Prospects for measuring |Vts| at the LHC

36th International Conference on High Energy Physics, July 4-11, 2012 Melbourne, AustraliaWe study the prospects of measuring the CKM matrix element |Vts| at the LHC with the top quarks produced in the processes pp → ttX¯ and pp → t/tX¯ , and the subsequent decays t → W+s and t¯ → W−s¯. To reduce the jet activity in top quark decays, we insist on tagging the W± leptonically, W± → ` ±ν` (` = e,µ, τ), and analyse the anticipated jet profiles in the signal process t → W s and the dominant background from the decay t → W b. To that end, we analyse the V0 (K 0 and Λ) distributions in the s- and b-quark jets concentrating on the energy and transverse momentum distributions of these particles. Noting that the V0s emanating from the t → W b branch have displaced decay vertexes from the interaction point due to the weak decays b → c → s, and that the b-quark jets are rich in charged leptons, the information from the secondary vertex distributions and the absence of energetic charged leptons in the jet provide additional (b-jet vs. s-jet) discrimination in top quark decays. These distributions are used to train a boosted decision tree (BDT), a technique used successfully in measuring the CKM matrix element |Vtb| in single top production at the Tevatron. We show that the BDT-response functions corresponding to the signal (t →W s) and background (t →W b) are very different. Detailed simulations undertaken by us with the Monte Carlo generator PYTHIA are used to estimate the background rejection versus signal efficiency for two representative LHC energies √ s = 7 TeV and 14 TeV. We argue that a benchmark with 5% signal (t → W s) efficiency and a background (t → bW ) rejection by a factor 103 (required due to the anticipated value of the ratio |Vts| 2/|Vtb| 2 ' 1.6×10−3 ) can be achieved at the LHC, given the promised luminosit

Religion, Alfar and Dvergar

En este artículo apuntamos a explorar la conexión entre dos tipos de entidades míticas colectivas, los álfar y los dvergar. Ponderamos críticamente la fiabilidad de las distintas fuentes y analizamos el modo en que esos seres son presentados en algunos documentos medievales. Finalmente, intentamos distinguirlos reconstruyendo (hipotéticamente) su respectivo lugar en la práctica religiosa, y su conexión con la más amplia moral de intercambio que impregnaba a las sociedades medievales escandinavas.In this article we aim to explore the connection between two types of collective mythical beings, the álfar and the dvergar. We assess critically the reliability of different sources and analyse the way in which those beings are depicted in some medieval documents. Finally, we attempt to distinguish them by reconstructing (hypothetically) their respective place in religious practice and their connection with the broader morality of exchange that pervaded medieval Scandinavian society.Fil: Barreiro, Santiago Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Saavedra 15. Instituto Multidisciplinario de Historia y Ciencias Humanas; Argentin

When do correlations increase with firing rates in recurrent networks?

A central question in neuroscience is to understand how noisy firing patterns are used to transmit information. Because neural spiking is noisy, spiking patterns are often quantified via pairwise correlations, or the probability that two cells will spike coincidentally, above and beyond their baseline firing rate. One observation frequently made in experiments, is that correlations can increase systematically with firing rate. Theoretical studies have determined that stimulus-dependent correlations that increase with firing rate can have beneficial effects on information coding; however, we still have an incomplete understanding of what circuit mechanisms do, or do not, produce this correlation-firing rate relationship. Here, we studied the relationship between pairwise correlations and firing rates in recurrently coupled excitatory-inhibitory spiking networks with conductance-based synapses. We found that with stronger excitatory coupling, a positive relationship emerged between pairwise correlations and firing rates. To explain these findings, we used linear response theory to predict the full correlation matrix and to decompose correlations in terms of graph motifs. We then used this decomposition to explain why covariation of correlations with firing rate—a relationship previously explained in feedforward networks driven by correlated input—emerges in some recurrent networks but not in others. Furthermore, when correlations covary with firing rate, this relationship is reflected in low-rank structure in the correlation matrix

Moduli as Inflatons in Heterotic M-theory

We consider different cosmological aspects of Heterotic M-theory. In particular we look at the dynamical behaviour of the two relevant moduli in the theory, namely the length of the eleventh segment (pi rho) and the volume of the internal six manifold (V) in models where supersymmetry is broken by multiple gaugino condensation. We look at different ways to stabilise these moduli, namely racetrack scenarios with or without non-perturbative corrections to the Kahler potential. The existence of different flat directions in the scalar potential, and the way in which they can be partially lifted, is discussed as well as their possible role in constructing a viable model of inflation. Some other implications such as the status of the moduli problem within these models are also studied.Comment: 16 pages, 8 Postscript figures. Final version to appear in JHE