Leptonic CP phases near the μ−τ\mu-\tau symmetric limit

The neutrino masses and mixings indicated by current neutrino oscillation experiments suggest that the neutrino mass matrix possesses an approximate $\mu-\tau$ exchange symmetry. In this study, we explore the neutrino parameter space and show that if a small $\mu-\tau$ symmetry breaking is considered, the Majorana $CP$ phases must be unequal and non-zero independently of the neutrino mass scale. Moreover, a small $\mu-\tau$ symmetry breaking favors quasi-degenerate masses. We also show that Majorana phases are strongly correlated with the Dirac $CP$ violating phase. Within this framework, we obtain robust predictions for the values of the Majorana phases when the experimental indications for the Dirac $CP$ phase are used.Comment: 13 pages, 3 figures. Version accepted for publication in PL

Temporal variations of monsoon systems

It has been proposed that the Asian-Australasian monsoon system is influenced by large-scale sea-surface temperature (SST) variability in the three tropical oceans although how this influence is manifested has remained a largely open question. Closure of this issue is important because it is needed to explain trends in monsoon precipitation and circulation that have occurred in the last 30 years. Using an atmospheric general circulation model, we run a series of experiments with different configurations of global SST relating to various epochs occurring during the last century to evaluate their influence on the monsoon. Comparisons of circulation fields show that a colder SST configuration generates a weaker large-scale monsoonal circulation. On the other hand, warmer SST states generate stronger large scale circulations with more vigorous centers of divergence and convergence. Warmer SST configurations are associated with positive anomalies of precipitation in the eastern Bay of Bengal, Eastern Indian Ocean and South East Asia. Cooler SST configurations are associated with negative anomalies of precipitation in the Arabian Sea and Indian peninsula, especially at the beginning of the summer. Since SST gradients determine, to a large degree, the low level flow, they are also going to influence the transport of atmospheric moisture. Comparison of vertically integrated moisture transport fields between the different experiments show that cold SST configuration favors an increased inter-hemispheric flow of moisture but decreases in the westerly moisture flow in to the Bay of Bengal and India. Warm SST configurations, on the other hand, strengthens westerly flow into the eastern Indian Ocean. An increasing availability of moisture in a region of stronger convergence constitutes a favorable environment for the production of monsoonal precipitation. African easterly waves (AEW) constitute an important component of the African and tropical Atlantic Ocean climate during the boreal summer. An understanding of this component is essential since AEW are closely related with tropical Atlantic storm activity. We adopt an idealized modeling approach using the WRF model initialized with ERA-40 reanalysis data to study the mechanisms that trigger the formation and maintenance of AEW. The model domain includes the African continent, central and eastern Atlantic Ocean and the western Indian Ocean. Experiments are designed to test the relative importance of the thermal effect of the eastern African topography and the influence of the cross-equatorial pressure gradient, induced by the sea surface temperature (SST) on the origins and maintenance of AEW. Topography and SST variation are selectively added and removed. The control experiment shows that the model reproduces many of the mean features observed during the boreal summer. Westward propagating disturbances of 3-8 day period that originate between 30 and 40E at the surface levels and in the mid troposphere are well depicted. In addition, the model provides a reasonable representation of the AEJ. When all topographic features are removed, there is a weakening of the AEJ over land and ocean, however, longitude-time sections of meridional velocity still exhibit westward propagating disturbances that reach the western African coast at the surface and at the jet level with the same 3-8 day period. Spectral analysis of meridional velocity show that the variability associated with AEWs is reduced over East Africa and West Africa at 850-hPa and is reduced west of 20E along the southern flank of the jet and over northern Africa at the jet level. Maximum amplitude of the disturbances occurs right at the coast. The spatial distribution of barotropic and baroclinic energy conversions explains the reduction in AEWs over land and the intensification of these features at the coast. When the zonal SST gradient is removed, a weaker AEJ displaces southward and a weaker monsoon flow ensues. Spectral analysis of meridional velocity displays a variance reduction in the 3-8 day band at the 850-hP a level in western and eastern Africa and at the coast. At the 650-hPa level significant changes are not observed at the latitude of the AEJ (15N), however, a decrease in the variance associated with AEW occurs at the southern flank of the jet. A southward displacement of the jet favors a weakening of the baroclinic energy conversions. Barotropic conversions also appear to be weaker when the SST gradient is removed. The present study suggests that orography plays an important role in determining the variability of meridional wind associated with AEW over Eastern Africa at the lower levels. Further, zonal SST gradients over the Atlantic favor intensification of waves when they reach the coast and the maintenance of disturbances across the Ocean. Also, results could suggest that SST gradients support genesis of AEW just off the coast of Africa.Ph.D.Committee Chair: Peter Webster; Committee Member: Annalisa Bracco; Committee Member: Judith A. Curry; Committee Member: Marshall Shepherd; Committee Member: Yi Den

The VLT-FLAMES Tarantula Survey

We present a number of notable results from the VLT-FLAMES Tarantula Survey (VFTS), an ESO Large Program during which we obtained multi-epoch medium-resolution optical spectroscopy of a very large sample of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). This unprecedented data-set has enabled us to address some key questions regarding atmospheres and winds, as well as the evolution of (very) massive stars. Here we focus on O-type runaways, the width of the main sequence, and the mass-loss rates for (very) massive stars. We also provide indications for the presence of a top-heavy initial mass function (IMF) in 30 Dor.Comment: 7 Figures, 8 pages. Invited talk: IAUS 329: "The Lives and Death-Throes of Massive Stars

Deviations to μ−τ\mu-\tau symmetry in a cobimaximal scenario

Within the different patterns of the neutrino mixing matrix, the cobimaximal mixing remains a plausible possibility for understanding the flavor structure of neutrinos as it is consistent with current experimental data. Such a pattern is related to a concrete form of the mass matrix, displaying a $\mu-\tau$ reflection symmetry, which has motivated many theoretical investigations in recent years. In this paper, we discuss the effects of the Majorana phases over a $\mu-\tau$ symmetric mass matrix obtained from a cobimaximal mixing matrix. We investigate how these phases could be restricted through deviations from the symmetric scenario and the most precise determinations of the mixing parameters. Some of our relations could be tested with future results.Comment: 8 pages, 2 figure

Rendimiento ex-dividendo como indicador de eficiencia en un mercado emergente: caso colombiano 1999-2007

Se evalúa el rendimiento ex-dividendo en acciones colombianas entre 1999 y 2007, período que incluye la conformación en Julio de 2001 de la Bolsa de Valores de Colombia resultado de la integración de tres bolsas previamente existentes. Contrario a la hipótesis de eficiencia de mercado, se encontraron rendimientos ex-dividendo positivos y estadísticamente significativos, similares a los evidenciados en diversos mercados internacionales. Se comprueba que los rendimientos ex-dividendo no son explicados en su totalidad por costos de transacción ni por efectos impositivos. Una estrategia limitada de captura de dividendos, substrayendo dichos costos, habría entregado rendimientos positivos y económicamente importantes entre 2006 y 2007 en las acciones más liquidas del mercado. Sin embargo, estos rendimientos tienden a disminuir en el período de estudio, consistentes con un avance hacia una mayor eficiencia en el mercado accionario colombiano después de la integración. Este estudio pone de relieve la importancia de considerar las fricciones en estudios académicos de eficiencia y de evaluación de estrategias especulativas.We study the ex-dividend return in the Colombian stock market between 1999 and 2007, period that includes the merger of the former three Colombian stock exchanges in the Bolsa de Valores de Colombia in July 2001. Contrary to the Efficient Market Hypothesis, we found positive and statistically significant ex-dividend returns in the sampled period, only in part explained by transaction cost and tax effects. Moreover, even subtracting transaction costs and tax effects, a dividend capture strategy would have got positive and economically sizable returns between 2006 and 2007 in the most liquid stocks. The decrease of those ex-dividend returns is also reported along the studied period, providing evidence of increasing informational efficiency after the merger of the three stock exchanges. Methodologically, this study highlights the importance of accounting for frictions in both academic efficiency studies and in testing speculative strategies by practitioners