942 research outputs found
A simulated study of implicit feedback models
In this paper we report on a study of implicit feedback models for unobtrusively tracking the information needs of searchers. Such models use relevance information gathered from searcher interaction and can be a potential substitute for explicit relevance feedback. We introduce a variety of implicit feedback models designed to enhance an Information Retrieval (IR) system's representation of searchers' information needs. To benchmark their performance we use a simulation-centric evaluation methodology that measures how well each model learns relevance and improves search effectiveness. The results show that a heuristic-based binary voting model and one based on Jeffrey's rule of conditioning [5] outperform the other models under investigation
A Knob for Changing Light Propagation from Subluminal to Superluminal
We show how the application of a coupling field connecting the two lower
metastable states of a lambda-system can produce a variety of new results on
the propagation of a weak electromagnetic pulse. In principle the light
propagation can be changed from subluminal to superluminal. The negative group
index results from the regions of anomalous dispersion and gain in
susceptibility.Comment: 6 pages,5 figures, typed in RevTeX, accepted in Phys. Rev.
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Global observations of 2Â day wave coupling to the diurnal tide in a highâaltitude forecastâassimilation system
We examine wave components in a high-altitude forecast-assimilation system that arise from nonlinear interaction between the diurnal tide and the westward traveling quasi 2 day wave. The process yields a westward traveling âsumâ wave with zonal wave number 4 and a period of 16 h, and an eastward traveling âdifferenceâ wave with zonal wave number 2 and a period of 2 days. While the eastward 2 day wave has been reported in satellite temperatures, the westward 16 h wave lies outside the Nyquist limits of resolution of twice daily local time satellite sampling. Hourly output from a high-altitude forecast-assimilation model is used to diagnose the nonlinear quadriad. A steady state primitive equation model forced by tide-2 day wave advection is used to intepret the nonlinear wave products. The westward 16 h wave maximizes in the midlatitude winter mesosphere and behaves like an inertia-gravity wave. The nonlinearly generated component of the eastward 2 day wave maximizes at high latitudes in the lower thermosphere, and only weakly penetrates to low latitudes. The 16 h and the eastward 2 day waves are of comparable amplitude and alias to the same apparent frequency when viewed from a satellite perspective
NaIrO3 - A pentavalent post-perovskite
Sodium iridium(V) oxide, NaIrO3, was synthesized by a high pressure solid
state method and recovered to ambient conditions. It is found to be
isostructural with CaIrO3, the much-studied structural analogue of the
high-pressure post-perovskite phase of MgSiO3. Among the oxide
post-perovskites, NaIrO3 is the first example with a pentavalent cation. The
structure consists of layers of corner- and edge-sharing IrO6 octahedra
separated by layers of NaO8 bicapped trigonal prisms. NaIrO3 shows no magnetic
ordering and resistivity measurements show non-metallic behavior. The crystal
structure, electrical and magnetic properties are discussed and compared to
known post-perovskites and pentavalent perovskite metal oxides.Comment: 22 pages, 5 figures. Submitted to Journal of Solid State Chemistr
Measurement of the Proton and Deuteron Spin Structure Functions g2 and Asymmetry A2
We have measured the spin structure functions g2p and g2d and the virtual
photon asymmetries A2p and A2d over the kinematic range 0.02 < x < 0.8 and 1.0
< Q^2 < 30(GeV/c)^2 by scattering 38.8 GeV longitudinally polarized electrons
from transversely polarized NH3 and 6LiD targets.The absolute value of A2 is
significantly smaller than the sqrt{R} positivity limit over the measured
range, while g2 is consistent with the twist-2 Wandzura-Wilczek calculation. We
obtain results for the twist-3 reduced matrix elements d2p, d2d and d2n. The
Burkhardt-Cottingham sum rule integral - int(g2(x)dx) is reported for the range
0.02 < x < 0.8.Comment: 12 pages, 4 figures, 1 tabl
Complementarity, quantum erasure and delayed choice with modified Mach-Zehnder interferometers
Often cited dictums in Quantum Mechanics include "observation disturbance
causes loss of interference" and "ignorance is interference". In this paper we
propose and describe a series of experiments with modified Mach-Zehnder
interferometers showing that one has to be careful when applying such dictums.
We are able to show that without interacting in any way with the light quantum
(or quanta) expected to behave "wave-like", interference fringes can be lost by
simply gaining (or having the potential to gain) the which-path knowledge.
Erasing this information may revive the interference fringes. Delayed choice
can be added, arriving to an experiment in line with Wheeler's original
proposal. We also show that ignorance is not always synonym with having the
interference fringes. The often-invoked "collapse of the wavefunction" is found
to be a non-necessary ingredient to describe our experiments.Comment: 8 pages, 3 figures; to appear in EPJ
Isospin-Violating Meson-Nucleon Vertices as an Alternate Mechanism of Charge-Symmetry Breaking
We compute isospin-violating meson-nucleon coupling constants and their
consequent charge-symmetry-breaking nucleon-nucleon potentials. The couplings
result from evaluating matrix elements of quark currents between nucleon states
in a nonrelativistic constituent quark model; the isospin violations arise from
the difference in the up and down constituent quark masses. We find, in
particular, that isospin violation in the omega-meson--nucleon vertex dominates
the class IV CSB potential obtained from these considerations. We evaluate the
resulting spin-singlet--triplet mixing angles, the quantities germane to the
difference of neutron and proton analyzing powers measured in elastic
scattering, and find them commensurate to those computed
originally using the on-shell value of the - mixing amplitude.
The use of the on-shell - mixing amplitude at has been
called into question; rather, the amplitude is zero in a wide class of models.
Our model possesses no contribution from - mixing at , and
we find that omega-meson exchange suffices to explain the measured
analyzing power difference~at~183 MeV.Comment: 20 pages, revtex, 3 uuencoded PostScript figure
Precision Measurement of the Proton and Deuteron Spin Structure Functions g2 and Asymmetries A2
We have measured the spin structure functions g2p and g2d and the virtual
photon asymmetries A2p and A2d over the kinematic range 0.02 < x < 0.8 and 0.7
< Q^2 < 20 GeV^2 by scattering 29.1 and 32.3 GeV longitudinally polarized
electrons from transversely polarized NH3 and 6LiD targets. Our measured g2
approximately follows the twist-2 Wandzura-Wilczek calculation. The twist-3
reduced matrix elements d2p and d2n are less than two standard deviations from
zero. The data are inconsistent with the Burkhardt-Cottingham sum rule if there
is no pathological behavior as x->0. The Efremov-Leader-Teryaev integral is
consistent with zero within our measured kinematic range. The absolute value of
A2 is significantly smaller than the sqrt[R(1+A1)/2] limit.Comment: 12 pages, 4 figures, 2 table
Measurements of the -Dependence of the Proton and Neutron Spin Structure Functions g1p and g1n
The structure functions g1p and g1n have been measured over the range 0.014 <
x < 0.9 and 1 < Q2 < 40 GeV2 using deep-inelastic scattering of 48 GeV
longitudinally polarized electrons from polarized protons and deuterons. We
find that the Q2 dependence of g1p (g1n) at fixed x is very similar to that of
the spin-averaged structure function F1p (F1n). From a NLO QCD fit to all
available data we find at
Q2=5 GeV2, in agreement with the Bjorken sum rule prediction of 0.182 \pm
0.005.Comment: 17 pages, 3 figures. Submitted to Physics Letters
Scaling up local energy infrastructure; An agent-based model of the emergence of district heating networks
The potential contribution of local energy infrastructure â such as heat networks â to the transition to a low carbon economy is increasingly recognised in international, national and municipal policy. Creating the policy environment to foster the scaling up of local energy infrastructure is, however, still challenging; despite national policy action and local authority interest the growth of heat networks in UK cities remains slow. Technoeconomic energy system models commonly used to inform policy are not designed to address institutional and governance barriers. We present an agent-based model of heat network development in UK cities in which policy interventions aimed at the institutional and governance barriers faced by diverse actors can be explored. Three types of project instigators are included â municipal, commercial and community â which have distinct decision heuristics and capabilities and follow a multi-stage development process. Scenarios of policy interventions developed in a companion modelling approach indicate that the effect of interventions differs between actors depending on their capabilities. Successful interventions account for the specific motivations and capabilities of different actors, provide a portfolio of support along the development process and recognise the important strategic role of local authorities in supporting low carbon energy infrastructure
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