109 research outputs found
Tensorial Reconstruction at the Integrand Level
We present a new approach to the reduction of one-loop amplitudes obtained by
reconstructing the tensorial expression of the scattering amplitudes. The
reconstruction is performed at the integrand level by means of a sampling in
the integration momentum. There are several interesting applications of this
novel method within existing techniques for the reduction of one-loop multi-leg
amplitudes: to deal with numerically unstable points, such as in the vicinity
of a vanishing Gram determinant; to allow for a sampling of the numerator
function based on real values of the integration momentum; to optimize the
numerical reduction in the case of long expressions for the numerator
functions.Comment: 20 pages, 2 figure
Feynman Rules for the Rational Part of the Standard Model One-loop Amplitudes in the 't Hooft-Veltman Scheme
We study Feynman rules for the rational part of the Standard Model
amplitudes at one-loop level in the 't Hooft-Veltman scheme.
Comparing our results for quantum chromodynamics and electroweak 1-loop
amplitudes with that obtained based on the Kreimer-Korner-Schilcher (KKS)
scheme, we find the latter result can be recovered when our
scheme becomes identical (by setting in our expressions)
with the KKS scheme. As an independent check, we also calculate Feynman rules
obtained in the KKS scheme, finding our results in complete agreement with
formulae presented in the literature. Our results, which are studied in two
different schemes, may be useful for clarifying the
problem in dimensional regularization. They are helpful to eliminate or find
ambiguities arising from different dimensional regularization schemes.Comment: Version published in JHEP, presentation improved, 41 pages, 10
figure
Scattering AMplitudes from Unitarity-based Reduction Algorithm at the Integrand-level
SAMURAI is a tool for the automated numerical evaluation of one-loop
corrections to any scattering amplitudes within the dimensional-regularization
scheme. It is based on the decomposition of the integrand according to the
OPP-approach, extended to accommodate an implementation of the generalized
d-dimensional unitarity-cuts technique, and uses a polynomial interpolation
exploiting the Discrete Fourier Transform. SAMURAI can process integrands
written either as numerator of Feynman diagrams or as product of tree-level
amplitudes. We discuss some applications, among which the 6- and 8-photon
scattering in QED, and the 6-quark scattering in QCD. SAMURAI has been
implemented as a Fortran90 library, publicly available, and it could be a
useful module for the systematic evaluation of the virtual corrections oriented
towards automating next-to-leading order calculations relevant for the LHC
phenomenology.Comment: 35 pages, 7 figure
Automation of one-loop QCD corrections
We present the complete automation of the computation of one-loop QCD
corrections, including UV renormalization, to an arbitrary scattering process
in the Standard Model. This is achieved by embedding the OPP integrand
reduction technique, as implemented in CutTools, into the MadGraph framework.
By interfacing the tool so constructed, which we dub MadLoop, with MadFKS, the
fully automatic computation of any infrared-safe observable at the
next-to-leading order in QCD is attained. We demonstrate the flexibility and
the reach of our method by calculating the production rates for a variety of
processes at the 7 TeV LHC.Comment: 64 pages, 12 figures. Corrected the value of m_Z in table 1. In table
2, corrected the values of cross sections in a.4 and a.5 (previously computed
with mu=mtop/2 rather than mu=mtop/4). In table 2, corrected the values of
NLO cross sections in b.3, b.6, c.3, and e.7 (the symmetry factor for a few
virtual channels was incorrect). In sect. A.4.3, the labeling of the
four-momenta was incorrec
Single Cut Integration
We present an analytic technique for evaluating single cuts for one-loop
integrands, where exactly one propagator is taken to be on shell. Our method
extends the double-cut integration formalism of one-loop amplitudes to the
single-cut case. We argue that single cuts give meaningful information about
amplitudes when taken at the integrand level. We discuss applications to the
computation of tadpole coefficients.Comment: v2: corrected typo in abstrac
On the Integrand-Reduction Method for Two-Loop Scattering Amplitudes
We propose a first implementation of the integrand-reduction method for
two-loop scattering amplitudes. We show that the residues of the amplitudes on
multi-particle cuts are polynomials in the irreducible scalar products
involving the loop momenta, and that the reduction of the amplitudes in terms
of master integrals can be realized through polynomial fitting of the
integrand, without any apriori knowledge of the integral basis. We discuss how
the polynomial shapes of the residues determine the basis of master integrals
appearing in the final result. We present a four-dimensional constructive
algorithm that we apply to planar and non-planar contributions to the 4- and
5-point MHV amplitudes in N=4 SYM. The technique hereby discussed extends the
well-established analogous method holding for one-loop amplitudes, and can be
considered a preliminary study towards the systematic reduction at the
integrand-level of two-loop amplitudes in any gauge theory, suitable for their
automated semianalytic evaluation.Comment: 26 pages, 11 figure
Cosmology with Ultra-light Pseudo-Nambu-Goldstone Bosons
We explore the cosmological implications of an ultra-light
pseudo-Nambu-Goldstone boson. With global spontaneous symmetry breaking scale
GeV and explicit breaking scale comparable to MSW neutrino
masses, eV, such a field, which acquires a mass , would have become dynamical at recent epochs and currently
dominate the energy density of the universe. The field acts as an effective
cosmological constant for several expansion times and then relaxes into a
condensate of coherent non-relativistic bosons. Such a model can reconcile
dynamical estimates of the density parameter, , with a
spatially flat universe, and can yield an expansion age
while remaining consistent with limits from gravitational lens statistics.Comment: 15 pages (including 2 figs.), uuencoded compressed postscript; also
available on www at http://www-astro-theory.fnal.gov/ under Publication
NLO QCD corrections to top anti-top bottom anti-bottom production at the LHC: 2. full hadronic results
We present predictions for top anti-top bottom anti-bottom production at the
LHC in next-to-leading order QCD. The precise description of this background
process is a prerequisite to observe associated top anti-top Higgs production
in the Higgs -> bottom anti-bottom decay channel and to directly measure the
top-quark Yukawa coupling at the LHC. The leading-order cross section is
extremely sensitive to scale variations. We observe that the traditional scale
choice adopted in ATLAS simulations underestimates the top anti-top bottom
anti-bottom background by a factor two and introduce a new dynamical scale that
stabilizes the perturbative predictions. We study various kinematic
distributions and observe that the corrections have little impact on their
shapes if standard cuts are applied. In the regime of highly boosted Higgs
bosons, which offers better perspectives to observe the top anti-top Higgs
signal, we find significant distortions of the kinematic distributions. The
one-loop amplitudes are computed using process-independent algebraic
manipulations of Feynman diagrams and numerical tensor reduction. We find that
this approach provides very high numerical stability and CPU efficiency.Comment: 42 pages, LaTeX, 44 postscript figure
Stabilizing Salt-Bridge Enhances Protein Thermostability by Reducing the Heat Capacity Change of Unfolding
Most thermophilic proteins tend to have more salt bridges, and achieve higher thermostability by up-shifting and broadening their protein stability curves. While the stabilizing effect of salt-bridge has been extensively studied, experimental data on how salt-bridge influences protein stability curves are scarce. Here, we used double mutant cycles to determine the temperature-dependency of the pair-wise interaction energy and the contribution of salt-bridges to ΔCp in a thermophilic ribosomal protein L30e. Our results showed that the pair-wise interaction energies for the salt-bridges E6/R92 and E62/K46 were stabilizing and insensitive to temperature changes from 298 to 348 K. On the other hand, the pair-wise interaction energies between the control long-range ion-pair of E90/R92 were negligible. The ΔCp of all single and double mutants were determined by Gibbs-Helmholtz and Kirchhoff analyses. We showed that the two stabilizing salt-bridges contributed to a reduction of ΔCp by 0.8–1.0 kJ mol−1 K−1. Taken together, our results suggest that the extra salt-bridges found in thermophilic proteins enhance the thermostability of proteins by reducing ΔCp, leading to the up-shifting and broadening of the protein stability curves
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