107,243 research outputs found
SAGE 1 and SAM 2 measurements of 1 micron aerosol extinction in the free troposphere
The SAGE 1 and SAM 2 satellite sensors were designed to measure, with global coverage, the 1 micron extinction produced by the stratospheric aerosol. In the absence of high altitude cloud, similar measurements may be made for the free tropospheric aerosol. Median extinction values in the Northern Hemisphere, for altitudes between 5 and 10 km, are found to be one-half to one order of magnitude greater than values at corresponding latitudes in the Southern Hemisphere. In addition, a seasonal increase by a factor of 1.5 yields 2 is observed in both hemispheres in local spring and summer. Following major volcanic eruptions, a long-lived enhancement of the aerosol extinction is observed for altitudes above 5 km
A schematic model for QCD at finite temperature
The simplest version of a class of toy models for QCD is presented. It is a
Lipkin-type model, for the quark-antiquark sector, and, for the gluon sector,
gluon pairs with spin zero are treated as elementary bosons. The model
restricts to mesons with spin zero and to few baryonic states. The
corresponding energy spectrum is discussed. We show that ground state
correlations are essential to describe physical properties of the spectrum at
low energies. Phase transitions are described in an effective manner, by using
coherent states. The appearance of a Goldstone boson for large values of the
interaction strength is discussed, as related to a collective state. The
formalism is extended to consider finite temperatures. The partition function
is calculated, in an approximate way, showing the convenience of the use of
coherent states. The energy density, heat capacity and transitions from the
hadronic phase to the quark-gluon plasma are calculated.Comment: 33 pages, 11 figure
Dilute Birman--Wenzl--Murakami Algebra and models
A ``dilute'' generalisation of the Birman--Wenzl--Murakami algebra is
considered. It can be ``Baxterised'' to a solution of the Yang--Baxter algebra.
The vertex models are examples of corresponding solvable
lattice models and can be regarded as the dilute version of the
vertex models.Comment: 11 page
Infrared electron modes in light deformed clusters
Infrared quadrupole modes (IRQM) of the valence electrons in light deformed
sodium clusters are studied by means of the time-dependent local-density
approximation (TDLDA). IRQM are classified by angular momentum components
20, 21 and 22 whose branches are separated by cluster
deformation. In light clusters with a low spectral density, IRQM are
unambiguously related to specific electron-hole excitations, thus giving access
to the single-electron spectrum near the Fermi surface (HOMO-LUMO region). Most
of IRQM are determined by cluster deformation and so can serve as a sensitive
probe of the deformation effects in the mean field. The IRQM branch 21 is coupled with the magnetic scissors mode, which gives a chance to detect
the latter. We discuss two-photon processes, Raman scattering (RS), stimulated
emission pumping (SEP), and stimulated adiabatic Raman passage (STIRAP), as the
relevant tools to observe IRQM. A new method to detect the IRQM population in
clusters is proposed.Comment: 22 pages, 6 figure
SIMBOL-X : a new generation hard X-ray telescope
SIMBOL-X is a hard X-ray mission, operating in the 0.5-70 keV range, which is
proposed by a consortium of European laboratories for a launch around 2010.
Relying on two spacecraft in a formation flying configuration, SIMBOL-X uses a
30 m focal length X-ray mirror to achieve an unprecedented angular resolution
(30 arcsec HEW) and sensitivity (100 times better than INTEGRAL below 50 keV)
in the hard X-ray range. SIMBOL-X will allow to elucidate fundamental questions
in high energy astrophysics, such as the physics of accretion onto Black Holes,
of acceleration in quasar jets and in supernovae remnants, or the nature of the
hard X-ray diffuse emission. The scientific objectives and the baseline
concepts of the mission and hardware design are presented.Comment: 12 pages, 16 fig., Proc. SPIE conf. 5168, San Diego, Aug. 200
Perturbation theory for self-gravitating gauge fields I: The odd-parity sector
A gauge and coordinate invariant perturbation theory for self-gravitating
non-Abelian gauge fields is developed and used to analyze local uniqueness and
linear stability properties of non-Abelian equilibrium configurations. It is
shown that all admissible stationary odd-parity excitations of the static and
spherically symmetric Einstein-Yang-Mills soliton and black hole solutions have
total angular momentum number , and are characterized by
non-vanishing asymptotic flux integrals. Local uniqueness results with respect
to non-Abelian perturbations are also established for the Schwarzschild and the
Reissner-Nordstr\"om solutions, which, in addition, are shown to be linearly
stable under dynamical Einstein-Yang-Mills perturbations. Finally, unstable
modes with are also excluded for the static and spherically
symmetric non-Abelian solitons and black holes.Comment: 23 pages, revtex, no figure
Calibration and measurement uncertainties of a continuous-flow cloud condensation nuclei counter (DMT-CCNC): CCN activation of ammonium sulfate and sodium chloride aerosol particles in theory and experiment
International audienceExperimental and theoretical uncertainties in the measurement of cloud condensation nuclei (CCN) with a continuous-flow thermal-gradient CCN counter from Droplet Measurement Technologies (DMT-CCNC) have been assessed by model calculations and calibration experiments with ammonium sulfate and sodium chloride aerosol particles in the diameter range of 20?220 nm. Experiments have been performed in the laboratory and during field measurement campaigns, extending over a period of more than one year and covering a wide range of operating conditions (650?1020 hPa ambient pressure, 0.5?1.0 L min?1 aerosol flow rate, 20?30°C inlet temperature, 4?34 K m?1 temperature gradient). For each set of conditions, the effective water vapor supersaturation (Seff) in the CCNC was determined from the measured CCN activation spectra and Köhler model calculations. High measurement precision was achieved under stable laboratory conditions, where relative variations of Seff in the CCNC were generally less than ±2%. During field measurements, however, the relative variability increased up to ±5?7%, which can be mostly attributed to variations of the CCNC column top temperature with ambient temperature. To assess the accuracy of the Köhler models used to calculate Seff, we have performed a comprehensive comparison and uncertainty analysis of the various Köhler models and thermodynamic parameterizations commonly used in CCN studies. For the relevant supersaturation range (0.05?2%), the relative deviations between different modeling approaches were as high as 25% for (NH4)2SO4 and 16% for NaCl. The deviations were mostly caused by the different parameterizations for the activity of water in aqueous solutions of (NH4)2SO4 and NaCl (activity parameterization, osmotic coefficient, and van't Hoff factor models). The uncertainties related to the model parameterizations of water activity clearly exceeded the CCNC measurement precision. Relative deviations caused by different ways of calculating or approximating solution density and surface tension did not exceed 3% for (NH4)2SO4 and 1.5% for NaCl. Nevertheless, they did exceed the CCNC measurement precision under well-defined operating conditions and should not be neglected in studies aimed at high accuracy. To ensure comparability of results, we suggest that CCN studies should always report exactly which Köhler model equations and parameterizations of solution properties were used. Substantial differences between the CCNC calibration results obtained with (NH4)2SO4 and NaCl aerosols under equal experimental conditions (relative deviations of Seff up to ~10%) indicate inconsistencies between widely used activity parameterizations derived from electrodynamic balance (EDB) single particle experiments (Tang and Munkelwitz, 1994; Tang, 1996) and hygroscopicity tandem differential mobility analyzer (HTDMA) aerosol experiments (Kreidenweis et al., 2005). Therefore, we see a need for further evaluation and experimental confirmation of preferred data sets and parameterizations for the activity of water in dilute aqueous (NH4)2SO4 and NaCl solutions. The experimental results were also used to test the CCNC flow model of Lance et al.~(2006), which describes the dependence of Seff on temperature, pressure, and flow rate in the CCN counter. This model could be applied after subtraction of a near-constant temperature offset and derivation of an instrument-specific thermal resistance parameter (RT?1.8 K W?1). At Seff>0.1% the relative deviations between the flow model and experimental results were mostly less than 5%, when the same Köhler model approach was used. At Seff?.1%, however, the deviations exceeded 20%, which can be attributed to non-idealities which also caused the near-constant temperature offset. Therefore, we suggest that the CCNC flow model can be used to extrapolate calibration results, but should generally be complemented by calibration experiments performed under the relevant operating conditions ? during field campaigns as well as in laboratory studies
Electric Dipole Moments of Light Nuclei From Chiral Effective Field Theory
We set up the framework for the calculation of electric dipole moments (EDMs)
of light nuclei using the systematic expansion provided by chiral effective
field theory (EFT). We take into account parity (P) and time-reversal (T)
violation which, at the quark-gluon level, originates from the QCD vacuum angle
and dimension-six operators capturing physics beyond the Standard Model. We
argue that EDMs of light nuclei can be expressed in terms of six low-energy
constants that appear in the P- and T-violating nuclear potential and electric
current. As examples, we calculate the EDMs of the deuteron, the triton, and
3He in leading order in the EFT expansion
Star formation in galaxies at z~4-5 from the SMUVS survey: a clear starburst/main-sequence bimodality for Halpha emitters on the SFR-M* plane
We study a large galaxy sample from the Spitzer Matching Survey of the
UltraVISTA ultra-deep Stripes (SMUVS) to search for sources with enhanced 3.6
micron fluxes indicative of strong Halpha emission at z=3.9-4.9. We find that
the percentage of "Halpha excess" sources reaches 37-40% for galaxies with
stellar masses log10(M*/Msun) ~ 9-10, and decreases to <20% at log10(M*/Msun) ~
10.7. At higher stellar masses, however, the trend reverses, although this is
likely due to AGN contamination. We derive star formation rates (SFR) and
specific SFR (sSFR) from the inferred Halpha equivalent widths (EW) of our
"Halpha excess" galaxies. We show, for the first time, that the "Halpha excess"
galaxies clearly have a bimodal distribution on the SFR-M* plane: they lie on
the main sequence of star formation (with log10(sSFR/yr^{-1})<-8.05) or in a
starburst cloud (with log10(sSFR/yr^{-1}) >-7.60). The latter contains ~15% of
all the objects in our sample and accounts for >50% of the cosmic SFR density
at z=3.9-4.9, for which we derive a robust lower limit of 0.066 Msun yr^{-1}
Mpc^{-3}. Finally, we identify an unusual >50sigma overdensity of z=3.9-4.9
galaxies within a 0.20 x 0.20 sq. arcmin region. We conclude that the SMUVS
unique combination of area and depth at mid-IR wavelengths provides an
unprecedented level of statistics and dynamic range which are fundamental to
reveal new aspects of galaxy evolution in the young Universe.Comment: 18 pages, 11 figures, 1 table. Re-submitted to the ApJ, after
addressing referee report. Main changes with respect to v1: a new section and
a new appendix have been added to investigate further the origin and
robustness of the sSFR bimodality. No conclusion change
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