Comments on discrete time in quantum mechanics

The possibility that time can be regarded as a discrete parameter is re-examined. We study the dynamics of the free particle and find in some cases superluminal propagation

Structure of the Coulomb and unitarity corrections to the cross section of e+e−e^+e^- pair production in ultra-relativistic nuclear collisions

We analyze the structure of the Coulomb and unitarity corrections to the single pair production as well as the cross section for the multiple pair production. In the external field approximation we consider the probability of $e^+e^-$ pair production at fixed impact parameter $\rho$ between colliding ultra-relativistic heavy nuclei. We obtain the analytical result for this probability at large $\rho$ as compared to the electron Compton wavelength. We estimate also the unitary corrections to the total cross section of the process.Comment: 10 pages, 2 figures, RevTeX, references correcte

Modelling species presence–absence in the ecological niche theory framework using shape-constrained generalized additive models

According to ecological niche theory, species response curves are unimodal with respect to environmental gradients. A variety of statistical methods have been developed for species distribution modelling. A general problem with most of these habitat modelling approaches is that the estimated response curves can display biologically implausible shapes which do not respect ecological niche theory. This work proposes using shapeconstrained generalized additive models (SC-GAMs) to build species distribution models under the ecological niche theory framework, imposing concavity constraints in the linear predictor scale. Based on a simulation study and a real data application, we compared performance with respect to other regression models without shape-constraints (such as standard GLMs and GAMs with varying degrees of freedom) and also to models based on so-called “Plateau” climate-envelopes. The imposition of concavity for response curves resulted in a good balance between the goodness of fit (GOF) and agreement with ecological niche theory. The approach has been applied to fit distribution models for three fish species given several environmental variables

Laser heating of a sintered oxide superconductor

Raman spectroscopy, in which a laser beam serves the dual role of exciting the Raman spectrum and annealing the sample, shows promise as a means of investigating oxygen effects in the oxide superconductors. A technique is described, based on measurements of the ratios of the areas of corresponding peaks in the anti-Stokes and Stokes spectra, whereby the temperature of the illuminated region of the sample can be determined as a function of the power in the incident laser beam. It is found that, for sintered samples of bismuth 2122, a small correction must be made for the departure from thermodynamic equilibrium induced by the pumping effect of the laser beam

Shutdown of an offshore wind power plant without using a brake to meet the required ramp rate in various storm-driven conditions

This paper proposes an offshore WPP (wind power plant) shutdown algorithm that does not use a braking system and meets the required ramp rate in the grid code in various storm-driven conditions. The proposed algorithm determines the number of WGs (wind generators) to shut down simultaneously to achieve this requirement without using brakes. Based on the storm speed and direction measured at a WM (wind mast) installed several kilometers away from the WPP, the storm-arrival time from the WM to each WG is calculated. Then, an arrival-ordered sequence is generated for the WGs based on these storm-arrival times. The WGs are grouped in a predetermined number to shut down simultaneously. The shutdown start- and end-times of the WGs are determined by considering the storm-arrival time and the shutdown duration time. The algorithm re-calculates the storm-arrival times and the shutdown start- and end-times of the WGs if the storm speed and/or direction change. The various test results demonstrate that the algorithm successfully shuts down the WPP without using a brake by meeting the required ramp rate even when the storm speed and direction change

Density waves in dry granular media falling through a vertical pipe

We report experimental measurements of density waves in granular materials flowing down in a capillary tube. The density wave regime occurs at intermediate flow rates between a low density free fall regime and a high compactness slower flow.Comment: LaTeX file, 17 pages, 6 EPS figures, Phys.Rev.E (Feb.1996

Future Directions in Parity Violation: From Quarks to the Cosmos

I discuss the prospects for future studies of parity-violating (PV) interactions at low energies and the insights they might provide about open questions in the Standard Model as well as physics that lies beyond it. I cover four types of parity-violating observables: PV electron scattering; PV hadronic interactions; PV correlations in weak decays; and searches for the permanent electric dipole moments of quantum systems.Comment: Talk given at PAVI 06 workshop on parity-violating interactions, Milos, Greece (May, 2006); 10 page