Gluon Tree Amplitudes in Open Twistor String Theory

We show how the link variables of Arkani-Hamed, Cachazo, Cheung and Kaplan (ACCK), can be used to compute general gluon tree amplitudes in the twistor string. They arise from instanton sectors labelled by d, with d=n-1, where n is the number of negative helicities. Read backwards, this shows how the various forms for the tree amplitudes studied by ACCK can be grouped into contour integrals whose structure implies the existence of an underlying string theory.Comment: 36 page

Movement-based subgrouping in low back pain: synergy and divergence in approaches

Background Classification systems for low back pain (LBP) aim to guide treatment decisions. In physiotherapy, there are five classification schemes for LBP which consider responses to clinical movement examination. Little is known of the relationship between the schemes

Switchable lasing in coupled multimode microcavities

We propose the new concept of a switchable multimode microlaser. As a generic, realistic model of a multimode microresonator a system of two coupled defects in a two-dimensional photonic crystal is considered. We demonstrate theoretically that lasing of the cavity into one selected resonator mode can be caused by injecting an appropriate optical pulse at the onset of laser action (injection seeding). Temporal mode-to-mode switching by re-seeding the cavity after a short cool-down period is demonstrated by direct numerical solution. A qualitative analytical explanation of the mode switching in terms of the laser bistability is presented.Comment: Accepted for publication in Physical Review Letters. Published, somewhat shortened versio

Braneworld inflation from an effective field theory after WMAP three-year data

In light of the results from the WMAP three-year sky survey, we study an inflationary model based on a single-field polynomial potential, with up to quartic terms in the inflaton field. Our analysis is performed in the context of the Randall-Sundrum II braneworld theory, and we consider both the high-energy and low-energy (i.e. the standard cosmology case) limits of the theory. We examine the parameter space of the model, which leads to both large-field and small-field inflationary type solutions. We conclude that small field inflation, for a potential with a negative mass square term, is in general favored by current bounds on the tensor-to-scalar perturbation ratio rs.Comment: 11 pages, 5 figures; references updated and a few comments added; final version to appear in Phys. Rev.

Constructing the Tree-Level Yang-Mills S-Matrix Using Complex Factorization

A remarkable connection between BCFW recursion relations and constraints on the S-matrix was made by Benincasa and Cachazo in 0705.4305, who noted that mutual consistency of different BCFW constructions of four-particle amplitudes generates non-trivial (but familiar) constraints on three-particle coupling constants --- these include gauge invariance, the equivalence principle, and the lack of non-trivial couplings for spins >2. These constraints can also be derived with weaker assumptions, by demanding the existence of four-point amplitudes that factorize properly in all unitarity limits with complex momenta. From this starting point, we show that the BCFW prescription can be interpreted as an algorithm for fully constructing a tree-level S-matrix, and that complex factorization of general BCFW amplitudes follows from the factorization of four-particle amplitudes. The allowed set of BCFW deformations is identified, formulated entirely as a statement on the three-particle sector, and using only complex factorization as a guide. Consequently, our analysis based on the physical consistency of the S-matrix is entirely independent of field theory. We analyze the case of pure Yang-Mills, and outline a proof for gravity. For Yang-Mills, we also show that the well-known scaling behavior of BCFW-deformed amplitudes at large z is a simple consequence of factorization. For gravity, factorization in certain channels requires asymptotic behavior ~1/z^2.Comment: 35 pages, 6 figure

ESR study of the single-ion anisotropy in the pyrochlore antiferromagnet Gd2Sn2O7

Single-ion anisotropy is of importance for the magnetic ordering of the frustrated pyrochlore antiferromagnets Gd2Ti2O7 and Gd2Sn2O7. The anisotropy parameters for the Gd2Sn2O7 were measured using the electron spin resonance (ESR) technique. The anisotropy was found to be of the easy plane type, with the main constant D=140mK. This value is 35% smaller than the value of the corresponding anisotropy constant in the related compound Gd2Ti2O7.Comment: 8 pages, 3 figure

Hg-supported phospholipid monolayer as rapid screening device for low molecular weight narcotic compounds in water

This study positions the fabricated Pt/Hg-supported phospholipid sensor element in the context of more conventional biomembrane-based screening platforms. The technology has been used together with immobilised artificial membrane (IAM) chromatography and COSMOmic simulation methods to screen the interaction of a series of low molecular weight narcotic organic compounds in water with phosphatidylcholine (PC) membranes. For these chemicals it is shown that toxicity to aquatic species is related to compound hydrophobicity which is associated with compound accumulation in the phospholipid membrane as modelled by IAM chromatography measurements and COSMOmic simulations. In contrast, the Hg-supported dioleoyl phosphatidylcholine (DOPC) sensor element records membrane damage/modification which is indirectly related to general toxicity and directly related to compound structure. Electrochemical limit of detection (LoD) values depend on molecular structure and range from 20 μmolL−1 for substituted phenols to 23 mmolL−1 for aliphatics. Rapid cyclic voltammetry (RCV) “fingerprints” showed that the major structural classes of compounds: alkyl/chlorobenzenes, substituted phenols, quaternary ammonium compounds and neutral amines interacted distinctively with the DOPC on Hg and that these observations correlated with and supported those predicted by the COSMOmic simulations of the compound/DMPC association. In addition, the compatibility of the electrochemical and COSMOmic methods validates the electrochemical device as a meaningful high throughput technology to screen compounds in water and report on the mechanistic details of their interaction with phospholipid layers

From Momentum Amplituhedron Boundaries to Amplitude Singularities and Back

20 pages, 7 figuresThe momentum amplituhedron is a positive geometry encoding tree-level scattering amplitudes in $\mathcal{N}=4$ super Yang-Mills directly in spinor-helicity space. In this paper we classify all boundaries of the momentum amplituhedron $\mathcal{M}_{n,k}$ and explain how these boundaries are related to the expected factorization channels, and soft and collinear limits of tree amplitudes. Conversely, all physical singularities of tree amplitudes are encoded in this boundary stratification. Finally, we find that the momentum amplituhedron $\mathcal{M}_{n,k}$ has Euler characteristic equal to one, which provides a first step towards proving that it is homeomorphic to a ball.Peer reviewedFinal Published versio

Interpretations of Presburger Arithmetic in Itself

Presburger arithmetic PrA is the true theory of natural numbers with addition. We study interpretations of PrA in itself. We prove that all one-dimensional self-interpretations are definably isomorphic to the identity self-interpretation. In order to prove the results we show that all linear orders that are interpretable in (N,+) are scattered orders with the finite Hausdorff rank and that the ranks are bounded in terms of the dimension of the respective interpretations. From our result about self-interpretations of PrA it follows that PrA isn't one-dimensionally interpretable in any of its finite subtheories. We note that the latter was conjectured by A. Visser.Comment: Published in proceedings of LFCS 201

Amplitudes at Weak Coupling as Polytopes in AdS_5

We show that one-loop scalar box functions can be interpreted as volumes of geodesic tetrahedra embedded in a copy of AdS_5 that has dual conformal space-time as boundary. When the tetrahedron is space-like, it lies in a totally geodesic hyperbolic three-space inside AdS_5, with its four vertices on the boundary. It is a classical result that the volume of such a tetrahedron is given by the Bloch-Wigner dilogarithm and this agrees with the standard physics formulae for such box functions. The combinations of box functions that arise in the n-particle one-loop MHV amplitude in N=4 super Yang-Mills correspond to the volume of a three-dimensional polytope without boundary, all of whose vertices are attached to a null polygon (which in other formulations is interpreted as a Wilson loop) at infinity.Comment: 16 pages, 5 figure