1,062 research outputs found
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
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
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
Integral method coefficients for the ring-core technique to evaluate non-uniform residual stresses
The ring-core technique allows for the determination of non-uniform residual stresses from the surface up to relatively higher depths as compared to the hole-drilling technique. The integral method, which is usually applied to hole-drilling, can also be used for elaborating the results of the ring-core test since these two experimental techniques share the axisymmetric geometry and the 0°â45°â90° layout of the strain gage rosette. The aim of this article is to provide accurate coefficients which can be used for evaluating the residual stress distribution by the ring-core integral method. The coefficients have been obtained by elaborating the results of a very refined plane harmonic axisymmetric finite element model and verified with an independent three-dimensional model. The coefficients for small depth steps were initially provided, and then the values for multiple integer step depths were also derived by manipulating the high-resolution coefficient matrices, thus showing how the present results can be practically used for obtaining the residual stresses according to different depth sequences, even non-uniform. This analysis also allowed the evaluation of the eccentricity effect which turned out to be negligible due to the symmetry of the problem. An applicative example was reported in which the input of the experimentally measured relaxed strains was elaborated with different depth resolutions, and the obtained residual stress distributions were compared
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
Holographic bulk viscosity: GPR vs EO
Recently Eling and Oz (EO) proposed a formula for the holographic bulk
viscosity, in arXiv:1103.1657, derived from the null horizon focusing equation.
This formula seems different from that obtained earlier by Gubser, Pufu and
Rocha (GPR) in arXiv:0806.0407 calculated from the IR limit of the two-point
function of the trace of the stress tensor. The two were shown to agree only
for some simple scaling cases. We point out that the two formulae agree in two
non-trivial holographic theories describing RG flows. The first is the strongly
coupled N=2* gauge theory plasma. The second is the semi-phenomenological model
of Improved Holographic QCD.Comment: 21 pages, 2 figure
X-Ray Diffraction and Hole-Drilling residual stress measurements of shot peening treatments validated on a calibration bench
The inverse problem of determining residual stresses from diffraction or relaxation methods is notoriously affected by a high sensitivity to errors in input data. A particular care must be devoted to ensuring that their input errors are minimized, and results shall come with a quantification of the corresponding uncertainties. Residual stress measurements are often validated by comparing the results of different techniques. Although this approach can strengthen the measurement confidence, it does not highlight potential biases of the methods. The authors presented a calibration bench [1, 2] which can impose a known bending distribution on a specimen while simultaneously performing an X-Ray Diffraction (XRD) or Hole-Drilling Method (HDM) residual stress measurement. Since the external load can freely be applied and removed, Buecknerâs superposition principle [3] can be exploited to simultaneously identify both the reference bending distribution and the actual residual stress distribution with the same experimental setup. As the first is accurately known, the bench provides a direct estimation of the achieved accuracy. Moreover, it can reveal systematic errors in the chosen procedures. Two shot peening treatments were analyzed on the calibration bench with both XRD and HDM. First, residual stresses on the surface were evaluated with XRD measurements, then electrochemical material removal was performed to investigate stresses at higher depths. After that, HDM measurements were carried out and compared with the results of XRD. Both methods were also used to identify the known bending stresses: that provided an additional validation of the residual stress results
Live Fast, Die Young: GMC lifetimes in the FIRE cosmological simulations of Milky Way-mass galaxies
We present the first measurement of the lifetimes of giant molecular clouds (GMCs) in cosmological simulations at z = 0, using the Latte suite of FIRE-2 simulations of Milky Way (MW) mass galaxies. We track GMCs with total gas mass âł10â” Mâ at high spatial (âŒ1 pc), mass (7100 Mâ), and temporal (1 Myr) resolution. Our simulated GMCs are consistent with the distribution of masses for massive GMCs in the MW and nearby galaxies. We find GMC lifetimes of 5â7 Myr, or 1â2 freefall times, on average, with less than 2 perâcent of clouds living longer than 20 Myr. We find decreasing GMC lifetimes with increasing virial parameter, and weakly increasing GMC lifetimes with galactocentric radius, implying that environment affects the evolutionary cycle of GMCs. However, our GMC lifetimes show no systematic dependence on GMC mass or amount of star formation. These results are broadly consistent with inferences from the literature and provide an initial investigation into ultimately understanding the physical processes that govern GMC lifetimes in a cosmological setting
Consistency Conditions on S-Matrix of Spin 1 Massless Particles
Motivated by new techniques in the computation of scattering amplitudes of
massless particles in four dimensions, like BCFW recursion relations, the
question of how much structure of the S-matrix can be determined from purely
S-matrix arguments has received new attention. The BCFW recursion relations for
massless particles of spin 1 and 2 imply that the whole tree-level S-matrix can
be determined in terms of three-particle amplitudes (evaluated at complex
momenta). However, the known proofs of the validity of the relations rely on
the Lagrangian of the theory, either by using Feynman diagrams explicitly or by
studying the effective theory at large complex momenta. This means that a
purely S-matrix theoretic proof of the relations is still missing. The aim of
this paper is to provide such a proof for spin 1 particles by extending the
four-particle test introduced by P. Benincasa and F. Cachazo in
arXiv:0705.4305[hep-th] to all particles. We show how n-particle tests imply
that the rational function built from the BCFW recursion relations possesses
all the correct factorization channels including holomorphic and
anti-holomorphic collinear limits. This in turn implies that they give the
correct S-matrix of the theory.Comment: 24 pages, 4 figure
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