Highly excited negative parity baryons in the 1/Nc1/N_c expansion

The masses of experimentally known highly excited baryons of negative parity supposed to belong to the $[{\bf 70},\ell^-]$ multiplets ($\ell$ = 1,2,3) of the $N = 3$ band are calculated in the $1/N_c$ expansion method to order $1/N_c$ by using a procedure which allows to considerably reduce the number of linearly independent operators entering the mass formula. The numerical fits to present data show that the coefficients encoding the QCD dynamics have large, comparable values, for the flavor and spin operators. It implies that these operators contribute dominantly to the flavor-spin SU(6) symmetry breaking, like for the $[{\bf 70},1^-]$ multiplet of the $N = 1$ band.Comment: 15 pages, accepted for publication in Phys.Rev.

Extraction of Resonances from Meson-Nucleon Reactions

We present a pedagogical study of the commonly employed Speed-Plot (SP) and Time-delay (TD) methods for extracting the resonance parameters from the data of two particle coupled-channels reactions. Within several exactly solvable models, it is found that these two methods find poles on different Riemann sheets and are not always valid. We then develop an analytic continuation method for extracting the nucleon resonances within a dynamical coupled-channel formulation of the $\pi N$ and $\gamma N$ reactions. The main focus is on resolving the complications due to the coupling with the unstable $\pi \Delta, \rho N, \sigma N$ channels which decay into $\pi \pi N$ states. By using the results from the considered exactly solvable models, explicit numerical procedures are presented and verified. As a first application of the developed analytic continuation method, we present the nucleon resonances in the $S_{11}$ and $S_{31}$ partial waves extracted within a recently developed coupled-channels model of $\pi N$ reactions. The results from this realistic $\pi N$ model, which includes $\pi N$, $\eta N$, $\pi\Delta$, $\rho N$, and $\sigma N$ channels, also show that the simple pole parametrization of the resonant propagator using the poles extracted from SP and TD methods works poorly.Comment: 32 pages, 14 figure

Pion parameters in nuclear medium from chiral perturbation theory and virial expansion

We consider two methods to find the effective parameters of the pion traversing a nuclear medium. One is the first order chiral perturbation theoretic evaluation of the pion pole contribution to the two-point function of the axial-vector current. The other is the exact, first order virial expansion of the pion self-energy. We find that, although the results of chiral perturbation theory are not valid at normal nuclear density, those from the virial expansion may be reliable at such density. The latter predicts both the mass-shift and the in-medium decay width of the pion to be small, of about a few MeV.Comment: 9 Pages RevTex, 3 eps figure

Metabolic variability in seafloor brines revealed by carbon and sulphur dynamics

Brine fluids that upwell from deep, hot reservoirs below the sea bed supply the sea floor with energy-rich substrates and nutrients that are used by diverse microbial ecosystems. Contemporary hypersaline environments formed by brine seeps may provide insights into the metabolism and distribution of microorganisms on the early Earth or on extraterrestrial bodies. Here we use geochemical and genetic analyses to characterize microbial community composition and metabolism in two seafloor brines in the Gulf of Mexico: an active mud volcano and a quiescent brine pool. Both brine environments are anoxic and hypersaline. However, rates of sulphate reduction and acetate production are much higher in the brine pool, whereas the mud volcano supports much higher rates of methane production. We find no evidence of anaerobic oxidation of methane, despite high methane fluxes at both sites. We conclude that the contrasting microbial community compositions and metabolisms are linked to differences in dissolved-organic-matter input from the deep subsurface and different fluid advection rates between the two sites. DOI: 10.1038/NGEO47

Investigation of the Neutron Form Factors by Inclusive Quasi-Elastic Scattering of Polarized Electrons off Polarized 3^{3}He: A Theoretical Overview

The theory of quasi-elastic inclusive scattering of polarized leptons off polarized $^3$He is critically reviewed and the origin of different expressions for the polarized nuclear response function appearing in the literature is explained. The sensitivity of the longitudinal asymmetry upon the neutron form factors is thoroughly investigated and the role played by the polarization angle for minimizing the proton contribution is illustrated.Comment: Phys. Rev C in press; 9 figs. (available upon request

A model for two-proton emission induced by electron scattering

A model to study two-proton emission processes induced by electron scattering is developed. The process is induced by one-body electromagnetic operators acting together with short-range correlations, and by two-body $\Delta$ currents. The model includes all the diagrams containing a single correlation function. A test of the sensitivity of the model to the various theoretical inputs is done. An investigation of the relevance of the $\Delta$ currents is done by changing the final state angular momentum, excitation energy and momentum transfer. The sensitivity of the cross section to the details of the correlation function is studied by using realistic and schematic correlations. Results for $^{12}$C, $^{16}$O and $^{40}$Ca nuclei are presented.Comment: 30 pages, 18 figures, 3 table

Relativistic resonances: Their masses, widths, lifetimes, superposition, and causal evolution

Whether one starts form the analytic S-matrix definition or the requirement of gauge parameter independence in renormalization theory, a relativistic resonance is given by a pole at a complex value s of energy squared. The complex number s does not define the mass and width separately and this definition does not lead to interfering Breit-Wigner if two or more resonances are involved. To accomplish both we invoke the decaying particle aspect of a resonance and associate to each pole a space of relativistic Gamow kets which transform irreducibly under causal Poincare transformations. A Gamow state has an exponential time evolution and one can choose of the many possible width parameters, that parameter as the width of the relativistic resonance which equals the inverse lifetime. This uniquely defines the mass and width parameters for a relativistic resonance. Two or more poles in the same partial wave are given by the sum of Breit-Wigners in the scattering amplitude and by a superposition of Gamow vectors with each Gamow vector corresponding to one Breit-Wigner. In addition to the sum of Breit-Wigners the scattering amplitude contains a background amplitude representing direct production of the final state (contact terms).This contact amplitude is associated to a background vector which is a continuous superposition of Lippmann-Schwinger states. Omitting this continuum gives the Weisskopf-Wigner approximation.Comment: 22 pages, REVTe