474 research outputs found
AdS/QCD Phenomenological Models from a Back-Reacted Geometry
We construct a fully back-reacted holographic dual of a four-dimensional
field theory which exhibits chiral symmetry breaking. Two possible models are
considered by studying the effects of a five-dimensional field, dual to the
operator. One model has smooth geometry at all radii and the other
dynamically generates a cutoff at finite radius. Both of these models satisfy
Einstein's field equations. The second model has only three free parameters, as
in QCD, and we show that this gives phenomenologically consistent results. We
also discuss the possibility that in order to obtain linear confinement from a
back-reacted model it may be necessary to consider the condensate of a
dimension two operator.Comment: 13 pages, 4 figures, Replaced with minor correction
Aqueous Alteration and Hydrogen Generation on Parent Bodies of Unequilibrated Ordinary Chondrites: Thermodynamic Modeling for the Semarkona Composition
Ordinary chondrites are the most abundant class of meteorites that could represent rocky parts of solar system bodies. However, even the most primitive unequilibrated ordinary chondrites (UOC) reveal signs of mild alteration that affected the matrix and peripheral zones of chondrules. Major chemical changes include oxidation of kamacite, alteration of glass, removal of alkalis, Al, and Si from chondrules, and formation of phases enriched in halogens, alkalis, and hydrogen. Secondary mineralogical changes include formation of magnetite, ferrous olivine, fayalite, pentlandite, awaruite, smectites, phosphates, carbonates, and carbides. Aqueous alteration is consistent with the oxygen isotope data for magnetite. The presence of secondary magnetite, Ni-rich metal alloys, and ferrous silicates in UOC implies that H2O was the oxidizing agent. However, oxidation by H2O means that H2 is produced in each oxidative pathway. In turn, production of H2, and its redistribution and possible escape should have affected total pressure, as well as the oxidation state of gas, aqueous and mineral phases in the parent body. Here we use equilibrium thermodynamic modeling to explore water-rock reactions in UOC. The chemical composition of gas, aqueous, and mineral phases is considered
Carbon Oxidation State in Microbial Polar Lipids Suggests Adaptation to Hot Spring Temperature and Redox Gradients
The influence of oxidation-reduction (redox) potential on the expression of biomolecules is a topic of ongoing exploration in geobiology. In this study, we investigate the novel possibility that structures and compositions of lipids produced by microbial communities are sensitive to environmental redox conditions. We extracted lipids from microbial biomass collected along the thermal and redox gradients of four alkaline hot springs in Yellowstone National Park (YNP) and investigated patterns in the average oxidation state of carbon (ZC), a metric calculated from the chemical formulae of lipid structures. Carbon in intact polar lipids (IPLs) and their alkyl chains becomes more oxidized (higher ZC) with increasing distance from each of the four hot spring sources. This coincides with decreased water temperature and increased concentrations of oxidized inorganic solutes, such as dissolved oxygen, sulfate, and nitrate. Carbon in IPLs is most reduced (lowest ZC) in the hot, reduced conditions upstream, with abundance-weighted ZC values between −1.68 and −1.56. These values increase gradually downstream to around −1.36 to −1.33 in microbial communities living between 29.0 and 38.1◦C. This near-linear increase in ZC can be attributed to a shift from ether-linked to ester-linked alkyl chains, a decrease in average aliphatic carbons per chain (nC), an increase in average degree of unsaturation per chain (nUnsat), and increased cyclization in tetraether lipids. The ZC of lipid headgroups and backbones did not change significantly downstream. Expression of lipids with relatively reduced carbon under reduced conditions and oxidized lipids under oxidized conditions may indicate microbial adaptation across environmental gradients in temperature and electron donor/acceptor supply
An examination of protist diversity in serpentinization-hosted ecosystems of the Samail Ophiolite of Oman
In the Samail Ophiolite of Oman, the geological process of serpentinization produces reduced, hydrogen rich, hyperalkaline (pH > 11) fluids. These fluids are generated through water reacting with ultramafic rock from the upper mantle in the subsurface. On Earth’s continents, serpentinized fluids can be expressed at the surface where they can mix with circumneutral surface water and subsequently generate a pH gradient (∼pH 8 to pH > 11) in addition to variations in other chemical parameters such as dissolved CO2, O2, and H2. Globally, archaeal and bacterial community diversity has been shown to reflect geochemical gradients established by the process of serpentinization. It is unknown if the same is true for microorganisms of the domain Eukarya (eukaryotes). In this study, using 18S rRNA gene amplicon sequencing, we explore the diversity of microbial eukaryotes called protists in sediments of serpentinized fluids in Oman. We demonstrate that protist community composition and diversity correlate significantly with variations in pH, with protist richness being significantly lower in sediments of hyperalkaline fluids. In addition to pH, the availability of CO2 to phototrophic protists, the composition of potential food sources (prokaryotes) for heterotrophic protists and the concentration of O2 for anaerobic protists are factors that likely shape overall protist community composition and diversity along the geochemical gradient. The taxonomy of the protist 18S rRNA gene sequences indicates the presence of protists that are involved in carbon cycling in serpentinized fluids of Oman. Therefore, as we evaluate the applicability of serpentinization for carbon sequestration, the presence and diversity of protists should be considered
Evidence for mid-Holocene rice domestication in the Americas
This is the author accepted manuscript. The final version is available from Springer Nature via the DOI in this recordThe development of agriculture is one of humankind's most pivotal achievements. Questions about plant domestication and the origins of agriculture have engaged scholars for well over a century, with implications for understanding its legacy on global subsistence strategies, plant distribution, population health and the global methane budget. Rice is one of the most important crops to be domesticated globally, with both Asia (Oryza sativa L.) and Africa (Oryza glaberrima Steud.) discussed as primary centres of domestication. However, until now the pre-Columbian domestication of rice in the Americas has not been documented. Here we document the domestication of Oryza sp. wild rice by the mid-Holocene residents of the Monte Castelo shell mound starting at approximately 4,000 cal. yr BP, evidenced by increasingly larger rice husk phytoliths. Our data provide evidence for the domestication of wild rice in a region of the Amazon that was also probably the cradle of domestication of other major crops such as cassava (Manihot esculenta), peanut (Arachis hypogaea) and chilli pepper (Capsicum sp.). These results underline the role of wetlands as prime habitats for plant domestication worldwide.The research was funded by the European Research Council project ‘Pre-Columbian Amazon-Scale Transformations’ (ERC-CoG 616179) to J.I. L.M.H. was funded by CAPES (Ministry of Education, Brazil) and Monte Castelo fieldwork was funded by grants from the Brazilian National Science Development Council (CNPq-307179/2013-3) and The National Geographic Society (W243-12) to E.G.N
Canonical Coordinates and Meson Spectra for Scalar Deformed N=4 SYM from the AdS/CFT Correspondence
Five supersymmetric scalar deformations of the AdS_5xS^5 geometry are
investigated. By switching on condensates for the scalars in the N=4 multiplet
with a form which preserves a subgroup of the original R-symmetry, disk and
sphere configurations of D3-branes are formed in the dual supergravity
background. The analytic, canonical metric for each geometry is formulated and
the singularity structure is studied. Quarks are introduced into two of the
corresponding field theories using D7-brane probes and the pseudoscalar meson
spectrum is calculated. For one of the condensate configurations, a mass gap is
found and shown analytically to be present in the massless limit. It is also
found that there is a stepped spectrum with eigenstate degeneracy in the limit
of small quark masses. In the case of a second, similar deformation, it is
necessary to understand the full D3-D7 brane interaction to study the limit of
small quark masses. It is seen that simple solutions to the equations of motion
for the other three geometries are unlikely to exist.Comment: 16 pages, 7 figures, references added, typos correcte
Three Flavour QCD from the Holographic Principle
Building on recent research into five-dimensional holographic models of QCD,
we extend this work by including the strange quark with an SU(3)_L\times
SU(3)_R gauge symmetry in the five-dimensional theory. In addition we deform
the naive metric with a single parameter, thereby breaking the conformal
symmetry at low energies. The vector and axial vector sectors are studied in
detail and both the masses and decay constants are calculated with the
additional parameters. It is shown that with a single extra degree of freedom,
exceptional agreement with experimental results can be obtained in the light
quark sector while the kaon sector is found to give around 10% agreement with
lattice results. We propose some simple extensions to this work to be taken up
in future research.Comment: 9 pages, 1 figure, references adde
Linear square-mass trajectories of radially and orbitally excited hadrons in holographic QCD
We consider a new approach towards constructing approximate holographic duals
of QCD from experimental hadron properties. This framework allows us to derive
a gravity dual which reproduces the empirically found linear square-mass
trajectories of universal slope for radially and orbitally excited hadrons.
Conformal symmetry breaking in the bulk is exclusively due to infrared
deformations of the anti-de Sitter metric and governed by one free mass scale
proportional to Lambda_QCD. The resulting background geometry exhibits dual
signatures of confinement and provides the first examples of holographically
generated linear trajectories in the baryon sector. The predictions for the
light hadron spectrum include new relations between trajectory slopes and
ground state masses and are in good overall agreement with experiment.Comment: 33 pages, 5 figures, updated to the extended version published in
JHEP, vector meson bulk potential and metric corrected, comments and
references added, phenomenology and conclusions unchange
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Challenges facing holographic models of QCD
This paper, written in memory of Manoj Banerjee, takes a critical look at
holographic models of QCD focusing on ``practical'' models in which the five
dimensional theory is treated classically. A number of theoretical and
phenomenological challenges to the approach are discussed.Comment: This paper was written for an issue in memory of Manoj Banerjee in
the Indian Journal of Physic
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