1,079 research outputs found
New Representations of the Perturbative S-Matrix
We propose a new framework to represent the perturbative S-matrix which is
well-defined for all quantum field theories of massless particles, constructed
from tree-level amplitudes and integrable term-by-term. This representation is
derived from the Feynman expansion through a series of partial fraction
identities, discarding terms that vanish upon integration. Loop integrands are
expressed in terms of "Q-cuts" that involve both off-shell and on-shell
loop-momenta, defined with a precise contour prescription that can be evaluated
by ordinary methods. This framework implies recent results found in the
scattering equation formalism at one-loop, and it has a natural extension to
all orders---even non-planar theories without well-defined forward limits or
good ultraviolet behavior.Comment: 4+1 pages, 4 figure
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
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
Heavy flavor diffusion in weakly coupled N=4 Super Yang-Mills theory
We use perturbation theory to compute the diffusion coefficient of a heavy
quark or scalar moving in N=4 SU(N_c) Super Yang-Mills plasma to leading order
in the coupling and the ratio T/M<<1. The result is compared both to recent
strong coupling calculations in the same theory and to the corresponding weak
coupling result in QCD. Finally, we present a compact and simple formulation of
the Lagrangian of our theory, N=4 SYM coupled to a massive fundamental N=2
hypermultiplet, which is well-suited for weak coupling expansions.Comment: 22 pages, 4 figures; v3: error corrected in calculations, figures and
discussion modified accordingl
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
Towards flavour diffusion coefficient and electrical conductivity without ultraviolet contamination
By subtracting from a recent lattice measurement of the thermal
vector-current correlator the known 5-loop vacuum contribution, we demonstrate
that the remainder is small and shows no visible short-distance divergence. It
can therefore in principle be subjected to model-independent analytic
continuation. Testing a particular implementation, we obtain estimates for the
flavour-diffusion coefficient (2 pi T D \gsim 0.8) and electrical conductivity
which are significantly smaller than previous results. Although systematic
errors remain beyond control at present, some aspects of our approach could be
of a wider applicability.Comment: 7 pages. v2: clarifications added, published versio
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