7,415 research outputs found
Loop integration results using numerical extrapolation for a non-scalar integral
Loop integration results have been obtained using numerical integration and
extrapolation. An extrapolation to the limit is performed with respect to a
parameter in the integrand which tends to zero. Results are given for a
non-scalar four-point diagram. Extensions to accommodate loop integration by
existing integration packages are also discussed. These include: using
previously generated partitions of the domain and roundoff error guards.Comment: 4 pages, 3 figures, revised, contribution to ACAT03 (Dec. 2003
GRACE at ONE-LOOP: Automatic calculation of 1-loop diagrams in the electroweak theory with gauge parameter independence checks
We describe the main building blocks of a generic automated package for the
calculation of Feynman diagrams. These blocks include the generation and
creation of a model file, the graph generation, the symbolic calculation at an
intermediate level of the Dirac and tensor algebra, implementation of the loop
integrals, the generation of the matrix elements or helicity amplitudes,
methods for the phase space integrations and eventually the event generation.
The report focuses on the fully automated systems for the calculation of
physical processes based on the experience in developing GRACE-loop. As such, a
detailed description of the renormalisation procedure in the Standard Model is
given emphasizing the central role played by the non-linear gauge fixing
conditions for the construction of such automated codes. The need for such
gauges is better appreciated when it comes to devising efficient and powerful
algorithms for the reduction of the tensorial structures of the loop integrals.
A new technique for these reduction algorithms is described. Explicit formulae
for all two-point functions in a generalised non-linear gauge are given,
together with the complete set of counterterms. We also show how infrared
divergences are dealt with in the system. We give a comprehensive presentation
of some systematic test-runs which have been performed at the one-loop level
for a wide variety of two-to-two processes to show the validity of the gauge
check. These cover fermion-fermion scattering, gauge boson scattering into
fermions, gauge bosons and Higgs bosons scattering processes. Comparisons with
existing results on some one-loop computation in the Standard Model show
excellent agreement. We also briefly recount some recent development concerning
the calculation of mutli-leg one-loop corrections.Comment: 131 pages. Manuscript expanded quite substantially with the inclusion
of an overview of automatic systems for the calculation of Feynman diagrams
both at tree-level and one-loop. Other additions include issues of
regularisation, width effects and renormalisation with unstable particles and
reduction of 5- and 6-point functions. This is a preprint version, final
version to appear as a Phys. Re
Spectroscopic Constraints on the Surface Magnetic Field of the Accreting Neutron Star EXO 0748-676
Gravitationally redshifted absorption lines of Fe XXVI, Fe XXV, and O VIII
were inferred recently in the X-ray spectrum of the bursting neutron star EXO
0748-676. We place an upper limit on the stellar magnetic field based on the
iron lines. The oxygen absorption feature shows a multiple component profile
that is consistent with Zeeman splitting in a magnetic field of ~(1-2)x10^9
gauss, and for which the corresponding Zeeman components of the iron lines are
expected to be blended together. In other systems, a field strength >5x10^{10}
gauss could induce a blueshift of the line centroids that would counteract
gravitational redshift and complicate the derivation of constraints on the
equation of state of the neutron star.Comment: 5 pages, submitted to Phys. Rev. Let
Cohomologically hyperbolic endomorphisms of complex manifolds
We show that if a compact Kahler manifold X admits a cohomologically
hyperbolic surjective endomorphism then its Kodaira dimension is non-positive.
This gives an affirmative answer to a conjecture of Guedj in the holomorphic
case. The main part of the paper is to determine the geometric structure and
the fundamental groups (up to finite index) for those X of dimension 3.Comment: International Journal of Mathematics (to appear
Kondo Problem and Related One-Dimensional Quantum Systems: Bethe Ansatz Solution and Boundary Conformal Field Theory
We review some exact results on Kondo impurity systems derived from
Bethe-ansatz solutions and boundary conformal field theory with particular
emphasis on universal aspects of the phenomenon. The finite-size spectra
characterizing the low-energy fixed point are computed from the Bethe-ansatz
solutions of various models related to the Kondo problem. Using the finite-size
scaling argument, we investigate their exact critical properties. We also
discuss that a universal relation between the Kondo effect and the impurity
effect in one-dimensional quantum systems usefully expedites our understanding
of these different phenomena.Comment: 6 pages, no figure
Full one-loop electroweak radiative corrections to single Higgs production in e+ e-
We present the full electroweak radiative corrections to
single Higgs production in \epemt. This takes into account the full one-loop
corrections as well as the effects of hard photon radiation. We include both
the fusion and Higgs-strahlung processes. The computation is performed with the
help of {\tt GRACE-loop} where we have implemented a generalised non-linear
gauge fixing condition. The latter includes 5 gauge parameters that can be used
for checks on our results. Besides the UV, IR finiteness and gauge parameter
independence checks it proves also powerful to test our implementation of the
5-point function. We find that for a 500GeV machine and a light Higgs of mass
150GeV, the total correction is small when the results are
expressed in terms of . The total correction decreases
slightly for higher energies. For moderate centre of mass energies the total
decreases as the Higgs mass increases, reaching -10% for
GeV and GeV. In order to quantify the genuine weak
corrections we have subtracted the universal virtual and bremsstrahlung
correction from the full . We find, for GeV, a
weak correction slowly decreasing from -2% to -4% as the energy increases from
GeV to TeV after expressing the tree-level results in
terms of Comment: 16 pages, 3 figures. Only correction is a reference to a web-pag
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