Baryonic Operators for Lattice Simulations

The construction of baryonic operators for determining the N* excitation spectrum is discussed. The operators are designed with one eye towards maximizing overlaps with the low-lying states of interest, and the other eye towards minimizing the number of sources needed in computing the required quark propagators. Issues related to spin identification are outlined. Although we focus on tri-quark baryon operators, the construction method is applicable to both mesons and penta-quark operators.Comment: 3 pages, poster presented at Lattice2003(spectrum), Tsukuba, Japan, July 15-19, 200

A Deep VLA survey at 6 cm in the Lockman Hole

This is the published version, also available here: http://dx.doi.org/10.1051/0004-6361:20021721.We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is ~ $11~\mu$Jy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The differential source counts are in good agreement with those obtained by other surveys. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to ~21% of the radio sample. Most of these associations (8 out of 13) are classified as type II AGN. Using optical CCD ( V and I) and $K^{\prime}$ band data with approximate limits of $V\sim25.5$ mag, $I\sim 24.5$ mag and $K^{\prime}\sim20.2$ mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ~92%, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux - optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift ( z>0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population ( $\gtrsim$50%) in a 5 GHz selected sample with a flux limit as low as 50 $\mu$Jy. We also find evidence that at these faint radio limits a large fraction (~60%) of the faintest optical counterparts (i.e. sources in the magnitude range 22.54$ and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ~2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus

Conserved currents of massless fields of spin s>0

A complete and explicit classification of all locally constructed conserved currents and underlying conserved tensors is obtained for massless linear symmetric spinor fields of any spin s>0 in four dimensional flat spacetime. These results generalize the recent classification in the spin s=1 case of all conserved currents locally constructed from the electromagnetic spinor field. The present classification yields spin s>0 analogs of the well-known electromagnetic stress-energy tensor and Lipkin's zilch tensor, as well as a spin s>0 analog of a novel chiral tensor found in the spin s=1 case. The chiral tensor possesses odd parity under a duality symmetry (i.e., a phase rotation) on the spin s field, in contrast to the even parity of the stress-energy and zilch tensors. As a main result, it is shown that every locally constructed conserved current for each s>0 is equivalent to a sum of elementary linear conserved currents, quadratic conserved currents associated to the stress-energy, zilch, and chiral tensors, and higher derivative extensions of these currents in which the spin s field is replaced by its repeated conformally-weighted Lie derivatives with respect to conformal Killing vectors of flat spacetime. Moreover, all of the currents have a direct, unified characterization in terms of Killing spinors. The cases s=2, s=1/2 and s=3/2 provide a complete set of conserved quantities for propagation of gravitons (i.e., linearized gravity waves), neutrinos and gravitinos, respectively, on flat spacetime. The physical meaning of the zilch and chiral quantities is discussed.Comment: 26 pages; final version with minor changes, accepted in Proc. Roy. Soc. A (London

Coarse-graining microscopic strains in a harmonic, two-dimensional solid and its implications for elasticity: non-local susceptibilities and non-affine noise

In soft matter systems the local displacement field can be accessed directly by video microscopy enabling one to compute local strain fields and hence the elastic moduli using a coarse-graining procedure. We study this process for a simple triangular lattice of particles connected by harmonic springs in two-dimensions. Coarse-graining local strains obtained from particle configurations in a Monte Carlo simulation generates non-trivial, non-local strain correlations (susceptibilities), which may be understood within a generalized, Landau type elastic Hamiltonian containing up to quartic terms in strain gradients (K. Franzrahe et al., Phys. Rev. E 78, 026106 (2008)). In order to demonstrate the versatility of the analysis of these correlations and to make our calculations directly relevant for experiments on colloidal solids, we systematically study various parameters such as the choice of statistical ensemble, presence of external pressure and boundary conditions. We show that special care needs to be taken for an accurate application of our results to actual experiments, where the analyzed area is embedded within a larger system, to which it is mechanically coupled. Apart from the smooth, affine strain fields, the coarse-graining procedure also gives rise to a noise field made up of non-affine displacements. Several properties of this noise field may be rationalized for the harmonic solid using a simple "cell model" calculation. Furthermore the scaling behavior of the probability distribution of the noise field is studied and a master curve is obtained.Comment: 16 pages, 12 figure