40,112 research outputs found
Monopoles, strings and dark matter
We develop a scenario whereby monopoles in a hidden sector yield a decaying
dark matter candidate of interest for the PAMELA and FERMI excesses.
The monopoles are not completely hidden due to a very small kinetic mixing and
a hidden photon mass. The latter also causes the monopoles and anti-monopoles
to be connected by strings. The resulting long-lived objects eventually decay
to hidden photons which tend to escape galactic cores before decaying. The mass
scales are those of the hidden photon ( MeV), the monopole
( TeV) and the mixing scale (close to the Planck scale). A gauge
coupling in the hidden sector is the only other parameter. This coupling must
be strong and this results in light point-like monopoles and light thin
strings.Comment: 21 pages, various improvements and additional reference
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
Child education and work choices in the presence of a conditional cash transfer programme in rural Colombia
This research is part of a large evaluation effort, undertaken by a consortium formed by IFS, Econometria and SEI, which has considered the effects of Familias en Accion on a variety of outcomes one year after its implementation. In early reports, we focussed on the effects of the programme on school enrolment. In this paper, we both expand those results, by carefully analysing anticipation effects along with other issues, and complement them with an analysis of child labour - both paid and unpaid (including domestic) work. The child labour analysis is made possible due to a rich time use module of the surveys that has not previously been analysed. We find that the programme increased the school participation rates of 14 to 17 year old children quite substantially, by between 5 and 7 percentage points, and had lower, but non-negligible effects on the enrolment of younger children of between 1.4 and 2.4 percentage points. In terms of work, the effects are generally largest for younger children whose participation in domestic work decreased by around 10 to 12 percentage points after the programme but whose participation in income-generating work remained largely unaffected by the programme. We also find evidence of school and work time not being fully substitutable, suggesting that some, but not all, of the increased time at school may be drawn from children's leisure time
Charged-Lepton-Flavour Violation in Kaon Decays in Supersymmetric Theories
We discuss rare kaon decays that violate charged-lepton flavour conservation
in supersymmetric theories with and without R parity, in view of possible
experiments using an intense proton source as envisaged for a neutrino factory.
In the Minimal Supersymmetric Standard Model, such decays are generated by box
diagrams involving charginos and neutralinos, but the limits from \mu -> e
\gamma, \mu-e conversion and \Delta m_K constrain the branching ratios to
challengingly small values. However, this is no longer the case in R-violating
theories, where such decays may occur at tree level at rates close to the
present experimental limits. Within this framework, we obtain bounds on
products of LL\bar{E} and LQ\bar{D} operators from the experimental upper
limits on K^0 -> \mu^\pm e^\mp and K^{\pm,0} -> \pi^{\pm,0} \mu^\pm e^\mp
decays. We also note the possibility of like-sign lepton decays K^\pm ->
\pi^\mp \ell^\pm \ell^\pm in the presence of non-zero \tilde{b}_L-\tilde{b}_R
mixing. We conclude that rare kaon decays violating charged-lepton flavour
conservation could be an interesting signature of R violation.Comment: 25 pages, 6 figures, Prepared for the Kaon Physics Working Group as
part of the ECFA/CERN study of Neutrino Factory & Muon Storage Rings At CERN.
Referencies added, constraints update
Control of dissipation in superconducting films by magnetic stray fields
Hybrid superconducting/magnetic nanostructures on Si substrates have been
built with identical physical dimensions but different magnetic configurations.
By constructing arrays based on Co-dots with in-plane, out-of-plane, and vortex
state magnetic configurations, the stray fields are systematically tuned.
Dissipation in the mixed state of superconductors can be decreased (increased)
by several orders of magnitude by decreasing (increasing) the stray magnetic
fields. Furthermore, ordering of the stray fields over the entire array helps
to suppress dissipation and enhance commensurability effects increasing the
number of dissipation minima.Comment: 16 pages including 4 figures; accepted in Applied Physics Letter
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