31,033 research outputs found
Inexactness of the Hydro-Thermal Coordination Semidefinite Relaxation
Hydro-thermal coordination is the problem of determining the optimal economic
dispatch of hydro and thermal power plants over time. The physics of
hydroelectricity generation is commonly simplified in the literature to account
for its fundamentally nonlinear nature. Advances in convex relaxation theory
have allowed the advent of Shor's semidefinite programming (SDP) relaxations of
quadratic models of the problem. This paper shows how a recently published SDP
relaxation is only exact if a very strict condition regarding turbine
efficiency is observed, failing otherwise. It further proposes the use of a set
of convex envelopes as a strategy to successfully obtain a stricter lower bound
of the optimal solution. This strategy is combined with a standard iterative
convex-concave procedure to recover a stationary point of the original
non-convex problem.Comment: Submitted to IEEE PES General Meeting 201
Probability distribution of the order parameter
The probability distribution of the order parameter is exploited in order to
obtain the criticality of magnetic systems. Monte Carlo simulations have been
employed by using single spin flip Metropolis algorithm aided by finite-size
scaling and histogram reweighting techniques. A method is proposed to obtain
this probability distribution even when the transition temperature of the model
is unknown. A test is performed on the two-dimensional spin-1/2 and spin-1
Ising model and the results show that the present procedure can be quite
efficient and accurate to describe the criticality of the system.Comment: 5 pages, 7 figures, to appear in Braz. J. Phys. 34, June 200
Controlled Shock Shells and Intracluster Fusion Reactions in the Explosion of Large Clusters
The ion phase-space dynamics in the Coulomb explosion of very large ( atoms) deuterium clusters can be tailored using two consecutive
laser pulses with different intensities and an appropriate time delay. For
suitable sets of laser parameters (intensities and delay), large-scale shock
shells form during the explosion, thus highly increasing the probability of
fusion reactions within the single exploding clusters. In order to analyze the
ion dynamics and evaluate the intracluster reaction rate, a one-dimensional
theory is used, which approximately accounts for the electron expulsion from
the clusters. It is found that, for very large clusters (initial radius
100 nm), and optimal laser parameters, the intracluster fusion yield becomes
comparable to the intercluster fusion yield. The validity of the results is
confirmed with three-dimensional particle-in-cell simulations.Comment: 25 pages, 11 figures, to appear in Physical Review
Probing the gluon density of the proton in the exclusive photoproduction of vector mesons at the LHC: A phenomenological analysis
The current uncertainty on the gluon density extracted from the global parton
analysis is large in the kinematical range of small values of the Bjorken -
variable and low values of the hard scale . An alternative to reduces this
uncertainty is the analysis of the exclusive vector meson photoproduction in
photon - hadron and hadron - hadron collisions. This process offers a unique
opportunity to constrain the gluon density of the proton, since its cross
section is proportional to the gluon density squared. In this paper we consider
current parametrizations for the gluon distribution and estimate the exclusive
vector meson photoproduction cross section at HERA and LHC using the leading
logarithmic formalism. We perform a fit of the normalization of the
cross section and the value of the hard scale for the process and demonstrate
that the current LHCb experimental data are better described by models that
assume a slow increasing of the gluon distribution at small - and low
.Comment: 8 pages, 6 figures, 1 table. Version published in European Physical
Journal
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