2,059 research outputs found
Realisations of Symmetry
We perform a systematic investigation of free-scalar realisations of the
Za\-mo\-lod\-chi\-kov algebra in which the operator product of two
spin-three generators contains a non-zero operator of spin four which has
vanishing norm. This generalises earlier work where such an operator was
required to be absent. By allowing this spin-four null operator we obtain
several realisations of the algebra both in terms of two scalars as well
as in terms of an arbitrary number of free scalars. Our analysis is
complete for the case of two-scalar realisations.Comment: 14 pages, LATEX, UG-6/9
Mirror effect induced by the dilaton field on the Hawking radiation
We discuss the string creation in the near-extremal NS1 black string
solution. The string creation is described by an effective field equation
derived from a fundamental string action coupled to the dilaton field in a
conformally invariant manner. In the non-critical string model the dilaton
field causes a timelike mirror surface outside the horizon when the size of the
black string is comparable to the Planck scale. Since the fundamental strings
are reflected by the mirror surface, the negative energy flux does not
propagate across the surface. This means that the evaporation stops just before
the naked singularity of the extremal black string appears even though the
surface gravity is non-zero in the extremal limit.Comment: 15 page
Improving novelty detection using the reconstructions of nearest neighbours
We show that using nearest neighbours in the latent space of autoencoders
(AE) significantly improves performance of semi-supervised novelty detection in
both single and multi-class contexts. Autoencoding methods detect novelty by
learning to differentiate between the non-novel training class(es) and all
other unseen classes. Our method harnesses a combination of the reconstructions
of the nearest neighbours and the latent-neighbour distances of a given input's
latent representation. We demonstrate that our nearest-latent-neighbours (NLN)
algorithm is memory and time efficient, does not require significant data
augmentation, nor is reliant on pre-trained networks. Furthermore, we show that
the NLN-algorithm is easily applicable to multiple datasets without
modification. Additionally, the proposed algorithm is agnostic to autoencoder
architecture and reconstruction error method. We validate our method across
several standard datasets for a variety of different autoencoding architectures
such as vanilla, adversarial and variational autoencoders using either
reconstruction, residual or feature consistent losses. The results show that
the NLN algorithm grants up to a 17% increase in Area Under the Receiver
Operating Characteristics (AUROC) curve performance for the multi-class case
and 8% for single-class novelty detection
Learning to detect radio frequency interference in radio astronomy without seeing it
Radio Frequency Interference (RFI) corrupts astronomical measurements, thus
affecting the performance of radio telescopes. To address this problem,
supervised segmentation models have been proposed as candidate solutions to RFI
detection. However, the unavailability of large labelled datasets, due to the
prohibitive cost of annotating, makes these solutions unusable. To solve these
shortcomings, we focus on the inverse problem; training models on only
uncontaminated emissions thereby learning to discriminate RFI from all known
astronomical signals and system noise. We use Nearest-Latent-Neighbours (NLN) -
an algorithm that utilises both the reconstructions and latent distances to the
nearest-neighbours in the latent space of generative autoencoding models for
novelty detection. The uncontaminated regions are selected using weak-labels in
the form of RFI flags (generated by classical RFI flagging methods) available
from most radio astronomical data archives at no additional cost. We evaluate
performance on two independent datasets, one simulated from the HERA telescope
and another consisting of real observations from LOFAR telescope. Additionally,
we provide a small expert-labelled LOFAR dataset (i.e., strong labels) for
evaluation of our and other methods. Performance is measured using AUROC, AUPRC
and the maximum F1-score for a fixed threshold. For the simulated data we
outperform the current state-of-the-art by approximately 1% in AUROC and 3% in
AUPRC for the HERA dataset. Furthermore, our algorithm offers both a 4%
increase in AUROC and AUPRC at a cost of a degradation in F1-score performance
for the LOFAR dataset, without any manual labelling
The Conformal Penrose Limit and the Resolution of the pp-curvature Singularities
We consider the exact solutions of the supergravity theories in various
dimensions in which the space-time has the form M_{d} x S^{D-d} where M_{d} is
an Einstein space admitting a conformal Killing vector and S^{D-d} is a sphere
of an appropriate dimension. We show that, if the cosmological constant of
M_{d} is negative and the conformal Killing vector is space-like, then such
solutions will have a conformal Penrose limit: M^{(0)}_{d} x S^{D-d} where
M^{(0)}_{d} is a generalized d-dimensional AdS plane wave. We study the
properties of the limiting solutions and find that M^{(0)}_{d} has 1/4
supersymmetry as well as a Virasoro symmetry. We also describe how the
pp-curvature singularity of M^{(0)}_{d} is resolved in the particular case of
the D6-branes of D=10 type IIA supergravity theory. This distinguished case
provides an interesting generalization of the plane waves in D=11 supergravity
theory and suggests a duality between the SU(2) gauged d=8 supergravity of
Salam and Sezgin on M^{(0)}_{8} and the d=7 ungauged supergravity theory on its
pp-wave boundary.Comment: 20 pages, LaTeX; typos corrected, journal versio
Variation in occupational exposure associated with musculoskeletal complaints:a cross-sectional study among professional bassists
Variation in occupational exposure is assumed to have a protective effect against the development of musculoskeletal complaints (MSC), but this common assumption is not strongly supported by the literature. Among musicians, who have a high prevalence of MSC, many play more than one type of instrument (multi-instrumentalism) for many hours a day. Since multi-instrumentalism implies greater variation in ergonomic load of specific musculoskeletal areas than mono-instrumentalism, musicians are a suitable study population to test whether the above assumption is true. To investigate in a sample of professional bass players whether multi-instrumentalists are less likely to have MSC than mono-instrumentalists. Participants were 141 professional and professional student double bassists and bass guitarists. Demographic, MSC and exposure characteristics were collected online with self-constructed and existing questionnaires. Logistic regression analysis was used to test the association between multi- versus mono-instrumentalism and MSC, adjusted for confounders. The prevalence of having MSC in the neck, back, right shoulder area and both wrist areas did not differ significantly between the two groups. Further analysis revealed that the likelihood of having MSC in the left shoulder area was higher in multi-instrumentalists compared to mono-instrumentalists (Odds ratio 0.30, 95% CI 0.119-0.753, p = 0.010). In this sample of professional bass players, no protective effect of multi-instrumentalism against MSC was found. Multi-instrumentalism was associated with a higher prevalence of MSC in the left shoulder. This result challenges theoretical and clinical assumptions in occupational and pain medicine
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