11,308 research outputs found
Complex Networks and Symmetry II: Reciprocity and Evolution of World Trade
We exploit the symmetry concepts developed in the companion review of this
article to introduce a stochastic version of link reversal symmetry, which
leads to an improved understanding of the reciprocity of directed networks. We
apply our formalism to the international trade network and show that a strong
embedding in economic space determines particular symmetries of the network,
while the observed evolution of reciprocity is consistent with a symmetry
breaking taking place in production space. Our results show that networks can
be strongly affected by symmetry-breaking phenomena occurring in embedding
spaces, and that stochastic network symmetries can successfully suggest, or
rule out, possible underlying mechanisms.Comment: Final accepted versio
Absence of resonant enhancements in some inclusive rates
A toy model is defined and solved perturbatively with the aim of examining
some claimed "resonant" enhancements of certain reaction rates that enter
popular models of leptogenesis. We find: a) that such enhancements are absent;
and b) that the perturbative solution, as done correctly using finite-
temperature field theory, is well defined without the "resumming" procedures
found in the literature. The pathologies that led to the perceived need for
these procedures are an artifact of uncritical use of weighted vacuum cross-
sections in the determination of rates, without adequate attention to the
effects of the medium upon the single particle states within it.Comment: 11 pages, no figures. Some typos corrected. More typos correcte
From Hadrons to Nuclei: Crossing the Border
The study of nuclei predates by many years the theory of quantum
chromodynamics. More recently, effective field theories have been used in
nuclear physics to ``cross the border'' from QCD to a nuclear theory. We are
now entering the second decade of efforts to develop a perturbative theory of
nuclear interactions using effective field theory. This work describes the
current status of these efforts.Comment: 141 pages, 58 figs, latex. To appear in the Boris Ioffe Festschrift,
ed. by M. Shifman, World Scientifi
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