12,620 research outputs found
Two phase transitions in the fully frustrated model
The fully frustrated model on a square lattice is studied by means of
Monte Carlo simulations. A Kosterlitz-Thouless transition is found at , followed by an ordinary Ising transition at a slightly
higher temperature, . The non-Ising exponents reported by
others, are explained as a failure of finite size scaling due to the screening
length associated with the nearby Kosterlitz-Thouless transition.Comment: REVTEX file, 8 pages, 5 figures in uuencoded postscrip
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Mars simulated exposure and the characteristic Raman biosignatures of amino acids and halophilic microbes
Though Raman bands of α-amino acids (AA) are well documented, often only the strongest intensity bands are quoted as identifiers (e.g. Jenkins et al., 2005; De Gelder et al., 2007; Zhu et al., 2011). Unknown regolith mixtures on Mars-sampling missions could obscure these bands. Here the case is made for determining, via a statistical method, sets of characteristic bands to be used as identifiers, independent of band intensity or number of bands (Rolfe et al., 2016). AA have upwards of 25 potentially identifying bands and this method defines sets of 10–19 bands per AA. Examination of AA-doped Mars-like basalt resulted in a maximum of eight bands being identified, as some characteristic bands were obscured by mineral bands, including the strongest intensity band in some cases. This proved the need for characteristic bands to be defined, enabling successful identification of AA. The ESA ExoMars Rover mission will crush and then pass the sample to the Raman Laser Spectrometer. We crushed a Mars-like basalt to a similar grain size expected to be created by the rover. Our samples were doped with 1 % (by weight) AA samples, resulting in no detection of AA, because of loss of original spatial context and spaces between the grains. We recommend that Raman spectroscopy on future missions should be conducted before the sample is crushed. Halite-entombed halophilic microbes, known to survive being entombed, were exposed to Mars-like surface (including temperature, pressure, atmospheric composition and UV) and freeze-thaw cycle (plus pressure and atmospheric composition) conditions. This test on the survival of the microbes showed that survival rates quickly deteriorated in surface conditions, but freeze-thaw cycle samples had well preserved Raman biosignatures, indicating that similar signatures could be detectable on Mars if similar life persists in evaporitic material or brines today
The Contribution of the Light Quark Condensate to the Pion-Nucleon Sigma Term
There has been a discrepancy between values of the pion-nucleon sigma term
extracted by two different methods for many years. Analysis of recent high
precision pion-nucleon data has widened the gap between the two determinations.
It is argued that the two extractions correspond to different quantities and
that the difference between them can be understood and calculated.Comment: Modern Physics Letters A (in press
Unitary ambiguity in the extraction of the E2/M1 ratio for the transition
The resonant electric quadrupole amplitude in the transition is of great interest for the understanding of
baryon structure. Various dynamical models have been developed to extract it
from the corresponding photoproduction multipole of pions on nucleons. It is
shown that once such a model is specified, a whole class of unitarily
equivalent models can be constructed, all of them providing exactly the same
fit to the experimental data. However, they may predict quite different
resonant amplitudes. Therefore, the extraction of the E2/M1() ratio (bare or dressed) which is based on a dynamical
model using a largely phenomenological interaction is not unique.Comment: 10 pages revtex including 4 postscript figure
Bayes' theorem and its applications in animal behaviour
Bayesian decision theory can be used to model animal behaviour. In this paper we give an overview of the theoretical concepts in such models. We also review the biological contexts in which Bayesian models have been applied, and outline some directions where future studies would be useful. Bayesian decision theory, when applied to animal behaviour, is based on the assumption that the individual has some sort of "prior opinion" of the possible states of the world. This may, for example, be a previously experienced distribution of qualities of food patches, or qualities of potential mates. The animal is then assumed to be able use sampling information to arrive at a "posterior opinion", concerning e.g. the quality of a given food patch, or the average qualities of mates in a year. A correctly formulated Bayesian model predicts how animals may combine previous experience with sampling information to make optimal decisions. We argue that the assumption that animals may have "prior opinions" is reasonable. Their priors may come from one or both of two sources: either from their own individual experience, gained while sampling the environment, or from an adaptation to the environment experienced by previous generations. This means that we should often expect to see "Bayesian-like" decision-making in nature
From scalar to string confinement
We outline a connection between scalar quark confinement, a
phenomenologically successful concept heretofore lacking fundamental
justification, and QCD. Although scalar confinement does not follow from QCD,
there is an interesting and close relationship between them. We develop a
simple model intermediate between scalar confinement and the QCD string for
illustrative purposes. Finally, we find the bound state masses of scalar,
time-component vector, and string confinement analytically through
semi-classical quantization.Comment: ReVTeX, 9 pages, 5 figure
Analytic Quantization of the QCD String
We perform an analytic semi-classical quantization of the straight QCD string
with one end fixed and a massless quark on the other, in the limits of orbital
and radial dominant motion. We compare our results to the exact numerical
semi-classical quantization. We observe that the numerical semi-classical
quantization agrees well with our exact numerical canonical quantization.Comment: RevTeX, 10 pages, 9 figure
Current-voltage characteristics of the two-dimensional XY model with Monte Carlo dynamics
Current-voltage characteristics and the linear resistance of the
two-dimensional XY model with and without external uniform current driving are
studied by Monte Carlo simulations. We apply the standard finite-size scaling
analysis to get the dynamic critical exponent at various temperatures. From
the comparison with the resistively-shunted junction dynamics, it is concluded
that is universal in the sense that it does not depend on details of
dynamics. This comparison also leads to the quantification of the time in the
Monte Carlo dynamic simulation.Comment: 5 pages in two columns including 5 figures, to appear in PR
Semi-leptonic B decays into higher charmed resonances
We apply HQET to semi-leptonic meson decays into a variety of excited
charm states. Using three realistic meson models with fermionic light degrees
of freedom, we examine the extent that the sum of exclusive single charmed
states account for the inclusive semi-leptonic decay rate. The consistency
of form factors with the Bjorken and Voloshin sum rules is also investigated.Comment: Latex, 27 pages. A few references and errors corrected, to appear in
Phys. Rev.
Critical Behavior of the Meissner Transition in the Lattice London Superconductor
We carry out Monte Carlo simulations of the three dimensional (3D) lattice
London superconductor in zero applied magnetic field, making a detailed finite
size scaling analysis of the Meissner transition. We find that the magnetic
penetration length \lambda, and the correlation length \xi, scale as \lambda ~
\xi ~ |t|^{-\nu}, with \nu = 0.66 \pm 0.03, consistent with ordinary 3D XY
universality, \nu_XY ~ 2/3. Our results confirm the anomalous scaling dimension
of magnetic field correlations at T_c.Comment: 4 pages, 5 ps figure
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