Kasper Peeters

Holographic melting and related properties of mesons in a quark-gluon plasma

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.

Supersymmetric higher-derivative actions in ten and eleven dimensions, the associated superalgebras and their formulation in superspace

Higher-derivative terms in the string and M-theory effective actions are strongly constrained by supersymmetry. Using a mixture of techniques, involving both string-amplitude calculations and an analysis of supersymmetry requirements, we determine the supersymmetric completion of the R4 action in 11 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.

Cadabra: a field-theory motivated symbolic computer algebra system

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 TEX-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. 1. Field theory versus general-purpose computer algebra For 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 Email address: kasper.peeters@aei.mpg.de (Kasper Peeters).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 TEX-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. 1. Field theory versus general-purpose computer algebra

Superspace geometry for supermembrane backgrounds

Abstract We construct part of the superspace vielbein and tensor gauge field in terms of the component fields of 11-dimensional on-shell supergravity. The result can be utilized to describe supermembranes and corresponding matrix models for Dirichlet particles in non-trivial supergravity backgrounds to second order in anticommuting coordinates. We exhibit the κ-invariance of the corresponding supermembrane action, which at this order holds for unrestricted supergravity backgrounds, the supersymmetry covariance and the corresponding surface terms in the action.

D-branes in a plane wave from covariant open strings

We derive boundary conditions for the covariant open string corresponding to D-branes in an Hpp-wave by requiring kappa-symmetry of its bulk action. Both half-supersymmetric and quarter-supersymmetric branes are seen to arise in this way, and the analysis furthermore agrees fully with the existing probe brane and supergravity computations. We elaborate on the origin of dynamical and kinematical supersymmetries from the covariant point of view. In particular we focus on the D-string which only preserves half of the dynamical supersymmetries and none of the kinematical ones. We discuss its origin in AdS5×S5 and its world-volume spectrum.

The M-theory two-brane in AdS4×S7 and AdS7×S4

Abstract We construct the supermembrane action in an AdS4×S7 and AdS7×S4 background to all orders in anticommuting coordinates. The result is compared to and agrees completely with results obtained earlier for generic supergravity backgrounds through gauge completion at low orders in θ.

Splitting spinning strings in AdS/CFT

We study the semiclassical decay of macroscopic spinning strings in AdS5 ×S 5 through spontaneous splitting of the folded string worldsheet. Based on similar considera- tions in flat space this decay channel is expected to dominate the full quantum computation. The outgoing strings are uniquely specified by an infinite set of conserved (local) charges with a regular expansion in inverse powers of the initial angular momentum. We compute these charges and determine functional relations between them. Finally, a preliminary dis- cussion of the corresponding calculation in the non-planar sector of the dual gauge theory is presented.

The string/gauge theory correspondence in QCD

Abstract.Ideas about a duality between gauge fields and strings have been around for many decades. During the last ten years, these ideas have taken a much more concrete mathematical form. String descriptions of the strongly coupled dynamics of semi-realistic gauge theories, exhibiting confinement and chiral symmetry breaking, are now available. These provide remarkably simple ways to compute properties of the strongly coupled quark-gluon fluid phase, and also shed new light on various phenomenological models of hadron fragmentation. We present a review and highlight some exciting recent developments.

Chiral splitting and world-sheet gravitinos in higher-derivative string amplitudes

We report on progress made in the construction of higher-derivative superinvariants for type-II theories in ten dimensions. The string amplitude calculations required for this analysis exhibit interesting features which have received little attention in the literature so far. We discuss two examples from a forthcoming publication: the construction of the (HNS)2 R3 terms and the fermionic completion of the R4 terms. We show that a correct answer requires very careful treatment of the chiral splitting theorem, implies unexpected new relations between fermionic correlators, and most interestingly, necessitates the use of world-sheet gravitino zero modes in the string vertex operators. In addition we compare the relation of our results to the predictions of the linear scalar superfield of the type-IIB theory.

Supermembranes with winding

We study supermembranes in the light-cone gauge in eleven flat dimensions with compact directions. The membrane states are subject to only the central charges associated with closed two-branes, which, in this case, are generated by the winding itself. We present arguments why this winding leaves the mass spectrum continuous. The lower bound on the mass spectrum is set by the winding number and corresponds to a BPS state.