38,958 research outputs found
The formation of spiral arms and rings in barred galaxies
In this and in a previous paper (Romero-Gomez et al. 2006) we propose a
theory to explain the formation of both spirals and rings in barred galaxies
using a common dynamical framework. It is based on the orbital motion driven by
the unstable equilibrium points of the rotating bar potential. Thus, spirals,
rings and pseudo-rings are related to the invariant manifolds associated to the
periodic orbits around these equilibrium points. We examine the parameter space
of three barred galaxy models and discuss the formation of the different
morphological structures according to the properties of the bar model. We also
study the influence of the shape of the rotation curve in the outer parts, by
making families of models with rising, flat, or falling rotation curves in the
outer parts. The differences between spiral and ringed structures arise from
differences in the dynamical parameters of the host galaxies. The results
presented here will be discussed and compared with observations in a
forthcoming paper.Comment: 16 pages, 13 figures, accepted in A&A. High resolution version
available at http://www.oamp.fr/dynamique/pap/merce.htm
-Flavour Violation at the LHC
We study the conditions required for
decays to yield observable tau flavour violation at the LHC, for cosmologically
interesting values of the neutralino relic density. These condition can be
achieved in the framework of a SU(5) model with a see-saw mechanism that allows
a possible coexistence of a LHC signal a low prediction for radiative LFV
decays.Comment: 7 pages, 5 figures, Prepared for the proceedings of the workshop:
"LC09: Physics at the TeV Scale and the Dark Matter Connection",
21-24 September 2009, Perugia, Ital
Double Pion Photoproduction in Nuclei
The inclusive A(gamma,pi+ pi-)X reaction is studied theoretically. A sizeable
enhancement of the cross section is found, in comparison with the scaling of
the deuteron cross section (sigma_deuteron * A/2). This enhancement is due to
the modifications in the nuclear medium of the gamma N ----> pi pi N amplitude
and the pion dispersion relation. The enhancement is found to be bigger than
the one already observed in the (pi,pi pi) reaction in nuclei.Comment: 11 pages, 7 figures (figures available from authors); TeX, Version
3.141 [PD VMS 3.4/CERN 1.0
New Factorization Relations for Yang Mills Amplitudes
A double-cover extension of the scattering equation formalism of Cachazo, He
and Yuan (CHY) leads us to conjecture covariant factorization formulas of
n-particle scattering amplitudes in Yang-Mills theories. Evidence is given that
these factorization relations are related to Berends-Giele recursions through
repeated use of partial fraction identities involving linearized propagators.Comment: 7 pages, 3 figures, version to appear in PR
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