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Open Access Publications from the University of California

Scholarly Works (256 results)

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UC Davis Previously Published Works (2005)

In temperate regions, the adoption of no-tillage (NT) often stimulates the sequestration of soil C and N and improves soil structural stability. The aim of this study was to investigate if NT has similar impacts on the stability of water-stable aggregates and C and N stabilization in two Oxisols (Typic Haplorthox) under different crop rotations. Slaking-resistant aggregates were isolated by wet sieving and analyzed for C and N concentrations at two agricultural experiment sites (Passo Fundo and Londrina) in southern Brazil. At both sites, the total organic C and N in the 0- to 5-cm depth interval, decreased in order native vegetation (NV) > NT > conventional tillage (CT). The mean weight diameter (MWD) of the aggregates was on average 0.5 mm greater under NT compared with CT, and was also greater (approximately 0.2 mm) under more diverse rotations, which included a leguminous green-manure crop, in comparison with continuous wheat (Triticam aestivum)-soybean (Glycine Max L.). The aggregate-size distribution was dominated (60-90%) by macroaggregates (> 250 mu m). At both sites, CT decreased the proportion of the largest macroaggregate class (> 2000 mu m) by approximately 10% in comparison with NT management, and there was a corresponding increase in the proportion of the 53- to 250-mu m aggregate class. In the 0- to 5-cm soil layer of both sites, the C and N concentrations were significantly higher in the macroaggregates of the NT than of the CT systems. The lack of differences in C, N content, and ON ratio across aggregate-size classes indicated that these soils dominated by 1:1 clays and Fe/Al oxides do not express an aggregate hierarchy and that an increase in aggregation does not explain the increase in C and N under NT. In conclusion, CT decreased aggregation similarly in Oxisols as in temperate soils, but this decrease does not explain the loss of C and N because the tight feedback between soil organic matter (SOM) and aggregate stability (i.e., SOM being a major binding agent) observed for temperate soils was not found for Oxisols.

    Cover page: Impact of tillage and crop rotation on aggregate-associated carbon in two oxisols
    • Article
    • Peer Reviewed
    UC San Francisco Previously Published Works (2014)
    Amantadine inhibits the M2 proton channel of influenza A virus, yet most of the currently circulating strains of the virus carry mutations in the M2 protein that render the virus amantadine-resistant. While most of the research on novel amantadine analogue
      • Article
      • Peer Reviewed
      International Organization of Citrus Virologists Conference Proceedings (1957-2010), Volume 16, Issue 16 (2005)
        Cover page: Genetic Transformation of Sour Orange and Alemow with Non-translatable CP, p27-CP and 3’-end Sequences from Citrus tristeza virus Found in Veracruz, Mexico
        • Article
        • Peer Reviewed
        International Organization of Citrus Virologists Conference Proceedings (1957-2010), Volume 16, Issue 16 (2005)
          Cover page: Molecular Characterization of Citrus tristeza virus Isolates from Veracruz and Tamaulipas States, Mexico
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2014)
          The High Altitude Water Cherenkov (HAWC) observatory is a wide field-of-view detector sensitive to gamma rays of 100 GeV to a few hundred TeV. Located in central Mexico at 19° North latitude and 4100 m above sea level, HAWC will observe gamma rays and cosmic rays with an array of water Cherenkov detectors. The full HAWC array is scheduled to be operational in Spring 2015. In this paper, we study the HAWC sensitivity to the gamma-ray signatures of high-mass (multi-TeV) dark matter annihilation. The HAWC observatory will be sensitive to diverse searches for dark matter annihilation, including annihilation from extended dark matter sources, the diffuse gamma-ray emission from dark matter annihilation, and gamma-ray emission from nonluminous dark matter subhalos. Here we consider the HAWC sensitivity to a subset of these sources, including dwarf galaxies, the M31 galaxy, the Virgo cluster, and the Galactic center. We simulate the HAWC response to gamma rays from these sources in several well-motivated dark matter annihilation channels. If no gamma-ray excess is observed, we show the limits HAWC can place on the dark matter cross section from these sources. In particular, in the case of dark matter annihilation into gauge bosons, HAWC will be able to detect a narrow range of dark matter masses to cross sections below thermal. HAWC should also be sensitive to nonthermal cross sections for masses up to nearly 1000 TeV. The constraints placed by HAWC on the dark matter cross section from known sources should be competitive with current limits in the mass range where HAWC has similar sensitivity. HAWC can additionally explore higher dark matter masses than are currently constrained.
            Cover page: Sensitivity of HAWC to high-mass dark matter annihilations
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            • Peer Reviewed
            UC Irvine Previously Published Works (2015)
            The first limits on the prompt emission from the long gamma-ray burst (GRB) 130427A in the >100 GeV energy band are reported. GRB 130427A was the most powerful burst ever detected with a redshift z ≲ 0.5 and featured the longest lasting emission above 100 MeV. The energy spectrum extends at least up to 95 GeV, clearly in the range observable by the High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, a new extensive air shower detector currently under construction in central Mexico. The burst occurred under unfavorable observation conditions, low in the sky and when HAWC was running 10% of the final detector. Based on the observed light curve at MeV-GeV energies, eight different time periods have been searched for prompt and delayed emission from this GRB. In all cases, no statistically significant excess of counts has been found and upper limits have been placed. It is shown that a similar GRB close to zenith would be easily detected by the full HAWC detector, which will be completed soon. The detection rate of the full HAWC detector may be as high as one to two GRBs per year. A detection could provide important information regarding the high energy processes at work and the observation of a possible cut-off beyond the Fermi Large Area Telescope energy range could be the signature of gamma-ray absorption, either in the GRB or along the line of sight due to the extragalactic background light.
              Cover page: SEARCH FOR GAMMA-RAYS FROM THE UNUSUALLY BRIGHT GRB 130427A WITH THE HAWC GAMMA-RAY OBSERVATORY
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              • Peer Reviewed
              UC Irvine Previously Published Works (2014)
              The High-Altitude Water Cherenkov (HAWC) Observatory is sensitive to gamma rays and charged cosmic rays at TeV energies. The detector is still under construction, but data acquisition with the partially deployed detector started in 2013. An analysis of the cosmic-ray arrival direction distribution based on 4.9 × 1010 events recorded between 2013 June and 2014 February shows anisotropy at the 10-4 level on angular scales of about 10°. The HAWC cosmic-ray sky map exhibits three regions of significantly enhanced cosmic-ray flux; two of these regions were first reported by the Milagro experiment. A third region coincides with an excess recently reported by the ARGO-YBJ experiment. An angular power spectrum analysis of the sky shows that all terms up to l = 15 contribute significantly to the excesses.
                Cover page: OBSERVATION OF SMALL-SCALE ANISOTROPY IN THE ARRIVAL DIRECTION DISTRIBUTION OF TeV COSMIC RAYS WITH HAWC
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                • Peer Reviewed
                UC Irvine Previously Published Works (2015)
                Primordial Black Holes (PBHs) are gravitationally collapsed objects that may have been created by density fluctuations in the early universe and could have arbitrarily small masses down to the Planck scale. Hawking showed that due to quantum effects, a black hole has a temperature inversely proportional to its mass and will emit all species of fundamental particles thermally. PBHs with initial masses of ∼5.0 × 1014 g should be expiring in the present epoch with bursts of high-energy particles, including gamma radiation in the GeV-TeV energy range. The Milagro high energy observatory, which operated from 2000 to 2008, is sensitive to the high end of the PBH evaporation gamma-ray spectrum. Due to its large field-of-view, more than 90% duty cycle and sensitivity up to 100 TeV gamma rays, the Milagro observatory is well suited to perform a search for PBH bursts. Based on a search on the Milagro data, we report new PBH burst rate density upper limits over a range of PBH observation times. In addition, we report the sensitivity of the Milagro successor, the High Altitude Water Cherenkov (HAWC) observatory, to PBH evaporation events.
                  Cover page: Milagro limits and HAWC sensitivity for the rate-density of evaporating Primordial Black Holes
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2015)
                  VAMOS1 was a prototype detector built in 2011 at an altitude of 4100 m a.s.l. in the state of Puebla, Mexico. The aim of VAMOS was to finalize the design, construction techniques and data acquisition system of the HAWC observatory. HAWC is an air-shower array currently under construction at the same site of VAMOS with the purpose to study the TeV sky. The VAMOS setup included six water Cherenkov detectors and two different data acquisition systems. It was in operation between October 2011 and May 2012 with an average live time of 30%. Besides the scientific verification purposes, the eight months of data were used to obtain the results presented in this paper: the detector response to the Forbush decrease of March 2012, and the analysis of possible emission, at energies above 30 GeV, for long gamma-ray bursts GRB111016B and GRB120328B.
                    Cover page: VAMOS: A pathfinder for the HAWC gamma-ray observatory
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                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2022)
                    In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD)1. These partons subsequently emit further partons in a process that can be described as a parton shower2, which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass mQ and energy E, within a cone of angular size mQ/E around the emitter3. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques4,5 to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.
                      Cover page: Direct observation of the dead-cone effect in quantum chromodynamics
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