4,213 research outputs found
Dispersion Relations in String Theory
We analyze the analytic continuation of the formally divergent one-loop
amplitude for scattering of the graviton multiplet in the Type II Superstring.
In particular we obtain explicit double and single dispersion relations,
formulas for all the successive branch cuts extending out to plus infinity, as
well as for the decay rate of a massive string state of arbitrary mass 2N into
two string states of lower mass. We compare our results with the box diagram in
a superposition of -like field theories. The stringy effects are traced
to a convergence problem in this superposition.Comment: 17 pages, COLUMBIA-YITP-UCLA/93/TEP/45 (figures fixed up
A critical Mach number for electron injection in collisionless shocks
Electron acceleration in collisionless shocks with arbitrary magnetic field
orientations is discussed. It is shown that the injection of thermal electrons
into diffusive shock acceleration process is achieved by an electron beam with
a loss-cone in velocity space that is reflected back upstream from the shock
through shock drift acceleration mechanism. The electron beam is able to excite
whistler waves which can scatter the energetic electrons themselves when the
Alfven Mach number of the shock is sufficiently high. A critical Mach number
for the electron injection is obtained as a function of upstream parameters.
The application to supernova remnant shocks is discussed.Comment: 4 pages, 2 figure, accepted for publication in Physical Review
Letter
Current-feedback-stabilized laser system for quantum simulation experiments using Yb clock transition at 578 nm
We developed a laser system for the spectroscopy of the clock transition in
ytterbium (Yb) atoms at 578 nm based on an interference-filter stabilized
external-cavity diode laser (IFDL) emitting at 1156 nm. Owing to the improved
frequency-to-current response of the laser-diode chip and the less sensitivity
of the IFDL to mechanical perturbations, we succeeded in stabilizing the
frequency to a high-finesse ultra-low-expansion glass cavity with a simple
current feedback system. Using this laser system, we performed high-resolution
clock spectroscopy of Yb and found that the linewidth of the stabilized laser
was less than 320 Hz.Comment: 5 pages, 7 figure
Momentum Analyticity and Finiteness of the 1-Loop Superstring Amplitude
The Type II Superstring amplitude to 1-loop order is given by an integral of
-functions over the moduli space of tori, which diverges for real
momenta. We construct the analytic continuation which renders this amplitude
well defined and finite, and we find the expected poles and cuts in the complex
momentum plane.Comment: 10pp, /UCLA/93/TEP/
âFake Tanâ or âFake Newsâ?
We estimated Trumpâs skin colour from 70 internet images and also from the âtwitter tan lineâ image (February 8, 2020; Twitter). We then compared the estimated skin colours with two existing data sets of skin colours: the range of skin tans that occur naturally in the Caucasian population and the range skin colours brought about by a sunless tan. We find that Trumpâs skin colour is close to the edge of the natural skin tan gamut and firmly within the gamut of a sunless skin tan. The skin colour above Trumpâs tan line is outside of the naturally occurring range of skin colours, even outside the skin tan of nonmelanized albinos. The latter finding is consistent with the hypothesis that part of the image may have been digitally distorted
Spatial and taxonomic biases in bat records: drivers and conservation implications in a megadiverse country
Biases in data availability have serious consequences on scientific inferences that can be derived. The potential consequences of these biases could be more detrimental in the lessâstudied megadiverse regions, often characterized by high biodiversity and serious risks of human threats, as conservation and management actions could be misdirected. Here, focusing on 134 bat species in Mexico, we analyze spatial and taxonomic biases and their drivers in occurrence data; and identify priority areas for further data collection which are currently underâsampled or at future environmental risk. We collated a comprehensive database of 26,192 presenceâonly bat records in Mexico to characterize taxonomic and spatial biases and relate them to species' characteristics (range size and foraging behavior). Next, we examined variables related to accessibility, species richness and security to explain the spatial patterns in occurrence records. Finally, we compared the spatial distributions of existing data and future threats to these species to highlight those regions that are likely to experience an increased level of threats but are currently underâsurveyed. We found taxonomic biases, where species with wider geographical ranges and narrowâspace foragers (species easily captured with traditional methods), had more occurrence data. There was a significant oversampling toward tropical regions, and the presence and number of records was positively associated with areas of high topographic heterogeneity, road density, urban, and protected areas, and negatively associated with areas which were predicted to have future increases in temperature and precipitation. Sampling efforts for Mexican bats appear to have focused disproportionately on easily captured species, tropical regions, areas of high species richness and security; leading to underâsampling in areas of high future threats. These biases could substantially influence the assessment of current status of, and future anthropogenic impacts on, this diverse species group in a tropical megadiverse country
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