515 research outputs found
On Gauge Invariance of Breit-Wigner Propagators
We present an approach to bosonic () as well as fermionic
(top-quark) Breit-Wigner propagators which is consistent with gauge invariance
arguments. In particular, for the -boson propagator we extend previous
analyses and show that the part proportional to must be
modified near the resonance. We derive a mass shift which agrees with results
obtained elsewhere by different methods. The modified form of a resonant heavy
fermion propagator is also given.Comment: 16 p., TeX, (final version
Quest for a Nuclear Georeactor
Knowledge about the interior of our planet is mainly based on the
interpretation of seismic data from earthquakes and nuclear explosions, and of
composition of meteorites. Additional observations have led to a wide range of
hypotheses on the heat flow from the interior to the crust, the abundance of
certain noble gases in gasses vented from volcanoes and the possibility of a
nuclear georeactor at the centre of the Earth. This paper focuses on a proposal
for an underground laboratory to further develop antineutrinos as a tool to map
the distribution of radiogenic heat sources, such as the natural radionuclides
and the hypothetical nuclear georeactor.Comment: Invited talk presented at the International Symposium on Radiation
Physics, Cape Town, 2003. Manuscript is submitted to Radiation Physics and
Chemistr
Extracting Br(omega->pi^+ pi^-) from the Time-like Pion Form-factor
We extract the G-parity-violating branching ratio Br(omega->pi^+ pi^-) from
the effective rho-omega mixing matrix element Pi_{rho omega}(s), determined
from e^+e^- -> pi^+ pi^- data. The omega->pi^+ pi^- partial width can be
determined either from the time-like pion form factor or through the constraint
that the mixed physical propagator D_{rho omega}^{mu nu}(s) possesses no poles.
The two procedures are inequivalent in practice, and we show why the first is
preferred, to find finally Br(omega->pi^+ pi^-) = 1.9 +/- 0.3%.Comment: 12 pages (published version
The Dominant Source of CD4+ and CD8+ T-Cell Activation in HIV Infection Is Antigenic Stimulation
To distinguish between antigenic stimulation and CD4+ T-cell homeostasis
as the cause of T-cell hyperactivation in HIV infection, we studied T-cell activation in
47 patients before and during highly active antiretroviral therapy (HAART). We show
that expression of human leukocyte antigen (HLA)-DR, CD38, and Ki67 on T cells
decreased during HAART but remained elevated over normal values until week 48 of
therapy. We confirm previous reports that T-cell activation correlates positively with
plasma HIV RNA levels (suggesting antigenic stimulation), and negatively with CD4
count (suggesting CD4+ T-cell homeostasis). However, these correlations may be
spurious, because misleading, due to the well-established negative correlation between
CD4 count and plasma HIV RNA levels. To resolve this conflict, we computed partial
correlation coefficients. Correcting for CD4 counts, we show that plasma HIV RNA
levels contributed to T-cell hyperactivation. Correcting for plasma HIV RNA levels,
we show that CD4+ T-cell depletion contributed to T-cell activation. Correcting for
both, activation of CD4+ and CD8+ T cells remained positively correlated. Because this
suggests that CD4^+ and CD8^+ T-cell activation is caused by a common additional
factor, we conclude that antigenic stimulation by HIV or other (opportunistic) infections
is the most parsimonious explanation for T-cell activation in HIV infection.
Persistence of HIV antigens may explain why T-cell activation fails to revert to levels
found in healthy individuals after 48 weeks of therapy
H-ATLAS/GAMA and HeViCS - dusty early-type galaxies in different environments
NKA acknowledges the support of the Science and Technology Facilities Council. LD, RJI and SJM acknowledge support from the European Research Council Advanced Grant COSMICISM. IDL gratefully acknowledges the support of the Flemish Fund for Scientific Research (FWO-Vlaanderen). KR acknowledges support from the European Research Council Starting Grant SEDmorph (P.I. V. Wild). Date of acceptance: 22/05/2015The Herschel Space Observatory has had a tremendous impact on the study of extragalactic dust. Specifically, early-type galaxies (ETG) have been the focus of several studies. In this paper, we combine results from two Herschel studies -a Virgo cluster study Herschel Virgo Cluster Survey (HeViCS) and a broader, low-redshift Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS)/Galaxy and Mass Assembly (GAMA) study -and contrast the dust and associated properties for similar mass galaxies. This comparison is motivated by differences in results exhibited between multiple Herschel studies of ETG. A comparison between consistent modified blackbody derived dust mass is carried out, revealing strong differences between the two samples in both dust mass and dust-to-stellar mass ratio. In particular, the HeViCS sample lacks massive ETG with as high a specific dust content as found in H-ATLAS. This is most likely connected with the difference in environment for the two samples. We calculate nearest neighbour environment densities in a consistent way, showing that H-ATLAS ETG occupy sparser regions of the local Universe, whereas HeViCS ETG occupy dense regions. This is also true for ETG that are not Herschel-detected but are in the Virgo and GAMA parent samples. Spectral energy distributions are fit to the panchromatic data. From these, we find that in H-ATLAS the specific star formation rate anticorrelates with stellar mass and reaches values as high as in our Galaxy. On the other hand HeViCS ETG appear to have little star formation. Based on the trends found here, H-ATLAS ETG are thought to have more extended star formation histories and a younger stellar population than HeViCS ETG.Publisher PDFPeer reviewe
Hsp70 in mitochondrial biogenesis
The family of hsp70 (70 kilodalton heat shock protein) molecular chaperones plays an essential and diverse role in cellular physiology, Hsp70 proteins appear to elicit their effects by interacting with polypeptides that present domains which exhibit non-native conformations at distinct stages during their life in the cell. In this paper we review work pertaining to the functions of hsp70 proteins in chaperoning mitochondrial protein biogenesis. Hsp70 proteins function in protein synthesis, protein translocation across mitochondrial membranes, protein folding and finally the delivery of misfolded proteins to proteolytic enzymes in the mitochondrial matrix
Assessing pesticide pollution of groundwater: current knowledge and remaining gaps
This paper summarises recent research on pesticides in groundwater in both temperate and tropical regions. Results of field, laboratory and modelling studies highlight the factors which determine the fate and behaviour of pesticides in groundwater systems. These include transport pathways from the soil to the water table and thence to supply sources, and the processes such as adsorption and degradation which can help to attenuate pesticide movement and reduce concentrations. Studies of degradation show that most compounds are likely to be much more persistent in aquifers than in soils, but below the water table the long travel times and potential for dilution may greatly reduce concentrations. The greatest risks are likely to occur in fractured aquifers with their potential for much more rapid flow.
Important uncertainties and gaps in knowledge remain. Laboratory studies of degradation present difficulties of extrapolation to field conditions and provide evidence of wide variations in half-lives spatially and with time; making the choice of values for transport modelling problematic. Further work is required to improve understanding of such variations. Studies can also indicate that different degradation pathways can occur and different pesticide metabolites produced, depending onenvironmental conditions. The occurrence and behaviour of metabolites in groundwater systems is also poorly known
Observation of vortex dynamics in arrays of nanomagnets
Vortex dynamics within arrays of square ferromagnetic nanoelements have been studied by time-resolved
scanningKerr microscopy (TRSKM),while x-ray photoemission electronmicroscopy has been used to investigate
the equilibrium magnetic state of the arrays. An alternating field demagnetization process was found to initialize
a distribution of equilibrium states within the individual elements of the array, including quasiuniform states
and vortex states of different chirality and core polarization. Repeated initialization revealed some evidence of
stochastic behavior during the formation of the equilibrium state. TRSKM with a spatial resolution of ∼300 nm
was used to detect vortex gyration within arrays of square nanoelements of 250-nm lateral size. Two arrays were
studied consisting of a 9 × 9 and 5 × 5 arrangement of nanoelements with 50- and 500-nm element edge-to-edge
separation to encourage strong and negligible dipolar interactions, respectively. In the 5 × 5 element array,
TRSKM images, acquired at a fixed phase of the driving microwave magnetic field, revealed differences in the
gyrotropic phase within individual elements. While some phase variation is attributed to the dispersion in the
size and shape of elements, the vortex chirality and core polarization are also shown to influence the phase. In
the 9 × 9 array, strong magneto-optical response due to vortex gyration was observed across regions with length
equal to either one or two elements. Micromagnetic simulations performed for 2 × 2 arrays of elements suggest
that particular combinations of vortex chirality and polarization in neighboring elements are required to generate
the observed magneto-optical contrast.Engineering and Physical Sciences Research Council (EPSRC
Phase-resolved x-ray ferromagnetic resonance measurements in fluorescence yield
Copyright © 2011 American Institute of PhysicsPhase-resolved x-ray ferromagnetic resonance (XFMR) has been measured in fluorescence yield, extending the application of XFMR to opaque samples on opaque substrates. Magnetization dynamics were excited in a Co50Fe50(0.7)/Ni90Fe10(5) bilayer by means of a continuous wave microwave excitation, while x-ray magnetic circular dichroism (XMCD) spectra were measured stroboscopically at different points in the precession cycle. By tuning the x-ray energy to the L-3 edges of Ni and Fe, the dependence of the real and imaginary components of the element specific magnetic susceptibility on the strength of an externally applied static bias field was determined. First results from measurements on a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1) sample confirm that enhanced damping results from the addition of the Dy cap
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