54 research outputs found
Photon Generalized Parton Distributions
We present a calculation of the generalized parton distributions of the
photon using overlaps of photon light-front wave functions.Comment: Talk given at LightCone 2011, 4 pages, 3 figure
The S-parameter in Holographic Technicolor Models
We study the S parameter, considering especially its sign, in models of
electroweak symmetry breaking (EWSB) in extra dimensions, with fermions
localized near the UV brane. Such models are conjectured to be dual to 4D
strong dynamics triggering EWSB. The motivation for such a study is that a
negative value of S can significantly ameliorate the constraints from
electroweak precision data on these models, allowing lower mass scales (TeV or
below) for the new particles and leading to easier discovery at the LHC. We
first extend an earlier proof of S>0 for EWSB by boundary conditions in
arbitrary metric to the case of general kinetic functions for the gauge fields
or arbitrary kinetic mixing. We then consider EWSB in the bulk by a Higgs VEV
showing that S is positive for arbitrary metric and Higgs profile, assuming
that the effects from higher-dimensional operators in the 5D theory are
sub-leading and can therefore be neglected. For the specific case of AdS_5 with
a power law Higgs profile, we also show that S ~ + O(1), including effects of
possible kinetic mixing from higher-dimensional operator (of NDA size) in the
theory. Therefore, our work strongly suggests that S is positive in
calculable models in extra dimensions.Comment: 21 pages, 2 figures. v2: references adde
Duality violations and spectral sum rules
We study the issue of duality violations in the VV-AA vacuum polarization
function in the chiral limit. This is done with the help of a model with an
expansion in inverse powers of the number of colors, Nc, allowing us to
consider resonances with a finite width. Due to these duality violations, the
Operator Product Expansion (OPE) and the moments of the spectral function (e.g.
the Weinberg sum rules) do not match at finite momentum, and we analyze this
difference in detail. We also perform a comparative study of many of the
different methods proposed in the literature for the extraction of the OPE
parameters and find that, when applied to our model, they all fare quite
similarly. In fact, the model strongly suggests that a significant improvement
in precision can only be expected after duality violations are included. To
this end, we propose a method to parameterize these duality violations. The
method works quite well for the model, and we hope that it may also be useful
in future determinations of OPE parameters in QCD.Comment: 29 pages, 9 figures, LateX file. Small changes to match journal
versio
Chiral Condensates, Q_7 and Q_8 Matrix Elements and Large-N_c QCD
The correlation function of a current with a current is discussed
within the framework of QCD in the limit of a large number of colours .
Applications to the evaluation of chiral condensates of dimension six and
higher, as well as to the matrix elements of the and electroweak
penguin operators are discussed. A critical comparison with previous
determinations of the same parameters has also been made.Comment: Layout modified, size of first figure correcte
La paille de riz dans l'alimentation animale au Sénégal. II. Biochimie du rumen - Embouche intensive - Conclusions
Dans une première partie, les auteurs étudient les résultats des analyses bromatologiques des digestibilités in vivo et de bilans minéraux effectués à partir de la paille de riz. La 2e partie intéresse la production des nutriments obtenus au niveau du rumen avec des rations à base de paille de riz, les résultats d'expérimentation d'embouche intensive avec des rations comportant ce fourrage. La paille de riz correctement supplémentée en azote et en sels minéraux peut constituer de 30 à 70 p. 100 de l'apport énergétique de rations d'embouche capables de produire de 400 à 800 g de croît journalier. A ce titre, les 150 000 tonnes de paille de riz disponibles au Sénégal pourraient contribuer dans une large mesure à la préparation bouchère de plus de 100 000 têtes de bétai
Growth modes of Fe(110) revisited: a contribution of self-assembly to magnetic materials
We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and
propose an overview or our contribution to the field. We show that the
Stranski-Krastanov growth mode, recognized for a long time in these systems, is
in fact characterized by a bimodal distribution of islands for growth
temperature in the range 250-700°C. We observe firstly compact islands
whose shape is determined by Wulff-Kaischev's theorem, secondly thin and flat
islands that display a preferred height, ie independant from nominal thickness
and deposition procedure (1.4nm for Mo, and 5.5nm for W on the average). We
used this effect to fabricate self-organized arrays of nanometers-thick stripes
by step decoration. Self-assembled nano-ties are also obtained for nucleation
of the flat islands on Mo at fairly high temperature, ie 800°C. Finally,
using interfacial layers and solid solutions we separate two effects on the
preferred height, first that of the interfacial energy, second that of the
continuously-varying lattice parameter of the growth surface.Comment: 49 pages. Invited topical review for J. Phys.: Condens. Matte
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
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