70 research outputs found
Instantaneous Shape Sampling - a model for the -absorption cross section of transitional nuclei
The influence of the quadrupole shape fluctuations on the dipole vibrations
in transitional nuclei is investigated in the framework of the Instantaneous
Shape Sampling Model, which combines the Interacting Boson Model for the slow
collective quadrupole motion with the Random Phase Approximation for the rapid
dipole vibrations. Coupling to the complex background configurations is taken
into account by folding the results with a Lorentzian with an energy dependent
width. The low-energy energy portion of the - absorption cross section,
which is important for photo-nuclear processes, is studied for the isotopic
series of Kr, Xe, Ba, and Sm. The experimental cross sections are well
reproduced. The low-energy cross section is determined by the Landau
fragmentation of the dipole strength and its redistribution caused by the shape
fluctuations. Collisional damping only wipes out fluctuations of the absorption
cross section, generating the smooth energy dependence observed in experiment.
In the case of semi-magic nuclei, shallow pygmy resonances are found in
agreement with experiment
Monodromy--like Relations for Finite Loop Amplitudes
We investigate the existence of relations for finite one-loop amplitudes in
Yang-Mills theory. Using a diagrammatic formalism and a remarkable connection
between tree and loop level, we deduce sequences of amplitude relations for any
number of external legs.Comment: 24 pages, 6 figures, v2 typos corrected, reference adde
On BCFW shifts of integrands and integrals
In this article a first step is made towards the extension of
Britto-Cachazo-Feng-Witten (BCFW) tree level on-shell recursion relations to
integrands and integrals of scattering amplitudes to arbitrary loop order.
Surprisingly, it is shown that the large BCFW shift limit of the integrands has
the same structure as the corresponding tree level amplitude in any minimally
coupled Yang-Mills theory in four or more dimensions. This implies that these
integrands can be reconstructed from a subset of their `single cuts'. The main
tool is powercounting Feynman graphs in a special lightcone gauge choice
employed earlier at tree level by Arkani-Hamed and Kaplan. The relation between
shifts of integrands and shifts of its integrals is investigated explicitly at
one loop. Two particular sources of discrepancy between the integral and
integrand are identified related to UV and IR divergences. This is
cross-checked with known results for helicity equal amplitudes at one loop. The
nature of the on-shell residue at each of the single-cut singularities of the
integrand is commented upon. Several natural conjectures and opportunities for
further research present themselves.Comment: 43 pages, 6 figures, v2: minor improvement in exposition, typos
fixed, bibliography update
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
Epistemic and Ontic Quantum Realities
Quantum theory has provoked intense discussions about its interpretation since its pioneer days. One of the few scientists who have been continuously engaged in this development from both physical and philosophical perspectives is Carl Friedrich von Weizsaecker. The questions he posed were and are inspiring for many, including the authors of this contribution. Weizsaecker developed Bohr's view of quantum theory as a theory of knowledge. We show that such an epistemic perspective can be consistently complemented by Einstein's ontically oriented position
Rational Terms in Theories with Matter
We study rational remainders associated with gluon amplitudes in gauge
theories coupled to matter in arbitrary representations. We find that these
terms depend on only a small number of invariants of the matter-representation
called indices. In particular, rational remainders can depend on the second and
fourth order indices only. Using this, we find an infinite class of
non-supersymmetric theories in which rational remainders vanish for gluon
amplitudes. This class includes all the "next-to-simplest" quantum field
theories of arXiv:0910.0930. This provides new examples of amplitudes in which
rational remainders vanish even though naive power counting would suggest their
presence.Comment: 10+4 pages. (v2) typos corrected, references adde
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
Random-phase approximation and its applications in computational chemistry and materials science
The random-phase approximation (RPA) as an approach for computing the
electronic correlation energy is reviewed. After a brief account of its basic
concept and historical development, the paper is devoted to the theoretical
formulations of RPA, and its applications to realistic systems. With several
illustrating applications, we discuss the implications of RPA for computational
chemistry and materials science. The computational cost of RPA is also
addressed which is critical for its widespread use in future applications. In
addition, current correction schemes going beyond RPA and directions of further
development will be discussed.Comment: 25 pages, 11 figures, published online in J. Mater. Sci. (2012
The GAIA theory: from Lovelock to Margulis. From a homeostatic to a cognitive autopoietic worldview
n/
Acetylation Regulates WRN Catalytic Activities and Affects Base Excision DNA Repair
Background: The Werner protein (WRN), defective in the premature aging disorder Werner syndrome, participates in a number of DNA metabolic processes, and we have been interested in the possible regulation of its function in DNA repair by post-translational modifications. Acetylation mediated by histone acetyltransferases is of key interest because of its potential importance in aging, DNA repair and transcription. Methodology/Principal Findings: Here, we have investigated the p300 acetylation mediated changes on the function of WRN in base excision DNA repair (BER). We show that acetylation of WRN increases in cells treated with methyl methanesulfonate (MMS), suggesting that acetylation of WRN may play a role in response to DNA damage. This hypothesis is consistent with our findings that acetylation of WRN stimulates its catalytic activities in vitro and in vivo, and that acetylated WRN enhances pol b-mediated strand displacement DNA synthesis more than unacetylated WRN. Furthermore, we show that cellular exposure to the histone deacetylase inhibitor sodium butyrate stimulates long patch BER in wild type cells but not in WRN depleted cells, suggesting that acetylated WRN participates significantly in this process. Conclusion/Significance: Collectively, these results provide the first evidence for a specific role of p300 mediated WRN acetylation in regulating its function during BER
- …