23,072 research outputs found
Angular momentum non-conserving decays in isotropic media
Various processes that are forbidden in the vacuum due to angular momentum
conservation can occur in a medium that is isotropic and does not carry any
angular momentum. We illustrate this by considering explicitly two examples.
The first one is the decay of a spin-0 particle into a photon and another
spin-0 particle, using a model involving the Yukawa interactions of the scalar
particles with a charged fermion field. The second one involves the decay of a
neutrino into another neutrino and a graviton, in the standard model of
particle interactions augmented with the linearized gravitational couplings.Comment: 22 pages, Latex, uses axodraw.sty. 3 figures embedded in the tex
file. This paper contains the same material as that presented in two earlier
papers, arXiv:0901.2981 and arXiv:0901.2982, written by us. This version
supersedes those two paper
Remarks on the Coulomb and Covariant Gauges in Finite Temperature QED
We compare the use of the Coulomb gauge in finite temperature QED with a
recently proposed prescription for covariant gauges, in which only the
transverse photon degrees of freedom are thermalized. Using the Landau rule as
a guide, we clarify the relation between the retarded electron self-energy and
the elements of the self-energy matrix in the real-time formulation of . The
general results are illustrated by means of the one-loop expressions for the
electron self-energy in a QED plasma.Comment: LaTeX, 10 pages, no figures; to be published in Phys. Lett.
Did PROGRESA send drop-outs back to school?
This paper analyzes the effect of PROGRESA education grants on school enrollment. It looks at
its effect on total school enrollment and in particular on school enrollment of drop-outs, i.e.
those children who face a re-enrollment decision since they were not enrolled in school the year
prior to the implementation of the PROGRESA program. Estimates of the impact of
PROGRESA education grants on drop-outs and non-drop-outs are obtained applying difference
estimation and maximum likelihood estimation of a reduced form equation for schooling
decision. Differences in results between both groups of children are discussed looking at the
distribution of marginal effects. PROGRESA did send drop-outs back to school. It had a larger
effect on drop-outs than on non-drop-outs. However, for the particular group of girls who
dropped out of school just before attending secondary school PROGRESA grants only had a
minor effect. This last finding highlights the fact that determinants of the schooling decision are
different for young girls and that PROGRESA grants do not provide a strong enough incentive
to send them back to school
Behind the Lines
This creative writing thesis contains fiction by Idalis Nieves
Renormalization of the 1S0 One-Pion-Exchange NN Interaction in Presence of Derivative Contact Interactions
We use standard distorted wave theory techniques and dimensional
regularization to find out solutions of the nucleon-nucleon Lippman--Schwinger
equation with a kernel determined by the Weinberg's next-to-leading potential,
which consists of one--pion exchange and additional contact terms with
derivatives. Though for simplicity, we restrict the discussion to the
channel and to contact terms containing up to two derivatives, the
generalization to higher waves and/or number of derivatives is straightforward.
The undetermined low energy constants emerging out of the renormalization
procedure are fitted to data.Comment: 7 pages, revtex4, Phys. Lett. B in prin
Asymptotic analysis of solutions to transmission problems in solids with many inclusions
We construct an asymptotic approximation to the solution of a transmission
problem for a body containing a region occupied by many small inclusions. The
cluster of inclusions is characterised by two small parameters that determine
the nominal diameter of individual inclusions and their separation within the
cluster. These small parameters can be comparable to each other. Remainder
estimates of the asymptotic approximation are rigorously justified. Numerical
illustrations demonstrate the efficiency of the asymptotic approach when
compared with benchmark finite element algorithms.Comment: 30 pages, 5 figure
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