A Characterization of Graphs with Small Palette Index

Given an edge-coloring of a graph G, we associate to every vertex v of G the set of colors appearing on the edges incident with v. The palette index of G is defined as the minimum number of such distinct sets, taken over all possible edge-colorings of G. A graph with a small palette index admits an edge-coloring which can be locally considered to be almost symmetric, since few different sets of colors appear around its vertices. Graphs with palette index 1 are r-regular graphs admitting an r-edge-coloring, while regular graphs with palette index 2 do not exist. Here, we characterize all graphs with palette index either 2 or 3 in terms of the existence of suitable decompositions in regular subgraphs. As a corollary, we obtain a complete characterization of regular graphs with palette index 3

An instrument for low-level measurements of the leakage current from high-voltage biased detectors

Resistive Plates Chambers (RPC) are detectors biased at High-Voltage (HV) in excess of 4 kV. When fired by a particle, they develop a large signal current that can be read across a small resistance, 100 Omega or so. A characterization has been made of their ageing as a function of the behaviour of their leakage current with time. An array of 10 detectors has been developed for this purpose. We present the instrument designed and built to perform a continuous and automatic monitoring of the leakage current from each detector of the array, while the system is taking data. For the particular biasing set-up adopted, the current has been measured in series to the terminal connected to the HV of every channel. Since the small value of the currents, order of tens of nA, a special circuit solution and special precautions have been adopte

Markedly Divergent Tree Assemblage Responses to Tropical Forest Loss and Fragmentation across a Strong Seasonality Gradient

We examine the effects of forest fragmentation on the structure and composition of tree assemblages within three seasonal and aseasonal forest types of southern Brazil, including evergreen, Araucaria, and deciduous forests. We sampled three southernmost Atlantic Forest landscapes, including the largest continuous forest protected areas within each forest type. Tree assemblages in each forest type were sampled within 10 plots of 0.1 ha in both continuous forests and 10 adjacent forest fragments. All trees within each plot were assigned to trait categories describing their regeneration strategy, vertical stratification, seed-dispersal mode, seed size, and wood density. We detected differences among both forest types and landscape contexts in terms of overall tree species richness, and the density and species richness of different functional groups in terms of regeneration strategy, seed dispersal mode and woody density. Overall, evergreen forest fragments exhibited the largest deviations from continuous forest plots in assemblage structure. Evergreen, Araucaria and deciduous forests diverge in the functional composition of tree floras, particularly in relation to regeneration strategy and stress tolerance. By supporting a more diversified light-demanding and stress-tolerant flora with reduced richness and abundance of shade-tolerant, old-growth species, both deciduous and Araucaria forest tree assemblages are more intrinsically resilient to contemporary human-disturbances, including fragmentation-induced edge effects, in terms of species erosion and functional shifts. We suggest that these intrinsic differences in the direction and magnitude of responses to changes in landscape structure between forest types should guide a wide range of conservation strategies in restoring fragmented tropical forest landscapes worldwide

A new concept for streamer quenching in resistive plate chambers

In this paper we propose a new concept for streamer quenching in Resistive Plate Chambers (RPCs). In our approach, the multiplication process is quenched by the appropriate design of a mechanical structure inserted between the two resistive electrodes. We show that stable performance is achieved with binary gas mixtures based on argon and a small fraction of isobutane. Fluorocarbons, deemed responsible for the degradation of the electrode inner surface of RPC detectors, are thus fully eliminated from the gas mixture. This design {also resulted} in a simplified assembly procedure. Preliminary results obtained with a few prototypes of ``Mechanically Quenched RPCs'' and some prospects for future developments are discussed

Response of microchannel plates in ionization mode to single particles and electromagnetic showers

Hundreds of concurrent collisions per bunch crossing are expected at future hadron colliders. Precision timing calorimetry has been advocated as a way to mitigate the pileup effects and, thanks to their excellent time resolution, microchannel plates (MCPs) are good candidate detectors for this goal. We report on the response of MCPs, used as secondary emission detectors, to single relativistic particles and to electromagnetic showers. Several prototypes, with different geometries and characteristics, were exposed to particle beams at the INFN-LNF Beam Test Facility and at CERN. Their time resolution and efficiency are measured for single particles and as a function of the multiplicity of particles. Efficiencies between 50% and 90% to single relativistic particles are reached, and up to 100% in presence of a large number of particles. Time resolutions between 20ps and 30ps are obtained.Comment: 20 pages, 9 figures. Paper submitted to NIM

Performance of a Tungsten-Cerium Fluoride Sampling Calorimeter in High-Energy Electron Beam Tests

A prototype for a sampling calorimeter made out of cerium fluoride crystals interleaved with tungsten plates, and read out by wavelength-shifting fibres, has been exposed to beams of electrons with energies between 20 and 150 GeV, produced by the CERN Super Proton Synchrotron accelerator complex. The performance of the prototype is presented and compared to that of a Geant4 simulation of the apparatus. Particular emphasis is given to the response uniformity across the channel front face, and to the prototype's energy resolution.Comment: 6 pages, 6 figures, Submitted to NIM

Response of microchannel plates to single particles and to electromagnetic showers

We report on the response of microchannel plates (MCPs) to single relativistic particles and to electromagnetic showers. Particle detection by means of secondary emission of electrons at the MCP surface has long been proposed and is used extensively in ion time-of-flight mass spectrometers. What has not been investigated in depth is their use to detect the ionizing component of showers. The time resolution of MCPs exceeds anything that has been previously used in calorimeters and, if exploited effectively, could aid in the event reconstruction at high luminosity colliders. Several prototypes of photodetectors with the amplification stage based on MCPs were exposed to cosmic rays and to 491 MeV electrons at the INFN-LNF Beam-Test Facility. The time resolution and the efficiency of the MCPs are measured as a function of the particle multiplicity, and the results used to model the response to high-energy showers.Comment: Paper submitted to NIM

Deviation of Atmospheric Mixing from Maximal and Structure in the Leptonic Flavor Sector

I attempt to quantify how far from maximal one should expect the atmospheric mixing angle to be given a neutrino mass-matrix that leads, at zeroth order, to a nu_3 mass-eigenstate that is 0% nu_e, 50% nu_mu, and 50% nu_tau. This is done by assuming that the solar mass-squared difference is induced by an "anarchical" first order perturbation, an approach than can naturally lead to experimentally allowed values for all oscillation parameters. In particular, both |cos 2theta_atm| (the measure for the deviation of atmospheric mixing from maximal) and |U_e3| are of order sqrt(Delta m^2_sol/Delta m^2_atm) in the case of a normal neutrino mass-hierarchy, or of order Delta m^2_sol/Delta m^2_atm in the case of an inverted one. Hence, if any of the textures analyzed here has anything to do with reality, next-generation neutrino experiments can see a nonzero cos 2theta_atm in the case of a normal mass-hierarchy, while in the case of an inverted mass-hierarchy only neutrino factories should be able to see a deviation of sin^2 2theta_atm from 1.Comment: 12 pages, no figures, references and acknowledgments adde