44,951 research outputs found
Semileptonic Decay Scalar Form Factor and from Lattice QCD
We present a new study of D semileptonic decays on the lattice which employs
the Highly Improved Staggered Quark (HISQ) action for both the charm and the
light valence quarks. We work with MILC unquenched lattices and
determine the scalar form factor for
semileptonic decays. The form factor is obtained from a scalar current matrix
element that does not require any operator matching. We develop a new approach
to carrying out chiral/continuum extrapolations of . The method uses
the kinematic "" variable instead of or the kaon energy and is
applicable over the entire physical range. We find in the chiral plus
continuum limit and hereby improve the theory error on this quantity by a
factor of 4 compared to previous lattice determinations. Combining the
new theory result with recent experimental measurements of the product from BaBar and CLEO-c leads to the most
precise direct determination of the CKM matrix element to date,
, where the first error comes from experiment and the
second is the lattice QCD theory error. We calculate the ratio and find GeV and show
that this agrees with experiment.Comment: 23 pages, 31 figures, 11 tables. Added a paragraph in sction VII, and
updated with PDG 2010 instead of PDG 200
Tomonaga-Luttinger liquid correlations and Fabry-Perot interference in conductance and finite-frequency shot noise in a single-walled carbon nanotube
We present a detailed theoretical investigation of transport through a
single-walled carbon nanotube (SWNT) in good contact to metal leads where weak
backscattering at the interfaces between SWNT and source and drain reservoirs
gives rise to electronic Fabry-Perot (FP) oscillations in conductance and shot
noise. We include the electron-electron interaction and the finite length of
the SWNT within the inhomogeneous Tomonaga-Luttinger liquid (TLL) model and
treat the non-equilibrium effects due to an applied bias voltage within the
Keldysh approach. In low-frequency transport properties, the TLL effect is
apparent mainly via power-law characteristics as a function of bias voltage or
temperature at energy scales above the finite level spacing of the SWNT. The
FP-frequency is dominated by the non-interacting spin mode velocity due to two
degenerate subbands rather than the interacting charge velocity. At higher
frequencies, the excess noise is shown to be capable of resolving the
splintering of the transported electrons arising from the mismatch of the
TLL-parameter at the interface between metal reservoirs and SWNT. This dynamics
leads to a periodic shot noise suppression as a function of frequency and with
a period that is determined solely by the charge velocity. At large bias
voltages, these oscillations are dominant over the ordinary FP-oscillations
caused by two weak backscatterers. This makes shot noise an invaluable tool to
distinguish the two mode velocities in the SWNT.Comment: 20 pages, 9 figure
Is the Lambda CDM Model Consistent with Observations of Large-Scale Structure?
The claim that large-scale structure data independently prefers the Lambda
Cold Dark Matter model is a myth. However, an updated compilation of
large-scale structure observations cannot rule out Lambda CDM at 95%
confidence. We explore the possibility of improving the model by adding Hot
Dark Matter but the fit becomes worse; this allows us to set limits on the
neutrino mass.Comment: To appear in Proceedings of "Sources and Detection of Dark
Matter/Energy in the Universe", ed. D. B. Cline. 6 pages, including 2 color
figure
Improved Semileptonic Form Factor Calculations in Lattice QCD
We investigate the computational efficiency of two stochastic based
alternatives to the Sequential Propagator Method used in Lattice QCD
calculations of heavy-light semileptonic form factors. In the first method, we
replace the sequential propagator, which couples the calculation of two of the
three propagators required for the calculation, with a stochastic propagator so
that the calculations of all three propagators are independent. This method is
more flexible than the Sequential Propagator Method but introduces stochastic
noise. We study the noise to determine when this method becomes competitive
with the Sequential Propagator Method, and find that for any practical
calculation it is competitive with or superior to the Sequential Propagator
Method. We also examine a second stochastic method, the so-called ``one-end
trick", concluding it is relatively inefficient in this context. The
investigation is carried out on two gauge field ensembles, using the
non-perturbatively improved Wilson-Sheikholeslami-Wohlert action with N_f=2
mass-degenerate sea quarks. The two ensembles have similar lattice spacings but
different sea quark masses. We use the first stochastic method to extract
-improved, matched lattice results for the semileptonic form
factors on the ensemble with lighter sea quarks, extracting f_+(0)
Twisted Masses and Enhanced Symmetries: the A&D Series
We study new symmetries between A and D type quiver gauge theories with
different numbers of colors. We realize these gauge theories with twisted
masses via a brane construction that reproduces all the parameters of the
Gauge/Bethe correspondence.Comment: 14 pages, 5 figure
Above-Room-Temperature Ferromagnetism in GaSb/Mn Digital Alloys
Digital alloys of GaSb/Mn have been fabricated by molecular beam epitaxy.
Transmission electron micrographs showed good crystal quality with individual
Mn-containing layers well resolved; no evidence of 3D MnSb precipitates was
seen in as-grown samples. All samples studied exhibited ferromagnetism with
temperature dependent hysteresis loops in the magnetization accompanied by
metallic p-type conductivity with a strong anomalous Hall effect (AHE) up to
400 K (limited by the experimental setup). The anomalous Hall effect shows
hysteresis loops at low temperatures and above room temperature very similar to
those seen in the magnetization. The strong AHE with hysteresis indicates that
the holes interact with the Mn spins above room temperature. All samples are
metallic, which is important for spintronics applications.
* To whom correspondence should be addressed. E-mail: [email protected]
Moho topography, ranges and folds of Tibet by analysis of global gravity models and GOCE data
The determination of the crustal structure is essential in geophysics, as it gives insight into the
geohistory, tectonic environment, geohazard mitigation, etc. Here we present the latest advance
on three-dimensional modeling representing the Tibetan Mohorovi\u10di\u107 discontinuity (topography and
ranges) and its deformation (fold), revealed by analyzing gravity data from GOCE mission. Our study
shows noticeable advances in estimated Tibetan Moho model which is superior to the results using
the earlier gravity models prior to GOCE. The higher quality gravity field of GOCE is reflected in the
Moho solution: we find that the Moho is deeper than 65 km, which is twice the normal continental
crust beneath most of the Qinghai-Tibetan plateau, while the deepest Moho, up to 82 km, is located
in western Tibet. The amplitude of the Moho fold is estimated to be ranging from 129 km to 9 km
with a standard deviation of ~2 km. The improved GOCE gravity derived Moho signals reveal a clear
directionality of the Moho ranges and Moho fold structure, orthogonal to deformation rates observed
by GPS. This geophysical feature, clearly more evident than the ones estimated using earlier gravity
models, reveals that it is the result of the large compressional tectonic process
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