379 research outputs found
The Chiral Magnetic Effect in Heavy Ion Collisions From Hydrodynamic Simulations
The quark-gluon plasma created in heavy ion collisions is an exotic state of matter in which many unusual phenomena are manifested. One such phenomenon is the Chiral-Magnetic Effect (CME), wherein the powerful magnetic fields generated by colliding ions spin-polarize chiral quarks, causing a net transport effect in the direction of the fields. The CME predicts specific charge-dependent correlation observables, for which experimental evidence was reported, although the evidence is subject to background contamination. Isobaric collision experiments have been planned for 2018 at RHIC, which will study this effect by comparing 96Ru-96Ru and 96Zr-96Zr collisions. The two colliding systems are expected to have nearly identical bulk properties (including background contamination), yet about 10% difference in their magnetic fields due to different nuclear charges. This provides a unique opportunity to disentangle the CME observable and background effects. By simulating this effect using anomalous hydrodynamic simulations, we make a quantitative prediction for the CME-induced signal for several centralities in each of these two colliding systems. Our results suggest a significant enough difference in the signal to be experimentally detected- on the order of 15-20%
Anomalous Chiral Transport in Heavy Ion Collisions from Anomalous-Viscous Fluid Dynamics
Chiral anomaly is a fundamental aspect of quantum theories with chiral
fermions. How such microscopic anomaly manifests itself in a macroscopic
many-body system with chiral fermions, is a highly nontrivial question that has
recently attracted significant interest. As it turns out, unusual transport
currents can be induced by chiral anomaly under suitable conditions in such
systems, with the notable example of the Chiral Magnetic Effect (CME) where a
vector current (e.g. electric current) is generated along an external magnetic
field. A lot of efforts have been made to search for CME in heavy ion
collisions, by measuring the charge separation effect induced by the CME
transport. A crucial challenge in such effort, is the quantitative prediction
for the CME signal. In this paper, we develop the Anomalous-Viscous Fluid
Dynamics (AVFD) framework, which implements the anomalous fluid dynamics to
describe the evolution of fermion currents in QGP, on top of the neutral bulk
background described by the VISH2+1 hydrodynamic simulations for heavy ion
collisions. With this new tool, we quantitatively and systematically
investigate the dependence of the CME signal to a series of theoretical inputs
and associated uncertainties. With realistic estimates of initial conditions
and magnetic field lifetime, the predicted CME signal is quantitatively
consistent with measured change separation data in 200GeV Au-Au collisions.
Based on analysis of Au-Au collisions, we further make predictions for the CME
observable to be measured in the planned isobaric (Ru-Ru v.s. Zr-Zr ) collision
experiment, which could provide a most decisive test of the CME in heavy ion
collisions.Comment: 28 pages, 13 figures; published versio
Quantification of Chiral Magnetic Effect from Event-by-Event Anomalous-Viscous Fluid Mechanics
Chiral Magnetic Effect (CME) is the macroscopic manifestation of the
fundamental chiral anomaly in a many-body system of chiral fermions, and
emerges as anomalous transport current in hydrodynamic framework. Experimental
observation of CME is of great interest and significant efforts have been made
to look for its signals in heavy ion collisions. Encouraging evidence of
CME-induced charge separation has been reported from both RHIC and LHC, albeit
with ambiguity due to potential background contributions. Crucial for
addressing such issue, is the need of quantitative predictions for both CME
signal and the non-CME background consistently, with sophisticated modeling
tool. In this contribution we report a recently developed Anomalous Viscous
Fluid Dynamics (AVFD) framework, which simulates the evolution of fermion
currents in QGP on top of the data-validated VISHNU bulk hydro evolution. In
particular, this framework has been extended to event-by-event simulations with
proper implementation of known flow-driven background contributions. We report
quantitative results from such simulations and evaluate the implications for
interpretations of current experimental measurements. Finally we give our
prediction for the CME signal in upcoming isobaric collisions.Comment: 5 pages, 7 figures; plenary talk at CPOD 2017 conference, Stony Brook
University, Stony Brook, NY. arXiv admin note: substantial text overlap with
arXiv:1704.05531; text overlap with arXiv:1611.0458
Quantifying the Chiral Magnetic Effect from Anomalous-Viscous Fluid Dynamics
In this contribution we report a recently developed Anomalous-Viscous Fluid
Dynamics (AVFD) framework, which simulates the evolution of fermion currents in
QGP on top of the bulk expansion from data-validated VISHNU hydrodynamics. With
reasonable estimates of initial conditions and magnetic field lifetime, the
predicted CME signal is quantitatively consistent with change separation
measurements in 200GeV Au-Au collisions at RHIC. We further develop the
event-by-event AVFD simulations that allow direct evaluation of two-particle
correlations arising from CME signal as well as the non-CME backgrounds.
Finally we report predictions from AVFD simulations for the upcoming isobaric
(Ru-Ru v.s. Zr-Zr ) collisions that could provide the critical test of the CME
in heavy ion collisions.Comment: Contribution to the Proceedings of the XXVIth International
Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter
2017), Feb 5-11, Chicago, U.S.A. 4 pages, 6 figure
Root biomass and production by two cushion plant species of tropical high-elevation peatlands in the andean páramo
High-elevation peatlands in the Andes are receiving increasing attention for their biodiversity and their high rates of carbon accumulation. However, the ecology of these peatlands and the environmental factors that control their carbon dynamics remain under-studied. Here we report on the patterns of root biomass productivity and turnover rates for two cushion plant species (Distichia muscoides, Plantago rigida) that commonly dominate high-elevation peatlands (\u3e 4200 m a.s.l.) in the Andean páramo landscape of Northern Ecuador. Root biomass for P. rigida ranged from 680 to 864 g m-2 and was approximately 40 % higher than for D. muscoides (507–620 g m-2). In contrast, root production was almost twice as high for D. muscoides (2000–2800 g m-2 yr-1) than for P. rigida (1030–1080 g m-2 yr-1). These patterns resulted in high root turnover rates, especially for D. muscoides (0.98–1.90 yr-1). Below-ground productivity (as C) at our sites conservatively ranged from 0.55 to 1.5 kg m–2 yr–1, representing approximately 30 % of the estimated total productivity for these species, which only accounts for root production down to 50 cm depth. These high productivity rates are in accordance with the extremely high rates of carbon accumulation that have been reported for high-elevation peatlands of the Andes
Eastern Temperate Forests
Human activity in the last century has led to a substantial increase in nitrogen (N) emissions and deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the level of pollution that would be harmful to ecosystems is the critical loads approach. The critical load is dei ned as the level of a pollutant below which no detrimental ecological effect occurs over the long term according to present knowledge.
The objective of this project was to synthesize current research relating atmospheric N deposition to effects on terrestrial and aquatic ecosystems in the United States and to identify empirical critical loads for atmospheric N deposition. The receptors that we evaluated included freshwater diatoms, mycorrhizal fungi and other soil microbes, lichens, herbaceous plants, shrubs, and trees. The main responses reported fell into two categories: (1) biogeochemical, and (2) individual species, population, and community responses.
The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1 to 39 kg N ha-1 y-1. This broad range spans the range of N deposition observed over most of the country. The empirical critical loads for N tend to increase in the following sequence for different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees.
The critical loads approach is an ecosystem assessment tool with great potential to simplify complex scientii c information and effectively communicate with the policy community and the public. This synthesis represents the i rst comprehensive assessment of empirical critical loads of N for ecoregions across the United States
Wetland and Hydric Soils
Soil and the inherent biogeochemical processes in wetlands contrast starkly with those in upland forests and rangelands. The differences stem from extended periods of anoxia, or the lack of oxygen in the soil, that characterize wetland soils; in contrast, upland soils are nearly always oxic. As a result, wetland soil biogeochemistry is characterized by anaerobic processes, and wetland vegetation exhibits specific adaptations to grow under these conditions. However, many wetlands may also have periods during the year where the soils are unsaturated and aerated. This fluctuation between aerated and nonaerated soil conditions, along with the specialized vegetation, gives rise to a wide variety of highly valued ecosystem services
Atmospheric nitrogen deposition impacts on the structure and function of forest mycorrhizal communities: a review
Humans have dramatically increased atmospheric nitrogen (N) deposition globally. At the coarsest resolution, N deposition is correlated with shifts from ectomycorrhizal (EcM) to arbuscular mycorrhizal (AM) tree dominance. At finer resolution, ectomycorrhizal fungal (EcMF) and arbuscular mycorrhizal fungal (AMF) communities respond strongly to long-term N deposition with the disappearance of key taxa. Conifer-associated EcMF are more sensitive than other EcMF, with current estimates of critical loads at 5–6 kg ha−1 yr−1 for the former and 10–20 kg ha−1 yr−1 for the latter. Where loads are exceeded, strong plant-soil and microbe-soil feedbacks may slow recovery rates after abatement of N deposition. Critical loads for AMF and tropical EcMF require additional study. In general, the responses of EcMF to N deposition are better understood than those of AMF because of methodological tractability. Functional consequences of EcMF community change are linked to decreases by fungi with medium-distance exploration strategies, hydrophobic walls, proteolytic capacity, and perhaps peroxidases for acquiring N from soil organic matter. These functional losses may contribute to declines in forest floor decomposition under N deposition. For AMF, limited capacity to directly access complexed organic N may reduce functional consequences, but research is needed to test this hypothesis. Mycorrhizal biomass often declines with N deposition, but the relative contributions of alternate mechanisms for this decline (lower C supply, higher C cost, physiological stress by N) have not been quantified. Furthermore, fungal biomass and functional responses to N inputs probably depend on ecosystem P status, yet how N deposition-induced P limitation interacts with belowground C flux and mycorrhizal community structure and function is still unclear. Current ‘omic analyses indicate potential functional differences among fungal lineages and should be integrated with studies of physiology, host nutrition, growth and health, fungal and plant community structure, and ecosystem processes
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