1,014 research outputs found
Collinear and Soft Limits of Multi-Loop Integrands in N=4 Yang-Mills
It has been argued in arXiv:1112.6432 that the planar four-point integrand in
N=4 super Yang-Mills theory is uniquely determined by dual conformal invariance
together with the absence of a double pole in the integrand of the logarithm in
the limit as a loop integration variable becomes collinear with an external
momentum. In this paper we reformulate this condition in a simple way in terms
of the amplitude itself, rather than its logarithm, and verify that it holds
for two- and three-loop MHV integrands for n>4. We investigate the extent to
which this collinear constraint and a constraint on the soft behavior of
integrands can be used to determine integrands. We find an interesting
complementarity whereby the soft constraint becomes stronger while the
collinear constraint becomes weaker at larger n. For certain reasonable choices
of basis at two and three loops the two constraints in unison appear strong
enough to determine MHV integrands uniquely for all n.Comment: 27 pages, 14 figures; v2: very minor change
Colour-electric spectral function at next-to-leading order
The spectral function related to the correlator of two colour-electric fields
along a Polyakov loop determines the momentum diffusion coefficient of a heavy
quark near rest with respect to a heat bath. We compute this spectral function
at next-to-leading order, O(alpha_s^2), in the weak-coupling expansion. The
high-frequency part of our result (omega >> T), which is shown to be
temperature-independent, is accurately determined thanks to asymptotic freedom;
the low-frequency part of our result (omega << T), in which Hard Thermal Loop
resummation is needed in order to cure infrared divergences, agrees with a
previously determined expression. Our result may help to calibrate the overall
normalization of a lattice-extracted spectral function in a perturbative
frequency domain T << omega << 1/a, paving the way for a non-perturbative
estimate of the momentum diffusion coefficient at omega -> 0. We also evaluate
the colour-electric Euclidean correlator, which could be directly compared with
lattice simulations. As an aside we determine the Euclidean correlator in the
lattice strong-coupling expansion, showing that through a limiting procedure it
can in principle be defined also in the confined phase of pure Yang-Mills
theory, even if a practical measurement could be very noisy there.Comment: 38 page
Gross community production and metabolic balance in the South Pacific Gyre, using a non intrusive bio-optical method
The very clear waters of the South Pacific Gyre likely constitute an end-member of oligotrophic conditions which remain essentially unknown with respect to its impact on carbon fixation and exportation. We describe a non-intrusive bio-optical method to quantify the various terms of a production budget (Gross community production, community losses, net community production) in this area. This method is based on the analysis of the diel cycle in Particulate Organic Carbon (POC), derived from high frequency measurements of the particle attenuation coefficient <i>c</i><sub>p</sub>. We report very high integrated rates of Gross Community Production within the euphotic layer (average of 846&plusmn;484 mg C m<sup>&minus;2</sup> d<sup>&minus;1</sup> for 17 stations) that are far above any rates determined using incubation techniques for such areas. Furthermore we show that the daily production of POC is essentially balanced by the losses so that the system cannot be considered as net heterotrophic. Our results thus agree well with geochemical methods, but not with incubation studies based on oxygen methods. We stress to the important role of deep layers, below the euphotic layer, in contributing to carbon fixation when incident irradiance at the ocean surface is high (absence of cloud coverage). These deep layers, not considered up to know, might fuel part of the heterotrophic processes in the upper layer, including through dissolved organic carbon. We further demonstrate that, in these extremely clear and stratified waters, integrated gross community production is proportional to the POC content and surface irradiance via an efficiency index &psi; <sub>GCP</sub><sup>*</sup>, the water column cross section for Gross Community Production. We finally discuss our results in the context of the role of oligotrophic gyre in the global carbon budget and of the possibility of using optical proxies from space for the development of growth community rather than primary production global models
Twistors, Harmonics and Holomorphic Chern-Simons
We show that the off-shell N=3 action of N=4 super Yang-Mills can be written
as a holomorphic Chern-Simons action whose Dolbeault operator is constructed
from a complex-real (CR) structure of harmonic space. We also show that the
local space-time operators can be written as a Penrose transform on the coset
SU(3)/(U(1) \times U(1)). We observe a strong similarity to ambitwistor space
constructions.Comment: 34 pages, 3 figures, v2: replaced with published version, v3: Added
referenc
Multi-Regge kinematics and the moduli space of Riemann spheres with marked points
We show that scattering amplitudes in planar N = 4 Super Yang-Mills in
multi-Regge kinematics can naturally be expressed in terms of single-valued
iterated integrals on the moduli space of Riemann spheres with marked points.
As a consequence, scattering amplitudes in this limit can be expressed as
convolutions that can easily be computed using Stokes' theorem. We apply this
framework to MHV amplitudes to leading-logarithmic accuracy (LLA), and we prove
that at L loops all MHV amplitudes are determined by amplitudes with up to L +
4 external legs. We also investigate non-MHV amplitudes, and we show that they
can be obtained by convoluting the MHV results with a certain helicity flip
kernel. We classify all leading singularities that appear at LLA in the Regge
limit for arbitrary helicity configurations and any number of external legs.
Finally, we use our new framework to obtain explicit analytic results at LLA
for all MHV amplitudes up to five loops and all non-MHV amplitudes with up to
eight external legs and four loops.Comment: 104 pages, six awesome figures and ancillary files containing the
results in Mathematica forma
Heavy Quark diffusion from lattice QCD spectral functions
We analyze the low frequency part of charmonium spectral functions on large
lattices close to the continuum limit in the temperature region as well as for . We present evidence for the
existence of a transport peak above and its absence below . The
heavy quark diffusion constant is then estimated using the Kubo formula. As
part of the calculation we also determine the temperature dependence of the
signature for the charmonium bound state in the spectral function and discuss
the fate of charmonium states in the hot medium.Comment: 4 pages, Proceedings for Quark Matter 2011 Conference, May 23-28,
2011, Annecy, Franc
Model and parameter dependence of heavy quark energy loss in a hot and dense medium
Within the framework of the Langevin equation, we study the energy loss of
heavy quark due to quasi-elastic multiple scatterings in a quark-gluon plasma
created by relativistic heavy-ion collisions. We investigate how the initial
configuration of the quark-gluon plasma as well as its properties affect the
final state spectra and elliptic flow of D meson and non-photonic electron. We
find that both the geometric anisotropy of the initial quark-gluon plasma and
the flow profiles of the hydrodynamic medium play important roles in the heavy
quark energy loss process and the development of elliptic flow. The relative
contribution from charm and bottom quarks is found to affect the transverse
momentum dependence of the quenching and flow patterns of heavy flavor decay
electron; such influence depends on the interaction strength between heavy
quark and the medium.Comment: 16 pages, 7 figure
Structure and Thermodynamics of the Mixed Alkali Alanates
The thermodynamics and structural properties of the hexahydride alanates
(M2M'AlH6) with the elpasolite structure have been investigated. A series of
mixed alkali alanates (Na2LiAlH6, K2LiAlH6 and K2NaAlH6) were synthesized and
found to reversibly absorb and desorb hydrogen without the need for a catalyst.
Pressure-composition isotherms were measured to investigate the thermodynamics
of the absorption and desorption reactions with hydrogen. Isotherms for
catalyzed (4 mol% TiCl3) and uncatalyzed Na2LiAlH6 exhibited an increase in
kinetics, but no change in the bulk thermodynamics with the addition of a
dopant. A structural analysis using synchrotron x-ray diffraction showed that
these compounds favor the Fm-3m space group with the smaller ion (M') occupying
an octahedral site. These results demonstrate that appropriate cation
substitutions can be used to stabilize or destabilize the material and may
provide an avenue to improving the unfavorable thermodynamics of a number of
materials with promising gravimetric hydrogen densities.Comment: 6 pages, 7 figures,3 tables, submitted to PR
The distribution and movement patterns of four woodland caribou herds in Quebec and Labrador
Recent studies of woodland caribou (Rangifer tarandus caribou) in northern Quebec and central Labrador have demonstrated similar patterns of seasonal movements and distribution among four herds. Aerial surveys and radio-telemetry indicated that animals occupied forest-wetland habitat at densities of 0.03 caribou km2, or lower, for most of the year. Although females were widely dispersed at calving individuals demonstrated fidelity toward specific calving locations, in successive years. Caribou did not form large post-calving aggregations. Movement was greatest in the spring, prior to calving, and in the fall, during or immediately after rutting. Caribou were generally sedentary during summer and winter, although some moved relatively long distances to late-winter range. Although the herds occupy continuous range across Quebec and Labrador, our data indicate that the herds are largely discreete and should be managed individually
Holographic dilepton production in a thermalizing plasma
We determine the out-of-equilibrium production rate of dileptons at rest in
strongly coupled N=4 Super Yang-Mills plasma using the AdS/CFT correspondence.
Thermalization is achieved via the gravitational collapse of a thin shell of
matter in AdS_5 space and the subsequent formation of a black hole, which we
describe in a quasistatic approximation. Prior to thermalization, the dilepton
spectral function is observed to oscillate as a function of frequency, but the
amplitude of the oscillations decreases when thermal equilibrium is approached.
At the same time, we follow the flow of the quasinormal spectrum of the
corresponding U(1) vector field towards its equilibrium limit.Comment: 21 pages, 7 figures. v2: Version accepted for publication in JHEP;
minor modifications, added reference
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