Excited nucleons with chirally improved fermions

We study positive and negative parity nucleons on the lattice using the chirally improved lattice Dirac operator. Our analysis is based on a set of three operators chi_i with the nucleon quantum numbers but in different representations of the chiral group and with different diquark content. We use a variational method to separate ground state and excited states and determine the mixing coefficients for the optimal nucleon operators in terms of the chi_i. We clearly identify the negative parity resonances N(1535) and N(1650) and their masses agree well with experimental data. The mass of the observed excited positive parity state is too high to be interpreted as the Roper state. Our results for the mixing coefficients indicate that chiral symmetry is important for N(1535) and N(1650) states. We confront our data for the mixing coefficients with quark models and provide insights into the physics of the nucleon system and the nature of strong decays.Comment: Tables added, small modifications in the tex

Melting and freezing of argon in a granular packing of linear mesopore arrays

Freezing and melting of Ar condensed in a granular packing of template-grown arrays of linear mesopores (SBA-15, mean pore diameter 8 nanometer) has been studied by specific heat measurements C as a function of fractional filling of the pores. While interfacial melting leads to a single melting peak in C, homogeneous and heterogeneous freezing along with a delayering transition for partial fillings of the pores result in a complex freezing mechanism explainable only by a consideration of regular adsorption sites (in the cylindrical mesopores) and irregular adsorption sites (in niches of the rough external surfaces of the grains, and at points of mutual contact of the powder grains). The tensile pressure release upon reaching bulk liquid/vapor coexistence quantitatively accounts for an upward shift of the melting/freeezing temperature observed while overfilling the mesopores.Comment: 4 pages, 4 figures, to appear as a Letter in Physical Review Letter

Reactive transport modeling to assess geochemical monitoring for detection of CO2 intrusion into shallow aquifers

AbstractThe hypothesis is tested if changes in electric conductivity of groundwater (EC) in response to gaseous CO2 intrusion are sufficient to be detected using probe measurements and geophysical electromagnetic measurements, e.g. airborne electromagnetic measurements. Virtual reactive scenario modelling is used to simulate the effects of the presence of calcite, CO2 intrusion rates, depth of the aquifer formation, initial salinity of groundwater and CO2 intrusion time on changes in EC. In all simulations, EC rises rapidly in response to CO2 intrusion, however in different magnitudes. When calcite is present, EC changes are strong (+1.11 mS/cm after 24 hours of CO2 intrusion) mainly due to calcite dissolution, whereas in aquifers without calcite changes are very low (+0.02 mS/cm after 24 hours) and close to the resolution range of probes. Increased depth (250 m / 500 m), i.e. higher temperature and pressure, and higher intrusion rates (up to full saturation) result in stronger rises in EC (+5.08 mS/cm in 500 m depth and 100 % saturation), and initial salinity has a negligible influence on changes in EC. Temporally limited CO2 intrusion leads to EC values close to pre- CO2-intrusion-levels in the long-term. Measurement resolution of commercial EC probes is sufficient to detect CO2 intrusion in almost all cases. In terms of geophysical electromagnetic measurements, applications in the field of monitoring saltwater-freshwater interfaces indicate a sufficient measurement resolution to detect changes in all simulations. However, practical limitations are expected due to the dependence of measurement resolutions on the applied measurement devices and site-specific geological settings