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Caltech Authors (138.832 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 51.655

  1. Using Predictions in Online Optimization: Looking Forward with an Eye on the Past

    Chen, Niangjun; Comden, Joshua; Liu, Zhenhua; Gandhi, Anshul; Wierman, Adam
    We consider online convex optimization (OCO) problems with switching costs and noisy predictions. While the design of online algorithms for OCO problems has received considerable attention, the design of algorithms in the context of noisy predictions is largely open. To this point, two promising algorithms have been proposed: Receding Horizon Control (RHC) and Averaging Fixed Horizon Control (AFHC). The comparison of these policies is largely open. AFHC has been shown to provide better worst-case performance, while RHC outperforms AFHC in many realistic settings. In this paper, we introduce a new class of policies, Committed Horizon Control (CHC), that generalizes both...

  2. Joint Data Purchasing and Data Placement in a Geo-Distributed Data Market

    Ren, Xiaoqi; London, Palma; Ziani, Juba; Wierman, Adam
    This paper studies design challenges faced by a geo-distributed cloud data market: which data to purchase (data purchasing) and where to place/replicate the data (data placement). We show that the joint problem of data purchasing and data placement within a cloud data market is NP-hard in general. However, we give a provably optimal algorithm for the case of a data market made up of a single data center, and then generalize the structure from the single data center setting and propose Datum, a near-optimal, polynomial-time algorithm for a geo-distributed data market.

  3. Sorting out Compositional Trends in Sedimentary Rocks of the Bradbury Group (Aeolus Palus), Gale Crater, Mars

    Siebach, K. L.; Baker, M. B.; Grotzinger, J. P.; McLennan, S. M.; Gellert, R.; Thompson, L. M.; Hurowitz, J. A.
    Sedimentary rocks are composed of detrital grains derived from source rocks, which are altered by chemical weathering, sorted during transport, and cemented during diagenesis. Fluvio-lacustrine sedimentary rocks of the Bradbury group, observed on the floor of Gale crater by the Curiosity rover during its first 860 sols, show trends in bulk chemistry that are consistent with sorting of mineral grains during transport. The Bradbury group rocks are uniquely suited for sedimentary provenance analysis because they appear to have experienced negligible cation-loss (i.e., open-system chemical weathering) at the scale of the Alpha Particle X-ray Spectrometer bulk chemistry analyses based on low...

  4. Physical cosmic strings do not generate closed timelike curves

    Deser, S.; Jackiw, R.; ’t Hooft, G.
    We reexamine the causal properties of geometries generated by parallel, moving cosmic strings, particularly our statement that closed timelike curves are forbidden there. Contrary to a recent claim, such acausal behavior cannot be realized by physical, timelike, sources.

  5. Removing excess topology from isosurfaces

    Wood, Zoë; Hoppe, Hugues; Desbrun, Mathieu; Schröder, Peter
    Many high-resolution surfaces are created through isosurface extraction from volumetric representations, obtained by 3D photography, CT, or MRI. Noise inherent in the acquisition process can lead to geometrical and topological errors. Reducing geometrical errors during reconstruction is well studied. However, isosurfaces often contain many topological errors in the form of tiny handles. These nearly invisible artifacts hinder subsequent operations like mesh simplification, remeshing, and parametrization. In this article we present a practical method for removing handles in an isosurface. Our algorithm makes an axis-aligned sweep through the volume to locate handles, compute their sizes, and selectively remove them. The algorithm...

  6. Theoretical problems in nonsymmetric gravitational theory

    Damour, T.; Deser, S.; McCarthy, J.
    It has recenty been noted that the nonsymmetric metric model of gravity faces severe observational constraints. We show here that it is also subject to physically unacceptable formal difficulties even as an effective field theory: When expanded about a Riemannian background, the model exhibits curvature-coupled negative-energy (ghost) modes and unacceptable asymptotic behavior.

  7. Nonsymmetric gravity theories: Inconsistencies and a cure

    Damour, T.; Deser, S.; McCarthy, J.
    Motivated by the apparent dependence of string σ models on the sum of spacetime metric and antisymmetric tensor fields, we reconsider gravity theories constructed from a nonsymmetric metric. We first show, by expanding in powers of the antisymmetric field, that all such ‘‘geometrical’’ theories homogeneous in second derivatives violate standard physical requirements: ghost freedom, absence of algebraic inconsistencies, or continuity of degree-of-freedom content. This no-go result applies in particular to the old unified theory of Einstein and its recent avatars. However, we find that the addition of nonderivative, ‘‘cosmological’’ terms formally restores consistency by giving a mass to the antisymmetric...

  8. Tree-level unitarity constraints on the gravitational couplings of higher-spin massive fields

    Cuccieri, A.; Porrati, M.; Deser, S.
    We analyze the high-energy behavior of tree-level graviton Compton amplitudes for particles of mass m and arbitrary spin, concentrating on a combination of forward amplitudes that will be unaffected by eventual cross couplings to other, higher spins. We first show that for any spin > 2, tree-level unitarity is already violated at energies ∼ √mM_(P1), rather than at the Planck scale M_(P1), even for m≪ M_(P1). We then restore unitarity to this amplitude up to M_(P1) by adding nonminimal couplings that depend on the curvature and its derivatives, and modify the minimal description, including particle gravitational quadrupole moments, at ∼m^(-1)...

  9. Electric-magnetic black hole duality

    Deser, S.; Henneaux, M.; Teitelboim, C.
    We generalize duality invariance for the free Maxwell action in an arbitrary background geometry to include the presence of electric and magnetic charges. In particular, it follows that the actions of equally charged electric and magnetic black holes are equal.

  10. Gauge Invariance, Finite Temperature, and Parity Anomaly in D = 3

    Deser, S.; Griguolo, L.; Seminara, D.
    The effective gauge field actions generated by charged fermions in QED_3 and QCD_3 can be made invariant under both small and large gauge transformations at any temperature by suitable regularization of the Dirac operator determinant, at the price of parity anomalies. We resolve the paradox that the perturbative expansion is not invariant, as manifested by the temperature dependence of the induced Chern-Simons term, by showing that large (unlike small) transformations and hence their Ward identities are not perturbative order preserving. Our results are illustrated through concrete examples of field configurations, where the interplay between gauge and parity anomalies is also...

  11. Effective QED actions: Representations, gauge invariance, anomalies, and mass expansions

    Deser, S.; Griguolo, L.; Seminara, D.
    We analyze and give explicit representations for the effective Abelian vector gauge field actions generated by charged fermions with particular attention to the thermal regime in odd dimensions, where spectral asymmetry can be present. We show, through ζ-function regularization, that both small and large gauge invariances are preserved at any temperature and for any number of fermions at the usual price of anomalies: helicity (parity) invariance will be lost in even (odd) dimensions, and in the latter even at zero mass. Gauge invariance dictates a very general “Fourier” representation of the action in terms of the holonomies that carry the...

  12. Counterterms and M-Theory Corrections to D = 11 Supergravity

    Deser, S.; Seminara, D.
    We construct a local on-shell invariant in D = 11 supergravity from the nonlocal four-point tree scattering amplitude. Its existence, together with earlier arguments, implies nonrenormalizability of the theory at lowest possible, two loop, level. This invariant, whose leading bosonic terms are exhibited, may also express the leading, “zero-slope,” M-theory corrections to its D = 11 supergravity limit.

  13. Mapping Hawking into Unruh thermal properties

    Deser, S.; Levin, Orit
    By globally embedding curved spaces into higher dimensional flat ones, we show that Hawking thermal properties map into their Unruh equivalents: The relevant curved space detectors become Rindler ones, whose temperature and entropy reproduce the originals. Specific illustrations include Schwarzschild, Schwarzschild–(anti-)de Sitter, Reissner-Nordström, and Bañados-Teitelboim-Zanelli spaces.

  14. Extraction of gravitational-wave energy in higher dimensional numerical relativity using the Weyl tensor

    Cook, William G.; Sperhake, Ulrich
    Gravitational waves are one of the most important diagnostic tools in the analysis of strong-gravity dynamics and have been turned into an observational channel with LIGO's detection of GW150914. Aside from their importance in astrophysics, black holes and compact matter distributions have also assumed a central role in many other branches of physics. These applications often involve spacetimes with D  >  4 dimensions where the calculation of gravitational waves is more involved than in the four dimensional case, but has now become possible thanks to substantial progress in the theoretical study of general relativity in D  >  4. Here, we develop a numerical implementation...

  15. Search for neutral resonances decaying into a Z boson and a pair of b jets or τ leptons

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A search is performed for a new resonance decaying into a lighter resonance and a Z boson. Two channels are studied, targeting the decay of the lighter resonance into either a pair of oppositely charged τ leptons or a View the MathML sourcebb‾ pair. The Z boson is identified via its decays to electrons or muons. The search exploits data collected by the CMS experiment at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.8 fb^(−1). No significant deviations are observed from the standard model expectation and limits are set on production cross sections and parameters...

  16. Polypyrrole-assisted oxygen electrocatalysis on perovskite oxides

    Lee, Dong-Gyu; Kim, Su Hwan; Joo, Se Hun; Ji, Ho-Il; Tavassol, Hadi; Jeon, Yuju; Choi, Sihyuk; Lee, Myeong-Hee; Kim, Chanseok; Kwak, Sang Kyu; Kim, Guntae; Song, Hyun-Kon
    Nitrogen-containing electrocatalysts, such as metal–nitrogen–carbon (M–N–C) composites and nitrogen-doped carbons, are known to exhibit high activities for an oxygen reduction reaction (ORR). Moreover, even if the mechanism by which nitrogen improves the activities is not completely understood, a strong electronic interaction between nitrogen and active sites has been found in these composites. Herein, we demonstrate a case in which nitrogen improves the electroactivity, but in the absence of a strong interaction with other components. The overpotentials of the ORR and oxygen evolution reaction (OER) on perovskite oxide catalysts were significantly reduced simply by mixing the catalyst particles with polypyrrole/carbon composites...

  17. The Depletion of Water During Dispersal of Planet-forming Disk Regions

    Banzatti, A.; Pontoppidan, K. M.; Salyk, C.; Herczeg, G. J.; van Dishoeck, E. F.; Blake, G. A.
    We present a new velocity-resolved survey of 2.9 μm spectra of hot H_2O and OH gas emission from protoplanetary disks, obtained with the Cryogenic Infrared Echelle Spectrometer at the VLT (R ~ 96,000). With the addition of archival Spitzer-IRS spectra, this is the most comprehensive spectral data set of water vapor emission from disks ever assembled. We provide line fluxes at 2.9–33 μm that probe from the dust sublimation radius at ~0.05 au out to the region of the water snow line. With a combined data set for 55 disks, we find a new correlation between H_2O line fluxes and...

  18. Study of Z boson production in pPb collisions at √SNN = 5.02 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.
    The production of Z bosons in pPb collisions at √SNN = 5.02 TeV is studied by the CMS experiment via the electron and muon decay channels. The inclusive cross section is compared to pp collision predictions, and found to scale with the number of elementary nucleon–nucleon collisions. The differential cross sections as a function of the Z boson rapidity and transverse momentum are measured. Though they are found to be consistent within uncertainty with theoretical predictions both with and without nuclear effects, the forward–backward asymmetry suggests the presence of nuclear effects at large rapidities. These results provide new data for...

  19. Fabrication of Active Surfaces with Metastable Microgel Layers Formed during Breath Figure Templating

    Zhou, Yuchen; Huang, Junjie; Sun, Wei; Ju, Yuanlai; Yang, Pinghui; Ding, Lingyun; Chen, Zhongren; Kornfield, Julia A.
    Patterned porous surfaces with responsive functionalities are fabricated by a thermo-responsive microgel-assisted breath figure (BF) process. When water droplets submerge into a polystyrene (PS) solution during formation of a porous surface by the bottom-up BF process, poly(N-isopropylacrylamide)-co-acrylic acid (PNIPAm-co-AA) microgels dispersed in the solution spontaneously assemble at the water-organic interfaces like “Pickering emulsions”, reinforced by capillary flow. The conformal layer of PNIPAm-co-AA microgels lining the pores appears in images of scanning electron microscope (SEM) either as a smooth surface layer (L) or as an array of dome-like protrusions (D), depending on the conditions at which the sample was dried for...

  20. What sets the size of current ripples?

    Lapotre, Mathieu G. A.; Lamb, Michael P.; McElroy, Brandon
    Water flowing over sand in fluvial and marine settings often results in the formation of current ripples. Found in modern and ancient deposits on Earth and Mars, ripple stratification records flow directions and fluid properties that are crucial to interpreting sedimentary records. Despite decades of observations of current ripples, there is no universal scaling relation to predict their size or to distinguish them from dunes. Here we use dimensional analysis and a new data compilation to develop a scaling relation that collapses data for equilibrium wavelengths of ripples forming under unidirectional flows. Results show that ripples are larger with more...

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