10,221 research outputs found
Dark photon searches with atomic transitions
Dark matter could be made up of dark photons, massive but very light
particles whose interactions with matter resemble those of usual photons but
suppressed by a small mixing parameter. We analyze the main approaches to dark
photon interactions and how they can be applied to direct detection experiments
which test different ranges of masses and mixings. A new experiment based on
counting dark photons from induced atomic transitions in a target material is
proposed. This approach appears to be particularly appropriate for dark photon
detection in the meV mass range, extending the constraints in the mixing
parameter by up to eight orders of magnitude with respect to previous
experiments.Comment: 16 pages, 3 figure
The classical double copy for Taub-NUT spacetime
The double copy is a much-studied relationship between gauge theory and
gravity amplitudes. Recently, this was generalised to an infinite family of
classical solutions to Einstein's equations, namely stationary Kerr-Schild
geometries. In this paper, we extend this to the Taub-NUT solution in gravity,
which has a double Kerr-Schild form. The single copy of this solution is a
dyon, whose electric and magnetic charges are related to the mass and NUT
charge in the gravity theory. Finally, we find hints that the classical double
copy extends to curved background geometries.Comment: 13 pages, no figures. Minor edits to match journal versio
High-resolution X-ray Spectra Of The Symbiotic Star SS73 17
SS73 17 was an innocuous Mira-type symbiotic star until Integral and Swift
discovered its bright hard X-ray emission, adding it to the small class of
"hard X-ray emitting symbiotics." Suzaku observations in 2006 then showed it
emits three bright iron lines as well, with little to no emission in the 0.3-2
keV bandpass. We present here followup observations with the Chandra HETG and
Suzaku that confirm the earlier detection of strong emission lines of Fe Kalpha
fluorescence, Fe XXV and Fe XXVI but also show significantly more soft X-ray
emission. The high resolution spectrum also shows emission lines of other
highly ionized ions as Si XIV and possibly S XVI. In addition, a reanalysis of
the 2006 Suzaku data using the latest calibration shows that the hard (15-50
keV) X-ray emission is brighter than previously thought and remains constant in
both the 2006 and 2008 data.
The G ratio calculated from the Fe XXV lines shows that these lines are
thermal, not photoionized, in origin. With the exception of the hard X-ray
emission, the spectra from both epochs can be fit using thermal radiation
assuming a differential emission measure based on a cooling flow model combined
with a full and partial absorber. We show that acceptable fits can be obtained
for all the data in the 1-10 keV band varying only the partial absorber. Based
on the temperature and accretion rate, the thermal emission appears to be
arising from the boundary layer between the accreting white dwarf and the
accretion disk.Comment: 7 pages, 5 figures. Accepted by the Astrophysical Journa
Crystallographic studies of the Escherichia coli quinol-fumarate reductase with inhibitors bound to the quinol-binding site
The quinol-fumarate reductase (QFR) respiratory complex of Escherichia coli is a four-subunit integral-membrane complex that catalyzes the final step of anaerobic respiration when fumarate is the terminal electron acceptor. The membrane-soluble redox-active molecule menaquinol (MQH(2)) transfers electrons to QFR by binding directly to the membrane-spanning region. The crystal structure of QFR contains two quinone species, presumably MQH(2), bound to the transmembrane-spanning region. The binding sites for the two quinone molecules are termed Q(P) and Q(D), indicating their positions proximal Q(P)) or distal (Q(D)) to the site of fumarate reduction in the hydrophilic flavoprotein and iron-sulfur protein subunits. It has not been established whether both of these sites are mechanistically significant. Co-crystallization studies of the E. coli QFR with the known quinol-binding site inhibitors 2-heptyl-4-hydroxyquinoline-N-oxide and 2-[1-(p-chlorophenyl)ethyl] 4,6-dinitrophenol establish that both inhibitors block the binding of MQH(2) at the Q(P) site. In the structures with the inhibitor bound at Q(P), no density is observed at Q(D), which suggests that the occupancy of this site can vary and argues against a structurally obligatory role for quinol binding to Q(D). A comparison of the Q(P) site of the E. coli enzyme with quinone-binding sites in other respiratory enzymes shows that an acidic residue is structurally conserved. This acidic residue, Glu-C29, in the E. coli enzyme may act as a proton shuttle from the quinol during enzyme turnover
cAMP Signaling Enhances HIV-1 Long Terminal Repeat (LTR)-directed Transcription and Viral Replication in Bone Marrow Progenitor Cells.
CD34+ hematopoietic progenitor cells have been shown to be susceptible to HIV-1 infection, possibly due to a low-level expression of CXCR4, a coreceptor for HIV-1 entry. Given these observations, we have explored the impact of forskolin on cell surface expression of CXCR4 in a cell line model (TF-1). The elevation of intracellular cyclic adenosine monophosphate (cAMP) by forskolin through adenylyl cyclase (AC) resulted in transcriptional upregulation of CXCR4 with a concomitant increase in replication of the CXCR4-utilizing HIV-1 strain IIIB. Transient expression analyses also demonstrated an increase in CXCR4-, CCR5-, and CXCR4-/CCR5-utilizing HIV-1 (LAI, YU2, and 89.6, respectively) promoter activity. Studies also implicated the protein kinase A (PKA) pathway and the downstream transcription factor CREB-1 in interfacing with cAMP response elements located in the CXCR4 and viral promoter. These observations suggest that the cAMP signaling pathway may serve as a regulator of CXCR4 levels and concomitantly of HIV-1 replication in bone marrow (BM) progenitor cells. © 2017, © The Author(s) 2017
Classical versus Quantum Structure of the Scattering Probability Matrix. Chaotic wave-guides
The purely classical counterpart of the Scattering Probability Matrix (SPM)
of the quantum scattering matrix is defined for 2D
quantum waveguides for an arbitrary number of propagating modes . We compare
the quantum and classical structures of for a waveguide
with generic Hamiltonian chaos. It is shown that even for a moderate number of
channels, knowledge of the classical structure of the SPM allows us to predict
the global structure of the quantum one and, hence, understand important
quantum transport properties of waveguides in terms of purely classical
dynamics. It is also shown that the SPM, being an intensity measure, can give
additional dynamical information to that obtained by the Poincar\`{e} maps.Comment: 9 pages, 9 figure
Alcohol-Based Adsorption Heat Pumps using Hydrophobic Metal-Organic Frameworks
The building climate industry and its influence on energy consumption have
consequences on the environment due to the emission of greenhouse gasses.
Improving the efficiency of this sector is essential to reduce the effect on
climate change. In recent years, the interest in porous materials in
applications such as heat pumps has increased for their promising potential. To
assess the performance of adsorption heat pumps and cooling systems, here we
discuss a multistep approach based on the processing of adsorption data
combined with a thermodynamic model. The process provides properties of
interest, such as the coefficient of performance, the working capacity, the
specific heat or cooling effect, or the released heat upon adsorption and
desorption cycles, and it also has the advantage of identifying the optimal
conditions for each adsorbent-fluid pair. To test this method, we select
several metal-organic frameworks that differ in topology, chemical composition,
and pore size, which we validate with available experiments. Adsorption
equilibrium curves were calculated using molecular simulations to describe the
adsorption mechanisms of methanol and ethanol as working fluids in the selected
adsorbents. Then, using a thermodynamic model we calculate the energetic
properties combined with iterative algorithms that simultaneously vary all the
required working conditions. We discuss the strong influence of operating
temperatures on the performance of heat pump devices. Our findings point to the
highly hydrophobic metal azolate framework MAF-6 as a very good candidate for
heating and cooling applications for its high working capacity and excellent
energy efficiency
Influence of diffractive interactions on cosmic ray air showers
A comparative study of commonly used hadronic collision simulation packages
is presented. The characteristics of the products of hadron-nucleus collisions
are analyzed from a general perspective, but focusing on their correlation with
diffractive processes. One of the purposes of our work is to give quantitative
estimations of the impact that different characteristics of the hadronic models
have on air shower observables. Several sets of shower simulations using
different settings for the parameters controlling the diffractive processes are
used to analyze the correlations between diffractivity and shower observables.
We find that the relative probability of diffractive processes during the
shower development have a non negligible influence over the longitudinal
profile as well as the distribution of muons at ground level. The implications
on experimental data analysis are discussed
The relationship between truncation and phosphorylation at the C-terminus of tau protein in the paired helical filaments of Alzheimer's disease
Acknowledgements: Authors want to express their gratitude to Dr. P. Davies (Albert Einstein College of Medicine, Bronx, NY, USA) and Lester I. Binder (NorthWestern, Chicago, IL, USA) for the generous gift of mAbs (TG-3, Alz-50, and MC1), and (TauC-3), respectively, and to M. en C. Ivan J. GalvĂĄn-Mendoza for his support in confocal microscopy, and Ms. Maricarmen De Lorenz for her secretarial assistance. We also want to express our gratitude to the Mexican Families who donate the brain of their loved ones affected with Alzheimer's disease, and made possible our research. This work was financially supported by CONACyT grant, No. 142293 (For R.M).Peer reviewedPublisher PD
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