On the branching ratio of the "second class" tau --> eta' pi nu_tau decay

We present an assessment of the standard model expectations for the branching ratio of the isotopic spin and G-parity violating decay tau --> eta' pi nu_tau. The estimate is based on a vector and scalar meson dominance parametrization of the relevant form factors, that explicitly accounts for pi^0-eta-eta' mixing. The numerical results obtained in this framework indicate a branching ratio one order of magnitude (or more) below the current experimental limit, and suggest the possibility of evidencing some novel interaction in high statistics studies of this decay.Comment: 8 page

Positron Polarization at the International Linear Collider

We review some recent arguments supporting the upgrade of the International Linear Collider by a polarized positron beam, in addition to the polarized electron beam. The examples presented here mainly focus on the impact of positron polarization on items relevant to new physics searches, such as the identification of novel interactions in fermion-pair production and the formulation of new CP-sensitive observables. In particular, in addition to the benefits from positron and electron longitudinal polarizations, the advantages in this field of having transverse polarization of both beams are emphasized.Comment: Invited talk at 2005 International Linear Collider Workshop, Stanford Ca (LCWS05) 6 pages, LaTeX, 2 eps figure

Investigating exudate- and habitat-mediated effects of phytoplankton on lake bacterial community dynamics

Correlated patterns of abundance, activity, and composition are commonly observed between phytoplankton and bacterial communities. There are a number of potential explanations for correlated dynamics. Recent observations of temporal succession in lake microbial communities provide evidence for phytoplankton populations acting as biological drivers structuring bacterial communities. The following thesis addresses two potentially important connections between phytoplankton and bacteria: resource-mediated linkage through production of labile carbon exudates and habitat-mediated linkage. To investigate phytoplankton exudates as a potential linkage, the first investigation focused on bacteria able to use the photorespiration-specific exudate glycolate. Diversity and dynamics of glycolate-utilizing bacteria were characterized in six lakes using functional gene glycolate oxidase subunit D (glcD). Freshwater glycolate-utilizing populations exhibited a range of taxonomic diversity and contained many sequences clustering with a glcD sequence from the cosmopolitan freshwater Polynucleobacter genus. Glycolate-utilizing and total bacterial community-level variation was largely explained by dynamics of phytoplankton populations (35-40%) and the interaction between these phytoplankton populations and the environment (17-18%). Population-level correlations between specific phytoplankton and glycolate-utilizing bacteria were also detected. These observations support the hypothesis that algal exudates are resource-based drivers of bacterial community composition and identify bacterial taxonomic groups with members capable of responding directly to a specific exudate. Contribution of epiphytic bacteria inhabiting algal cells to correlated community dynamics was investigated by comparing temporal community patterns in the particle-associated bacteria to those of whole bacteria and phytoplankton in Crystal and South Sparkling Bogs. Regular patterns of succession, in addition to correlations between phytoplankton and whole bacterial communities (ρ=0.514, p=0.001) and phytoplankton and particle-associated bacterial communities (ρ=0.739, p=0.001), were detected in Crystal, but not South Sparkling Bog (ρ=0.265, p=0.038; ρ=0.167, p=0.103, respectively). Attached and free-living bacterial assemblages were compared to classify bacterial taxa based on presence in attached and/ or free-living fractions. Despite mixed results for community-level correlations, bacterial populations were positively correlated to abundance of specific phytoplankton in both lakes. Algal-correlated bacteria ranged from primarily particle-associated bacteria in Crystal Bog to a mix of particle-associated and free-living bacteria in South Sparkling Bog. These observations provide support for habitat-mediated linkages, while also indicating the importance of other mechanisms that affect free-living and habitat generalist populations. Taken together, these investigations contribute to a growing body of research demonstrating the importance of biological interactions in shaping microbial community structure. Microbial community composition and function have implications for energy flow, carbon flux, and biogeochemical transformations with ecosystem-level consequences. Understanding biological interactions that structure bacterial communities may facilitate the building of a predictive framework for understanding compositional and functional responses of bacteria and microbially-mediated processes to changing environmental conditions

Bhabha vs. Moller scattering as a contact-interaction analyzer at a polarized Linear Collider

We discuss electron-electron contact-interaction searches in the Bhabha and Moller processes at planned Linear Colliders run in the electron-positron and electron-electron modes, with longitudinally polarized beams. Our analysis is based on the measurement, for the two processes, of polarized differential cross sections that allow to disentangle the different, independent, four-fermion couplings. We evaluate by a model-independent analysis the sensitivities to the contact coupling constants, emphasizing the role of the available beam polarization and the complementarity of the two processes in giving the best limits. We also make a comparison with the potential of $e^+e^-\to\mu^+\mu^-$ at the same energy and beams polarization.Comment: 18 latex pages, 15 eps figs., some misprints corrected and added ref

On meson dominance in the `second class' tau \to eta pi nu_tau decay

Motivated by recent estimates of the isospin-violating process tau \to eta pi nu_tau, mostly relying on the rho and a_0 dominance of the relevant form factors near threshold, we present an assessment for the branching ratio that accounts for additional, potential, effects from the lowest radial excitations rho(1450) and a_0(1450), respectively, also lying in the decay phase space.Comment: 8 page