95,280 research outputs found
Analyzing X-ray variability by State Space Models
In recent years, autoregressive models have had a profound impact on the
description of astronomical time series as the observation of a stochastic
process. These methods have advantages compared with common Fourier techniques
concerning their inherent stationarity and physical background. If
autoregressive models are used, however, it has to be taken into account that
real data always contain observational noise often obscuring the intrinsic time
series of the object. We apply the technique of a Linear State Space Model
which explicitly models the noise of astronomical data and allows to estimate
the hidden autoregressive process. As an example, we have analysed a sample of
Active Galactic Nuclei (AGN) observed with EXOSAT and found evidence for a
relationship between the relaxation timescale and the spectral hardness.Comment: 4 pages, Latex, uses Kluwer Style file crckapb.cls To appear in Proc.
of Astronomical Time Series, Tel Aviv, 199
Preliminary Results from Recent Measurements of the Antiprotonic Helium Hyperfine Structure
We report on preliminary results from a systematic study of the hyperfine
(HF) structure of antiprotonic helium. This precise measurement which was
commenced in 2006, has now been completed. Our initial analysis shows no
apparent density or power dependence and therefore the results can be averaged.
The statistical error of the observable M1 transitions is a factor of 60
smaller than that of three body quantum electrodynamic (QED) calculations,
while their difference has been resolved to a precision comparable to theory (a
factor of 10 better than our first measurement). This difference is sensitive
to the antiproton magnetic moment and agreement between theory and experiment
would lead to an increased precision of this parameter, thus providing a test
of CPT invariance.Comment: 6 pages, 4 figure
QCD axion and quintessential axion
The axion solution of the strong CP problem is reviewed together with the
other strong CP solutions. We also point out the quintessential
axion(quintaxion) whose potential can be extremely flat due to the tiny ratio
of the hidden sector quark mass and the intermediate hidden sector scale. The
quintaxion candidates are supposed to be the string theory axions, the model
independent or the model dependent axions.Comment: 15 pages. Talk presented at Castle Ringberg, June 9-14, 200
Parikh Image of Pushdown Automata
We compare pushdown automata (PDAs for short) against other representations.
First, we show that there is a family of PDAs over a unary alphabet with
states and stack symbols that accepts one single long word for
which every equivalent context-free grammar needs
variables. This family shows that the classical algorithm for converting a PDA
to an equivalent context-free grammar is optimal even when the alphabet is
unary. Moreover, we observe that language equivalence and Parikh equivalence,
which ignores the ordering between symbols, coincide for this family. We
conclude that, when assuming this weaker equivalence, the conversion algorithm
is also optimal. Second, Parikh's theorem motivates the comparison of PDAs
against finite state automata. In particular, the same family of unary PDAs
gives a lower bound on the number of states of every Parikh-equivalent finite
state automaton. Finally, we look into the case of unary deterministic PDAs. We
show a new construction converting a unary deterministic PDA into an equivalent
context-free grammar that achieves best known bounds.Comment: 17 pages, 2 figure
The anomalous U(1) global symmetry and flavors from an SU(5) x SU(5) GUT in orbifold compactification
In string compactifications, frequently there appears the anomalous U(1)
gauge symmetry which belonged to E8E8 of the heterotic string. This
anomalous U(1) gauge boson obtains mass at the compactification scale, just
below GeV, by absorbing one pseudoscalar (corresponding to the
model-independent axion) from the second rank anti-symmetric tensor field
.
Below the compactification scale, there results a global symmetry U(1) whose charge is the original gauge U(1) charge. This is
the most natural global symmetry, realizing the "invisible" axion. This global
symmetry U(1) is suitable for a flavor symmetry. In the simplest
compactification model with the flipped SU(5) grand unification, we calculate
all the low energy parameters in terms of the vacuum expectation values of the
standard model singlets.Comment: 18 pages, 4 figur
LR characterization of chirotopes of finite planar families of pairwise disjoint convex bodies
We extend the classical LR characterization of chirotopes of finite planar
families of points to chirotopes of finite planar families of pairwise disjoint
convex bodies: a map \c{hi} on the set of 3-subsets of a finite set I is a
chirotope of finite planar families of pairwise disjoint convex bodies if and
only if for every 3-, 4-, and 5-subset J of I the restriction of \c{hi} to the
set of 3-subsets of J is a chirotope of finite planar families of pairwise
disjoint convex bodies. Our main tool is the polarity map, i.e., the map that
assigns to a convex body the set of lines missing its interior, from which we
derive the key notion of arrangements of double pseudolines, introduced for the
first time in this paper.Comment: 100 pages, 73 figures; accepted manuscript versio
Normal Form and Nekhoroshev stability for nearly-integrable Hamiltonian systems with unconditionally slow aperiodic time dependence
The aim of this paper is to extend the results of Giorgilli and Zehnder for
aperiodic time dependent systems to a case of general nearly-integrable convex
analytic Hamiltonians. The existence of a normal form and then a stability
result are shown in the case of a slow aperiodic time dependence that, under
some smallness conditions, is independent on the size of the perturbation.Comment: Corrected typo in the title and statement of Lemma 3.
Quasienergy spectrum and tunneling current in ac-driven triple quantum dot shuttles
The dynamics of electrons in ac driven double quantum dots have been
extensively analyzed by means of Floquet theory. In these systems, coherent
destruction of tunneling has been shown to occur for certain ac field
parameters. In the present work we analyze, by means of Floquet theory, the
electron dynamics of a triple quantum dot in series attached to electric
contacts, where the central dot position oscillates. In particular, we analyze
the quasienergy spectrum of this ac driven nanoelectromechanical system, as a
function of the intensity and frequency of the ac field and of external dc
voltages. For strong driving fields, we derive, by means of perturbation
theory, analytical expressions for the quasienergies of the driven oscillator
system. From this analysis we discuss the conditions for coherent destruction
of tunneling (CDT) to occur as a function of detuning and field parameters. For
zero detuning, and from the invariance of the Floquet Hamiltonian under a
generalized parity transformation, we find analytical expressions describing
the symmetry properties of the Fourier components of the Floquet states under
such transformation. By using these expressions, we show that in the vicinity
of the CDT condition, the quasienergy spectrum exhibits exact crossings which
can be characterized by the parity properties of the corresponding
eigenvectors
Asymmetry of localised states in a single quantum ring: polarization dependence of excitons and biexcitons
We performed spectroscopic studies of a single GaAs quantum ring with an
anisotropy in the rim height. The presence of an asymmetric localised state was
suggested by the adiabatic potential. The asymmetry was investigated in terms
of the polarization dependence of excitons and biexcitons, where a large energy
di erence (0.8 meV) in the exciton emission energy for perpendicular
polarizations was observed and the oscillator strengths were also compared
using the photoluminescence decay rate. For perpendicular polarizations the
biexciton exhibits twice the energy di erence seen for the exciton, a fact that
may be attributed to a possible change in the selection rules for the lowered
symmetry.Comment: accepted in Applied physics Letter
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Theories of behaviour change synthesised into a set of theoretical groupings: Introducing a thematic series on the Theoretical Domains Framework
Behaviour change is key to increasing the uptake of evidence into healthcare practice. Designing behaviour-change interventions first requires problem analysis, ideally informed by theory. Yet the large number of partly overlapping theories of behaviour makes it difficult to select the most appropriate theory. The need for an overarching theoretical framework of behaviour change was addressed in research in which 128 explanatory constructs from 33 theories of behaviour were identified and grouped. The resulting Theoretical Domains Framework (TDF) appears to be a helpful basis for investigating implementation problems. Research groups in several countries have conducted TDF-based studies. It seems timely to bring together the experience of these teams in a thematic series to demonstrate further applications and to report key developments. This overview article describes the TDF, provides a brief critique of the framework, and introduces this thematic series.
In a brief review to assess the extent of TDF-based research, we identified 133 papers that cite the framework. Of these, 17 used the TDF as the basis for empirical studies to explore health professionalsâ behaviour. The identified papers provide evidence of the impact of the TDF on implementation research. Two major strengths of the framework are its theoretical coverage and its capacity to elicit beliefs that could signify key mediators of behaviour change. The TDF provides a useful conceptual basis for assessing implementation problems, designing interventions to enhance healthcare practice, and understanding behaviour-change processes. We discuss limitations and research challenges and introduce papers in this series
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