Next-to-leading Order SUSY-QCD Calculation of Associated Production of Gauginos and Gluinos

Results are presented of a next-to-leading order calculation in perturbative QCD of the production of charginos and neutralinos in association with gluinos at hadron colliders. Predictions for cross sections are shown at the energies of the Fermilab Tevatron and CERN Large Hadron Collider for a typical supergravity (SUGRA) model of the sparticle mass spectrum and for a light gluino model.Comment: 3 pages, latex, 4 figures, paper presented by E. L. Berger at ICHEP 2000, the XXXth International Conference on High Energy Physics July 27 - August 2, 2000, Osaka, Japa

Evolution and diversity of secretome genes in the apicomplexan parasite Theileria annulata

<b>BACKGROUND</b>: Little is known about how apicomplexan parasites have evolved to infect different host species and cell types. Theileria annulata and Theileria parva invade and transform bovine leukocytes but each species favours a different host cell lineage. Parasite-encoded proteins secreted from the intracellular macroschizont stage within the leukocyte represent a critical interface between host and pathogen systems. Genome sequencing has revealed that several Theileria-specific gene families encoding secreted proteins are positively selected at the inter-species level, indicating diversification between the species. We extend this analysis to the intra-species level, focusing on allelic diversity of two major secretome families. These families represent a well-characterised group of genes implicated in control of the host cell phenotype and a gene family of unknown function. To gain further insight into their evolution and function, this study investigates whether representative genes of these two families are diversifying or constrained within the T. annulata population. <b>RESULTS</b>: Strong evidence is provided that the sub-telomerically encoded SVSP family and the host-nucleus targeted TashAT family have evolved under contrasting pressures within natural T. annulata populations. SVSP genes were found to possess atypical codon usage and be evolving neutrally, with high levels of nucleotide substitutions and multiple indels. No evidence of geographical sub-structuring of allelic sequences was found. In contrast, TashAT family genes, implicated in control of host cell gene expression, are strongly conserved at the protein level and geographically sub-structured allelic sequences were identified among Tunisian and Turkish isolates. Although different copy numbers of DNA binding motifs were identified in alleles of TashAT proteins, motif periodicity was strongly maintained, implying conserved functional activity of these sites. <b>CONCLUSIONS</b>: This analysis provides evidence that two distinct secretome genes families have evolved under contrasting selective pressures. The data supports current hypotheses regarding the biological role of TashAT family proteins in the management of host cell phenotype that may have evolved to allow adaptation of T. annulata to a specific host cell lineage. We provide new evidence of extensive allelic diversity in representative members of the enigmatic SVSP gene family, which supports a putative role for the encoded products in subversion of the host immune response

Electroweak Corrections to the Top Quark Decay

We have calculated the one-loop electroweak corrections to the decay t-> bW+, including the counterterm for the CKM matrix elements V(tb). Previous calculations used an incorrect delta V(tb) that led to a gauge dependent amplitude. However, since the contribution stemming from delta V(tb) is small, those calculations only underestimate the width by roughly one part in 10^5.Comment: 7 pages, 2 figure

A study of top polarization in single-top production at the LHC

This paper complements the study of single top production at the LHC aiming to estimate the sensitivity of different observables to the magnitude of the effective couplings. In a previous paper the dominant $W$-gluon fusion mechanism was considered, while here we extend the analysis to the subdominant (10% with our set of experimental cuts) s-channel process. In order to distinguish left from right effective couplings it is required to consider polarized cross-sections and/or include $m_b$ effects. The spin of the top is accessible only indirectly by measuring the angular distribution of its decay products. We show that the presence of effective right-handed couplings implies necessarily that the top is not in a pure spin state. We discuss to what extent quantum interference terms can be neglected in the measurement and therefore simply multiply production and decay probabilities clasically. The coarsening involved in the measurement process makes this possible. We determine for each process the optimal spin basis where theoretical errors are minimized and, finally, discuss the sensitivity in the s-channel to the effective right-handed coupling. The results presented here are all analytical and include $m_b$ corrections. They are derived within the narrow width approximation for the top.Comment: 30 pages, 14 figure

Flavor changing scalar couplings and tγ(Z)t\gamma(Z) production at hadron colliders

We calculate the contributions of the flavor changing scalar ($FCS$) couplings arised from topcolor-assisted technicolor ($TC2$) models at tree-level to the $t\gamma$ and $tZ$ production at the Tevatron and $LHC$ experiments. We find that the production cross sections are very small at the Tevatron with $\sqrt{s}=1.96TeV$, which is smaller than 5 fb in most of the parameter space of $TC2$ models. However, the virtual effects of the $FCS$ couplings on the $t\gamma(Z)$ production can be easily detected at the $LHC$ with $\sqrt{s}=14TeV$ via the final state $\gamma l\bar{\nu}b$ ($l^{+}l^{-}l\bar{\nu}b$).Comment: 10 pages,5 figure