4,949 research outputs found
Instanton Contribution to the Proton and Neutron Electric Form Factors
We study the instanton contribution to the proton and neutron electric form
factors. Using the single instanton approximation, we perform the calculations
in a mixed time-momentum representation in order to obtain the form factors
directly in momentum space. We find good agreement with the experimentally
measured electric form factor of the proton. For the neutron, our result falls
short of the experimental data. We argue that this discrepancy is due to the
fact that we neglect the contribution of the sea quarks. We compare to lattice
calculations and a relativistic version of the quark-diquark model.Comment: 8 pages, 5 figures, updated references, to appear in Phys. Lett.
Simulating Stochastic Dynamics Using Large Time Steps
We present a novel approach to investigate the long-time stochastic dynamics
of multi-dimensional classical systems, in contact with a heat-bath. When the
potential energy landscape is rugged, the kinetics displays a decoupling of
short and long time scales and both
Molecular Dynamics (MD) or Monte Carlo (MC) simulations are generally
inefficient. Using a field theoretic approach, we perform analytically the
average over the short-time stochastic fluctuations. This way, we obtain an
effective theory, which generates the same long-time dynamics of the original
theory, but has a lower time resolution power. Such an approach is used to
develop an improved version of the MC algorithm, which is particularly suitable
to investigate the dynamics of rare conformational transitions. In the specific
case of molecular systems at room temperature, we show that elementary
integration time steps used to simulate the effective theory can be chosen a
factor ~100 larger than those used in the original theory. Our results are
illustrated and tested on a simple system, characterized by a rugged energy
landscape.Comment: 17 pager, 7 figure
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