938 research outputs found
The tensor reduction and master integrals of the two-loop massless crossed box with light-like legs
The class of the two-loop massless crossed boxes, with light-like external
legs, is the final unresolved issue in the program of computing the scattering
amplitudes of 2 --> 2 massless particles at next-to-next-to-leading order.
In this paper, we describe an algorithm for the tensor reduction of such
diagrams. After connecting tensor integrals to scalar ones with arbitrary
powers of propagators in higher dimensions, we derive recurrence relations from
integration-by-parts and Lorentz-invariance identities, that allow us to write
the scalar integrals as a combination of two master crossed boxes plus
simpler-topology diagrams.
We derive the system of differential equations that the two master integrals
satisfy using two different methods, and we use one of these equations to
express the second master integral as a function of the first one, already
known in the literature. We then give the analytic expansion of the second
master integral as a function of epsilon=(4-D)/2, where D is the space-time
dimension, up to order O(epsilon^0).Comment: 30 pages, 5 figure
Conductance of tubular nanowires with disorder
We calculate the conductance of tubular-shaped nanowires having many
potential scatterers at random positions. Our approach is based on the
scattering matrix formalism and our results analyzed within the scaling theory
of disordered conductors. When increasing the energy the conductance for a big
enough number of impurities in the tube manifests a systematic evolution from
the localized to the metallic regimes. Nevertheless, a conspicuous drop in
conductance is predicted whenever a new transverse channel is open. Comparison
with the semiclassical calculation leading to purely ohmic behavior is made.Comment: 8 pages, 5 figure
Field Theoretical Quantum Effects on the Kerr Geometry
We study quantum aspects of the Einstein gravity with one time-like and one
space-like Killing vector commuting with each other. The theory is formulated
as a \coset nonlinear -model coupled to gravity. The quantum analysis
of the nonlinear -model part, which includes all the dynamical degrees
of freedom, can be carried out in a parallel way to ordinary nonlinear
-models in spite of the existence of an unusual coupling. This means
that we can investigate consistently the quantum properties of the Einstein
gravity, though we are limited to the fluctuations depending only on two
coordinates. We find the forms of the beta functions to all orders up to
numerical coefficients. Finally we consider the quantum effects of the
renormalization on the Kerr black hole as an example. It turns out that the
asymptotically flat region remains intact and stable, while, in a certain
approximation, it is shown that the inner geometry changes considerably however
small the quantum effects may be.Comment: 16 pages, LaTeX. The hep-th number added on the cover, and minor
typos correcte
ac-Field-Controlled Anderson Localization in Disordered Semiconductor Superlattices
An ac field, tuned exactly to resonance with the Stark ladder in an ideal
tight binding lattice under strong dc bias, counteracts Wannier-Stark
localization and leads to the emergence of extended Floquet states. If there is
random disorder, these states localize. The localization lengths depend
non-monotonically on the ac field amplitude and become essentially zero at
certain parameters. This effect is of possible relevance for characterizing the
quality of superlattice samples, and for performing experiments on Anderson
localization in systems with well-defined disorder.Comment: 10 pages, Latex; figures available on request from [email protected]
Fitotoxicidade causada por herbicidas na fase inicial de desenvolvimento da cultura do sorgo
Energetics, forces, and quantized conductance in jellium modeled metallic nanowires
Energetics and quantized conductance in jellium modeled nanowires are
investigated using the local density functional based shell correction method,
extending our previous study of uniform in shape wires [C. Yannouleas and U.
Landman, J. Phys. Chem. B 101, 5780 (1997)] to wires containing a variable
shaped constricted region. The energetics of the wire (sodium) as a function of
the length of the volume conserving, adiabatically shaped constriction leads to
formation of self selecting magic wire configurations. The variations in the
energy result in oscillations in the force required to elongate the wire and
are directly correlated with the stepwise variations of the conductance of the
nanowire in units of 2e^2/h. The oscillatory patterns in the energetics and
forces, and the correlated stepwise variation in the conductance are shown,
numerically and through a semiclassical analysis, to be dominated by the
quantized spectrum of the transverse states at the narrowmost part of the
constriction in the wire.Comment: Latex/Revtex, 11 pages with 5 Postscript figure
Dimensional Crossover of Localisation and Delocalisation in a Quantum Hall Bar
The 2-- to 1--dimensional crossover of the localisation length of electrons
confined to a disordered quantum wire of finite width is studied in a
model of electrons moving in the potential of uncorrelated impurities. An
analytical formula for the localisation length is derived, describing the
dimensional crossover as function of width , conductance and
perpendicular magnetic field . On the basis of these results, the scaling
analysis of the quantum Hall effect in high Landau levels, and the
delocalisation transition in a quantum Hall wire are reconsidered.Comment: 12 pages, 7 figure
Isolating a light Higgs boson from the di-photon background at the LHC
We compute the QCD corrections to the gluon fusion subprocess gg to gamma
gamma, which forms an important component of the background to the search for a
light Higgs boson at the LHC. We study the dependence of the improved pp to
gamma gamma X background calculation on the factorization and renormalization
scales, on various choices for photon isolation cuts, and on the rapidities of
the photons. We also investigate ways to enhance the statistical significance
of the Higgs signal in the di-photon channel.Comment: Additional reference included, 17 pages, 16 figure files, revte
Observing the First Stars and Black Holes
The high sensitivity of JWST will open a new window on the end of the
cosmological dark ages. Small stellar clusters, with a stellar mass of several
10^6 M_sun, and low-mass black holes (BHs), with a mass of several 10^5 M_sun
should be directly detectable out to redshift z=10, and individual supernovae
(SNe) and gamma ray burst (GRB) afterglows are bright enough to be visible
beyond this redshift. Dense primordial gas, in the process of collapsing from
large scales to form protogalaxies, may also be possible to image through
diffuse recombination line emission, possibly even before stars or BHs are
formed. In this article, I discuss the key physical processes that are expected
to have determined the sizes of the first star-clusters and black holes, and
the prospect of studying these objects by direct detections with JWST and with
other instruments. The direct light emitted by the very first stellar clusters
and intermediate-mass black holes at z>10 will likely fall below JWST's
detection threshold. However, JWST could reveal a decline at the faint-end of
the high-redshift luminosity function, and thereby shed light on radiative and
other feedback effects that operate at these early epochs. JWST will also have
the sensitivity to detect individual SNe from beyond z=10. In a dedicated
survey lasting for several weeks, thousands of SNe could be detected at z>6,
with a redshift distribution extending to the formation of the very first stars
at z>15. Using these SNe as tracers may be the only method to map out the
earliest stages of the cosmic star-formation history. Finally, we point out
that studying the earliest objects at high redshift will also offer a new
window on the primordial power spectrum, on 100 times smaller scales than
probed by current large-scale structure data.Comment: Invited contribution to "Astrophysics in the Next Decade: JWST and
Concurrent Facilities", Astrophysics & Space Science Library, Eds. H.
Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008
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