On amplitudes in self-dual sector of Yang-Mills theory

Self-dual perturbiner in the Yang-Mills theory is constructed by the twistor methods both in topologically trivial and topologically nontrivial cases. Maximally helicity violating amplitudes and their instanton induced analogies are briefly discussed.Comment: 9 pages, Latex, a misguided reference is corrected, ps is also available at http://wwwtheor.itep.ru/preprints/1996

From Yang-Mills Lagrangian to MHV Diagrams

We prove the equivalence of a recently suggested MHV-formalism to the standard Yang-Mills theory. This is achieved by a formally non-local change of variables. In this note we present the explicit formulas while the detailed proofs are postponed to a future publication.Comment: Latex,11 pages, minor changes, reference added, version to appear in JHE

On Five-dimensional Superspaces

Recent one-loop calculations of certain supergravity-mediated quantum corrections in supersymmetric brane-world models employ either the component formulation (hep-th/0305184) or the superfield formalism with only half of the bulk supersymmetry manifestly realized (hep-th/0305169 and hep-th/0411216). There are reasons to expect, however, that 5D supergraphs provide a more efficient setup to deal with these and more involved (in particular, higher-loop) calculations. As a first step toward elaborating such supergraph techniques, we develop in this letter a manifestly supersymmetric formulation for 5D globally supersymmetric theories with eight supercharges. Simple rules are given to reduce 5D superspace actions to a hybrid form which keeps manifest only the 4D, N=1 Poincare supersymmetry. (Previously, such hybrid actions were carefully worked out by rewriting the component actions in terms of simple superfields). To demonstrate the power of this formalism for model building applications, two families of off-shell supersymmetric nonlinear sigma-models in five dimensions are presented (including those with cotangent bundles of Kahler manifolds as target spaces). We elaborate, trying to make our presentation maximally clear and self-contained, on the techniques of 5D harmonic and projective superspaces used at some stages in this letter.Comment: 46 pages, 3 figures. V5: version published in JHE

On Superspace Chern-Simons-like Terms

We search for superspace Chern-Simons-like higher-derivative terms in the low energy effective actions of supersymmetric theories in four dimensions. Superspace Chern-Simons-like terms are those gauge-invariant terms which cannot be written solely in terms of field strength superfields and covariant derivatives, but in which a gauge potential superfield appears explicitly. We find one class of such four-derivative terms with N=2 supersymmetry which, though locally on the Coulomb branch can be written solely in terms of field strengths, globally cannot be. These terms are classified by certain Dolbeault cohomology classes on the moduli space. We include a discussion of other examples of terms in the effective action involving global obstructions on the Coulomb branch.Comment: 23 pages; a reference and an author email correcte

Higher-Derivative Terms in N=2 Supersymmetric Effective Actions

We show how to systematically construct higher-derivative terms in effective actions in harmonic superspace despite the infinite redundancy in their description due to the infinite number of auxiliary fields. Making an assumption about the absence of certain superspace Chern-Simons-like terms involving vector multiplets, we write all 3- and 4-derivative terms on Higgs, Coulomb, and mixed branches. Among these terms are several with only holomorphic dependence on fields, and at least one satisfies a non-renormalization theorem. These holomorphic terms include a novel 3-derivative term on mixed branches given as an integral over 3/4 of superspace. As an illustration of our method, we search for Wess-Zumino terms in the low energy effective action of N=2 supersymmetric QCD. We show that such terms occur only on mixed branches. We also present an argument showing that the combination of space-time locality with supersymmetry implies locality in the anticommuting superspace coordinates of for unconstrained superfields.Comment: 30 pages. Added references and simplified final form of WZ ter

Anomalous Magnetic Properties of Sr2YRuO6

Anomalous magnetic properties of the double perovskite ruthenates compound Sr2YRuO6 are reported here. Magnetization measurements as a function of temperature in low magnetic fields show clear evidence for two components of magnetic order (TM1 ~ 32K and TM2 ~ 27K) aligned opposite to each other with respect to the magnetic field direction even though only Ru5+moments can order magnetically in this compound. The second component of the magnetic order at TM2 ~ 27K results only in a magnetization reversal, and not in the negative magnetization when the magnetization is measured in the field cooled (FC) mode. Isothermal magnetization (M-H) measurements show hysteresis with maximum coercivity (Hc) and remnant magnetization (Mr) at T ~ 27 K, corroborating the presence of the two oppositely aligned magnetic moments, each with a ferromagnetic component. The two components of magnetic ordering are further confirmed by the double peak structure in the heat capacity measurements. These anomalous properties have significance to some of the earlier results obtained for the Cu-substituted superconducting Sr2YRu1-xCuxO6 compounds.Comment: 6 figur

Gravity, Twistors and the MHV Formalism

We give a self-contained derivation of the MHV amplitudes for gravity and use the associated twistor generating function to define a twistor action for the MHV diagram approach to gravity. Starting from a background field calculation on a spacetime with anti self-dual curvature, we obtain a simple spacetime formula for the scattering of a single, positive helicity linearized graviton into one of negative helicity. Re-expressing our integral in terms of twistor data allows us to consider a spacetime that is asymptotic to a superposition of plane waves. Expanding these out perturbatively yields the gravitational MHV amplitudes of Berends, Giele & Kuijf. We go on to take the twistor generating function off-shell at the perturbative level. Combining this with a twistor action for the anti self-dual background, we obtain a twistor action for the MHV diagram approach to perturbative gravity. We finish by extending these results to supergravity, in particular N=4 and N=8.Comment: 39 pages, 3 figures. Minor typos corrected, some clarification adde

Supermoduli spaces

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