28,652 research outputs found
Upper Energy Limit of Heavy Baryon Chiral Perturbation Theory in Neutral Pion Photoproduction
With the availability of the new neutral pion photoproduction from the proton
data from the A2 and CB-TAPS Collaborations at Mainz it is mandatory to revisit
Heavy Baryon Chiral Perturbation Theory (HBChPT) and address the extraction of
the partial waves as well as other issues such as the value of the low-energy
constants, the energy range where the calculation provides a good agreement
with the data and the impact of unitarity. We find that, within the current
experimental status, HBChPT with the fitted LECs gives a good agreement with
the existing neutral pion photoproduction data up to 170 MeV and that
imposing unitarity does not improve this picture. Above this energy the data
call for further improvement in the theory such as the explicit inclusion of
the \Delta (1232). We also find that data and multipoles can be well described
up to 185 MeV with Taylor expansions in the partial waves up to first
order in pion energy.Comment: 6 pages, 5 figures, version to be published in Physics Letters
Selection of the Argentine indicator region
Determined from available Argentine crop statistics, selection of the Indicator Region was based on the highest wheat, corn, and soybean producing provinces, which were: Buenos Aires, Cordoba, Entre Rios, and Santa Fe. Each province in Argentina was examined for the availability of LANDSAT data; area, yield and production statistics; crop calendars; and other ancillary data. The Argentine Indicator Region is described
The phase diagram of ice: a quasi-harmonic study based on a flexible water model
The phase diagram of ice is studied by a quasi-harmonic approximation. The
free energy of all experimentally known ice phases has been calculated with the
flexible q-TIP4P/F model of water. The only exception is the high pressure ice
X, in which the presence of symmetric O-H-O bonds prevents its modeling with
this empirical interatomic potential. The simplicity of our approach allows us
to study ice phases at state points of the T-P plane that have been omitted in
previous simulations using free energy methods based on thermodynamic
integration. The effect in the phase diagram of averaging the proton disorder
that appears in several ice phases has been studied. It is found particularly
relevant for ice III, at least for cell sizes typically used in phase
coexistence simulations. New insight into the capability of the employed water
model to describe the coexistence of ice phases is presented. We find that the
H-ordered ices IX and XIV, as well as the H-disordered ice XII, are
particularly stable for this water model. This fact disagrees with experimental
data. The unexpected large stability of ice IX is a property related to the
TIP4P-character of the water model. Only after omission of these three stable
ice phases, the calculated phase diagram becomes in reasonable qualitative
agreement to the experimental one in the T-P region corresponding to ice Ih,
II, III, V, and VI. The calculation of the phase diagram in the quantum and
classical limits shows that the most important quantum effect is the
stabilization of ice II due to its lower zero-point energy when compared to
that one of ices Ih, III, and V.Comment: 13 pages, 8 figures, 5 table
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