12 research outputs found
The extended gaussian ensemble and metastabilities in the Blume-Capel model
The Blume-Capel model with infinite-range interactions presents analytical
solutions in both canonical and microcanonical ensembles and therefore, its
phase diagram is known in both ensembles. This model exhibits nonequivalent
solutions and the microcanonical thermodynamical features present peculiar
behaviors like nonconcave entropy, negative specific heat, and a jump in the
thermodynamical temperature. Examples of nonequivalent ensembles are in general
related to systems with long-range interactions that undergo canonical
first-order phase transitions. Recently, the extended gaussian ensemble (EGE)
solution was obtained for this model. The gaussian ensemble and its extended
version can be considered as a regularization of the microcanonical ensemble.
They are known to play the role of an interpolating ensemble between the
microcanonical and the canonical ones. Here, we explicitly show how the
microcanonical energy equilibrium states related to the metastable and unstable
canonical solutions for the Blume-Capel model are recovered from EGE, which
presents a concave "extended" entropy as a function of energy.Comment: 6 pages, 5 eps figures. Presented at the XI Latin American Workshop
on Nonlinear Phenomena, October 05-09 (2009), B\'uzios (RJ), Brazil. To
appear in JPC
Comparison among HB-inspired algorithms for continuous-spin systems and gauge fields
We propose a new local algorithm for the thermalization of n-vector spin
models, which can also be used in the numerical simulation of SU(N) lattice
gauge theories. The algorithm combines heat-bath (HB) and micro-canonical
updates in a single step -- as opposed to the hybrid overrelaxation method,
which alternates between the two kinds of update steps -- while preserving
ergodicity. We test our proposed algorithm in the case of the one-dimensional
4-vector spin model and compare its performance with the standard HB algorithm
and with other HB-inspired algorithms.Comment: 6 pages, 4 figures. Work presented at the IV Brazilian Meeting on
Simulational Physics -- Ouro Preto - MG/Brazil, August 200
Methods for calculating nonconcave entropies
Five different methods which can be used to analytically calculate entropies
that are nonconcave as functions of the energy in the thermodynamic limit are
discussed and compared. The five methods are based on the following ideas and
techniques: i) microcanonical contraction, ii) metastable branches of the free
energy, iii) generalized canonical ensembles with specific illustrations
involving the so-called Gaussian and Betrag ensembles, iv) restricted canonical
ensemble, and v) inverse Laplace transform. A simple long-range spin model
having a nonconcave entropy is used to illustrate each method.Comment: v1: 22 pages, IOP style, 7 color figures, contribution for the JSTAT
special issue on Long-range interacting systems. v2: Open problem and
references added, minor typos corrected, close to published versio
Hadron Spectroscopy with Dynamical Chirally Improved Fermions
We simulate two dynamical, mass degenerate light quarks on 16^3x32 lattices
with a spatial extent of 2.4 fm using the Chirally Improved Dirac operator. The
simulation method, the implementation of the action and signals of
equilibration are discussed in detail. Based on the eigenvalues of the Dirac
operator we discuss some qualitative features of our approach. Results for
ground state masses of pseudoscalar and vector mesons as well as for the
nucleon and delta baryons are presented.Comment: 26 pages, 17 figures, 10 table
Excitations of single-beauty hadrons
In this work we study the predominantly orbital and radial excitations of
hadrons containing a single heavy quark. We present meson and baryon mass
splittings and ratios of meson decay constants (e.g., and
) resulting from quenched and dynamical two-flavor
configurations. Light quarks are simulated using the chirally improved (CI)
lattice Dirac operator at valence masses as light as MeV.
The heavy quark is approximated by a static propagator, appropriate for the
quark on our lattices ( GeV). We also include some preliminary
calculations of the kinetic corrections to the states, showing,
in the process, a viable way of applying the variational method to three-point
functions involving excited states. We compare our results with recent
experimental findings.Comment: 23 pages, 18 figures, 17 tables; slight title change (Ed. killjoy);
reference added; version to appear in Phys Rev
Extended gaussian ensemble solution and tricritical points of a system with long-range interactions
The gaussian ensemble and its extended version theoretically play the
important role of interpolating ensembles between the microcanonical and the
canonical ensembles. Here, the thermodynamic properties yielded by the extended
gaussian ensemble (EGE) for the Blume-Capel (BC) model with infinite-range
interactions are analyzed. This model presents different predictions for the
first-order phase transition line according to the microcanonical and canonical
ensembles. From the EGE approach, we explicitly work out the analytical
microcanonical solution. Moreover, the general EGE solution allows one to
illustrate in details how the stable microcanonical states are continuously
recovered as the gaussian parameter is increased. We found out that it
is not necessary to take the theoretically expected limit
to recover the microcanonical states in the region between the canonical and
microcanonical tricritical points of the phase diagram. By analyzing the
entropy as a function of the magnetization we realize the existence of
unaccessible magnetic states as the energy is lowered, leading to a treaking of
ergodicity.Comment: 8 pages, 5 eps figures. Title modified, sections rewritten,
tricritical point calculations added. To appear in EPJ
Couplings of light I=0 scalar mesons to simple operators in the complex plane
The flavour and glue structure of the light scalar mesons in QCD are probed
by studying the couplings of the I=0 mesons and to the
operators , and to two photons. The Roy dispersive
representation for the amplitude is used to determine the
pole positions as well as the residues in the complex plane. On the real axis,
is constrained to solve the Roy equation together with elastic
unitarity up to the K\Kbar threshold leading to an improved description of
the . The problem of using a two-particle threshold as a matching
point is discussed. A simple relation is established between the coupling of a
scalar meson to an operator and the value of the related pion form-factor
computed at the resonance pole. Pion scalar form-factors as well as two-photon
partial-wave amplitudes are expressed as coupled-channel Omn\`es dispersive
representations. Subtraction constants are constrained by chiral symmetry and
experimental data. Comparison of our results for the couplings with
earlier determinations of the analogous couplings of the lightest I=1 and
scalar mesons are compatible with an assignment of the ,
, , into a nonet. Concerning the gluonic operator
we find a significant coupling to both the and the
.Comment: 31 pages, 5 figure