16,865 research outputs found
On free energies of the Ising model on the Cayley tree
We present, for the Ising model on the Cayley tree, some explicit formulae of
the free energies (and entropies) according to boundary conditions (b.c.). They
include translation-invariant, periodic, Dobrushin-like b.c., as well as those
corresponding to (recently discovered) weakly periodic Gibbs states. The later
are defined through a partition of the tree that induces a 4-edge-coloring. We
compute the density of each color.Comment: 18 pages, 4 figure
The Pulse Scale Conjecture and the Case of BATSE Trigger 2193
The pulses that compose gamma-ray bursts (GRBs) are hypothesized to have the
same shape at all energies, differing only by scale factors in time and
amplitude. This "Pulse Scale Conjecture" is confirmed here between energy
channels of the dominant pulse in GRB 930214c (BATSE trigger 2193), the single
most fluent single-pulsed GRB that occurred before May 1998. Furthermore,
pulses are hypothesized to start at the same time independent of energy. This
"Pulse Start Conjecture" is also confirmed in GRB 930214c. Analysis of GRB
930214c also shows that, in general, higher energy channels show shorter
temporal scale factors. Over the energy range 100 KeV - 1 MeV, it is found that
the temporal scale factors between a pulse measured at different energies are
related to that energy by a power law, possibly indicating a simple
relativistic mechanism is at work. To test robustness, the Pulse Start and
Pulse Scale Conjectures were also tested on the four next most fluent
single-pulse GRBs. Three of the four clearly passed, with a second smaller
pulse possibly confounding the discrepant test. Models where the pulse rise and
decay are created by different phenomena do not typically predict pulses that
satisfy both the Pulse Start Conjecture and the Pulse Scale Conjecture, unless
both processes are seen to undergo common time dilation.Comment: 19 pages, 9 figures, analysis revised and extended, accepted to Ap
Feynman-Hellmann theorem for resonances and the quest for QCD exotica
The generalization of the Feynman-Hellmann theorem for resonance states in
quantum field theory is derived. On the basis of this theorem, a criterion is
proposed to study the possible exotic nature of certain hadronic states
emerging in QCD. It is shown that this proposal is supported by explicit
calculations in Chiral Perturbation Theory and by large- arguments.
Analyzing recent lattice data on the quark mass dependence in the pseudoscalar,
vector meson, baryon octet and baryon decuplet sectors, we conclude that, as
expected, these are predominately quark-model states, albeit the corrections
are non-negligible.Comment: 26 pages, 2 figure
Molecular Gas and Star Formation in the Cartwheel
Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(J=1-0) observations
are used to study the cold molecular ISM of the Cartwheel ring galaxy and its
relation to HI and massive star formation (SF). CO moment maps find
M of H associated with the inner ring
(72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor
(). The spokes and disk are not detected. Analysis of the
inner ring's CO kinematics show it to be expanding ( km
s) implying an Myr age. Stack averaging reveals CO emission
in the starburst outer ring for the first time, but only where HI surface
density () is high, representing M for a metallicity appropriate
, giving small ( M
pc), molecular fraction (), and H depletion
timescales ( Myr). Elsewhere in the outer ring
M pc,
and Myr (all ). The inner ring and
nucleus are H-dominated and are consistent with local spiral SF laws.
in the outer ring appears independent of ,
or . The ISM's long confinement in the
robustly star forming rings of the Cartwheel and AM0644-741 may result in
either a large diffuse H component or an abundance of CO-faint low column
density molecular clouds. The H content of evolved starburst rings may
therefore be substantially larger. Due to its lower and age
the Cartwheel's inner ring has yet to reach this state. Alternately, the outer
ring may trigger efficient SF in an HI-dominated ISM.Comment: 10-pages text; 5-figure
Reconciling threshold and subthreshold expansions for pion-nucleon scattering
Heavy-baryon chiral perturbation theory (ChPT) at one loop fails in relating
the pion-nucleon amplitude in the physical region and for subthreshold
kinematics due to loop effects enhanced by large low-energy constants. Studying
the chiral convergence of threshold and subthreshold parameters up to fourth
order in the small-scale expansion, we address the question to what extent this
tension can be mitigated by including the as an explicit degree
of freedom and/or using a covariant formulation of baryon ChPT. We find that
the inclusion of the indeed reduces the low-energy constants to more
natural values and thereby improves consistency between threshold and
subthreshold kinematics. In addition, even in the -less theory the
resummation of corrections in the covariant scheme improves the results
markedly over the heavy-baryon formulation, in line with previous observations
in the single-baryon sector of ChPT that so far have evaded a profound
theoretical explanation.Comment: 10 pages, 4 tables, Mathematica notebook with the analytic
expressions for threshold and subthreshold parameters included as
supplementary material; journal versio
On the Four-Dimensional Diluted Ising Model
In this letter we show strong numerical evidence that the four dimensional
Diluted Ising Model for a large dilution is not described by the Mean Field
exponents. These results suggest the existence of a new fixed point with
non-gaussian exponents.Comment: 9 pages. compressed ps-file (uufiles
Critical properties of Ising model on Sierpinski fractals. A finite size scaling analysis approach
The present paper focuses on the order-disorder transition of an Ising model
on a self-similar lattice. We present a detailed numerical study, based on the
Monte Carlo method in conjunction with the finite size scaling method, of the
critical properties of the Ising model on some two dimensional deterministic
fractal lattices with different Hausdorff dimensions. Those with finite
ramification order do not display ordered phases at any finite temperature,
whereas the lattices with infinite connectivity show genuine critical behavior.
In particular we considered two Sierpinski carpets constructed using different
generators and characterized by Hausdorff dimensions d_H=log 8/log 3 = 1.8927..
and d_H=log 12/log 4 = 1.7924.., respectively.
The data show in a clear way the existence of an order-disorder transition at
finite temperature in both Sierpinski carpets.
By performing several Monte Carlo simulations at different temperatures and
on lattices of increasing size in conjunction with a finite size scaling
analysis, we were able to determine numerically the critical exponents in each
case and to provide an estimate of their errors.
Finally we considered the hyperscaling relation and found indications that it
holds, if one assumes that the relevant dimension in this case is the Hausdorff
dimension of the lattice.Comment: 21 pages, 7 figures; a new section has been added with results for a
second fractal; there are other minor change
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