29,620 research outputs found
Minimal mass-size of a stable 3He cluster
The minimal number of 3He atoms required to form a bound cluster has been
estimated by means of a Diffusion Monte Carlo procedure within the fixed-node
approximation. Several importance sampling wave functions have been employed in
order to consider different shell-model configurations. The resulting upper
bound for the minimal number is 32 atoms.Comment: 2 pages, no figure
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
Excited states of 4He droplets
We study low-lying excited states of 4He clusters up to a cluster size of 40 atoms in a variational framework. The ansatz wave function combines two- and three-body correlations, coming from a translationally invariant configuration interaction description, and Jastrow-type short-range correlation. We have previously used this scheme to determine the ground-state energies of 4He and 3He clusters. Here we present an extension of this ansatz wave function having a good quantum angular momentum L. The variational procedure is applied independently to the cases with L = 0,2,4, and upper bounds for the corresponding energies are thus obtained. Moreover, centroid energies for L excitations are calculated through the use of sum rules. A comparison with previous calculations is also made.Fil: Guardiola, R.. Facultad de Física / Dpto de Física Atómica y Nuclear; EspañaFil: Navarro, J.. Csic - Univ. de Valencia / Inst. de Física Corpuscular; EspañaFil: Portesi, Mariela Adelina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin
The spectra of mixed He-He droplets
The diffusion Monte Carlo technique is used to calculate and analyze the
excitation spectrum of He atoms bound to a cluster of He atoms, by
using a previously determined optimum filling of single-fermion orbits with
well defined orbital angular momentum , spin and parity quantum numbers.
The study concentrates on the energies and shapes of the three kinds of states
for which the fermionic part of the wave function is a single Slater
determinant: maximum or maximum states within a given orbit, and fully
polarized clusters. The picture that emerges is that of systems with strong
shell effects whose binding and excitation energies are essentially determined
over configuration at fixed number of particles and spin, i.e., by the monopole
properties of an effective Hamiltonian.Comment: 14 pages, 15 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
Symmetries in two-dimensional dilaton gravity with matter
The symmetries of generic 2D dilaton models of gravity with (and without)
matter are studied in some detail. It is shown that , one of the
symmetries of the matterless models, can be generalized to the case where
matter fields of any kind are present. The general (classical) solution for
some of these models, in particular those coupled to chiral matter, which
generalizes the Vaidya solution of Einstein Gravity, is also given.Comment: Minor changes have been made; the references have been updated and
some added; 11 pages. To appear in Phys. Rev.
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