University of Los Andes

About: University of Los Andes is a education organization based out in Bogotá, Colombia. It is known for research contribution in the topics: Population & Large Hadron Collider. The organization has 17616 authors who have published 25555 publications receiving 413463 citations.

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.

Direct measurement of thermal conductivity in solid iron at planetary core conditions

Abstract: The thermal conductivity of solid iron at the pressure and temperature conditions that prevail in the cores of planets is measured directly using a dynamically laser-heated diamond-anvil cell, yielding values that support findings from ancient magnetized rocks that suggest Earth’s magnetic field has persisted since the Earth’s earliest history. The thermal conductivity of iron and its alloys at high pressure and temperature is a critical factor in the evolution and dynamics of Earth-like planets. Recently, increasing uncertainty in these values has produced dramatically variable predictions for Earth's history that challenge traditional geophysical theories. Two groups reporting in this issue of Nature use laser-heated diamond-anvil cells to study the properties of iron at the extreme temperatures and pressures relevant to Earth's core, but using different methodologies, and they arrive at contrasting results. Kenji Ohta and co-authors measured the electrical resistivity of iron at up to 4,500 kelvin and obtained an estimate that is even lower than the low values predicted from recent ab initio studies. They conclude that this suggests a high thermal conductivity for Earth's core, which would imply rapid core cooling by conduction and a relatively young inner core. Zuzana Konopkova and co-authors measured heat pulses propagating through solid iron after heating with a laser pulse at pressures and temperatures relevant to the cores of planets ranging in size from Mercury to Earth. Their measurements place the thermal conductivity of Earth's core near the low end of previous estimates, implying that thermal convection in Earth's core could have driven the geodynamo for billions of years, and allowing for an ancient inner core. In a linked News & Views, David Dobson discusses the interpretation of these two tours de force of experimental geophysics. The conduction of heat through minerals and melts at extreme pressures and temperatures is of central importance to the evolution and dynamics of planets. In the cooling Earth’s core, the thermal conductivity of iron alloys defines the adiabatic heat flux and therefore the thermal and compositional energy available to support the production of Earth’s magnetic field via dynamo action1,2,3. Attempts to describe thermal transport in Earth’s core have been problematic, with predictions of high thermal conductivity4,5,6,7 at odds with traditional geophysical models and direct evidence for a primordial magnetic field in the rock record8,9,10. Measurements of core heat transport are needed to resolve this difference. Here we present direct measurements of the thermal conductivity of solid iron at pressure and temperature conditions relevant to the cores of Mercury-sized to Earth-sized planets, using a dynamically laser-heated diamond-anvil cell11,12. Our measurements place the thermal conductivity of Earth’s core near the low end of previous estimates, at 18–44 watts per metre per kelvin. The result is in agreement with palaeomagnetic measurements10 indicating that Earth’s geodynamo has persisted since the beginning of Earth’s history, and allows for a solid inner core as old as the dynamo.

Regional and seasonal patterns of litterfall in tropical South America

Abstract: . The production of aboveground soft tissue represents an important share of total net primary production in tropical rain forests. Here we draw from a large number of published and unpublished datasets (n=81 sites) to assess the determinants of litterfall variation across South American tropical forests. We show that across old-growth tropical rainforests, litterfall averages 8.61±1.91 Mg ha−1 yr−1 (mean ± standard deviation, in dry mass units). Secondary forests have a lower annual litterfall than old-growth tropical forests with a mean of 8.01±3.41 Mg ha−1 yr−1. Annual litterfall shows no significant variation with total annual rainfall, either globally or within forest types. It does not vary consistently with soil type, except in the poorest soils (white sand soils), where litterfall is significantly lower than in other soil types (5.42±1.91 Mg ha−1 yr−1). We also study the determinants of litterfall seasonality, and find that it does not depend on annual rainfall or on soil type. However, litterfall seasonality is significantly positively correlated with rainfall seasonality. Finally, we assess how much carbon is stored in reproductive organs relative to photosynthetic organs. Mean leaf fall is 5.74±1.83 Mg ha−1 yr−1 (71% of total litterfall). Mean allocation into reproductive organs is 0.69±0.40 Mg ha−1 yr−1 (9% of total litterfall). The investment into reproductive organs divided by leaf litterfall increases with soil fertility, suggesting that on poor soils, the allocation to photosynthetic organs is prioritized over that to reproduction. Finally, we discuss the ecological and biogeochemical implications of these results.