5,928 research outputs found
Dynamics of methane ebullition from a peat monolith revealed from a dynamic flux chamber system
Methane (CH4) ebullition in northern peatlands is poorly quantified in part due to its high spatiotemporal variability. In this study, a dynamic flux chamber (DFC) system was used to continuously measure CH4 fluxes from a monolith of near‐surface Sphagnum peat at the laboratory scale to understand the complex behavior of CH4 ebullition. Coincident transmission ground penetrating radar measurements of gas content were also acquired at three depths within the monolith. A graphical method was developed to separate diffusion, steady ebullition, and episodic ebullition fluxes from the total CH4 flux recorded and to identify the timing and CH4 content of individual ebullition events. The results show that the application of the DFC had minimal disturbance on air‐peat CH4 exchange and estimated ebullition fluxes were not sensitive to the uncertainties associated with the graphical model. Steady and episodic ebullition fluxes were estimated to be averagely 36 ± 24% and 38 ± 24% of the total fluxes over the study period, respectively. The coupling between episodic CH4 ebullition and gas content within the three layers supports the existence of a threshold gas content regulating CH4 ebullition. However, the threshold at which active ebullition commenced varied between peat layers with a larger threshold (0.14 m3 m−3) observed in the deeper layers, suggesting that the peat physical structure controls gas bubble dynamics in peat. Temperature variation (23°C to 27°C) was likely only responsible for small episodic ebullition events from the upper peat layer, while large ebullition events from the deeper layers were most likely triggered by drops in atmospheric pressure
Implications of the ALEPH tau-Lepton Decay Data for Perturbative and Non-Perturbative QCD
We use ALEPH data on hadronic decays in order to calculate Euclidean
coordinate space correlation functions in the vector and axial-vector channels.
The linear combination receives no perturbative contribution and is
quantitatively reproduced by the instanton liquid model. In the case of
the instanton calculation is in good agreement with the data once perturbative
corrections are included. These corrections clearly show the evolution of
. We also analyze the range of validity of the Operator Product
Expansion (OPE). In the channel we find a dimension contribution
which is comparable to the original SVZ estimate, but the instanton model
provides a different non-singular term of the same magnitude. In the case
both the OPE and the instanton model predict the same power correction
induced by the gluon condensate, but it is masked by much larger perturbative
contributions. We conclude that the range of validity of the OPE is limited to
x\lsim0.3 fm, whereas the instanton model describes the data over the entire
range.Comment: 4 pages, 6 figure
Singlet VA \tilde V correlator within the instanton vacuum model
The correlator of singlet axial-vector and vector currents in the external
electromagnetic field is studied within the instanton liquid model of QCD
vacuum. In the chiral limit we calculate the longitudinal w_L^0 and transversal
w_T^0 with respect to axial-vector index invariant amplitudes at arbitrary
momentum transfer q. It is demonstrated how the anomalous longitudinal part of
the correlator is renormalized at low momenta due to the presence of the U_A(1)
anomaly.Comment: 9 pages, 4 figure
Perspectivas para o monitoramento de Spodoptera frugiperda por meio de feromônio sexual sintético em áreas de várzea do Rio Grande do Sul.
bitstream/CPACT-2009-09/11949/1/documento_251.pd
The Ginzburg-Landau Free Energy Functional of Color Superconductivity at Weak Coupling
We derive the Ginzburg-Landau free energy functional of color
superconductivity in terms of the thermal diagrams of QCD in its perturbative
region. The zero mode of the quadratic term coefficient yields the same
transition temperature, including the pre-exponential factor, as the one
obtained previously from the Fredholm determinant of the two quark scattering
amplitude. All coefficients of the free energy can be made identical to those
of a BCS model by setting the Fermi velocity of the latter equal to the speed
of light. We also calculate the induced symmetric color condensate near
and find that it scales as the cubic power of the dominant antisymmetric color
component. We show that in the presence of an inhomogeneity and a nonzero gauge
potential, while the color-flavor locked condensate dominates in the bulk, the
unlocked condensate, the octet, emerges as a result of a simultaneous
color-flavor rotation in the core region of a vortex filament or at the
junction of super and normal phases.Comment: 32 pages, Plain Tex, 3 figure
Transient absorption and reshaping of ultrafast XUV light by laser-dressed helium
We present a theoretical study of transient absorption and reshaping of
extreme ultraviolet (XUV) pulses by helium atoms dressed with a moderately
strong infrared (IR) laser field. We formulate the atomic response using both
the frequency-dependent absorption cross section and a time-frequency approach
based on the time-dependent dipole induced by the light fields. The latter
approach can be used in cases when an ultrafast dressing pulse induces
transient effects, and/or when the atom exchanges energy with multiple
frequency components of the XUV field. We first characterize the dressed atom
response by calculating the frequency-dependent absorption cross section for
XUV energies between 20 and 24 eV for several dressing wavelengths between 400
and 2000 nm and intensities up to 10^12 W/cm^2. We find that for dressing
wavelengths near 1600 nm, there is an Autler-Townes splitting of the 1s ---> 2p
transition that can potentially lead to transparency for absorption of XUV
light tuned to this transition. We study the effect of this XUV transparency in
a macroscopic helium gas by incorporating the time-frequency approach into a
solution of the coupled Maxwell-Schr\"odinger equations. We find rich temporal
reshaping dynamics when a 61 fs XUV pulse resonant with the 1s ---> 2p
transition propagates through a helium gas dressed by an 11 fs, 1600 nm laser
pulse.Comment: 13 pages, 8 figures, 1 table, RevTeX4, revise
Evidence for fine tuning of fermionic modes in lattice gluodynamics
We consider properties of zero and near-zero fermionic modes in lattice
gluodynamics. The modes are known to be sensitive to the topology of the
underlying gluonic fields in the quantum vacuum state of the gluodynamics. We
find evidence that these modes are fine tuned, that is exhibit sensitivity to
both physical (one can say, hadronic) scale and to the ultraviolet cutoff.
Namely, the density of the states is in physical units while the localization
volume of the modes tends to zero in physical units with the lattice spacing
tending to zero. We discuss briefly possible theoretical implications and also
include some general, review-type remarks.Comment: 7 pages, 7 eps figures, uses JETP Letters style (included);
substantial stylistic changes, discussions added, conclusions unchanged.
Supplementary materials and computer animations are available at
http://lattice.itep.ru/overla
Vacuum Condensates in the Global Color Symmetry Model
Based on the quark propagator in the instanton dilute liquid approximation,
we calculate analytically the quark condensate , the mixed quark
gluon condensate $g_{s}$ and the four quark
condensate at the mean field level in the
framework of global color symmetry model. The numerical calculation shows that
the values of these condensates are compatible with the ranges determined by
other nonperturbative approaches. Moreover, we find that for nonlocal four
quark condensate the previous vacuum saturation assumption is not a good
approximation even at the mean field level.Comment: 8 latex pages, no figure, Submitted to Phys. Rev.
Short distance current correlators: Comparing lattice simulations to the instanton liquid
Point to point correlators of currents are computed in quenched QCD using a
chiral lattice fermion action, the overlap action. I compare correlators made
of exact quark propagators with correlators restricted to low (less than 500
MeV) eigenvalue eigenmodes of the Dirac operator. In many cases they show
qualitative resemblence (typically at small values of the quark mass and
distances larger than 0.4 fm) and they differ qualitatively at larger quark
masses or at very short distance. Lattice results are in qualitative agreement
(and in the difference of vector and axial vector channels, quantitative
agreement) with the expectations of instanton liquid models. The scalar channel
shows the effects of a quenched finite volume zero mode artifact, a negative
correlator.Comment: 18 pages, Revtex, 11 postscript figures. Some changes. Version to
appear in Phys. Rev.
Integrability and Transcendentality
We derive the two-loop Bethe ansatz for the sl(2) twist operator sector of
N=4 gauge theory directly from the field theory. We then analyze a recently
proposed perturbative asymptotic all-loop Bethe ansatz in the limit of large
spacetime spin at large but finite twist, and find a novel all-loop scaling
function. This function obeys the Kotikov-Lipatov transcendentality principle
and does not depend on the twist. Under the assumption that one may extrapolate
back to leading twist, our result yields an all-loop prediction for the
large-spin anomalous dimensions of twist-two operators. The latter also appears
as an undetermined function in a recent conjecture of Bern, Dixon and Smirnov
for the all-loop structure of the maximally helicity violating (MHV) n-point
gluon amplitudes of N=4 gauge theory. This potentially establishes a direct
link between the worldsheet and the spacetime S-matrix approach. A further
assumption for the validity of our prediction is that perturbative BMN
(Berenstein-Maldacena-Nastase) scaling does not break down at four loops, or
beyond. We also discuss how the result gets modified if BMN scaling does break
down. Finally, we show that our result qualitatively agrees at strong coupling
with a prediction of string theory.Comment: 45 pages LaTeX, 3 postscript figures. v2: Chapter on BMN scaling and
transcendentality added. v3: version accepted for publication in JSTA
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