149 research outputs found
A non trivial extension of the two-dimensional Ising model: the d-dimensional "molecular" model
A recently proposed molecular model is discussed as a non-trivial extension
of the Ising model. For d=2 the two models are shown to be equivalent, while
for d>2 the molecular model describes a peculiar second order transition from
an isotropic high temperature phase to a low-dimensional anisotropic low
temperature state. The general mean field analysis is compared with the results
achieved by a variational Migdal-Kadanoff real space renormalization group
method and by standard Monte Carlo sampling for d=3. By finite size scaling the
critical exponent has been found to be 0.44\pm 0.02 thus establishing that the
molecular model does not belong to the universality class of the Ising model
for d>2.Comment: 25 pages, 5 figure
Symmetry breaking and restoring under high pressure: the amazing behaviour of the "simple" alkali metals
We argue that an ionic lattice surrounded by a Fermi liquid changes phase
several times under pressure, oscillating between the symmetric phase and a
low-symmetry dimerized structure, as a consequence of Friedel oscillations in
the pair potential. Phase oscillations explain the tendency towards
dimerization which has been recently reported for the light alkali metals under
high pressure. Moreover, a restoring of the symmetric phase is predicted for
such elements at an even higher density.Comment: accepted in Eur. Phys. J.
Light Higgs bosons from a strongly interacting Higgs sector
The mass and the decay width of a Higgs boson in the minimal standard model
are evaluated by a variational method in the limit of strong self-coupling
interaction. The non-perturbative technique provides an interpolation scheme
between strong-coupling regime and weak-coupling limit where the standard
perturbative results are recovered. In the strong-coupling limit the physical
mass and the decay width of the Higgs boson are found to be very small as a
consequence of mass renormalization. Thus it is argued that the eventual
detection of a light Higgs boson would not rule out the existence of a strongly
interacting Higgs sector.Comment: 2 figure
Grand unification in the minimal left-right symmetric extension of the standard model
The simplest minimal left-right symmetric extension of the standard model is
studied in the high energy limit, and some consequences of the grand
unification hypothesis are explored assuming that the parity breaking scale is
the only relevant energy between the electro-weak scale and the unification
point. While the model is shown to be compatible with the observed neutrino
phenomenology, the parity breaking scale and the heavy boson masses are
predicted to be above 10^7 TeV, quite far from the reach of nowadays
experiments. Below that scale only an almost sterile right handed neutrino is
allowed with a mass M \approx 100 TeV
A Truly Minimal Left-Right Symmetric Extension of the Standard Model
By invoking the existence of a general custodial O(2) symmetry, a minimal
Left-Right symmetric model based on the gauge group G=SU(2)L SU(2)R U(1)BL is
shown to require the existence of only two physical Higgs bosons. The lighter
Higgs is predicted to have a small mass which could be evaluated by standard
perturbation theory. The fermionic mass matrices are recovered by insertion of
ad hoc fermion-Higgs interactions. The model is shown to be undistinguishable
from the standard model at the currently reachable energies.Comment: 1 figure in a separate ps fil
Gaussian Effective Potential and superconductivity
The Gaussian Effective Potential in a fixed transverse unitarity gauge is
studied for the static three-dimensional U(1) scalar electrodynamics
(Ginzburg-Landau phenomenological theory of superconductivity). In the
broken-symmetry phase the mass of the electromagnetic field (inverse
penetration depth) and the mass of the scalar field (inverse correlation
length) are both determined by solution of the coupled variational equations.
At variance with previous calculations, the choice of a fixed unitarity gauge
prevents from the occurrence of any unphysical degree of freedom. The theory
provides a nice interpolation of the experimental data when approaching the
critical region, where the standard mean-field method is doomed to failure
A variational method from the variance of energy
A variational method is studied based on the minimum of energy variance. The
method is tested on exactly soluble problems in quantum mechanics, and is shown
to be a useful tool whenever the properties of states are more relevant than
the eigenvalues. In quantum field theory the method provides a consistent
second order extension of the gaussian effective potential.Comment: 5 ps figure
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