Showing papers by "University of Los Andes published in 2008"

The CMS experiment at the CERN LHC

Abstract: The Compact Muon Solenoid (CMS) detector is described. The detector operates at the Large Hadron Collider (LHC) at CERN. It was conceived to study proton-proton (and lead-lead) collisions at a centre-of-mass energy of 14 TeV (5.5 TeV nucleon-nucleon) and at luminosities up to 10(34)cm(-2)s(-1) (10(27)cm(-2)s(-1)). At the core of the CMS detector sits a high-magnetic-field and large-bore superconducting solenoid surrounding an all-silicon pixel and strip tracker, a lead-tungstate scintillating-crystals electromagnetic calorimeter, and a brass-scintillator sampling hadron calorimeter. The iron yoke of the flux-return is instrumented with four stations of muon detectors covering most of the 4 pi solid angle. Forward sampling calorimeters extend the pseudo-rapidity coverage to high values (vertical bar eta vertical bar <= 5) assuring very good hermeticity. The overall dimensions of the CMS detector are a length of 21.6 m, a diameter of 14.6 m and a total weight of 12500 t.

Behavioural Development Economics: Lessons from Field Labs in the Developing World

Abstract: Explanations of poverty, growth and development depend on the assumptions made about individual preferences and the willingness to engage in strategic behaviour. Economic experiments, especially those conducted in the field, have begun to paint a picture of economic agents in developing communities that is at variance with the traditional portrait. We review this growing literature with an eye towards preference-related experiments conducted in the field. We also offer lessons on what development economists might learn from experiments. We conclude by sharing our thoughts on how to conduct experiments in the field and then offer a few ideas for future research.

The effect of maternal employment and child care on children's cognitive development*

Abstract: This article develops and estimates a dynamic model of employment and child care decisions of women after childbirth to evaluate the effects of these choices on children's cognitive ability. We use data from the National Longitudinal Survey of Youth to estimate it. Results indicate that the effects of maternal employment and child care on children's ability are negative and sizable. Having a mother that works full-time and uses child care during one year is associated with a reduction in ability test scores of approximately 1.8% (0.13 standard deviations). We assess the impact of policies related to parental leave and child care on children's outcomes.

Measurement of the inclusive jet cross section in pp̄ collisions at s=1.96TeV

Abstract: We report on a measurement of the inclusive jet cross section in p (p) over bar collisions at a center-of-mass energy root s = 1.96 TeV using data collected by the D0 experiment at the Fermilab Tevatron Collider corresponding to an integrated luminosity of 0: 70 fb(-1). The data cover jet transverse momenta from 50 to 600 GeV and jet rapidities in the range -2.4 to 2.4. Detailed studies of correlations between systematic uncertainties in transverse momentum and rapidity are presented, and the cross section measurements are found to be in good agreement with next-to-leading order QCD calculations.

Civil Conflict and Forced Migration: The Microdeterminants and the Welfare Losses of Displacement in Colombia

Abstract: During the last decade, forced internal displacement in Colombia has been a growing phenomenon closely linked to the escalation of the internal armed conflict, particularly in rural areas. The displacement problem has affected nearly every region and vulnerable groups of the population. Two emerging policy questions are whether the magnitude of the response to this problem has been proportional to its size and to what extent the instruments chosen are the most adequate to address it. The purpose of this paper is twofold. First, to identify the determinants of displacement behavior and to compare these findings with standard migration literature. Second, to estimate the burden or welfare losses of displacement. Empirical evidence shows that the welfare loss of displacement is considerable and amount to 37 percent of the net present value of rural lifetime aggregate consumption for the average household. This loss is estimated for each household with a method that derives welfare changes from behavioral model estimates, widely used in environmental economics. Our empirical findings also show that the level of violence at the origin site is not only the dominant factor of displacement behavior, but also that in a violent environment other migration determinants have the opposite effect, relative to the one expected by the migration literature in a non-violent context. That is, the violent environment modifies the migration incentives for risk aversion, access to information, the planning horizon, and location-specific assets, human and non-human.

Corporate Governance and Firm Value: The Case of Venezuela

Abstract: Manuscript Type: Empirical Research Question/Issue: We examine the relationship between corporate governance and firm value, and evaluate the relatively understudied governance practices in Venezuela. Research Findings/Results: We construct a corporate governance index (CGI) for publicly-listed firms that is free of self-selection and self-reported bias and find that its mean value is below the emerging market average in general, and below the Latin American average in particular. This weak investor protection environment makes Venezuela a good setting to study how corporate governance practices affect firm value. We show that an increase of 1 per cent in the CGI results in an average increase of 11.3 per cent in dividend payouts, 9.9 per cent in price-to-book, and 2.7 per cent in Tobin's Q. These findings are robust after considering the potential endogeneity of our regression variables. Theoretical Implications: Results contrast to those reported in the US due to the higher interfirm variations in CGI. Our findings are consistent with the theoretical models that relate good corporate governance practices to higher investor confidence, and with the agency model of dividend payout. Furthermore, we conjecture that our results are generalizable mainly to other countries where investor protection is low. Practical Implications: Two direct insights to policy makers and practitioners follow from our analysis: first, managers in weak investor protection environments could differentiate their firms adopting corporate policies to improve their governance structure; and second, our measure of governance practices gives investors a quantitative tool to better assess Venezuelan firms.

Measurement of Bs0 Mixing Parameters from the Flavor-Tagged Decay Bs0→J/ψ

Abstract: From an analysis of the flavor-tagged decay B-s(0)-> J/psi phi we obtain the width difference between the B-s(0) light and heavy mass eigenstates, Delta Gamma(s)=0.19 +/- 0.07(stat)(-0.01)(+0.02)(syst) ps(-1), and the CP-violating phase, phi(s)=-0.57(-0.30)(+0.24)(stat)(-0.02)(+0.08)(syst). The allowed 90% CL intervals of Delta Gamma(s) and phi(s) are 0.06

Measurement of the forward-backward charge asymmetry in top-quark pair production

Abstract: We present the first measurement of the integrated forward-backward charge asymmetry in top-quark-top-antiquark pair (t (t) over bar) production in proton-antiproton (p (p) over bar) collisions in the lepton+jets final state. Using a b-jet tagging algorithm and kinematic reconstruction assuming t (t) over bar +X production and decay, a sample of 0.9 fb(-1) of data, collected by the D0 experiment at the Fermilab Tevatron Collider, is used to measure the asymmetry for different jet multiplicities. The result is also used to set upper limits on t (t) over bar +X production via a Z' resonance.

A prospective, randomized, controlled trial of noninvasive ventilation in pediatric acute respiratory failure.

Abstract: Outcomes:To compare the benefits of noninvasive ventilation (NIV) plus standard therapy vs. standard therapy alone in children with acute respiratory failure; assess method effectiveness in improving gas exchange and vital signs; and assess method safety.Design:Prospective, randomized, controlled st

Phylogeography and Genetic Variation of Triatoma dimidiata, the Main Chagas Disease Vector in Central America, and Its Position within the Genus Triatoma

Abstract: Background: Among Chagas disease triatomine vectors, the largest genus, Triatoma, includes species of high public health interest. Triatoma dimidiata, the main vector throughout Central America and up to Ecuador, presents extensive phenotypic, genotypic, and behavioral diversity in sylvatic, peridomestic and domestic habitats, and non-domiciliated populations acting as reinfestation sources. DNA sequence analyses, phylogenetic reconstruction methods, and genetic variation approaches are combined to investigate the haplotype profiling, genetic polymorphism, phylogeography, and evolutionary trends of T. dimidiata and its closest relatives within Triatoma. This is the largest interpopulational analysis performed on a triatomine species so far. Methodology and Findings: Triatomines from Mexico, Guatemala, Honduras, Nicaragua, Panama, Cuba, Colombia, Ecuador, and Brazil were used. Triatoma dimidiata populations follow different evolutionary divergences in which geographical isolation appears to have had an important influence. A southern Mexican‐northern Guatemalan ancestral form gave rise to two main clades. One clade remained confined to the Yucatan peninsula and northern parts of Chiapas State, Guatemala, and Honduras, with extant descendants deserving specific status. Within the second clade, extant subspecies diversity was shaped by adaptive radiation derived from Guatemalan ancestral populations. Central American populations correspond to subspecies T. d. dimidiata. A southern spread into Panama and Colombia gave the T. d. capitata forms, and a northwestern spread rising from Guatemala into Mexico gave the T. d. maculipennis forms. Triatoma hegneri appears as a subspecific insular form. Conclusions: The comparison with very numerous Triatoma species allows us to reach highly supported conclusions not only about T. dimidiata, but also on different, important Triatoma species groupings and their evolution. The very large intraspecific genetic variability found in T. dimidiata sensu lato has never been detected in a triatomine species before. The distinction between the five different taxa furnishes a new frame for future analyses of the different vector transmission capacities and epidemiological characteristics of Chagas disease. Results indicate that T. dimidiata will offer problems for control, although dwelling insecticide spraying might be successful against introduced populations in Ecuador.

Homoploid hybrid speciation in animals

Abstract: Among animals, evidence for homoploid hybrid speciation (HHS, i.e. the creation of a hybrid lineage without a change in chromosome number) was limited until recently to the virgin chub, Gila seminuda, and some controversial data in support of hybrid status for the red wolf, Canis rufus. This scarcity of evidence, together with pessimistic attitudes among zoologists about the evolutionary importance of hybridisation, prompted the view that HHS is extremely rare among animals, especially as compared with plants. However, in recent years, the literature on animal HHS has expanded to include several new putative examples in butterflies, ants, flies and fishes. We argue that this evidence suggests that HHS is far more common than previously thought and use it to provide insights into some of the genetic and ecological aspects associated with this type of speciation among animals.

Growth and wood density predict tree mortality in Amazon forests

Abstract: Summary 1. Tree mortality is an important process in forest ecology. We explored the extent to which tropical tree death is a predictable outcome of taxon and individual level properties by means of mixedspecies logistic regression, for trees ≥ 10 cm in diameter. We worked in two lowland forest regions with markedly different floristic composition and dynamic regimes ‐ the high wood density, lowmortality northeastern (NE) Amazon (in eastern Venezuela), and the low wood density, highmortality northwestern (NW) Amazon (in northern Peru). 2. Among those genera that are shared between regions there were no detectable regional differences in mortality rates. This suggests that floristic compositional differences are a major driver of the twofold regional contrast in stand-level mortality. 3. In NE forests, mortality risk of individual trees is best predicted by low taxon-level wood density, slow relative growth, and large size, reflecting phylogenetically determined life-history strategy, physiological stress and senescence. 4. In NW forests, trees with low wood density and slow relative growth are also at most risk, but probability of death is independent of tree size, indicating that senescence is unimportant in this region. 5. Synthesis. This study shows that despite fundamental floristic and dynamic differences between the two Amazonian regions, mortality risk can be predicted with mixed-species, individual-based statistical models and that the predictors are remarkably similar, such that tree growth and wood density both play important roles.

Physical activity levels and counseling practices of U.S. medical students.

Abstract: Introduction:Some literature shows a positive relationship between physicians' personal physical activity (PA) levels and physicians' encouraging patients' PA, though it remains unclear how this evolves during medical training. In this paper, we describe U.S. medical students' PA levels and

The triglyceride/HDL-cholesterol ratio as a marker of cardiovascular risk in obese children; association with traditional and emergent risk factors.

Abstract: Objectives: To determine the presence of traditional and emergent cardiovascular risk factors and to evaluate the triglyceride/high-density lipoprotein cholesterol (Tg/HDL-C) ratio as a marker for cardiovascular disease and metabolic syndrome (MS) in obese children Material and methods: Sixty-seven prepubertal children of both sexes, between the ages of 6 and 12 yr, 20 normal-weight children, 18 overweight, and 29 obese subjects, were studied Anthropometric measures, blood pressure, body mass index (BMI), and fat mass (FM), were measured Plasma glucose, serum insulin, lipid profile, C-reactive protein (CRP), and leptin concentrations were quantified Glucose and insulin concentrations 2 h post-glucose load were determined The Tg/HDL-C ratio, homeostasis model assessment index (HOMA), and quantitative insulin sensitivity check index (QUICKI) were calculated Results: Systolic, diastolic, and mean blood pressures (MBP), low-density lipoprotein cholesterol (LDL-C), Tg/HDL-C, total cholesterol/HDL-C, LDL-C/HDL-C ratios, basal and 2 h postload insulin, CRP, and leptin were significantly higher and the QUICKI index were lower in the obese group MBP, Tg/HDL-C ratio, HOMA, CRP, and leptin levels showed a positive and significant correlation and QUICKI a negative correlation with abdominal circumference, BMI, and FM The Tg/HDL-C ratio correlated positively with MBP The frequency of MS in the obese group was 69% While Tg/HDL-C ratio, CRP, and leptin were higher and the values of QUICKI were lower in subjects with MS, it was the Tg/HDL-C ratio and the BMI that significantly explained the MS Conclusions: Obesity increases the cardiovascular risk in childhood The Tg/HDL-C ratio could be a useful index in identifying children at risk for dyslipidemia, hypertension, and MS

Evidence for production of single top quarks

Abstract: We present first evidence for the production of single top quarks in the D0 detector at the Fermilab Tevatron ppbar collider. The standard model predicts that the electroweak interaction can produce a top quark together with an antibottom quark or light quark, without the antiparticle top quark partner that is always produced from strong coupling processes. Top quarks were first observed in pair production in 1995, and since then, single top quark production has been searched for in ever larger datasets. In this analysis, we select events from a 0.9 fb-1 dataset that have an electron or muon and missing transverse energy from the decay of a W boson from the top quark decay, and two, three, or four jets, with one or two of the jets identified as originating from a b hadron decay. The selected events are mostly backgrounds such as W+jets and ttbar events, which we separate from the expected signals using three multivariate analysis techniques: boosted decision trees, Bayesian neural networks, and matrix element calculations. A binned likelihood fit of the signal cross section plus background to the data from the combination of the results from the three analysis methods gives a cross section for single top quark production of 4.7 +- 1.3 pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.014%, corresponding to a 3.6 standard deviation significance. The measured cross section value is compatible at the 10% level with the standard model prediction for electroweak top quark production.

Measurement of the shape of the boson-transverse momentum distribution in pp̄→Z/γ*→e+e-+X events produced at s=1.96TeV

Abstract: We present a measurement of the shape of the Z/gamma* boson transverse momentum (q(T)) distribution in p (p) over bar -> Z/gamma(*)-> e(+)e(-)+X events at a center-of-mass energy of 1.96 TeV using 0.98 fb(-1) of data collected with the D0 detector at the Fermilab Tevatron collider. The data are found to be consistent with the resummation prediction at low q(T), but above the perturbative QCD calculation in the region of q(T)> 30 GeV/c. Using events with q(T)< 30 GeV/c, we extract the value of g(2), one of the nonperturbative parameters for the resummation calculation. Data at large boson rapidity y are compared with the prediction of resummation and with alternative models that employ a resummed form factor with modifications in the small Bjorken x region of the proton wave function.

Thigmotactic responses in an open-field

Abstract: The goal of the present study was to investigate the role of thigmotaxis (the tendency to remain close to vertical surfaces) in rat exploratory behavior in an open-field. Thigmotaxis was investigated in a parametric way, using 24 experimentally adult naive male Wistar rats (210-230 g). Exploratory behavior was studied in an open-field (N = 12) in 5-min sessions and behavior was analyzed in terms of where it occurred: in areas surrounded by two, one, or no walls. Another group of rats (N = 12) was studied in an open-field with blocks placed near two of the corners so as to make these corner areas surrounded by three walls. The floor of the open-fields was divided into 20-cm squares in order to locate the exact place of occurrence of each behavior. The following behaviors were recorded: entries into the squares, rearings, and groomings. In both types of open-field the rats chose to remain longer in the squares surrounded by the largest possible number of walls. In one of the open-fields, the mean time (seconds) spent in squares surrounded by two walls was longer than the time spent in squares surrounded by one or no walls (37.2, 7.7, and 1.8 s, respectively). In the other open-field, the mean time spent in squares surrounded by three walls was longer than the time spent in squares surrounded by two, one or no walls (41.7, 20.4, 7.0, and 2.6 s, respectively). Other measures presented a similar profile. These results indicate that rats are sensitive to the number of walls in an environment and prefer to remain close to them.

Potential Effects of Ateline Extinction and Forest Fragmentation on Plant Diversity and Composition in the Western Orinoco Basin, Colombia

Abstract: Cattle and agricultural farming in the western Orinoco Basin began in 1555, and since then fragmentation of continuous forest has occurred. We evaluated the effects of the disturbances and the absence of large primates on plant community composition, diversity, and regeneration patterns. Atelines (Lagothrix and Ateles) inhabited the lowlands close to the Andean mountains, but no longer live in fragmented habitats. Their absence may have negative effects on plant populations because atelines play important roles as seed dispersers in neotropical forests, especially for large-seeded plants, which are rarely swallowed by other seed dispersers. We compared 2 1-ha vegetation plots in forest fragments north of the La Macarena Mountains with 7 plots in continuous forest in Tinigua National Park. Both sites share the same climatic conditions and have similar geological origins. There is floristic affinity between forests with similar ecological characteristics; the fragmented forests are also less diverse than the continuous forests. As predicted, the forest fragments have fewer individuals with large seeds. The results suggest that forest fragmentation and local ateline extinctions affect plant communities, reducing diversity and affecting large-seeded plants.

Observation of the Bc meson in the exclusive decay Bc→J/ψπ

Abstract: A fully reconstructed B-c -> J/psi pi signal is observed with the D0 detector at the Fermilab Tevatron p (p) over bar collider using 1.3 fb(-1) of integrated luminosity. The signal consists of 54 +/- 12 candidates with a significance that exceeds 5 standard deviations, and confirms earlier observations of this decay. The measured mass of the B-c meson is 6300 +/- 14(stat)+/- 5(syst) MeV/c(2).

Energy of digraphs

Abstract: We extend the concept of energy to directed graphs in such a way that Coulson's integral formula remains valid. As a consequence, it is shown that the energy is increasing over the set of digraphs with n vertices and cycles of length h, with respect to a quasi-order relation. Applications to the problem of extremal values of the energy in various classes of digraphs are considered.