14,399 research outputs found
Sturcture of the Goldstone Bosons
The feasibility of measuring the pion and kaon structure functions has been
investigated. A high luminosity electron-proton collider would make these
measurements feasible. Also, it appears feasible to measure these structure
functions in a nuclear medium. Simulations using the RAPGAP Monte Carlo of a
possible pion structure function measurement are presented.Comment: To appear in the proceedings of the Second Workshop on Physics with
an Electron Polarized Light-Ion Collider, 14-16 Sept 2000, Cambridge, Ma 6
pages, 5 figures, late
The suppression of electron correlations in the collapsed tetragonal phase of CaFe2As2 under ambient pressure demonstrated by 75As NMR-NQR measurements
The static and the dynamic spin correlations in the low temperature collapsed
tetragonal and the high temperature tetragonal phase in CaFe2As2 have been
investigated by 75As nuclear magnetic resonance (NMR) and nuclear quadrupole
resonance (NQR) measurements.
Through the temperature (T) dependence of the nuclear spin lattice relaxation
rates (1/T1) and the Knight shifts, although stripe-type antiferromagnetic
(AFM) spin correlations are realized in the high temperature tetragonal phase,
no trace of the AFM spin correlations can be found in the non-superconducting,
low temperature, collapsed tetragonal (cT) phase.
Given that there is no magnetic broadening in 75As NMR spectra, together with
the T-independent behavior of magnetic susceptibility (x) and the T dependence
of 1/T1Tx, we conclude that Fe spin correlations are completely quenched
statically and dynamically in the non-superconducting cT phase in CaFe2As2.Comment: 5 pages, 4 figures, submitted to PR
Model Counting of Query Expressions: Limitations of Propositional Methods
Query evaluation in tuple-independent probabilistic databases is the problem
of computing the probability of an answer to a query given independent
probabilities of the individual tuples in a database instance. There are two
main approaches to this problem: (1) in `grounded inference' one first obtains
the lineage for the query and database instance as a Boolean formula, then
performs weighted model counting on the lineage (i.e., computes the probability
of the lineage given probabilities of its independent Boolean variables); (2)
in methods known as `lifted inference' or `extensional query evaluation', one
exploits the high-level structure of the query as a first-order formula.
Although it is widely believed that lifted inference is strictly more powerful
than grounded inference on the lineage alone, no formal separation has
previously been shown for query evaluation. In this paper we show such a formal
separation for the first time.
We exhibit a class of queries for which model counting can be done in
polynomial time using extensional query evaluation, whereas the algorithms used
in state-of-the-art exact model counters on their lineages provably require
exponential time. Our lower bounds on the running times of these exact model
counters follow from new exponential size lower bounds on the kinds of d-DNNF
representations of the lineages that these model counters (either explicitly or
implicitly) produce. Though some of these queries have been studied before, no
non-trivial lower bounds on the sizes of these representations for these
queries were previously known.Comment: To appear in International Conference on Database Theory (ICDT) 201
Data-Intensive Computing in the 21st Century
The deluge of data that future applications must process—in domains ranging from science to business informatics—creates a compelling argument for substantially increased R&D targeted at discovering scalable hardware and software solutions for data-intensive problems
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