21,452 research outputs found
Insights on scalar mesons from their radiative decays
We estimate the rates for radiative transitions of the lightest scalar mesons
f_0(980) and a_0(980) to the vector mesons rho and omega. We argue that
measurements of the radiative decays of those scalar mesons can provide
important new information on their structure.Comment: 20 pages, 5 figures; appendix added, to be published in Phys. Rev.
Ion acceleration from laser-driven electrostatic shocks
Multi-dimensional particle-in-cell simulations are used to study the
generation of electrostatic shocks in plasma and the reflection of background
ions to produce high-quality and high-energy ion beams. Electrostatic shocks
are driven by the interaction of two plasmas with different density and/or
relative drift velocity. The energy and number of ions reflected by the shock
increase with increasing density ratio and relative drift velocity between the
two interacting plasmas. It is shown that the interaction of intense lasers
with tailored near-critical density plasmas allows for the efficient heating of
the plasma electrons and steepening of the plasma profile at the critical
density interface, leading to the generation of high-velocity shock structures
and high-energy ion beams. Our results indicate that high-quality 200 MeV
shock-accelerated ion beams required for medical applications may be obtained
with current laser systems.Comment: 33 pages, 12 figures, accepted for publication in Physics of Plasma
A yield-cost tradeoff governs Escherichia coli’s decision between fermentation and respiration in carbon-limited growth
Living cells react to changes in growth conditions by re-shaping their proteome. This accounts for different stress-response strategies, both specific (i.e., aimed at increasing the availability of stress-mitigating proteins) and systemic (such as large-scale changes in the use of metabolic pathways aimed at a more efficient exploitation of resources). Proteome re-allocation can, however, imply significant biosynthetic costs. Whether and how such costs impact the growth performance are largely open problems. Focusing on carbon-limited E. coli growth, we integrate genome-scale modeling and proteomic data to address these questions at quantitative level. After deriving a simple formula linking growth rate, carbon intake, and biosynthetic costs, we show that optimal growth results from the tradeoff between yield maximization and protein burden minimization. Empirical data confirm that E. coli growth is indeed close to Pareto-optimal over a broad range of growth rates. Moreover, we establish that, while most of the intaken carbon is diverted into biomass precursors, the efficiency of ATP synthesis is the key driver of the yield-cost tradeoff. These findings provide a quantitative perspective on carbon overflow, the origin of growth laws and the multidimensional optimality of E. coli metabolism
Efficient Quantum Polar Coding
Polar coding, introduced 2008 by Arikan, is the first (very) efficiently
encodable and decodable coding scheme whose information transmission rate
provably achieves the Shannon bound for classical discrete memoryless channels
in the asymptotic limit of large block sizes. Here we study the use of polar
codes for the transmission of quantum information. Focusing on the case of
qubit Pauli channels and qubit erasure channels, we use classical polar codes
to construct a coding scheme which, using some pre-shared entanglement,
asymptotically achieves a net transmission rate equal to the coherent
information using efficient encoding and decoding operations and code
construction. Furthermore, for channels with sufficiently low noise level, we
demonstrate that the rate of preshared entanglement required is zero.Comment: v1: 15 pages, 4 figures. v2: 5+3 pages, 3 figures; argumentation
simplified and improve
Ticks in the box: Argas persicus occurrence in nest boxes of secondary cavity-nesting bird species in Italy
Migratory organisms can be vectors of parasitic host organisms that can then be transported along seasonal migratory journeys and spread across continents. Ornithophilic nidicolous ticks (e.g. soft ticks) include species responsible for the transmission of pathogens and bacteria, thus representing a health problem not only for wild species that are directly parasitized, but also for those that share the same environments or reproductive sites with them. In this regard, artificial nests for birds may turn out to be site-sources of parasites. Here, we document the occurrence of different life stages of Argas persicus ticks in nest boxes of wild birds in a natural area (not associated with poultry activities) of central coastal Italy (Maremma Regional Park, Tuscany). Between 2018 and 2022, 168 ticks were collected from nest boxes occupied by different secondary cavity-nesting birds, such as European rollers Coracias garrulus and scops owl Otus scops. Ticks were analysed morphologically, and selected specimens were also identified by mitochondrial ribosomal 16S (16S) subunit gene to ascertain their taxonomic status. All ticks were identified as Argas persicus. This finding not only suggests that this tick species has formed a viable population in this Italian region, but also further confirms the previously doubtful natural origin of the species at country level and sheds new light on its underestimated and little investigated distribution. Possible pathways of introduction and its potential impacts on local avian community are discussed
Influence of conformational fluctuations on enzymatic activity: modelling the functional motion of beta-secretase
Considerable insight into the functional activity of proteins and enzymes can
be obtained by studying the low-energy conformational distortions that the
biopolymer can sustain. We carry out the characterization of these large scale
structural changes for a protein of considerable pharmaceutical interest, the
human -secretase. Starting from the crystallographic structure of the
protein, we use the recently introduced beta-Gaussian model to identify, with
negligible computational expenditure, the most significant distortion occurring
in thermal equilibrium and the associated time scales. The application of this
strategy allows to gain considerable insight into the putative functional
movements and, furthermore, helps to identify a handful of key regions in the
protein which have an important mechanical influence on the enzymatic activity
despite being spatially distant from the active site. The results obtained
within the Gaussian model are validated through an extensive comparison against
an all-atom Molecular Dynamics simulation.Comment: To be published in a special issue of J. Phys.: Cond. Mat. (Bedlewo
Workshop
NuSTAR discovery of a cyclotron line in the accreting X-ray pulsar IGR J16393-4643
The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was
observed by NuSTAR in the 3-79 keV energy band for a net exposure time of 50
ks. We present the results of this observation which enabled the discovery of a
cyclotron resonant scattering feature with a centroid energy of 29.3(+1.1/-1.3)
keV. This allowed us to measure the magnetic field strength of the neutron star
for the first time: B = (2.5+/-0.1)e12 G. The known pulsation period is now
observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone a
long-term spin-up trend at a rate of P' = -2e-8 s/s (-0.6 s per year, or a
frequency derivative of nu' = 3e-14 Hz/s ). In the power density spectrum, a
break appears at the pulse frequency which separates the zero slope at low
frequency from the steeper slope at high frequency. This addition of angular
momentum to the neutron star could be due to the accretion of a quasi-spherical
wind, or it could be caused by the transient appearance of a prograde accretion
disk that is nearly in corotation with the neutron star whose magnetospheric
radius is around 2e8 cm.Comment: Accepted for publication in the Astrophysical Journal, 7 pages, 8
figures, 2 table
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