628 research outputs found
On Tree Amplitudes in Gauge Theory and Gravity
The BCFW recursion relations provide a powerful way to compute tree
amplitudes in gauge theories and gravity, but only hold if some amplitudes
vanish when two of the momenta are taken to infinity in a particular complex
direction. This is a very surprising property, since individual Feynman
diagrams all diverge at infinite momentum. In this paper we give a simple
physical understanding of amplitudes in this limit, which corresponds to a hard
particle with (complex) light-like momentum moving in a soft background, and
can be conveniently studied using the background field method exploiting
background light-cone gauge. An important role is played by enhanced spin
symmetries at infinite momentum--a single copy of a "Lorentz" group for gauge
theory and two copies for gravity--which together with Ward identities give a
systematic expansion for amplitudes at large momentum. We use this to study
tree amplitudes in a wide variety of theories, and in particular demonstrate
that certain pure gauge and gravity amplitudes do vanish at infinity. Thus the
BCFW recursion relations can be used to compute completely general gluon and
graviton tree amplitudes in any number of dimensions. We briefly comment on the
implications of these results for computing massive 4D amplitudes by KK
reduction, as well understanding the unexpected cancelations that have recently
been found in loop-level gravity amplitudes.Comment: 22 pages, 3 figure
Effects of sub-optimal temperatures on seed germination of three warm-season turfgrasses with perspectives of cultivation in transition zone
Warm-season turfgrass species prevail in tropical and subtropical areas, but can also be grown in the transition zone. In this case, cold tolerance is a key aspect for germination and successful turfgrass establishment. The germination response to sub-optimal temperatures was investigated for Cynodon dactylon (cvs Jackpot, La Paloma, Transcontinental, Yukon, Riviera), Buchloe dactyloides (cv SWI 2000) and Paspalum vaginatum (cv Pure Dynasty). Four temperature regimes were applied, i.e., 20/30 °C, 15/25 °C, 10/20 °C and 5/15 °C, with a 12:12 h (light:dark) photoperiod. Germination assays were performed twice, with six replicates (Petri dishes) per treatment in each experiment, fifty seeds per dish. The final germinated percentages at last inspection time (FGP) were obtained for each Petri dish and processed by using a generalized linear mixed model (binomial error and logit link). Germination curves were fitted to each Petri dish by using time-to-event methods and germination rates (GR) for the 10th, 20th and 30th percentiles were derived and used to fit a linear thermal-time model. For all cultivars, FGP decreased with decreasing mean daily temperatures. Base temperatures (Tb) ranged between 11.4 °C and 17.0 °C, while the thermal time to obtain 30% germination ranged from 51.3 °C day for SWI 2000 to 144.0 °C day for Pure Dynasty. The estimated parameters were used to predict germination time in the field, considering the observed soil temperatures in Legnaro. The estimated date for the beginning of germination in the field would range from early April for SWI 2000 and Transcontinental to mid-May for Riviera. These results might be used as a practical support for planning spring sowing, which is crucial for successful turfgrass establishment, especially without irrigation
Effective action for Einstein-Maxwell theory at order RF**4
We use a recently derived integral representation of the one-loop effective
action in Einstein-Maxwell theory for an explicit calculation of the part of
the effective action containing the information on the low energy limit of the
five-point amplitudes involving one graviton, four photons and either a scalar
or spinor loop. All available identities are used to get the result into a
relatively compact form.Comment: 13 pages, no figure
The Viscosity Bound Conjecture and Hydrodynamics of M2-Brane Theory at Finite Chemical Potential
Kovtun, Son and Starinets have conjectured that the viscosity to entropy
density ratio is always bounded from below by a universal multiple of
i.e., for all forms of matter. Mysteriously, the
proposed viscosity bound appears to be saturated in all computations done
whenever a supergravity dual is available. We consider the near horizon limit
of a stack of M2-branes in the grand canonical ensemble at finite R-charge
densities, corresponding to non-zero angular momentum in the bulk. The
corresponding four-dimensional R-charged black hole in Anti-de Sitter space
provides a holographic dual in which various transport coefficients can be
calculated. We find that the shear viscosity increases as soon as a background
R-charge density is turned on. We numerically compute the few first corrections
to the shear viscosity to entropy density ratio and surprisingly
discover that up to fourth order all corrections originating from a non-zero
chemical potential vanish, leaving the bound saturated. This is a sharp signal
in favor of the saturation of the viscosity bound for event horizons even in
the presence of some finite background field strength. We discuss implications
of this observation for the conjectured bound.Comment: LaTeX, 26+1 Pages, 4 Figures, Version 2: references adde
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