Response of nucleons to external probes in hedgehog models: I. Electromagnetic polarizabilities

Electromagnetic polarizabilities of the nucleon are analyzed in a hedgehog model with quark and meson degrees of freedom.Comment: 27 pages, DOE/ER/40322-155, U. of MD PP \#92-193, (ReVTeX

Transitions between nonsymmetric and symmetric steady states near a triple eigenvalue

We examine the existence of nonuniform steady-state solutions of a certain class of reaction-diffusion equations. Our analysis concentrates on the case where the first bifurcation is near a triple eigenvalue. We derive the conditions for a continuous transition between nonsymmetric and symmetric solutions when the bifurcation parameter progressively increases from zero. Finally, we give an example of a four variables model which presents the possibility of a triple eigenvalue

Does one observe chiral symmetry restoration in baryon spectrum?

It has recently been suggested that the parity doublet structure seen in the spectrum of highly excited baryons may be due to effective chiral symmetry restoration for these states. We review the recent developments in this field. We demonstrate with a simple quantum-mechanical example that it is a very natural property of quantum systems that a symmetry breaking effect which is important for the low-lying spectrum of the system, can become unimportant for the highly-lying states; the highly lying states reveal a multiplet structure of nearly degenerate states. Using the well established concepts of quark-hadron duality, asymptotic freedom in QCD and validity of the operator product expansion in QCD we show that the spectral densities obtained with the local currents that are connected to each other via chiral transformations, very high in the spectrum must coincide. Hence effects of spontaneous breaking of chiral symmetry in QCD vacuum that are crucially important for the low-lying spectra, become irrelevant for the highly-lying states. Then to the extent that identifiable hadronic resonances still exist in the continuum spectrum at high excitations this implies that the highly excited hadrons must fall into multiplets associated with the representations of the chiral group. We demonstrate that this is indeed the case for meson spectra in the large $N_c$ limit. All possible parity-chiral multiplets are classified for baryons and it is demonstrated that the existing data on highly excited $N$ and $\Delta$ states at masses of 2 GeV and higher is consistent with approximate chiral symmetry restoration. However new experimental studies are needed to achieve any definitive conclusions.Comment: 30 pages, LaTeX . Review commissioned by Int. J. of Mod. Phys.

Context dependence of the event-related brain potential associated with reward and punishment

The error-related negativity (ERN) is an event-related brain potential elicited by error commission and by presentation of feedback stimuli indicating incorrect performance. In this study, the authors report two experiments in which participants tried to learn to select between response options by trial and error, using feedback stimuli indicating monetary gains and losses. The results demonstrate that the amplitude of the ERN is determined by the value of the eliciting outcome relative to the range of outcomes possible, rather than by the objective value of the outcome. This result is discussed in terms of a recent theory that holds that the ERN reflects a reward prediction error signal associated with a neural system for reinforcement learning

The structure of the pion and effective electroweak currents in soliton models of the nucleon

Nonminimal substitution terms in electroweak currents are studied in effective chiral soliton models. It is found that the terms describing the structure of the pion lead to sizable effects in form factors and polarizabilities of the nucleon.Comment: 9 pages, report DOE/ER/40322-170, U. of MD PP \#93-001, (uses RevTeX

Lick Slit Spectra of Thirty-Eight Objective Prism QSO Candidates and Low Metallicity Halo Stars

We present Lick Observatory slit spectra of 38 objects which were claimed to have pronounced ultraviolet excess and emission lines by Zhan \& Chen. Most of our spectra have FWHM spectral resolutions of about 4~\AA , and relatively high S/N of about 10 -- 50, although some have FWHM $\simeq 15$~\AA ~or lower S/N. We find eleven QSOs, four galaxies at $z \simeq 0.1$, twenty-two stars and one unidentified object with a low S/N spectrum. Six of the QSOs show absorption systems, including Q0000+027A with a relatively strong associated C~IV absorption system, and Q0008+008 (V$\simeq 18.9$) with a damped Ly$\alpha$ system with an H~I column density of $10^{21}$ cm$^{-2}$. The stars include a wide variety of spectral types. There is one new DA4 white dwarf at 170~pc, one sdB at 14~kpc, and three M stars. The rest are of types F, G and K. We have measured the equivalent widths of the Ca~II~K line, the G-band and the Balmer lines in ten stars with the best spectra, and we derive metallicities. Seven of them are in the range $-2.5 \leq$~[Fe/H]~$\leq -1.7$, while the others are less metal poor. If the stars are dwarfs, then they are at distances of 1 to 7~kpc, but if they are giants, typical distances will be about 10~kpc.Comment: (Plain Tex, 21 pages, including tables. Send email to 'travell_oir%[email protected]' for 12 pages of figures) To appear in the %%Astronomical Journal, August, 199

Imperfect Bifurcation Near a Double Eigenvalue: Transitions Between Nonsymmetric and Symmetric Patterns

We examine the existence of nonsymmetric and symmetric steady state solutions of a general class of reaction-diffusion equations. Our study consists of two parts: (i) By analyzing the bifurcation from a uniform reference state to nonuniform regimes, we demonstrate the existence of a unique symmetric solution (basic wave number two) which becomes linearly stable when it surpasses a critical amplitude. (We assume that the first bifurcation point corresponds to the emergence of the simplest nonsymmetric steady state solutions.) (ii) This result is not affected when a parameter is nonuniformly distributed in the system. However, one of the two possible branches of nonsymmetric solutions may disappear from the bifurcation diagram. Our analysis is motivated by the fact that experimental observations of pattern transitions during morphogenesis are interpreted in terms of the dynamics of stable concentration gradients. We have shown that in addition to the values of the physico-chemical parameters, these structures can be selected by two different mechanisms: (i) the linear stability of the nonuniform patterns, (ii) the effects of a small and nonuniform variation of a parameter in the spatial domain

New Relations for Excited Baryons in Large N_c QCD

We show that excited baryons in large N_c QCD form multiplets, within which masses are first split at O(1/N_c). The dominant couplings of resonances to various mesons are highly constrained: The N(1535) decays at leading 1/N_c order exclusively to eta-N rather than pi-N, and vice versa for the N(1650). This multiplet structure is reproduced by a simple large N_c quark model, well studied in the literature, that describes resonances as single-quark excitations.Comment: 4 pages, no figures, ReVTeX 4. Includes new discussion of previous work on excited baryon tower

Excited Baryons in Large NcN_c QCD

This talk reviews recent developments in the use of large $N_c$ QCD in the description of baryonic resonances. The emphasis is on the model-independent nature of the approach. Key issues discussed include the spin-flavor symmetry which emerges at large $N_c$ and the direct use of scattering observables. The connection to quark model approaches is stressed.Comment: Talk at "Baryons 04", Palaiseau, October 200