379 research outputs found
Recommended from our members
Climate variability and climate change vulnerability and adaptation. Workshop summary
Representatives from fifteen countries met in Prague, Czech Republic, on September 11-15, 1995, to share results from the analysis of vulnerability and adaptation to global climate change. The workshop focused on the issues of global climate change and its impacts on various sectors of a national economy. The U.N. Framework Convention on Climate Change (FCCC), which has been signed by more than 150 governments worldwide, calls on signatory parties to develop and communicate measures they are implementing to respond to global climate change. An analysis of a country`s vulnerability to changes in the climate helps it identify suitable adaptation measures. These analyses are designed to determine the extent of the impacts of global climate change on sensitive sectors such as agricultural crops, forests, grasslands and livestock, water resources, and coastal areas. Once it is determined how vulnerable a country may be to climate change, it is possible to identify adaptation measures for ameliorating some or all of the effects.The objectives of the vulnerability and adaptation workshop were to: The objectives of the vulnerability and adaptation workshop were to: Provide an opportunity for countries to describe their study results; Encourage countries to learn from the experience of the more complete assessments and adjust their studies accordingly; Identify issues and analyses that require further investigation; and Summarize results and experiences for governmental and intergovernmental organizations
Amplitudes and Spinor-Helicity in Six Dimensions
The spinor-helicity formalism has become an invaluable tool for understanding
the S-matrix of massless particles in four dimensions. In this paper we
construct a spinor-helicity formalism in six dimensions, and apply it to derive
compact expressions for the three, four and five point tree amplitudes of
Yang-Mills theory. Using the KLT relations, it is a straightforward process to
obtain amplitudes in linearized gravity from these Yang-Mills amplitudes; we
demonstrate this by writing down the gravitational three and four point
amplitudes. Because there is no conserved helicity in six dimensions, these
amplitudes describe the scattering of all possible polarization states (as well
as Kaluza-Klein excitations) in four dimensions upon dimensional reduction. We
also briefly discuss a convenient formulation of the BCFW recursion relations
in higher dimensions.Comment: 26 pages, 2 figures. Minor improvements of the discussio
Coulomb Phase Gluon Scattering at Strong Coupling
We calculate corrections to gluon scattering amplitudes in a Coulomb phase
using gauge/string duality. The Coulomb phase considered is a maximal rank
breaking of . This problem
therefore has 3 scales involved: 1) the scale of the massive fields
arising from the spontaneous breaking of the gauge group; 2) The scale of the
scattering, characterized by the Mandelstam variables ; 3) The IR
regulator . We find corrections in the hard scattering limit , and also find below threshold corrections
with . We find that the corrections in the second case
are finite, and so are IR regulator independent.Comment: 17+17 pages, 3 figure
Two-Loop g -> gg Splitting Amplitudes in QCD
Splitting amplitudes are universal functions governing the collinear behavior
of scattering amplitudes for massless particles. We compute the two-loop g ->
gg splitting amplitudes in QCD, N=1, and N=4 super-Yang-Mills theories, which
describe the limits of two-loop n-point amplitudes where two gluon momenta
become parallel. They also represent an ingredient in a direct x-space
computation of DGLAP evolution kernels at next-to-next-to-leading order. To
obtain the splitting amplitudes, we use the unitarity sewing method. In
contrast to the usual light-cone gauge treatment, our calculation does not rely
on the principal-value or Mandelstam-Leibbrandt prescriptions, even though the
loop integrals contain some of the denominators typically encountered in
light-cone gauge. We reduce the integrals to a set of 13 master integrals using
integration-by-parts and Lorentz invariance identities. The master integrals
are computed with the aid of differential equations in the splitting momentum
fraction z. The epsilon-poles of the splitting amplitudes are consistent with a
formula due to Catani for the infrared singularities of two-loop scattering
amplitudes. This consistency essentially provides an inductive proof of
Catani's formula, as well as an ansatz for previously-unknown 1/epsilon pole
terms having non-trivial color structure. Finite terms in the splitting
amplitudes determine the collinear behavior of finite remainders in this
formula.Comment: 100 pages, 33 figures. Added remarks about leading-transcendentality
argument of hep-th/0404092, and additional explanation of cut-reconstruction
uniquenes
Black Hole Entropy Associated with Supersymmetric Sigma Model
By means of an identity that equates elliptic genus partition function of a
supersymmetric sigma model on the -fold symmetric product of
(, is the symmetric group of elements) to the
partition function of a second quantized string theory, we derive the
asymptotic expansion of the partition function as well as the asymptotic for
the degeneracy of spectrum in string theory. The asymptotic expansion for the
state counting reproduces the logarithmic correction to the black hole entropy.Comment: 11 pages, no figures, version to appear in the Phys. Rev. D (2003
Unweighted event generation in hadronic WZ production at order(alpha_S)
We present an algorithm for unweighted event generation in the partonic
process pp -> WZ (j) with leptonic decays at next-to-leading order in alpha_S.
Monte Carlo programs for processes such as this frequently generate events with
negative weights in certain regions of phase space. For simulations of
experimental data one would like to have unweighted events only. We demonstrate
how the phase space from the matrix elements can be combined to achieve
unweighted event generation using a second stage Monte Carlo integration over a
volume of real emissions (jets). Observable quantities are kept fixed in the
laboratory frame throughout the integration. The algorithm is applicable to a
broader class of processes and is CPU intensive.Comment: 9 pages, 4 figure
Direct Extraction Of One Loop Rational Terms
We present a method for the direct extraction of rational contributions to
one-loop scattering amplitudes, missed by standard four-dimensional unitarity
techniques. We use generalised unitarity in D=4-2\e dimensions to write the
loop amplitudes in terms of products of massive tree amplitudes. We find that
the rational terms in 4-2\e dimensions can be determined from quadruple,
triple and double cuts without the need for independent pentagon contributions
using a massive integral basis. The additional mass-dependent integral
coefficients may then be extracted from the large mass limit which can be
performed analytically or numerically. We check the method by computing the
rational parts of all gluon helicity amplitudes with up to six external legs.
We also present a simple application to amplitudes with external massless
fermions.Comment: 35 pages, 6 figures. Major revisions: new analytic results for gluon
amplitudes and new section on treatment of massless fermions. References
added and typos corrected. Accepted for publication in JHE
Complete Order alpha_s^3 Results for e^+ e^- to (gamma,Z) to Four Jets
We present the next-to-leading order (O(alpha_s^3)) perturbative QCD
predictions for e^+e^- annihilation into four jets. A previous calculation
omitted the O(alpha_s^3) terms suppressed by one or more powers of 1/N_c^2,
where N_c is the number of colors, and the `light-by-glue scattering'
contributions. We find that all such terms are uniformly small, constituting
less than 10% of the correction. For the Durham clustering algorithm, the
leading and next-to-leading logarithms in the limit of small jet resolution
parameter y_{cut} can be resummed. We match the resummed results to our
fixed-order calculation in order to improve the small y_{cut} prediction.Comment: Latex2e, 17 pages with 5 encapsulated figures. Note added regarding
subsequent related work. To appear in Phys. Rev.
Scattering amplitudes with massive fermions using BCFW recursion
We study the QCD scattering amplitudes for \bar{q}q \to gg and \bar{q}q \to
ggg where q is a massive fermion. Using a particular choice of massive fermion
spinor we are able to derive very compact expressions for the partial spin
amplitudes for the 2 \to 2 process. We then investigate the corresponding 2 \to
3 amplitudes using the BCFW recursion technique. For the helicity conserving
partial amplitudes we again derive very compact expressions, but were unable to
treat the helicity-flip amplitudes recursively, except for the case where all
the gluon helicities are the same. We therefore evaluate the remaining partial
amplitudes using standard Feynman diagram techniques.Comment: 21 page
- âŠ