6,973 research outputs found
Damping of dHvA oscillations and vortex-lattice disorder in the peak-effect region of strong type-II superconductors
The phenomenon of magnetic quantum oscillations in the superconducting state
poses several questions that still defy satisfactory answers. A key
controversial issue concerns the additional damping observed in the vortex
state. Here, we show results of \mu SR, dHvA, and SQUID magnetization
measurements on borocarbide superconductors, indicating that a sharp drop
observed in the dHvA amplitude just below H_{c2} is correlated with enhanced
disorder of the vortex lattice in the peak-effect region, which significantly
enhances quasiparticle scattering by the pair potential.Comment: 4 pages 4 figure
An Overview of the Rotational Behavior of Metal--Poor Stars
The present paper describes the behavior of the rotational velocity in
metal--poor stars ([Fe/H]<-0.5 dex) in different evolutionary stages, based on
Vsini values from the literature. Our sample is comprised of stars in the field
and some Galactic globular clusters, including stars on the main sequence, the
red giant branch (RGB), and the horizontal branch (HB). The metal--poor stars
are, mainly, slow rotators, and their Vsini distribution along the HR diagram
is quite homogeneous. Nevertheless, a few moderate to high values of Vsini are
found in stars located on the main sequence and on the HB. We show that the
overall distribution of Vsini values is basically independent of metallicity
for the stars in our sample. In particular, the fast-rotating main sequence
stars in our sample present similar rotation rates as their metal-rich
counterparts, suggesting that some of them may actually be fairly young, in
spite of their low metallicity, or else that at least some of them would be
better classified as blue straggler stars. We do not find significant evidence
of evolution in Vsini values as a function of position on the RGB; in
particular, we do not confirm previous suggestions that stars close to the RGB
tip rotate faster than their less evolved counterparts. While the presence of
fast rotators among moderately cool blue HB stars has been suggested to be due
to angular momentum transport from a stellar core that has retained significant
angular momentum during its prior evolution, we find that any such transport
mechanisms must likely operate very fast as the star arrives on the zero-age HB
(ZAHB), since we do not find a link between evolution off the ZAHB and Vsini
values.
We present an extensive tabulation of all quantities discussed in this paper,
including rotation velocities, temperatures, gravitieComment: 22 pages, 10 figure
Boosting Higgs pair production in the final state with multivariate techniques
The measurement of Higgs pair production will be a cornerstone of the LHC
program in the coming years. Double Higgs production provides a crucial window
upon the mechanism of electroweak symmetry breaking and has a unique
sensitivity to the Higgs trilinear coupling. We study the feasibility of a
measurement of Higgs pair production in the final state at
the LHC. Our analysis is based on a combination of traditional cut-based
methods with state-of-the-art multivariate techniques. We account for all
relevant backgrounds, including the contributions from light and charm jet
mis-identification, which are ultimately comparable in size to the irreducible
QCD background. We demonstrate the robustness of our analysis strategy in
a high pileup environment. For an integrated luminosity of
ab, a signal significance of is obtained,
indicating that the final state alone could allow for the
observation of double Higgs production at the High-Luminosity LHC.Comment: 47 pages, 22 figures. v2: updated references, added comparison of
post-MVA kinematic distributions. v3: matches published version in EPJ
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Effect of Time-of-Flight and Regularized Reconstructions on Quantitative Measurements and Qualitative Assessments in Newly Diagnosed Prostate Cancer With 18F-Fluorocholine Dual Time Point PET/MRI.
Recent technical advances in positron emission tomography/magnetic resonance imaging (PET/MRI) technology allow much improved time-of-flight (TOF) and regularized iterative PET reconstruction regularized iterative reconstruction (RIR) algorithms. We evaluated the effect of TOF and RIR on standardized uptake values (maximum and peak SUV [SUVmax and SUVpeak]) and their metabolic tumor volume dependencies and visual image quality for 18F-fluorocholine PET/MRI in patients with newly diagnosed prostate cancer. Fourteen patients were administered with 3 MBq/kg of 18F-fluorocholine and scanned dynamically for 30 minutes. Positron emission tomography images were divided to early and late time points (1-6 minutes summed and 7-30 minutes summed). The values of the different SUVs were documented for dominant PET-avid lesions, and metabolic tumor volume was estimated using a 50% isocontour and SUV threshold of 2.5. Image quality was assessed via visual acuity scoring (VAS). We found that incorporation of TOF or RIR increased lesion SUVs. The lesion to background ratio was not improved by TOF reconstruction, while RIR improved the lesion to background ratio significantly ( P < .05). The values of the different VAS were all significantly higher ( P < .05) for RIR images over TOF, RIR over non-TOF, and TOF over non-TOF. In conclusion, our data indicate that TOF or RIR should be incorporated into current protocols when available
Neutron-irradiation effects in LaO0.9F0.1FeAs superconductor
The effect of atomic disorder induced by neutrons irradiation on
superconducting and normal state properties of polycrystalline LaFeAsO_0.9F_0.1
was investigated. The irradiation of the sample by a moderate neutron fluence F
= 1.6*1019 cm^-2 at Tirr = 50 +- 10 C leads to the suppression of
superconductivity which recovers almost completely after annealing at
temperatures Tann < 750 C. It is shown that the reduction of superconducting
transition temperature Tc under atomic disordering is not determined solely by
the value of Hall concentration nH, i.e. doping level, but is governed by the
reduction of electronic relaxation time. This behavior can be described
qualitatively by universal Abrikosov-Gorkov equation which presents evidence on
the anomalous type of electrons pairing in Fe-based superconductors.Comment: 8 pages, 11 figure
On Coordinate Transformations in Planar Noncommutative Theories
We consider planar noncommutative theories such that the coordinates verify a
space-dependent commutation relation. We show that, in some special cases, new
coordinates may be introduced that have a constant commutator, and as a
consequence the construction of Field Theory models may be carried out by an
application of the standard Moyal approach in terms of the new coordinates. We
apply these ideas to the concrete example of a noncommutative plane with a
curved interface. We also show how to extend this method to more general
situations.Comment: 20 pages, 1 figure. references adde
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