7,017 research outputs found
Implications for New Physics from Fine-Tuning Arguments: II. Little Higgs Models
We examine the fine-tuning associated to electroweak breaking in Little Higgs
scenarios and find it to be always substantial and, generically, much higher
than suggested by the rough estimates usually made. This is due to implicit
tunings between parameters that can be overlooked at first glance but show up
in a more systematic analysis. Focusing on four popular and representative
Little Higgs scenarios, we find that the fine-tuning is essentially comparable
to that of the Little Hierarchy problem of the Standard Model (which these
scenarios attempt to solve) and higher than in supersymmetric models. This does
not demonstrate that all Little Higgs models are fine-tuned, but stresses the
need of a careful analysis of this issue in model-building before claiming that
a particular model is not fine-tuned. In this respect we identify the main
sources of potential fine-tuning that should be watched out for, in order to
construct a successful Little Higgs model, which seems to be a non-trivial
goal.Comment: 39 pages, 26 ps figures, JHEP forma
Large mixing angles for neutrinos from infrared fixed points
Radiative amplification of neutrino mixing angles may explain the large
values required by solar and atmospheric neutrino oscillations. Implementation
of such mechanism in the Standard Model and many of its extensions (including
the Minimal Supersymmetric Standard Model) to amplify the solar angle, the
atmospheric or both requires (at least two) quasi-degenerate neutrino masses,
but is not always possible. When it is, it involves a fine-tuning between
initial conditions and radiative corrections. In supersymmetric models with
neutrino masses generated through the Kahler potential, neutrino mixing angles
can easily be driven to large values at low energy as they approach infrared
pseudo-fixed points at large mixing (in stark contrast with conventional
scenarios, that have infrared pseudo-fixed points at zero mixing). In addition,
quasi-degeneracy of neutrino masses is not always required.Comment: 36 pages, 7 ps figure
Theoretical Constraints on the Vacuum Oscillation Solution to the Solar Neutrino Problem
The vacuum oscillation (VO) solution to the solar anomaly requires an
extremely small neutrino mass splitting, Delta m^2_{sol}\leq 10^{-10} eV^2. We
study under which circumstances this small splitting (whatever its origin) is
or is not spoiled by radiative corrections. The results depend dramatically on
the type of neutrino spectrum. If m_1^2 \sim m_2^2 \geq m_3^2, radiative
corrections always induce too large mass splittings. Moreover, if m_1 and m_2
have equal signs, the solar mixing angle is driven by the renormalization group
evolution to very small values, incompatible with the VO scenario (however, the
results could be consistent with the small-angle MSW scenario). If m_1 and m_2
have opposite signs, the results are analogous, except for some small (though
interesting) windows in which the VO solution may be natural with moderate
fine-tuning. Finally, for a hierarchical spectrum of neutrinos, m_1^2 << m_2^2
<< m_3^2, radiative corrections are not dangerous, and therefore this scenario
is the only plausible one for the VO solution.Comment: 13 pages, LaTeX, 3 ps figures (psfig.sty
Semiconductor cavity QED: Bandgap induced by vacuum fluctuations
We consider theoretically a semiconductor nanostructure embedded in
one-dimensional microcavity and study the modification of its electron energy
spectrum by the vacuum fluctuations of the electromagnetic field. To solve the
problem, a non-perturbative diagrammatic approach based on the Green's function
formalism is developed. It is shown that the interaction of the system with the
vacuum fluctuations of the optical cavity opens gaps within the valence band of
the semiconductor. The approach is verified for the case of large photon
occupation numbers, proving the validity of the model by comparing to previous
studies of the semiconductor system excited by a classical electromagnetic
field. The developed theory is of general character and allows for unification
of quantum and classical descriptions of the strong light-matter interaction in
semiconductor structures
Self collimation of ultrasound in a 3D sonic crystal
We present the experimental demonstration of self-collimation (subdiffractive
propagation) of an ultrasonic beam inside a three-dimensional sonic crystal.
The crystal is formed by two crossed steel cylinders structures in a
woodpile-like geometry disposed in water. Measurements of the 3D field
distribution show that a narrow beam which diffractively spreads in the absence
of the sonic crystal is strongly collimated in propagation inside the crystal,
demonstrating the 3D self-collimation effect.Comment: 3 figures, submitted to Applied Physics Letter
Les locutores de rà dio a Catalunya 1924-1939: perfils, semblances i diferències
Les dones que van treballar a l'inici de la rà dio a l'Estat espanyol són les
grans oblidades de les diverses històries de la rà dio que s'han anat compilant.
Amb aquesta tesi es vol acabar amb aquesta injustÃcia històrica buscant
i entrevistant les dones que des de 1924 fins a 1941 van treballar a la
rà dio a Catalunya. L'autora ha aconseguit establir una cronologia exacta de
les dones que foren locutores de rà dio en aquesta etapa i donar a conèixer
els seus perfils professionals i vitals.The women who worked in Spanish radio at its beginning are the great
unsung protagonists — absent in the diverse radio stories complied. The
present thesis aims to reverse this historic injustice, at least in part, by locating
and interviewing the women who worked in Catalan radio from 1924
to 1941. The author has written an exact chronology of the women who
were radio commentators during this period and included their professional
and biographical profiles
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