RbR_b and RcR_c in the Two Higgs Doublet Model with Flavor Changing Neutral Currents

A study of $R_b$ and $R_c$ is presented in the context of a Two Higgs Doublet Model (2HDM) with flavor changing scalar currents (FCSC). Implications of the model for the $\rho$-parameter and for $b\to s\gamma$ are also considered. The experimental data on $R_b$ places stringent constraints on the model parameters. The configuration of the model needed to account for $R_b$ is found to be irreconcilable with constraints from $b\to s\gamma$ and $\rho$. In particular, if R^{\rm exp}_b>R^{\sss{\rm SM}}_b persists then this version of 2HDM will be ruled out or require significant modifications. Noting that aspects of the experimental analysis for $R_b$ and $R_c$ may be of some concern, we also disregard $R^{\rm exp}_b$ and $R^{\rm exp}_c$ and give predictions for these using constraints from $b\to s\gamma$ and $\rho$ parameter only. We emphasize the theoretical and experimental advantages of the observable R_{b+c}\equiv \Gamma(Z\to b\bar b\mbox{ or } c\bar c)/\Gamma(Z\to\mbox{hadrons}). We also stress the role of R_\ell\equiv \Gamma(Z\to\mbox{hadrons})/\Gamma(Z\to \ell^+\ell^-) in testing the Standard Model (SM) despite its dependence on QCD corrections. Noting that in models with FCNC the amplitude for $Z\to c\bar c$ receives a contribution which grows with $m^2_t$, the importance and uniqueness of precision $Z\to c\bar c$ measurements for constraining flavor changing $t\bar c$ currents is underscored.Comment: 35 pages, 5 Postscript figures, 10 Postscript files used in the tex file, uses epsf.st

On the sensitivity of the HAWC observatory to gamma-ray bursts

We present the sensitivity of HAWC to Gamma Ray Bursts (GRBs). HAWC is a very high-energy gamma-ray observatory currently under construction in Mexico at an altitude of 4100 m. It will observe atmospheric air showers via the water Cherenkov method. HAWC will consist of 300 large water tanks instrumented with 4 photomultipliers each. HAWC has two data acquisition (DAQ) systems. The main DAQ system reads out coincident signals in the tanks and reconstructs the direction and energy of individual atmospheric showers. The scaler DAQ counts the hits in each photomultiplier tube (PMT) in the detector and searches for a statistical excess over the noise of all PMTs. We show that HAWC has a realistic opportunity to observe the high-energy power law components of GRBs that extend at least up to 30 GeV, as it has been observed by Fermi LAT. The two DAQ systems have an energy threshold that is low enough to observe events similar to GRB 090510 and GRB 090902b with the characteristics observed by Fermi LAT. HAWC will provide information about the high-energy spectra of GRBs which in turn could help to understanding about e-pair attenuation in GRB jets, extragalactic background light absorption, as well as establishing the highest energy to which GRBs accelerate particles

New insights into the genetic etiology of Alzheimer's disease and related dementias

Characterization of the genetic landscape of Alzheimer's disease (AD) and related dementias (ADD) provides a unique opportunity for a better understanding of the associated pathophysiological processes. We performed a two-stage genome-wide association study totaling 111,326 clinically diagnosed/'proxy' AD cases and 677,663 controls. We found 75 risk loci, of which 42 were new at the time of analysis. Pathway enrichment analyses confirmed the involvement of amyloid/tau pathways and highlighted microglia implication. Gene prioritization in the new loci identified 31 genes that were suggestive of new genetically associated processes, including the tumor necrosis factor alpha pathway through the linear ubiquitin chain assembly complex. We also built a new genetic risk score associated with the risk of future AD/dementia or progression from mild cognitive impairment to AD/dementia. The improvement in prediction led to a 1.6- to 1.9-fold increase in AD risk from the lowest to the highest decile, in addition to effects of age and the APOE ε4 allele