Driven collective instabilities in magneto-optical traps: a fluid-dynamical approach

We present a theoretical model to describe an instability mechanism in ultra-cold gases, where long-range interactions are taken into account. Focusing on the nonlinear coupling between the collective (plasma-like) and the center-of-mass modes, we show that the resulting dynamics is governed by a parametric equation of the generalized Mathieu type and compute the corresponding stability chart. We apply our model to typical ranges of magneto-optical traps (MOT) parameters and find a good agreement with previous experimental observations.Comment: 4 pages, 3 figures. Some minor changes in the published version

Technical Note: Could benzalkonium chloride be a suitable alternative to mercuric chloride for preservation of seawater samples?

Instrumental equipment unsuitable or unavailable for fieldwork as well as lack of ship space can necessitate the preservation of seawater samples prior to analysis in a shore-based laboratory. Mercuric chloride (HgCl2) is routinely used for such preservation, but its handling and subsequent disposal incur environmental risks and significant expense. There is therefore a strong motivation to find less hazardous alternatives. Benzalkonium chloride (BAC) has been used previously as microbial inhibitor for freshwater samples. Here, we assess the use of BAC for marine samples prior to the measurement of oxygen-to-argon (O2 / Ar) ratios, as used for the determination of biological net community production. BAC at a concentration of 50 mg dm−3 inhibited microbial activity for at least 3 days in samples tested with chlorophyll a (Chl a) concentrations up to 1 mg m−3. BAC concentrations of 100 and 200 mg dm−3 were no more effective than 50 mg dm−3 . With fewer risks to human health and the environment, and no requirement for expensive waste disposal, BAC could be a viable alternative to HgCl2 for short-term preservation of seawater samples, but is not a replacement for HgCl2 in the case of oxygen triple isotope analysis, which requires storage over weeks to months. In any event, further tests on a case-by-case basis should be undertaken if use of BAC was considered, since its inhibitory activity may depend on concentration and composition of the microbial community

The time-evolution of bias

We study the evolution of the bias factor b and the mass-galaxy correlation coefficient r in a simple analytic model for galaxy formation and the gravitational growth of clustering. The model shows that b and r can be strongly time-dependent, but tend to approach unity even if galaxy formation never ends as the gravitational growth of clustering debiases the older galaxies. The presence of random fluctuations in the sites of galaxy formation relative to the mass distribution can cause large and rapidly falling bias values at high redshift.Comment: 4 pages, with 2 figures included. Typos corrected to match published ApJL version. Color figure and links at http://www.sns.ias.edu/~max/bias.html or from [email protected]

A wave-envelope of sound propagation in nonuniform circular ducts with compressible mean flows

An acoustic theory is developed to determine the sound transmission and attenuation through an infinite, hard-walled or lined circular duct carrying compressible, sheared, mean flows and having a variable cross section. The theory is applicable to large as well as small axial variations, as long as the mean flow does not separate. The technique is based on solving for the envelopes of the quasi-parallel acoustic modes that exist in the duct instead of solving for the actual wave, thereby reducing the computation time and the round-off error encountered in purely numerical techniques. The solution recovers the solution based on the method of multiple scales for slowly varying duct geometry. A computer program was developed based on the wave-envelope analysis for general mean flows. Results are presented for the reflection and transmission coefficients as well as the acoustic pressure distributions for a number of conditions: both straight and variable area ducts with and without liners and mean flows from very low to high subsonic speeds are considered

Transmission of sound through nonuniform circular ducts with compressible mean flows

An acoustic theory is developed to determine the sound transmission and attenuation through an infinite, hard-walled or lined, circular duct carrying compressible, sheared, mean flows and having a variable cross section. The theory is applicable to large as well as small axial variations, as long as the mean flow does not separate. Although the theory is described for circular ducts, it is applicable to other duct configurations - annular, two dimensional, and rectangular. The theory is described for the linear problem, but the technique is general and has the advantage of being applicable to the nonlinear case as well as the linear case. The technique is based on solving for the envelopes of the quasi-parallel acoustic modes that exist in the duct instead of solving for the actual wave. A computer program was developed. The mean flow model consists of a one dimensional flow in the core and a quarter-sine profile in the boundary layer. Results are presented for the reflection and transmission coefficients in ducts with varying slopes and carrying different mean flows

PRICE VERSUS STOCK EFFECT POLICIES FOR REDUCING EXCESS MILK PRODUCTION

This article presents a comparative dynamic analysis of the market impact of alternative U.S. policies designed to reduce excess capacity in milk production. Two policy options are examined based on an econometric model of the dairy industry and a dynamic simulation of the system. The stock effect policy relies on voluntary reductions in cow numbers to reduce milk supplies, while the price effect policy makes use of reductions in the support price levels to achieve the same goal. The simulation results are used to evaluate equilibrium prices and quantities for the farm and retail markets, government costs, and consumer and producer surpluses from 1986 to 1995 for each policy alternative. The analysis shows that farmers are better off under a voluntary supply control program, while consumers are better off under a support price reduction policy.Agricultural and Food Policy, Livestock Production/Industries,

A Kohn-Sham system at zero temperature

An one-dimensional Kohn-Sham system for spin particles is considered which effectively describes semiconductor {nano}structures and which is investigated at zero temperature. We prove the existence of solutions and derive a priori estimates. For this purpose we find estimates for eigenvalues of the Schr\"odinger operator with effective Kohn-Sham potential and obtain $W^{1,2}$-bounds of the associated particle density operator. Afterwards, compactness and continuity results allow to apply Schauder's fixed point theorem. In case of vanishing exchange-correlation potential uniqueness is shown by monotonicity arguments. Finally, we investigate the behavior of the system if the temperature approaches zero.Comment: 27 page

Nuclear energy density functional from chiral pion-nucleon dynamics: Isovector spin-orbit terms

We extend a recent calculation of the nuclear energy density functional in the systematic framework of chiral perturbation theory by computing the isovector spin-orbit terms: $(\vec \nabla \rho_p- \vec \nabla \rho_n)\cdot(\vec J_p-\vec J_n) G_{so}(k_f)+ (\vec J_p-\vec J_n)^2 G_J(k_f)$. The calculation includes the one-pion exchange Fock diagram and the iterated one-pion exchange Hartree and Fock diagrams. From these few leading order contributions in the small momentum expansion one obtains already a good equation of state of isospin-symmetric nuclear matter. We find that the parameterfree results for the (density-dependent) strength functions $G_{so}(k_f)$ and $G_J(k_f)$ agree fairly well with that of phenomenological Skyrme forces for densities $\rho > \rho_0/10$. At very low densities a strong variation of the strength functions $G_{so}(k_f)$ and $G_J(k_f)$ with density sets in. This has to do with chiral singularities $m_\pi^{-1}$ and the presence of two competing small mass scales $k_f$ and $m_\pi$. The novel density dependencies of $G_{so}(k_f)$ and $G_J(k_f)$ as predicted by our parameterfree (leading order) calculation should be examined in nuclear structure calculations.Comment: 9 pages, 3 figure, published in: Physical Review C68, 014323 (2003

Electroweak corrections and anomalous triple gauge-boson couplings in WW and WZ production at the LHC

We have analysed the production of WW and WZ vector-boson pairs at the LHC. These processes give rise to four-fermion final states, and are particularly sensitive to possible non-standard trilinear gauge-boson couplings. We have studied the interplay between the influence of these anomalous couplings and the effect of the complete logarithmic electroweak O(\alpha) corrections. Radiative corrections to the Standard Model processes in double-pole approximation and non-standard terms due to trilinear couplings are implemented into a Monte Carlo program for p p -> 4f (+\gamma) with final states involving four or two charged leptons. We numerically investigate purely leptonic final states and find that electroweak corrections can fake new-physics signals, modifying the observables by the same amount and shape, in kinematical regions of statistical significance.Comment: 19 pages, LaTex, 12 eps figure