1,788 research outputs found
Applications of Two-Body Dirac Equations to the Meson Spectrum with Three versus Two Covariant Interactions, SU(3) Mixing, and Comparison to a Quasipotential Approach
In a previous paper Crater and Van Alstine applied the Two Body Dirac
equations of constraint dynamics to the meson quark-antiquark bound states
using a relativistic extention of the Adler-Piran potential and compared their
spectral results to those from other approaches, ones which also considered
meson spectroscopy as a whole and not in parts. In this paper we explore in
more detail the differences and similarities in an important subset of those
approaches, the quasipotential approach. In the earlier paper, the
transformation properties of the quark-antiquark potentials were limited to a
scalar and an electromagnetic-like four vector, with the former accounting for
the confining aspects of the overall potential, and the latter the short range
portion. A part of that work consisted of developing a way in which the static
Adler-Piran potential was apportioned between those two different types of
potentials in addition to covariantization. Here we make a change in this
apportionment that leads to a substantial improvement in the resultant
spectroscopy by including a time-like confining vector potential over and above
the scalar confining one and the electromagnetic-like vector potential. Our fit
includes 19 more mesons than the earlier results and we modify the scalar
portion of the potential in such a way that allows this formalism to account
for the isoscalar mesons {\eta} and {\eta}' not included in the previous work.
Continuing the comparisons made in the previous paper with other approaches to
meson spectroscopy we examine in this paper the quasipotential approach of
Ebert, Faustov, and Galkin for a comparison with our formalism and spectral
results.Comment: Revisions of earlier versio
The Quantum Hall Effect in Drag: Inter-layer Friction in Strong Magnetic Fields
We study the Coulomb drag between two spatially separated electron systems in
a strong magnetic field, one of which exhibits the quantum Hall effect. At a
fixed temperature, the drag mimics the behavior of in the quantum
Hall system, in that it is sharply peaked near the transitions between
neighboring plateaux. We assess the impact of critical fluctuations near the
transitions, and find that the low temperature behavior of the drag measures an
exponent that characterizes anomalous low frequency dissipation; the
latter is believed to be present following the work of Chalker.Comment: 13 pages, Revtex 2.0, 1 figure upon request, P-93-11-09
Ecosystem resistance in the face of climate change: a case study from the freshwater marshes of the Florida Everglades
Shaped by the hydrology of the Kissimmee-Okeechobee-Everglades watershed, the Florida Everglades is composed of a conglomerate of wetland ecosystems that have varying capacities to sequester and store carbon. Hydrology, which is a product of the region’s precipitation and temperature patterns combined with water management policy, drives community composition and productivity. As shifts in both precipitation and air temperature are expected over the next 100 years as a consequence of climate change, CO2 dynamics in the greater Everglades are expected to change. To reduce uncertainties associated with climate change and to explore how projected changes in atmospheric CO2 concentration and climate can alter current CO2 exchange rates in Everglades freshwater marsh ecosystems, we simulated fluxes of carbon among the atmosphere, vegetation, and soil using the DAYCENT model. We explored the effects of low, moderate, and high scenarios for atmospheric CO2 (550, 850, and 950 ppm), mean annual air temperature (þ1, þ2.5, and þ4.28C) and precipitation (2, þ7, and þ14%), as predicted by the IPCC for the year 2100 for the region, on CO2 exchange rates in short- and long-hydroperiod wetland ecosystems. Under 100 years of current climate and atmospheric CO2 concentration, Everglades freshwater marsh ecosystems were estimated to be CO2-neutral. As atmospheric CO2 concentration increased and under climate change projections, there were slight shifts in the start and length of the wet season (1 to þ7 days) and a small enhancement in the sink capacity (by 169 to 573 g C m2 century1 ) occurred at both short- and longhydroperiod ecosystems compared to CO2 dynamics under the current climate regime. Over 100 years, rising temperatures increased net CO2 exchange rates (þ1 to 13 g C m2 century1 ) and shifts in precipitation patterns altered cumulative net carbon uptake by þ13 to 46 g C m2 century1 . While changes in ecosystem structure, species composition, and disturbance regimes were beyond the scope of this research, results do indicate that climate change will produce small changes in CO2 dynamics in Everglades freshwater marsh ecosystems and suggest that the hydrologic regime and oligotrophic conditions of Everglades freshwater marshes lowers the ecosystem sensitivity to climate change. Key word
Sigma-2 receptors as a biomarker of proliferation in solid tumours
Over the past several years, our group has provided considerable evidence that the expression of sigma-2 (σ2) receptors may serve as a biomarker of tumour cell proliferation. In these in vitro studies, σ2receptors were expressed 8–10 times more in proliferative (P) tumour cells than in quiescent (Q) tumour cells, and the extent and kinetics of their expression were independent of a number of biological, physiological and environmental factors often found in solid tumours. Moreover, the expression of σ2receptors followed both the population growth kinetics when Q-cells were recruited into the P-cell compartment and the proliferative status of human breast tumour cells treated with cytostatic concentrations of tamoxifen. However, these in vitro studies may or may not be indicative of what might occur in solid tumours. In the present study, the σ2receptor P:Q ratio was determined for the cells from subcutaneous 66 (diploid) and 67 (aneuploid) tumours grown in female nude mice. The σ2receptor P:Q ratio of the 66 tumours was 10.6 compared to the σ2receptor P:Q ratio of 9.5 measured for the 66 tissue culture model. The σ2receptor P:Q ratio of the 67 tumours was 4.5 compared to the σ2receptor P:Q ratio of ≈ 8 measured for the 67 tissue culture model. The agreement between the solid tumour and tissue culture data indicates that: (1) the expression of σ2receptors may be a reliable biomarker of the proliferative status of solid tumours and (2) radioligands with both high affinity and high selectivity for σ2receptors may have the potential to non-invasively assess the proliferative status of human solid tumours using imaging techniques such as positron emission tomography or single-photon emission computerized tomography. © 2000 Cancer Research Campaig
Elemental energy spectra of cosmic rays measured by CREAM-II
We present new measurements of the energy spectra of cosmic-ray (CR) nuclei
from the second flight of the balloon-borne experiment CREAM (Cosmic Ray
Energetics And Mass). The instrument (CREAM-II) was comprised of detectors
based on different techniques (Cherenkov light, specific ionization in
scintillators and silicon sensors) to provide a redundant charge identification
and a thin ionization calorimeter capable of measuring the energy of cosmic
rays up to several hundreds of TeV. The data analysis is described and the
individual energy spectra of C, O, Ne, Mg, Si and Fe are reported up to ~ 10^14
eV. The spectral shape looks nearly the same for all the primary elements and
can be expressed as a power law in energy E^{-2.66+/-0.04}. The nitrogen
absolute intensity in the energy range 100-800 GeV/n is also measured.Comment: 4 pages, 3 figures, presented at ICRC 2009, Lodz, Polan
Measurements of cosmic-ray energy spectra with the 2nd CREAM flight
During its second Antarctic flight, the CREAM (Cosmic Ray Energetics And
Mass) balloon experiment collected data for 28 days, measuring the charge and
the energy of cosmic rays (CR) with a redundant system of particle
identification and an imaging thin ionization calorimeter. Preliminary direct
measurements of the absolute intensities of individual CR nuclei are reported
in the elemental range from carbon to iron at very high energy.Comment: 4 pages, 3 figures, presented at XV International Symposium on Very
High Energy Cosmic Ray Interactions (ISVHECRI 2008
Search for flavor-changing neutral currents and lepton-family-number violation in two-body D0 decays
Results of a search for the three neutral charm decays, D0 -> mu e, D0 -> mu
mu, and D0 -> e e, are presented. This study was based on data collected in
Experiment 789 at the Fermi National Accelerator Laboratory using 800 GeV/c
proton-Au and proton-Be interactions. No evidence is found for any of the
decays. Upper limits on the branching ratios, at the 90% confidence level, are
obtained.Comment: 28 pages, 18 figures. Submitted to Physical Review
Whole Earth Telescope observations of the pulsating subdwarf B star PG 0014+067
PG 0014+067 is one of the most promising pulsating subdwarf B stars for
seismic analysis, as it has a rich pulsation spectrum. The richness of its
pulsations, however, poses a fundamental challenge to understanding the
pulsations of these stars, as the mode density is too complex to be explained
only with radial and nonradial low degree (l < 3) p-modes without rotational
splittings. One proposed solution, for the case of PG 0014+067 in particular,
assigns some modes with high degree (l=3). On the other hand, theoretical
models of sdB stars suggest that they may retain rapidly rotating cores, and so
the high mode density may result from the presence of a few rotationally-split
triplet (l=1), quintuplet (l=2) modes, along with radial (l=0) p-modes. To
examine alternative theoretical models for these stars, we need better
frequency resolution and denser longitude coverage. Therefore, we observed this
star with the Whole Earth Telescope for two weeks in October 2004. In this
paper we report the results of Whole Earth Telescope observations of the
pulsating subdwarf B star PG 0014+067. We find that the frequencies seen in PG
0014+067 do not appear to fit any theoretical model currently available;
however, we find a simple empirical relation that is able to match all of the
well-determined frequencies in this star.Comment: 19 pages, preprint of paper accepted for publication in The
Astrophysical Journa
Energy spectra of cosmic-ray nuclei at high energies
We present new measurements of the energy spectra of cosmic-ray (CR) nuclei
from the second flight of the balloon-borne experiment Cosmic Ray Energetics
And Mass (CREAM). The instrument included different particle detectors to
provide redundant charge identification and measure the energy of CRs up to
several hundred TeV. The measured individual energy spectra of C, O, Ne, Mg,
Si, and Fe are presented up to eV. The spectral shape looks
nearly the same for these primary elements and it can be fitted to an power law in energy. Moreover, a new measurement of the absolute
intensity of nitrogen in the 100-800 GeV/ energy range with smaller errors
than previous observations, clearly indicates a hardening of the spectrum at
high energy. The relative abundance of N/O at the top of the atmosphere is
measured to be (stat.)(sys.) at 800
GeV/, in good agreement with a recent result from the first CREAM flight.Comment: 32 pages, 10 figures. Accepted for publication in Astrophysical
Journa
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