249,776 research outputs found
Revisiting the Fradkin-Vilkovisky Theorem
The status of the usual statement of the Fradkin-Vilkovisky theorem, claiming
complete independence of the Batalin-Fradkin-Vilkovisky path integral on the
gauge fixing "fermion" even within a nonperturbative context, is critically
reassessed. Basic, but subtle reasons why this statement cannot apply as such
in a nonperturbative quantisation of gauge invariant theories are clearly
identified. A criterion for admissibility within a general class of gauge
fixing conditions is provided for a large ensemble of simple gauge invariant
systems. This criterion confirms the conclusions of previous counter-examples
to the usual statement of the Fradkin-Vilkovisky theorem.Comment: 21 pages, no figures, to appear in Jnl. Phys.
Evaluating Scoped Meaning Representations
Semantic parsing offers many opportunities to improve natural language
understanding. We present a semantically annotated parallel corpus for English,
German, Italian, and Dutch where sentences are aligned with scoped meaning
representations in order to capture the semantics of negation, modals,
quantification, and presupposition triggers. The semantic formalism is based on
Discourse Representation Theory, but concepts are represented by WordNet
synsets and thematic roles by VerbNet relations. Translating scoped meaning
representations to sets of clauses enables us to compare them for the purpose
of semantic parser evaluation and checking translations. This is done by
computing precision and recall on matching clauses, in a similar way as is done
for Abstract Meaning Representations. We show that our matching tool for
evaluating scoped meaning representations is both accurate and efficient.
Applying this matching tool to three baseline semantic parsers yields F-scores
between 43% and 54%. A pilot study is performed to automatically find changes
in meaning by comparing meaning representations of translations. This
comparison turns out to be an additional way of (i) finding annotation mistakes
and (ii) finding instances where our semantic analysis needs to be improved.Comment: Camera-ready for LREC 201
Isolating vacuum amplitudes in quantum field calculations at finite temperature
In calculating Feynman diagrams at finite temperature, it is sometimes
convenient to isolate subdiagrams which do not depend explicitly on the
temperature. We show that, in the imaginary time formalism, such a separation
can be achieved easily by exploiting a simple method, due to M. Gaudin, to
perform the sum over the Matsubara frequencies. In order to manipulate freely
contributions which may be individually singular, a regularization has to be
introduced. We show that, in some cases, it is possible to choose this
regularization in such a way that the isolated subdiagrams can be identified
with analytical continuations of vacuum n-point functions. As an aside
illustration of Gaudin's method, we use it to prove the main part of a recent
conjecture concerning the relation which exists in the imaginary time formalism
between the expressions of a Feynman diagram at zero and finite temperature.Comment: 37 pages, 12 figure
Towards gauge theories in four dimensions
The abundance of infrared singularities in gauge theories due to unresolved
emission of massless particles (soft and collinear) represents the main
difficulty in perturbative calculations. They are typically regularized in
dimensional regularization, and their subtraction is usually achieved
independently for virtual and real corrections. In this paper, we introduce a
new method based on the loop-tree duality (LTD) theorem to accomplish the
summation over degenerate infrared states directly at the integrand level such
that the cancellation of the infrared divergences is achieved simultaneously,
and apply it to reference examples as a proof of concept. Ultraviolet
divergences, which are the consequence of the point-like nature of the theory,
are also reinterpreted physically in this framework. The proposed method opens
the intriguing possibility of carrying out purely four-dimensional
implementations of higher-order perturbative calculations at next-to-leading
order (NLO) and beyond free of soft and final-state collinear subtractions.Comment: Final version to appear in JHE
Grover's algorithm on a Feynman computer
We present an implementation of Grover's algorithm in the framework of
Feynman's cursor model of a quantum computer. The cursor degrees of freedom act
as a quantum clocking mechanism, and allow Grover's algorithm to be performed
using a single, time-independent Hamiltonian. We examine issues of locality and
resource usage in implementing such a Hamiltonian. In the familiar language of
Heisenberg spin-spin coupling, the clocking mechanism appears as an excitation
of a basically linear chain of spins, with occasional controlled jumps that
allow for motion on a planar graph: in this sense our model implements the idea
of "timing" a quantum algorithm using a continuous-time random walk. In this
context we examine some consequences of the entanglement between the states of
the input/output register and the states of the quantum clock
The Hasse principle for lines on diagonal surfaces
Given a number field and a positive integer , in this paper we
consider the following question: does there exist a smooth diagonal surface of
degree in over which contains a line over every
completion of , yet no line over ? We answer the problem using Galois
cohomology, and count the number of counter-examples using a result of
Erd\H{o}s.Comment: 14 page
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