959 research outputs found
Constructing the Tree-Level Yang-Mills S-Matrix Using Complex Factorization
A remarkable connection between BCFW recursion relations and constraints on
the S-matrix was made by Benincasa and Cachazo in 0705.4305, who noted that
mutual consistency of different BCFW constructions of four-particle amplitudes
generates non-trivial (but familiar) constraints on three-particle coupling
constants --- these include gauge invariance, the equivalence principle, and
the lack of non-trivial couplings for spins >2. These constraints can also be
derived with weaker assumptions, by demanding the existence of four-point
amplitudes that factorize properly in all unitarity limits with complex
momenta. From this starting point, we show that the BCFW prescription can be
interpreted as an algorithm for fully constructing a tree-level S-matrix, and
that complex factorization of general BCFW amplitudes follows from the
factorization of four-particle amplitudes. The allowed set of BCFW deformations
is identified, formulated entirely as a statement on the three-particle sector,
and using only complex factorization as a guide. Consequently, our analysis
based on the physical consistency of the S-matrix is entirely independent of
field theory. We analyze the case of pure Yang-Mills, and outline a proof for
gravity. For Yang-Mills, we also show that the well-known scaling behavior of
BCFW-deformed amplitudes at large z is a simple consequence of factorization.
For gravity, factorization in certain channels requires asymptotic behavior
~1/z^2.Comment: 35 pages, 6 figure
On the supergravity description of boost invariant conformal plasma at strong coupling
We study string theory duals of the expanding boost invariant conformal gauge
theory plasmas at strong coupling. The dual supergravity background is
constructed as an asymptotic late-time expansion, corresponding to
equilibration of the gauge theory plasma. The absence of curvature
singularities in the first few orders of the late-time expansion of the dual
gravitational background unambiguously determines the equilibrium equation of
the state, and the shear viscosity of the gauge theory plasma. While the
absence of the leading pole singularities in the gravitational curvature
invariants at the third order in late-time expansion determines the relaxation
time of the plasma, the subleading logarithmic singularity can not be canceled
within a supergravity approximation. Thus, a supergravity approximation to a
dual description of the strongly coupled boost invariant expanding plasma is
inconsistent. Nevertheless we find that the relaxation time determined from
cancellation of pole singularities is quite robust.Comment: 26 pages, no figures; v2: references adde
Transport properties of N=4 supersymmetric Yang-Mills theory at finite coupling
Gauge theory-string theory duality describes strongly coupled N=4
supersymmetric SU(n) Yang-Mills theory at finite temperature in terms of near
extremal black 3-brane geometry in type IIB string theory. We use this
correspondence to compute the leading correction in inverse 't Hooft coupling
to the shear diffusion constant, bulk viscosity and the speed of sound in the
large-n N=4 supersymmetric Yang-Mills theory plasma. The transport coefficients
are extracted from the dispersion relation for the shear and the sound wave
lowest quasinormal modes in the leading order alpha'-corrected black D3 brane
geometry. We find the shear viscosity extracted from the shear diffusion
constant to agree with result of [hep-th/0406264]; also, the leading correction
to bulk viscosity and the speed of sound vanishes. Our computation provides a
highly nontrivial consistency check on the hydrodynamic description of the
alpha'-corrected nonextremal black branes in string theory.Comment: 19 pages, LaTe
A Superbubble Feedback Model for Galaxy Simulations
We present a new stellar feedback model that reproduces superbubbles.
Superbubbles from clustered young stars evolve quite differently to individual
supernovae and are substantially more efficient at generating gas motions. The
essential new components of the model are thermal conduction, sub-grid
evaporation and a sub-grid multi-phase treatment for cases where the simulation
mass resolution is insufficient to model the early stages of the superbubble.
The multi-phase stage is short compared to superbubble lifetimes. Thermal
conduction physically regulates the hot gas mass without requiring a free
parameter. Accurately following the hot component naturally avoids overcooling.
Prior approaches tend to heat too much mass, leaving the hot ISM below K
and susceptible to rapid cooling unless ad-hoc fixes were used. The hot phase
also allows feedback energy to correctly accumulate from multiple, clustered
sources, including stellar winds and supernovae.
We employ high-resolution simulations of a single star cluster to show the
model is insensitive to numerical resolution, unresolved ISM structure and
suppression of conduction by magnetic fields. We also simulate a Milky Way
analog and a dwarf galaxy. Both galaxies show regulated star formation and
produce strong outflows.Comment: 13 pages, 13 figures; replaced with version accepted to MNRA
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
Note on graviton MHV amplitudes
Two new formulas which express n-graviton MHV tree amplitudes in terms of
sums of squares of n-gluon amplitudes are discussed. The first formula is
derived from recursion relations. The second formula, simpler because it
involves fewer permutations, is obtained from the variant of the Berends,
Giele, Kuijf formula given in Arxiv:0707.1035.Comment: 10 page
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