High-energy physics

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 (in the form of ``holographic duals'' involving strings in extra dimensions). 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.

For some more detailed information, see e.g. our review

The string/gauge theory correspondence in QCD arXiv:0708.1502, with Marija Zamaklar.

I am working on the application of the string/gauge theory correspondence to strongly-coupled QCD-like theories, at finite temperature and finite density; see e.g.

A Non-homogeneous ground state of the low-temperature Sakai-Sugimoto model arXiv:1104.2291 with C.A. Ballon Bayona and Marija Zamaklar.

Holographic decays of large-spin mesons hep-th/0511044 with Jacob Sonnenschein and Marija Zamaklar.

Rho meson condensation at finite isospin chemical potential in a holographic model for QCD arXiv:0709.3948 with Ofer Aharony, Jacob Sonnenschein and Marija Zamaklar.

During the last few years, holographic techniques have also been applied to superfluids and superconductors. This again is an interesting new approach to study strongly coupled condensed matter systems. We have looked at so-called `colourful' superconductors,

Exploring colourful holographic superconductors arXiv:0907.1508 with Jonathan Powell and Marija Zamaklar.

A large part of my work deals with string theory and supersymmetry. String theory predicts that all the known elementary particles correspond, at even smaller distance scales, to excitations of a single, string-like object. The quantum gravitational force arises automagically (read much more about it at superstringtheory.com). String theory effects can be studied in the form of higher-derivative corrections to the Einstein-Hilbert action. Those quantum corrections are constrained by a variety of symmetries. See e.g.

Supersymmetric higher-derivative actions in ten and eleven dimensions, the associated superalgebras and their formulation in superspace hep-th/0010167, with Pierre Vanhove and Anders Westerberg.

We also used string scattering amplitudes to analyse such effective actions; see for instance

The Ramond-Ramond sector of string theory beyond leading order hep-th/0307298, with Anders Westerberg,

in which we worked out various string amplitudes with external Ramond-Ramond gauge field state. See the citation list of the big paper above for pointers to the literature. These higher-derivative corrections have various applications in the study of stringy black holes.

Another string topic on which I have been working is the study of large strings in the context of the AdS/CFT correspondence (see the picture). This work has appeared in

Splitting spinning strings in AdS/CFT hep-th/0410275 with Jan Plefka and Marija Zamaklar.

In many of these computations, my symbolic computer algebra software Cadabra has been useful.

Solitons in field theory typically exhibit very complicated dynamics. By restricting attention only to the fluctuations of their zero-energy deformations, one arrives at the celebrated "moduli space approximation". While this is a relatively good approximation for half-BPS solitons, it fails to qualitatively describe the dynamics for systems with less supersymmetry. The analytical and numerical tools to study quarter-BPS solitons are described in

Motion on moduli spaces with potentials hep-th/0107164, with Marija Zamaklar.

You can find the associated programs (which also produced the image on the left) on a separate page.

In the large tension limit, the string world-sheet genus expansion of string theory reduces to a world-line perturbation series based on Schwinger propagators. This provides a very interesting alternative perspective on computations field theory. I worked mainly on relations of this formalism to index theorems:

Spinors on manifolds with boundary: APS index theorems with torsion, hep-th/9901016 with Andrew Waldron.

I am also interested in the applications of these ideas in the context of finite-temperature physics.