Casimir energy for a scalar field with a frequency dependent boundary condition

We consider the vacuum energy for a scalar field subject to a frequency dependent boundary condition. The effect of a frequency cut-off is described in terms of an {\it incomplete} $\zeta$-function. The use of the Debye asymptotic expansion for Bessel functions allows to determine the dominant (volume, area, >...) terms in the Casimir energy. The possible interest of this kind of models for dielectric media (and its application to sonoluminescence) is also discussed.Comment: 7 pages, RevTeX. Version to appear in PRD (Introduction enlarged, references added

Electromagnetic nucleon form factors from QCD sum rules

The electromagnetic form factors of the nucleon, in the space-like region, are determined from three-point function Finite Energy QCD Sum Rules. The QCD calculation is performed to leading order in perturbation theory in the chiral limit, and to leading order in the non-perturbative power corrections. The results for the Dirac form factor, $F_1(q^2)$, are in very good agreement with data for both the proton and the neutron, in the currently accessible experimental region of momentum transfers. This is not the case, though, for the Pauli form factor $F_2(q^2)$, which has a soft $q^2$-dependence proportional to the quark condensate $$.Comment: Replaced Version. An error has been corrected in the numerical evaluation of the Pauli form factor. This changes the results for F_2(q^2), as well as the conclusion

About the Dirac Equation with a δ\delta potential

An elementary treatment of the Dirac Equation in the presence of a three-dimensional spherically symmetric $\delta (r-r_0)$-potential is presented. We show how to handle the matching conditions in the configuration space, and discuss the occurrence of supercritical effects.Comment: 8 pages, 1 postscript figure, Latex, Revise

On the algebraic structure of rotationally invariant two-dimensional Hamiltonians on the noncommutative phase space

We study two-dimensional Hamiltonians in phase space with noncommutativity both in coordinates and momenta. We consider the generator of rotations on the noncommutative plane and the Lie algebra generated by Hermitian rotationally invariant quadratic forms of noncommutative dynamical variables. We show that two quantum phases are possible, characterized by the Lie algebras $sl(2,\mathbb{R})$ or $su(2)$ according to the relation between the noncommutativity parameters, with the rotation generator related with the Casimir operator. From this algebraic perspective, we analyze the spectrum of some simple models with nonrelativistic rotationally invariant Hamiltonians in this noncommutative phase space, as the isotropic harmonic oscillator, the Landau problem and the cylindrical well potential. PACS: 03.65.-w; 03.65.Fd MSC: 81R05; 20C35; 22E70Comment: 49 pages. No figures. Version to appear in JP

Chiral Condensates in Quark and nuclear Matter

We present a novel treatment for calculating the in-medium quark condensates. The advantage of this approach is that one does not need to make further assumptions on the derivatives of model parameters with respect to the quark current mass. The normally accepted model-independent result in nuclear matter is naturally reproduced. The change of the quark condensate induced by interactions depends on the incompressibility of nuclear matter. When it is greater than 260 MeV, the density at which the condensate vanishes is higher than that from the linear extrapolation. For the chiral condensate in quark matter, a similar model-independent linear behavior is found at lower densities, which means that the decreasing speed of the condensate in quark matter is merely half of that in nuclear matter if the pion-nucleon sigma commutator is six times the average current mass of u and d quarks. The modification due to QCD-like interactions is found to slow the decreasing speed of the condensate, compared with the linear extrapolation.Comment: 12 pages, 7 figures, revtex4 styl

Change in Selected Sensitive Psychological Areas of the Only Child When a Sibling is Born

A study was undertaken to see if, through the use of story projective fables, data could be obtained about the change in selected sensitive psychological areas of an only child when a sibling is born. Projective stories from the Fine Revision of the Despert Fables were used to form brief tests of the sensitive psychological areas of parental rejection/sibling rivalry, dependency and aggression/hostility. These tests were given, both before and six weeks after the birth of a sibling, to fifty children from families in the Santa Clara Valley. Criteria used in selecting the children limited the subjects to those who were old enough to talk, English speaking, under the age of six years and having no known deformity. Each was the only child of an intact family who anticipated the birth of a baby. The economic level of the families was about the same since the fathers were employed as skilled workmen in industry. The children\u27s responses to the story projective fables were studied for the presence or absence of the psychodynamic representing the selected sensitive psychological area, judgment being based on the work of Peixotto with young elementary school children. If the psychodynamic were present a score of 1 was given for the projective story and if not a score of 0. The scores were added together for the projective stories of each test or psychodynamic for every child and the score before the birth of the sibling was paired with the score after the birth of the baby to find the mean difference of the paired responses for each test or psychodynamic. From the mean difference the significance of difference was obtained by using the t score. The increase in all three psychodynamics or selected sensitive psychological areas was highly significant and at the 1 percent level. This increase in the presence of all three psychodynamics, as determined through the use of story projective fables, was probably not caused by chance, but, since many other variables were held constant, by the birth of the sibling. This would suggest that children do change at the birth of a sibling and that this change can be measured. Nurses are in a position to initiate and nurture parents\u27 interest in helping their child during this great change in his environment and in himself. Nurses should assist parents in finding out how their older child feels and in utilizing factors indicated by research to assist the child with his feelings at this time. Additional research about the feelings and adjustment of a child at the birth of a sibling is needed

Complementary approaches to understanding the plant circadian clock

Circadian clocks are oscillatory genetic networks that help organisms adapt to the 24-hour day/night cycle. The clock of the green alga Ostreococcus tauri is the simplest plant clock discovered so far. Its many advantages as an experimental system facilitate the testing of computational predictions. We present a model of the Ostreococcus clock in the stochastic process algebra Bio-PEPA and exploit its mapping to different analysis techniques, such as ordinary differential equations, stochastic simulation algorithms and model-checking. The small number of molecules reported for this system tests the limits of the continuous approximation underlying differential equations. We investigate the difference between continuous-deterministic and discrete-stochastic approaches. Stochastic simulation and model-checking allow us to formulate new hypotheses on the system behaviour, such as the presence of self-sustained oscillations in single cells under constant light conditions. We investigate how to model the timing of dawn and dusk in the context of model-checking, which we use to compute how the probability distributions of key biochemical species change over time. These show that the relative variation in expression level is smallest at the time of peak expression, making peak time an optimal experimental phase marker. Building on these analyses, we use approaches from evolutionary systems biology to investigate how changes in the rate of mRNA degradation impacts the phase of a key protein likely to affect fitness. We explore how robust this circadian clock is towards such potential mutational changes in its underlying biochemistry. Our work shows that multiple approaches lead to a more complete understanding of the clock

QED vacuum fluctuations and induced electric dipole moment of the neutron

Quantum fluctuations in the QED vacuum generate non-linear effects, such as peculiar induced electromagnetic fields. In particular, we show here that an electrically neutral particle, possessing a magnetic dipole moment, develops an induced electric dipole-type moment with unusual angular dependence, when immersed in a quasistatic, constant external electric field. The calculation of this effect is done in the framework of the Euler-Heisenberg effective QED Lagrangian, corresponding to the weak field asymptotic expansion of the effective action to one-loop order. It is argued that the neutron might be a good candidate to probe this signal of non-linearity in QED.Comment: A misprint has been corrected, and three new references have been adde