18,906 research outputs found
The tipping point: a mathematical model for the profit-driven abandonment of restaurant tipping
The custom of voluntarily tipping for services rendered has gone in and out
of fashion in America since its introduction in the 19th century. Restaurant
owners that ban tipping in their establishments often claim that social justice
drives their decisions, but we show that rational profit-maximization may also
justify the decisions. Here, we propose a conceptual model of restaurant
competition for staff and customers, and we show that there exists a critical
conventional tip rate at which restaurant owners should eliminate tipping to
maximize profit. Because the conventional tip rate has been increasing steadily
for the last several decades, our model suggests that restaurant owners may
abandon tipping en masse when that critical tip rate is reached.Comment: 14 pages, 5 figures, supplementary material include
PAMELA Positron Excess as a Signal from the Hidden Sector
The recent positron excess observed in the PAMELA satellite experiment
strengthens previous experimental findings. We give here an analysis of this
excess in the framework of the Stueckelberg extension of the standard model
which includes an extra gauge field and matter in the hidden sector.
Such matter can produce the right amount of dark matter consistent with the
WMAP constraints. Assuming the hidden sector matter to be Dirac fermions it is
shown that their annihilation can produce the positron excess with the right
positron energy dependence seen in the HEAT, AMS and the PAMELA experiments.
Further test of the proposed model can come at the Large Hadron Collider. The
predictions of the flux ratio also fit the data.Comment: 9 pages,3 figures; Breit-Wigner enhancement emphasized; published in
PR
Genetic disruption of the core circadian clock impairs hippocampus-dependent memory
Perturbing the circadian system by electrolytically lesioning the suprachiasmatic nucleus (SCN) or varying the environmental light:dark schedule impairs memory, suggesting that memory depends on the circadian system. We used a genetic approach to evaluate the role of the molecular clock in memory. Bmal1−/− mice, which are arrhythmic under constant conditions, were examined for hippocampus-dependent memory, LTP at the Schaffer-collateral synapse, and signal transduction activity in the hippocampus. Bmal1−/− mice exhibit impaired contextual fear and spatial memory. Furthermore, LTP in hippocampal slices from Bmal1−/− mice is also significantly decreased relative to that from wild-type mice. Activation of Erk1,2 MAP kinase (MAPK) during training for contextual fear memory and diurnal oscillation of MAPK activity and cAMP in the hippocampus is also lost in Bmal1−/− mice, suggesting that the memory defects are due to reduction of the memory consolidation pathway in the hippocampus. We conclude that critical signaling events in the hippocampus required for memory depend on BMAL1
In Vivo Evolution of Butane Oxidation by Terminal Alkane Hydroxylases AlkB and CYP153A6
Enzymes of the AlkB and CYP153 families catalyze the first step in the catabolism of medium-chain-length alkanes, selective oxidation of the alkane to the 1-alkanol, and enable their host organisms to utilize alkanes as carbon sources. Small, gaseous alkanes, however, are converted to alkanols by evolutionarily unrelated methane monooxygenases. Propane and butane can be oxidized by CYP enzymes engineered in the laboratory, but these produce predominantly the 2-alkanols. Here we report the in vivo-directed evolution of two medium-chain-length terminal alkane hydroxylases, the integral membrane di-iron enzyme AlkB from Pseudomonas putida GPo1 and the class II-type soluble CYP153A6 from Mycobacterium sp. strain HXN-1500, to enhance their activity on small alkanes. We established a P. putida evolution system that enables selection for terminal alkane hydroxylase activity and used it to select propane- and butane-oxidizing enzymes based on enhanced growth complementation of an adapted P. putida GPo12(pGEc47{Delta}B) strain. The resulting enzymes exhibited higher rates of 1-butanol production from butane and maintained their preference for terminal hydroxylation. This in vivo evolution system could be useful for directed evolution of enzymes that function efficiently to hydroxylate small alkanes in engineered hosts
Testing minimal lepton flavor violation with extra vector-like leptons at the LHC
Models of minimal lepton flavor violation where the seesaw scale is higher
than the relevant flavor scale predict that all lepton flavor violation is
proportional to the charged lepton Yukawa matrix. If extra vector-like leptons
are within the reach of the LHC, it will be possible to test the resulting
predictions in ATLAS/CMS.Comment: 19 pages, 8 figure
NuSTAR observations of the young, energetic radio pulsar PSR B1509-58
We report on Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray
observations of the young rotation-powered radio pulsar PSR B150958 in the
supernova remnant MSH 1552. We confirm the previously reported curvature in
the hard X-ray spectrum, showing that a log parabolic model provides a
statistically superior fit to the spectrum compared with the standard power
law. The log parabolic model describes the NuSTAR data, as well as previously
published gamma-ray data obtained with COMPTEL and AGILE, all together spanning
3 keV through 500 MeV. Our spectral modelling allows us to constrain the peak
of the broadband high energy spectrum to be at 2.60.8 MeV, an improvement
of nearly an order of magnitude in precision over previous measurements. In
addition, we calculate NuSTAR spectra in 26 pulse phase bins and confirm
previously reported variations of photon indices with phase. Finally, we
measure the pulsed fraction of PSR B150958 in the hard X-ray energy band for
the first time. Using the energy resolved pulsed fraction results, we estimate
that the pulsar's off-pulse emission has a photon index value between 1.26 and
1.96. Our results support a model in which the pulsar's lack of GeV emission is
due to viewing geometry, with the X-rays originating from synchrotron emission
from secondary pairs in the magnetosphere.Comment: 10 pages, 8 figures, 6 tables, ApJ accepte
How the First Stars Regulated Star Formation. II. Enrichment by Nearby Supernovae
Metals from Population III (Pop III) supernovae led to the formation of less
massive Pop II stars in the early universe, altering the course of evolution of
primeval galaxies and cosmological reionization. There are a variety of
scenarios in which heavy elements from the first supernovae were taken up into
second-generation stars, but cosmological simulations only model them on the
largest scales. We present small-scale, high-resolution simulations of the
chemical enrichment of a primordial halo by a nearby supernova after partial
evaporation by the progenitor star. We find that ejecta from the explosion
crash into and mix violently with ablative flows driven off the halo by the
star, creating dense, enriched clumps capable of collapsing into Pop II stars.
Metals may mix less efficiently with the partially exposed core of the halo, so
it might form either Pop III or Pop II stars. Both Pop II and III stars may
thus form after the collision if the ejecta do not strip all the gas from the
halo. The partial evaporation of the halo prior to the explosion is crucial to
its later enrichment by the supernova.Comment: Accepted to Ap
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