased by late April/May because of the decrease in NO and the increase in H2O. In the latitude range of 20°-30°N, P(O3) in the boundary layer was positive in January and turned negative by March. The earlier transition was mainly due to lower NO and higher H2O concentrations, combined with weaker transport and higher temperatures than those at 30°-45°N. The upper troposphere (6-12 km) has been shown to be a region of net O3 formation throughout most of the year because of high NO and low H2O. The present study illustrates that a decrease in the net O3 formation rate at 20°-45°N latitude from winter to late spring is explained systematically by the increases in J(O1D), H2O, OH, and HO2 (primarily due to increases in temperature and solar radiation) and the decrease in NO (primarily due to decrease in transport from the Asian continent). Differences in the seasonal variation of O3 photochemistry observed over the North American continent are interpreted in terms of the differences in factors controlling O3 formation and destruction. Copyright 2004 by the American Geophysical Union

Uranium on uranium collisions at relativistic energies

Deformation and orientation effects on compression, elliptic flow and particle production in uranium on uranium collisions (UU) at relativistic energies are studied within the transport model ART. The density compression in tip-tip UU collisions is found to be about 30% higher and lasts approximately 50% longer than in body-body or spherical UU reactions. The body-body UU collisions have the unique feature that the nucleon elliptic flow is the highest in the most central collisions and remain a constant throughout the reaction. We point out that the tip-tip UU collisions are more probable to create the QGP at AGS and SPS energies while the body-body UU collisions are more useful for studying properties of the QGP at higher energies.Comment: 8 pages + 4 figure