4,978 research outputs found
Perturbative expansion of N<8 Supergravity
We characterise the one-loop amplitudes for N=6 and N=4 supergravity in four
dimensions. For N=6 we find that the one-loop n-point amplitudes can be
expanded in terms of scalar box and triangle functions only. This
simplification is consistent with a loop momentum power count of n-3, which we
would interpret as being n+4 for gravity with a further -7 from the N=6
superalgebra. For N=4 we find that the amplitude is consistent with a loop
momentum power count of n, which we would interpret as being n+4 for gravity
with a further -4 from the N=4 superalgebra. Specifically the N=4 amplitudes
contain non-cut-constructible rational terms.Comment: 13 pages. v2 adds analytic expression for rational parts of 5-pt
1-loop N=4 SUGRA amplitude; v3 normalisations clarifie
Obtaining One-loop Gravity Amplitudes Using Spurious Singularities
The decomposition of a one-loop scattering amplitude into elementary
functions with rational coefficients introduces spurious singularities which
afflict individual coefficients but cancel in the complete amplitude. These
cancellations create a web of interactions between the various terms. We
explore the extent to which entire one-loop amplitudes can be determined from
these relationships starting with a relatively small input of initial
information, typically the coefficients of the scalar integral functions as
these are readily determined. In the context of one-loop gravity amplitudes, of
which relatively little is known, we find that some amplitudes with a small
number of legs can be completely determined from their box coefficients. For
increasing numbers of legs, ambiguities appear which can be determined from the
physical singularity structure of the amplitude. We illustrate this with the
four-point and N=1,4 five-point (super)gravity one-loop amplitudes.Comment: Minor corrections. Appendix adde
The n-point MHV one-loop Amplitude in N=4 Supergravity
We present an explicit formula for the n-point MHV one-loop amplitude in a
N=4 supergravity theory. This formula is derived from the soft and collinear
factorisations of the amplitude.Comment: 8 pages; v2 References added. Minor changes to tex
Andreev's Theorem on hyperbolic polyhedra
In 1970, E. M. Andreev published a classification of all three-dimensional
compact hyperbolic polyhedra having non-obtuse dihedral angles. Given a
combinatorial description of a polyhedron, , Andreev's Theorem provides five
classes of linear inequalities, depending on , for the dihedral angles,
which are necessary and sufficient conditions for the existence of a hyperbolic
polyhedron realizing with the assigned dihedral angles. Andreev's Theorem
also shows that the resulting polyhedron is unique, up to hyperbolic isometry.
Andreev's Theorem is both an interesting statement about the geometry of
hyperbolic 3-dimensional space, as well as a fundamental tool used in the proof
for Thurston's Hyperbolization Theorem for 3-dimensional Haken manifolds. It is
also remarkable to what level the proof of Andreev's Theorem resembles (in a
simpler way) the proof of Thurston.
We correct a fundamental error in Andreev's proof of existence and also
provide a readable new proof of the other parts of the proof of Andreev's
Theorem, because Andreev's paper has the reputation of being ``unreadable''.Comment: To appear les Annales de l'Institut Fourier. 47 pages and many
figures. Revision includes significant modification to section 4, making it
shorter and more rigorous. Many new references include
Glaucoma and the Optic Nerve
In summary, the anatomic characteristics of the optic nerve head have been described along the ophthalmoscopic interpretation of these characteristics. One hopes that this information combined with a knowledge of what constitutes glaucomatous abnormality in the optic disc will encourage the ophthalmic as well as non-ophthalmic practitioner to evaluate the optic nerve head and recognize those which are suspicious of glaucoma
Analytic results for two-loop Yang-Mills
Recent Developments in computing very specific helicity amplitudes in two
loop QCD are presented. The techniques focus upon the singular structure of the
amplitude rather than on a diagramatic and integration approachComment: Talk presented at 13th International Symposium on Radiative
Corrections, 24-29 September, 2017,St. Gilgen, Austria, 9 page
Constructing Gravity Amplitudes from Real Soft and Collinear Factorisation
Soft and collinear factorisations can be used to construct expressions for
amplitudes in theories of gravity. We generalise the "half-soft" functions used
previously to "soft-lifting" functions and use these to generate tree and
one-loop amplitudes. In particular we construct expressions for MHV tree
amplitudes and the rational terms in one-loop amplitudes in the specific
context of N=4 supergravity. To completely determine the rational terms
collinear factorisation must also be used. The rational terms for N=4 have a
remarkable diagrammatic interpretation as arising from algebraic link diagrams.Comment: 18 pages, axodraw, Proof of eq. 4.3 adde
Clinical Perspectives and Trends: Microperimetry as a trial endpoint in retinal disease
Endpoint development trials are underway across the spectrum of retinal disease. New validated endpoints are urgently required for the assessment of emerging gene therapies and in preparation for the arrival of novel therapeutics targeting early stages of common sight-threatening conditions such as age-related macular degeneration. Visual function measures are likely to be key candidates in this search. Over the last two decades, microperimetry has been used extensively to characterize functional vision in a wide range of retinal conditions, detecting subtle defects in retinal sensitivity that precede visual acuity loss and tracking disease progression over relatively short periods. Given these appealing features, microperimetry has already been adopted as an endpoint in interventional studies, including multicenter trials, on a modest scale. A review of its use to date shows a concurrent lack of consensus in test strategy and a wealth of innovative disease and treatment-specific metrics which may show promise as clinical trial endpoints. There are practical issues to consider, but these have not held back its popularity and it remains a widely used psychophysical test in research. Endpoint development trials will undoubtedly be key in understanding the validity of microperimetry as a clinical trial endpoint, but existing signs are promising
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