24,064 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
Diamagnetic Interactions in Disordered Suspensions of Metastable Superconducting Granules
The simulation of the transition sequence of superheated Type I
superconducting granules (SSG) in disordered suspensions when an external
magnetic field is slowly increased from zero has been studied. Simulation takes
into account diamagnetic interactions and the presence of surface defects.
Results have been obtained for the transition sequence and surface fields
distribution covering a wide range of densities. These results are compared
with previous analytical perturbative theory, which provides qualitative
information on transitions and surface magnetic fields during transitions, but
with a range of validity apparently limited to extremely dilute samples.
Simulations taking into account the complete diamagnetic interactions between
spheres appear to be a promising tool in interpreting SSG experiments, in
applications such as particle detectors, and in some fundamental calculations
of Solid State Physics.Comment: 25 pages, 11 figures, accepted for publication in European Physics
Journal
Semiclassical ordering in the large-N pyrochlore antiferromagnet
We study the semiclassical limit of the generalization of the
pyrochlore lattice Heisenberg antiferromagnet by expanding about the saddlepoint in powers of a generalized inverse spin. To leading order,
we write down an effective Hamiltonian as a series in loops on the lattice.
Using this as a formula for calculating the energy of any classical ground
state, we perform Monte-Carlo simulations and find a unique collinear ground
state. This state is not a ground state of linear spin-wave theory, and can
therefore not be a physical (N=1) semiclassical ground state.Comment: 4 pages, 4 eps figures; published versio
Realizing Colloidal Artificial Ice on Arrays of Optical Traps
We demonstrate how a colloidal version of artificial ice can be realized on
optical trap lattices. Using numerical simulations, we show that this system
obeys the ice rules and that for strong colloid-colloid interactions, an
ordered ground state appears. We show that the ice rule ordering can occur for
systems with as few as twenty-four traps and that the ordering transition can
be observed at constant temperature by varying the barrier strength of the
traps.Comment: 4 pages, 3 postscript figures; version to appear in Phys. Rev. Let
- …