34,533 research outputs found
Two observers calculate the trace anomaly
We adapt a calculation due to Massacand and Schmid to the coordinate
independent definition of time and vacuum given by Capri and Roy in order to
compute the trace anomaly for a massless scalar field in a curved spacetime in
1+1 dimensions. The computation which requires only a simple regulator and
normal ordering yields the well-known result in a
straightforward manner.Comment: RevTeX, 13 pages, some typos corrected and an appendix added, this is
the version to appear in Class. and Quantum Gavit
Global analysis by hidden symmetry
Hidden symmetry of a G'-space X is defined by an extension of the G'-action
on X to that of a group G containing G' as a subgroup. In this setting, we
study the relationship between the three objects:
(A) global analysis on X by using representations of G (hidden symmetry);
(B) global analysis on X by using representations of G';
(C) branching laws of representations of G when restricted to the subgroup
G'.
We explain a trick which transfers results for finite-dimensional
representations in the compact setting to those for infinite-dimensional
representations in the noncompact setting when is -spherical.
Applications to branching problems of unitary representations, and to spectral
analysis on pseudo-Riemannian locally symmetric spaces are also discussed.Comment: Special volume in honor of Roger Howe on the occasion of his 70th
birthda
Real Space Imaging of Spin Polarons in Zn Doped SrCu2(BO3)2
We report on the real space profile of spin polarons in the quasi
two-dimensional frustrated dimer spin system SrCu2(BO3)2 doped with 0.16% of
Zn. The 11B nuclear magnetic resonance spectrum exhibits 15 additional boron
sites near non-magnetic Zn impurities. With the help of exact diagonalizations
of finite clusters, we have deduced from the boron spectrum the distribution of
local magnetizations at the Cu sites with fine spatial resolution, providing
direct evidence for an extended spin polaron. The results are confronted with
those of other experiments performed on doped and undoped samples of
SrCu2(BO3)2.Comment: 9 pages, 11 figures, including supplemental materials. accepted for
publication in PR
Co-NMR Knight Shift of NaxCoO2 \dot yH2O Studied in Both Superconducting Regions of the Tc-nuQ3 Phase Diagram Divided by the Nonsuperconducting Phase
In the temperature (T)-nuQ3 phase diagram of NaxCoO2 \dot yH2O, there exist
two superconducting regions of nuQ3 separated by the nonsuperconducting region,
where nuQ3 is usually estimated from the peak position of the 59Co-NQR spectra
of the 5/2-7/2 transition and well-approximated here as nuQ3~3nuQ,nuQ being the
interaction energy between the nuclear quadrupole moment and the electric field
gradient. We have carried out measurements of the 59Co-NMR Knight shift (K) for
a single crystal in the higher-nuQ3 superconducting phase and found that K
begins to decrease with decreasing T at Tc for both magnetic field directions
parallel and perpendicular to CoO2-planes. The result indicates together with
the previous ones that the superconducting pairs are in the spin-singlet state
in both phases, excluding the possibility of the spin-triplet superconductivity
in this phase diagram. The superconductivity of this system spreads over the
wide nuQ3 regions, but is suppressed in the narrow region located at the middle
point of the region possibly due to charge instability.Comment: 8 pages, 5 figures, submitted to J. Phys. Soc. Jp
Predictions for the unitarity triangle angles in a new parametrization
A new approach to the parametrization of the CKM matrix, , is considered
in which is written as a linear combination of the unit matrix and a
non-diagonal matrix which causes intergenerational-mixing, that is
. Such a depends on 3 real parameters
including the parameter . It is interesting that a value of
is required to fit the available data on the CKM-matrix
including CP-violation. Predictions of this fit for the angles ,
and for the unitarity triangle corresponding to
, are given. For
=, we obtain , and
. These values are just about in agreement, within errors,
with the present data. It is very interesting that the unitarity triangle is
expected to be approximately a right-angled, isosceles triangle. Our prediction
is in excellent agreement with the value reported by the Belle collaboration at the Lepton-Photon 2001 meeting.Comment: 11 pages, latex, no figure
Influence of firing mechanisms on gain modulation
We studied the impact of a dynamical threshold on the f-I curve-the
relationship between the input and the firing rate of a neuron-in the presence
of background synaptic inputs. First, we found that, while the leaky
integrate-and-fire model cannot reproduce the f-I curve of a cortical neuron,
the leaky integrate-and-fire model with dynamical threshold can reproduce it
very well. Second, we found that the dynamical threshold modulates the onset
and the asymptotic behavior of the f-I curve. These results suggest that a
cortical neuron has an adaptation mechanism and that the dynamical threshold
has some significance for the computational properties of a neuron.Comment: 7 pages, 4 figures, conference proceeding
Specialized physiological studies in support of manned space flight
The effects of a diuretic (Lasix) induced dehydration on the cardiovascular and hematological responses to lower body negative pressure (LBNP) were analyzed and compared to previous observations on dehydration following exercise in the heat. During LBNP runs the subjects were monitored for changes in blood volume, heart rate, blood pressure, and variations in the volume of the left calf. It was concluded that Lasix dehydration produced a depletion of the body electrolytes at the expense of both the plasma and extravascular compartments. Striking differences were found between those subjects who were physically active (Runners: R) and those who did not engage in any regular physical activity (Non-runners: NR). Tolerance to LBNP (Torr x min) was significantly lower in the R's than the NR's before and after dehydration, however the R's lost more of their tolerance after dehydration with Lasix than after exercise in the heat for about the same fluid loss. The opposite was true for the NR's. Two factors appear to be responsible for the lower LBNP tolerance in R's: parasympathetic inhibition of cardiac activity during LBNP and a greater propensity to pool blood in the lower extremities
Baryogenesis from the Kobayashi-Maskawa Phase
The Standard Model fulfills the three Sakharov conditions for baryogenesis.
The smallness of quark masses suppresses, however, the CP violation from the
Kobayashi-Maskawa phase to a level that is many orders of magnitude below what
is required to explain the observed baryon asymmetry. We point out that if, as
a result of time variation in the Yukawa couplings, quark masses were large at
the time of the electroweak phase transition, then the Kobayashi-Maskawa
mechanism could be the source of the asymmetry. The Froggatt-Nielsen mechanism
provides a plausible framework where the Yukawa couplings could all be of order
one at that time, and settle to their present values before nucleosynthesis.
The problems related to a strong first order electroweak phase transition may
also be alleviated in this framework. Our scenario reveals a loophole in the
commonly held view that the Kobayashi-Maskawa mechanism cannot be the dominant
source of CP violation to play a role in baryogenesis.Comment: 4 page
Predicting and verifying transition strengths from weakly bound molecules
We investigated transition strengths from ultracold weakly bound 41K87Rb
molecules produced via the photoassociation of laser-cooled atoms. An accurate
potential energy curve of the excited state (3)1Sigma+ was constructed by
carrying out direct potential fit analysis of rotational spectra obtained via
depletion spectroscopy. Vibrational energies and rotational constants extracted
from the depletion spectra of v'=41-50 levels were combined with the results of
the previous spectroscopic study, and they were used for modifying an ab initio
potential. An accuracy of 0.14% in vibrational level spacing and 0.3% in
rotational constants was sufficient to predict the large observed variation in
transition strengths among the vibrational levels. Our results show that
transition strengths from weakly bound molecules are a good measure of the
accuracy of an excited state potential.Comment: 7 pages, 7 figure
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