1,349 research outputs found
The impact of Covid-19 on US firms
We use survey data on an opt-in panel of around 2,500 US small businesses to assess the impact of COVID-19. We find a significant negative sales impact that peaked in Quarter 2 of 2020, with an average loss of 29% in sales. The large negative impact masks significant heterogeneity, with over 40% of firms reporting zero or a positive impact, while almost a quarter report losses of more than 50%. These impacts also appear to be persistent, with firms reporting the largest sales drops in mid-2020 still forecasting large sales losses a year later in mid-2021. In terms of business types, we find that the smallest offline firms experienced sales drops of over 40% compared to less than 10% for the largest online firms. Finally, in terms of owners, we find female and black owners reported significantly larger drops in sales. Owners with a humanities degree also experienced far larger losses, while those with a STEM degree saw the least impact
Three-Body Recombination and Rydberg Atoms in Ultracold Plasmas
Ultracold neutral plasmas, created by photoionizing samples of laser-cooled atoms, have well-controlled initial density and temperature parameters. With initial particle peak densities of ~10^15 m^-3, initial ion temperatures in the tens of micro-Kelvin range, and initial electron temperatures with a controllable range of 1-1000 K, these systems provide a means to study otherwise laboratory-inaccessible parameter ranges for plasma research. Furthermore, these plasmas are inhomogeneous, unconfined, and freely expanding into a vacuum. Despite the extraordinarily low electron temperatures, the electron system remains weakly coupled, although the ion system exhibits strong coupling behavior.
While the initial electron temperatures are very low in ultracold plasmas, the temperature evolution has only been measured indirectly, in the earliest ~5% of the plasma lifetime, and often with large uncertainties. We present a technique that, with further theoretical support, can provide straightforward temperature measurements throughout the first fifth of the plasma lifetime. By making use of collective modes of the plasma, we fit a model of Tonks-Dattner resonances (electron sound wave propagating in the plasma) to measurements of these resonances and obtain a time-dependent electron temperature measurement for the ultracold plasma.
Three-body recombination, a plasma loss process that has a rate scaling with the -9/2 power of the electron temperature, is of obvious interest in these ultracold plasma systems. Several theoretical works have predicted that the three-body recombination rate expression would need to be modified at these low electron temperatures, although the validity of these changes often hinges on the electron system being strongly coupled. We have performed the lowest temperature measurements of three-body recombination rates in a plasma and show that these measurements potentially provide a low-uncertainty means to calculate electron temperatures
Making Connections:John Darwin and his Histories of Empire
John Darwin and his Histories of Empir
Redox Signaling in Colonial Hydroids: Many Pathways for Peroxide
Studies of mitochondrial redox signaling predict that
the colonial hydroids Eirene viridula and Podocoryna
carnea should respond to manipulations of reactive oxygen
species (ROS). Both species encrust surfaces with feeding
polyps connected by networks of stolons; P. carnea is more
‘sheet-like’ with closely spaced polyps and short stolons,
while E. viridula is more ‘runner-like’ with widely spaced
polyps and long stolons. Treatment with the chemical antioxidant
vitamin C diminishes ROS in mitochondrion-rich
epitheliomuscular cells (EMCs) and produces phenotypic
effects (sheet-like growth) similar to uncouplers of
oxidative phosphorylation. In peripheral stolon tips,
treatment with vitamin C triggers a dramatic increase of
ROS that is followed by tissue death and stolon regression.
The enzymatic anti-oxidant catalase is probably not taken
up by the colony but, rather, converts hydrogen peroxide
in the medium to water and oxygen. Exogenous catalase
does not affect ROS in mitochondrion-rich EMCs, but
does increase the amounts of ROS emitted from
peripheral stolons, resulting in rapid, runner-like growth.
Treatment with exogenous hydrogen peroxide increases
ROS levels in stolon tips and results in somewhat faster
colony growth. Finally, untreated colonies of E. viridula
exhibit higher levels of ROS in stolon tips than untreated
colonies of P. carnea. ROS may participate in a number of
putative signaling pathways: (1) high levels of ROS may
trigger cell and tissue death in peripheral stolon tips; (2)
more moderate levels of ROS in stolon tips may trigger
outward growth, inhibit branching and, possibly, mediate
the redox signaling of mitochondrion-rich EMCs; and (3)
ROS may have an extra-colony function, perhaps in
suppressing the growth of bacteria
The statistical mechanics of complex signaling networks : nerve growth factor signaling
It is becoming increasingly appreciated that the signal transduction systems
used by eukaryotic cells to achieve a variety of essential responses represent
highly complex networks rather than simple linear pathways. While significant
effort is being made to experimentally measure the rate constants for
individual steps in these signaling networks, many of the parameters required
to describe the behavior of these systems remain unknown, or at best,
estimates. With these goals and caveats in mind, we use methods of statistical
mechanics to extract useful predictions for complex cellular signaling
networks. To establish the usefulness of our approach, we have applied our
methods towards modeling the nerve growth factor (NGF)-induced differentiation
of neuronal cells. Using our approach, we are able to extract predictions that
are highly specific and accurate, thereby enabling us to predict the influence
of specific signaling modules in determining the integrated cellular response
to the two growth factors. We show that extracting biologically relevant
predictions from complex signaling models appears to be possible even in the
absence of measurements of all the individual rate constants. Our methods also
raise some interesting insights into the design and possible evolution of
cellular systems, highlighting an inherent property of these systems wherein
particular ''soft'' combinations of parameters can be varied over wide ranges
without impacting the final output and demonstrating that a few ''stiff''
parameter combinations center around the paramount regulatory steps of the
network. We refer to this property -- which is distinct from robustness -- as
''sloppiness.''Comment: 24 pages, 10 EPS figures, 1 GIF (makes 5 multi-panel figs + caption
for GIF), IOP style; supp. info/figs. included as brown_supp.pd
Quantum Error Correction via Convex Optimization
We show that the problem of designing a quantum information error correcting
procedure can be cast as a bi-convex optimization problem, iterating between
encoding and recovery, each being a semidefinite program. For a given encoding
operator the problem is convex in the recovery operator. For a given method of
recovery, the problem is convex in the encoding scheme. This allows us to
derive new codes that are locally optimal. We present examples of such codes
that can handle errors which are too strong for codes derived by analogy to
classical error correction techniques.Comment: 16 page
The mechanism of resistance to favipiravir in influenza.
Favipiravir is a broad-spectrum antiviral that has shown promise in treatment of influenza virus infections. While emergence of resistance has been observed for many antiinfluenza drugs, to date, clinical trials and laboratory studies of favipiravir have not yielded resistant viruses. Here we show evolution of resistance to favipiravir in the pandemic H1N1 influenza A virus in a laboratory setting. We found that two mutations were required for robust resistance to favipiravir. We demonstrate that a K229R mutation in motif F of the PB1 subunit of the influenza virus RNA-dependent RNA polymerase (RdRP) confers resistance to favipiravir in vitro and in cell culture. This mutation has a cost to viral fitness, but fitness can be restored by a P653L mutation in the PA subunit of the polymerase. K229R also conferred favipiravir resistance to RNA polymerases of other influenza A virus strains, and its location within a highly conserved structural feature of the RdRP suggests that other RNA viruses might also acquire resistance through mutations in motif F. The mutations identified here could be used to screen influenza virus-infected patients treated with favipiravir for the emergence of resistance
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