41,221 research outputs found
Impulsive cylindrical gravitational wave: one possible radiative form emitted from cosmic strings and corresponding electromagnetic response
The cosmic strings(CSs) may be one important source of gravitational
waves(GWs), and it has been intensively studied due to its special properties
such as the cylindrical symmetry. The CSs would generate not only usual
continuous GW, but also impulsive GW that brings more concentrated energy and
consists of different GW components broadly covering low-, intermediate- and
high-frequency bands simultaneously. These features might underlie interesting
electromagnetic(EM) response to these GWs generated by the CSs. In this paper,
with novel results and effects, we firstly calculate the analytical solutions
of perturbed EM fields caused by interaction between impulsive cylindrical GWs
(would be one of possible forms emitted from CSs) and background celestial high
magnetic fields or widespread cosmological background magnetic fields, by using
rigorous Einstein - Rosen metric. Results show: perturbed EM fields are also in
the impulsive form accordant to the GW pulse, and asymptotic behaviors of the
perturbed EM fields are fully consistent with the asymptotic behaviors of the
energy density, energy flux density and Riemann curvature tensor of
corresponding impulsive cylindrical GWs. The analytical solutions naturally
give rise to the accumulation effect which is proportional to the term of
distance^1/2, and based on it, we for the first time predict potentially
observable effects in region of the Earth caused by the EM response to GWs from
the CSs.Comment: 34 pages, 12 figure
Determination of and Extraction of from Semileptonic Decays
By globally analyzing all existing measured branching fractions and partial
rates in different four momentum transfer-squared bins of decays, we obtain the product of the form factor and magnitude of
CKM matrix element to be . With this
product, we determine the semileptonic form factor
in conjunction with the value of
determined from the SM global fit. Alternately, with the product together with
the input of the form factor calculated in lattice QCD recently, we
extract , where the error is
still dominated by the uncertainty of the form factor calculated in lattice
QCD. Combining the
extracted from all existing measurements of decays and
together, we find the most
precisely determined to be , which improves
the accuracy of the PDG'2014 value by
Moderate deviations in Poisson approximation: a first attempt
Poisson approximation using Stein's method has been extensively studied in
the literature. The main focus has been on bounding the total variation
distance. This paper is a first attempt on moderate deviations in Poisson
approximation for right-tail probabilities of sums of dependent indicators. We
obtain results under certain general conditions for local dependence as well as
for size-bias coupling. These results are then applied to independent
indicators, 2-runs, and the matching problem.Comment: 21 page
Investigation on thermal-hydraulic performance of outdoor heat exchanger in air sources heat pump
A plethora of findings exists on the decreasing heat transfer performance of ASHP under frosting condition. This study investigates, using field and laboratory experiments, the frosting behavior and heat transfer performance of fin-tube exchangers. Findings from our initial observation revealed the severity of frosting phenomenon during winter in hot summer and cold winter zone of China; for avoiding the lower COP, defrosting period of ASHP was not longer than 60 min. The results showed that the basic tube surface temperature decreases with running time and remains stable after 70 min, and an adverse pressure drop ensued. The findings, thereby suggests a 70 min defrosting period in ASHP. Comparing jH/fH and COP between exchangers revealed an improved performance of plain fin-tube exchanger over that of louver fin-tube exchanger in all the test conditions. Therefore, under frosting condition, plain fin-tube heat exchanger provides a superior thermal-hydraulic performance over louver fin-tube heat exchanger. Findings from this study will help designers and facility managers in taking a more informed decision when selecting heat exchanger types for ASHP
Temperature dependence and resonance effects in Raman scattering of phonons in NdFeAsOF single crystals
We report plane-polarized Raman scattering spectra of iron oxypnictide
superconductor NdFeAsOF single crystals with varying fluorine
content. The spectra exhibit sharp and symmetrical phonon lines with a weak
dependence on fluorine doping . The temperature dependence does not show any
phonon anomaly at the superconducting transition. The Fe related phonon
intensity shows a strong resonant enhancement below 2 eV. We associate the
resonant enhancement to the presence of an interband transition around 2 eV
observed in optical conductivity. Our results point to a rather weak coupling
between Raman-active phonons and electronic excitations in iron oxypnictides
superconductors.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
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