12,510 research outputs found
Dynamical generation and dynamical reconstruction
A definition of `dynamical generation', a hotly debated topic at present, is
proposed and its implications are discussed. This definition, in turn, leads to
a method allowing to distinguish in principle tetraquark and molecular states.
The different concept of `dynamical reconstruction' is also introduced and
applies to the generation of preexisting mesons (quark-antiquark, glueballs,
>...) via unitarization methods applied to low-energy effective Lagrangians.
Large arguments play an important role in all these investigations. A
simple toy model with two scalar fields is introduced to elucidate these
concepts. The large behavior of the parameters is chosen in order that
the two scalar fields behave as quark-antiquark mesons. When the heavier field
is integrated out, one is left with an effective Lagrangian with the lighter
field only. A unitarization method applied to the latter allows to
`reconstruct' the heavier `quarkonium-like' field, which was previously
integrated out. It is shown that a Bethe-Salpeter (BS) analysis is capable to
reproduce the preformed quark-antiquark state. However, when only the lowest
term of the effective Lagrangian is retained, the large limit of the
reconstructed state is not reproduced: instead of the correct large
quarkonium limit, it fades out as a molecular state would do. Implications of
these results are presented: it is proposed that axial-vector, tensor and
(some) scalar mesons just above 1 GeV obtained via the BS approach from the
corresponding low-energy, effective Lagrangian in which only the lowest term is
kept, are quarkonia states, in agreement with the constituent quark model,
although they might fade away as molecular states in the large limit.Comment: 14 pages, 3 figure
The Instability Strip for Pre--Main-Sequence Stars
We investigate the pulsational properties of Pre--Main-Sequence (PMS) stars
by means of linear and nonlinear calculations. The equilibrium models were
taken from models evolved from the protostellar birthline to the ZAMS for
masses in the range 1 to 4 solar masses. The nonlinear analysis allows us to
define the instability strip of PMS stars in the HR diagram. These models are
used to constrain the internal structure of young stars and to test
evolutionary models. We compare our results with observations of the best case
of a pulsating young star, HR~5999, and we also identify possible candidates
for pulsational variability among known Herbig Ae/Be stars which are located
within or close to the instability strip boundaries.Comment: 14 pages, three postscript figures, accepted for publication on the
Astrophysical Journal Letter
An Exact Monte Carlo Method for Continuum Fermion Systems
We offer a new proposal for the Monte Carlo treatment of many-fermion systems
in continuous space. It is based upon Diffusion Monte Carlo with significant
modifications: correlated pairs of random walkers that carry opposite signs;
different functions ``guide'' walkers of different signs; the Gaussians used
for members of a pair are correlated; walkers can cancel so as to conserve
their expected future contributions. We report results for free-fermion systems
and a fermion fluid with 14 He atoms, where it proves stable and correct.
Its computational complexity grows with particle number, but slowly enough to
make interesting physics within reach of contemporary computers.Comment: latex source, 3 separated figures (2 in jpg format, 1 in eps format
Folding of the Triangular Lattice with Quenched Random Bending Rigidity
We study the problem of folding of the regular triangular lattice in the
presence of a quenched random bending rigidity + or - K and a magnetic field h
(conjugate to the local normal vectors to the triangles). The randomness in the
bending energy can be understood as arising from a prior marking of the lattice
with quenched creases on which folds are favored. We consider three types of
quenched randomness: (1) a ``physical'' randomness where the creases arise from
some prior random folding; (2) a Mattis-like randomness where creases are
domain walls of some quenched spin system; (3) an Edwards-Anderson-like
randomness where the bending energy is + or - K at random independently on each
bond. The corresponding (K,h) phase diagrams are determined in the hexagon
approximation of the cluster variation method. Depending on the type of
randomness, the system shows essentially different behaviors.Comment: uses harvmac (l), epsf, 17 figs included, uuencoded, tar compresse
Folding of the Triangular Lattice in the FCC Lattice with Quenched Random Spontaneous Curvature
We study the folding of the regular two-dimensional triangular lattice
embedded in the regular three-dimensional Face Centered Cubic lattice, in the
presence of quenched random spontaneous curvature. We consider two types of
quenched randomness: (1) a ``physical'' randomness arising from a prior random
folding of the lattice, creating a prefered spontaneous curvature on the bonds;
(2) a simple randomness where the spontaneous curvature is chosen at random
independently on each bond. We study the folding transitions of the two models
within the hexagon approximation of the Cluster Variation Method. Depending on
the type of randomness, the system shows different behaviors. We finally
discuss a Hopfield-like model as an extension of the physical randomness
problem to account for the case where several different configurations are
stored in the prior pre-folding process.Comment: 12 pages, Tex (harvmac.tex), 4 figures. J.Phys.A (in press
Photon Wave-packet Manipulation via Dynamic Electromagnetically Induced Transparency in Multilayer Structures
We present a Maxwell-Bloch description of the dynamics of a light pulse
propagating through a spatially inhomogeneous system consisting of alternating
layers of EIT media and vacuum. We study the effect of a dynamical modulation
of the EIT control field on the shape of the wave packet: interesting effects
due to the presence of interfaces with group velocity mismatch are found. An
effective description based on a continuity equation is developed. Modulation
schemes that can be realized in ultracold atomic samples with standard
experimental techniques are proposed and discussed
Effects of dislocation density on injection and temperature sensitivity of InGaN LED emission spectra: a combined experimental and simulation approach
The aim of this paper is to describe a combined simulation and characterization activity carried out on blue LEDs grown on templates with different threading dislocation densities (TDDs)
Lattice two-point functions and conformal invariance
A new realization of the conformal algebra is studied which mimics the
behaviour of a statistical system on a discrete albeit infinite lattice. The
two-point function is found from the requirement that it transforms covariantly
under this realization. The result is in agreement with explicit lattice
calculations of the Ising model and the dimensional spherical
model. A hard core is found which is not present in the continuum. For a
semi-infinite lattice, profiles are also obtained.Comment: 5 pages, plain Tex with IOP macros, no figure
Quantum Knizhnik-Zamolodchikov equation: reflecting boundary conditions and combinatorics
We consider the level 1 solution of quantum Knizhnik-Zamolodchikov equation
with reflecting boundary conditions which is relevant to the Temperley--Lieb
model of loops on a strip. By use of integral formulae we prove conjectures
relating it to the weighted enumeration of Cyclically Symmetric Transpose
Complement Plane Partitions and related combinatorial objects
Generalized modularity matrices
Various modularity matrices appeared in the recent literature on network
analysis and algebraic graph theory. Their purpose is to allow writing as
quadratic forms certain combinatorial functions appearing in the framework of
graph clustering problems. In this paper we put in evidence certain common
traits of various modularity matrices and shed light on their spectral
properties that are at the basis of various theoretical results and practical
spectral-type algorithms for community detection
- …