A Simple Mode on a Highly Excited Background: Collective Strength and Damping in the Continuum

Simple states, such as isobaric analog states or giant resonances, embedded into continuum are typical for mesoscopic many-body quantum systems. Due to the coupling to compound states in the same energy range, a simple mode acquires a damping width ("internal" dynamics). When studied experimentally with the aid of various reactions, such states reveal enhanced cross sections in specific channels at corresponding resonance energies ("external" dynamics which include direct decay of a simple mode and decays of intrinsic compound states through their own channels). We consider the interplay between internal and external dynamics using a general formalism of the effective nonhermitian hamiltonian and looking at the situation both from "inside" (strength functions and spreading widths) and from "outside" (S-matrix, cross sections and delay times). The restoration of isospin purity and disappearance of the collective strength of giant resonances at high excitation energy are discussed as important particular manifestations of this complex interplay.Comment: 23 pages, LaTeX, 5 ps-figures included, to appear in PRC (Jule 1997

"Super-radiance" and the width of exotic baryons

It is suggested that the narrow width of the recently observed resonance $\Theta^{+}(1540)$ with strangeness $S=+1$ could be a result of the super-radiance mechanism of the redistribution of the widths of overlapping resonances due to their coupling through common decay channels.Comment: This is an update of the original version submitted on October 08, 2003; it includes consideration of an additional model and one new figur

How changing physical constants and violation of local position invariance may occur?

Light scalar fields very naturally appear in modern cosmological models, affecting such parameters of Standard Model as electromagnetic fine structure constant $\alpha$, dimensionless ratios of electron or quark mass to the QCD scale, $m_{e,q}/\Lambda_{QCD}$. Cosmological variations of these scalar fields should occur because of drastic changes of matter composition in Universe: the latest such event is rather recent (redshift $z\sim 0.5$), from matter to dark energy domination. In a two-brane model (we use as a pedagogical example) these modifications are due to changing distance to "the second brane", a massive companion of "our brane". Back from extra dimensions, massive bodies (stars or galaxies) can also affect physical constants. They have large scalar charge $Q_d$ proportional to number of particles which produces a Coulomb-like scalar field $\phi=Q_d/r$. This leads to a variation of the fundamental constants proportional to the gravitational potential, e.g. $\delta \alpha/ \alpha = k_\alpha \delta (GM/ r c^2)$. We compare different manifestations of this effect. The strongest limits $k_\alpha +0.17 k_e= (-3.5\pm 6) * 10^{-7}$ are obtained from the measurements of dependence of atomic frequencies on the distance from Sun (the distance varies due to the ellipticity of the Earth's orbit).Comment: reference adde

Schiff Theorem Revisited

We carefully rederive the Schiff theorem and prove that the usual expression of the Schiff moment operator is correct and should be applied for calculations of atomic electric dipole moments. The recently discussed corrections to the definition of the Schiff moment are absent.Comment: 6 page