We analyze mesons at finite temperature in a chiral, confining string dual. The temperature dependence of low-spin as well as high-spin meson masses is shown to exhibit a pattern familiar from the lattice. Furthermore, we find the dissociation temperature of mesons as a function of their spin, showing that at a fixed quark mass, mesons with larger spins dissociate at lower temperatures. The Goldstone bosons associated with chiral symmetry breaking are shown to disappear above the chiral symmetry restoration temperature. Finally, we show that holographic considerations imply that large-spin mesons do not experience drag effects when moving through the quark-gluon plasma. They do, however, have a maximum velocity for fixed spin, beyond which they dissociate.

Abstract:We review aspects of loop quantum gravity in a pedagogical manner, with the aim of enabling a precise but critical assessment of its achievements so far. We emphasise that the off-shell ('strong') closure of the constraint algebra is a crucial test of quantum space-time covariance, and thereby of the consistency, of the theory. Special attention is paid to the appearance of a large number of ambiguities, in particular in the formulation of the Hamiltonian constraint. Developing suitable approximation methods to establish a connection with classical gravity on the one hand, and with the physics of elementary particles on the other, remains a major challenge.

Field theory is an area in physics with a deceptively compact notation. Although general purpose computer algebra systems, built around generic list-based data structures, can be used to represent and manipulate field-theory expressions, this often leads to cumbersome input formats, unexpected side-effects, or the need for a lot of special-purpose code. This makes a direct translation of problems from paper to computer and back needlessly time-consuming and error-prone. A prototype computer algebra system is presented which features T E X-like input, graph data structures, lists with Young-tableaux symmetries and a multiple-inheritance property system. The usefulness of this approach is illustrated with a number of explicit field-theory problems. © 2007 Elsevier B.V. All rights reserved.Keywords: Field theory; Computer algebra Field theory versus general-purpose computer algebraFor good reasons, the area of general-purpose computer algebra programs has historically been dominated by what one could call "list-based" systems. These are systems which are centred on the idea that, at the lowest level, mathematical expressions are nothing else but nested lists (or equivalently: nested functions, trees, directed acyclic graphs, . . .). There is no doubt that a lot of mathematics indeed maps elegantly to problems concerning the manipulation of nested lists, as the success of a large class of LISP-based computer algebra systems illustrates (either implemented in LISP itself or in another language with appropriate list data structures). However, there are certain problems for which a pure list-based approach may not be the most elegant, efficient or robust one.That a pure list-based approach does not necessarily lead to the fastest algorithms is of course well known. For, e.g., polynomial manipulation, there exists a multitude of other representations which are often more appropriate for the problem at hand. An area for which the limits of a pure list-based approach have received less attention consists of what one might call "field theory" problems. Without attempting to define this term rigorously, one can have in mind problems such as the manipulation of Lagrangians, field equations or symmetry algebras; the examples discussed later will define the class of problems more explicitly. The standard physics notation in this field is deceptively compact, and as a result it is easy to overlook the amount of information that is being manipulated when one handles these problems with pencil and paper. As a consequence, problems such as deriving the equations of motion from an action, or verifying supersymmetry or BRST invariance, often become a tedious transcription exercise when one attempts to do them with existing general-purpose computer algebra systems. Here, the inadequateness of simple lists is not so much that it leads to sub-optimal, slow solutions (although this certainly also plays a role at some stage), but rather that it prevents solutions from being developed at all.

Higher-derivative terms in the string and M-theory effective actions are strongly constrained by supersymmetry. Using a mixture of techniques, involving both stringamplitude calculations and an analysis of supersymmetry requirements, we determine the supersymmetric completion of the R 4 action in eleven dimensions to second order in the fermions, in a form compact enough for explicit further calculations. Using these results, we obtain the modifications to the field transformation rules and determine the resulting field-dependent modifications to the coefficients in the supersymmetry algebra. We then make the link to the superspace formulation of the theory and discuss the mechanism by which higher-derivative interactions lead to modifications to the supertorsion constraints. For the particular interactions under discussion we find that no such modifications are induced.