Steuard Jensen's Research

Thus far, most of what's here is aimed at a general audience: people who know a little bit about physics and would like to know more. (Links to that material can be found in the general audience section below.) Ideally, I would like to write a reasonably complete and detailed site giving a comprehensive introduction to string theory and to my current work in particular, but in practice actually doing research takes priority over trying to explain it to non-specialists. Still, I've made a start at it below.

Discussions for a general audience

An Introduction to String Theory: This is a talk that I gave in Feb. 2004 to the Chicago chapter of the MIT alumni club. It's aimed at an audience who've had a year or two of college physics, possibly a long time ago, and who want to see at least a few of the equations of string theory (presumably without being overwhelmed). I don't know if that's possible, but I tried.
Jeff Harvey and I studied "Worldsheet Instanton Corrections to the Kaluza-Klein Monopole". More recently, I extended those results to a new formalism in a paper called "The KK-Monopole/NS5-Brane in Doubled Geometry". On the pages linked here, I've pointed to the papers themselves (and to related talks that I've given) and tried to explain what we did in a way that may be understandable by (knowledgeable!) non-scientists.
String Theory and Duality: This is a variation on my introduction to string theory talk that I gave at Skidmore College in Apr. 2006. There isn't really much new material, but the content has been reorganized a fair bit (and I cut a number of topics as well). This is slides only for now; links to explanatory text from my MIT Club talk are provided. I have continued to develop related presentations for more recent colloquia.

My research history

What follows a quick overview of topics that I have worked on in the past. (My apologies to non-experts, but for now this list is heavy on jargon and light on explanations.) I've given a very small number of links to representative papers related to each of these subjects. Some might also be interested in a more official statement of research interests, or in a related list of proposed undergraduate research projects; my curriculum vitae is also available.

The KK-Monopole/NS5-Brane in Doubled Geometry, with results published in arXiv:1106.1174 (officially published as JHEP07(2011)088). I showed how to express the string theory backgrounds for these objects within the doubled geometry formalism (in the form introduced by Hull). This appears to be well-defined not just for the "smeared" versions of these solutions but also for their true "localized" forms, despite a broken isometry that would naively appear to make T-duality (and doubled geometry) inapplicable. The gauged linear sigma model formulation and worldsheet instanton calculation from my earlier work on this system also appear to generalize to the doubled formalism in a straightforward way.
NJL and QCD from string theory, with results published in arXiv:hep-th/0604017 by Eduard Antonyan, Jeff Harvey, myself, and David Kutasov. Again, the paper describes this work in detail; in brief, we study a configuration of intersecting D4, D8, and anti-D8 branes whose low energy field theory description involves dynamically broken chiral symmetry in four dimensions. At weak coupling it corresponds to a non-local version of the Nambu-Jona-Lasinio model, while at strong coupling it can be understood geometrically in terms of the near-horizon geometry of the D4-branes. With one additional dimension compactified, the same brane configuration can describe large N_C QCD, so these results provide the first direct connection between NJL and QCD physics. Related work is ongoing.
Worldsheet instanton corrections to the Kaluza-Klein monopole, with initial results published in arXiv:hep-th/0507204 by Jeff Harvey and myself (officially published as JHEP10(2005)028). This is described in detail in the paper; in brief, we showed that the usual gravitational solution for the Kaluza-Klein monpole is not the proper solution in string theory, but rather is "smeared" in the "winding space" associated with the Kaluza-Klein dyon coordinate. (I have written an overview of this work for non-specialists as well.) Work on related topics is ongoing.
Another topic I've studied relates to particular membrane states in M-theory. I have completed some work on this, but it's not clear that there is enough there to publish.
Open membrane instanton effects in heterotic M-theory with M5-branes. This work was most directly inspired by a paper by Moore, Peradze, and Saulina which studied the superpotential generated by open membrane instantons in a background of M5-branes in the heterotic M-theory interval (and also compactified on a Calabi-Yau 3-fold). Such an M5-brane has a complex modulus whose real part gives its position on the interval and whose axionic imaginary part comes from a Kaluza-Klein reduction of the self-dual 2-form on its worldvolume.

We studied the instanton-generated potential for this axion in more detail to see if it could be a candidate for a "quintessence" field. Instead, we discovered that the potential actually had a non-zero minimum whose magnitude was exponentially suppressed, which provided a possible natural origin for the observed cosmological constant. Unfortunately, between discovering this result and publishing it, we discovered a paper by Curio and Krause from just a few months earlier that already contained that conclusion. We continued to study other aspects of the same system for some time, in particular considering compactifications on K3xT² that would preserve more supersymmetry, but the technical challenges involved led to slow progress, and we set the project aside.
Boundary states on the string worldsheet, in collaboration with Li-Sheng Tseng. We hoped to find a complete classification of string boundary states at special points in the moduli space of compactifications. At those points, it is possible to decompose the conformal field theory into a product of theories with lower central charge (so, for instance, at the self-dual radius of a circle, the c=1 theory decomposes into a product of two c=1/2 theories). If the boundary states in the lower central charge theories are known (as they are, e.g., for minimal models like c=1/2), this allows a full classification for the full theory. Unfortunately, after substantial effort it turned out that the full classification (for c=1) gave nothing new: just the well known Dirichlet and Neumann boundary conditions of the usual D-brane picture. We concluded that "a much more complicated road to the same old result" was not worth publishing.
Tachyon condensation in string theory with non-commutative geometry, related to a paper by Harvey, Kraus, and Larsen. We were looking for a toy model that would capture some of the features of the full string theory picture of D-branes as non-commutative solitons, in the hope that we could understand how the "solution generating technique" in that paper related to supersymmetry. This was another brief project; the idea never really panned out, and I moved on.
Thermodynamic phases of black holes and branes, a small project based on Vatche Sahakian's thesis work suggested by Prof. Martinec. I made some progress toward an extension of earlier work, but this project was interrupted by my candidacy exam and second year classes, and after settling on Jeff Harvey as my thesis advisor I did not come back to it.
Weyl curvature at the Cauchy horizon of a charged black hole, undergraduate thesis in general relativity at Harvey Mudd College under the direction of Prof. Thomas Helliwell. Our particular project focused on the stability of curvature in the Reissner-Nordstrom black holes in the presence of infalling matter.
Scattering from neutron-rich halo nuclei, research that I did at a summer REU program at Michigan State University working with Prof. P.\ Gregers Hansen. We modeled collisions in which test particles struck He⁸ with large impact parameters, so that the scattering depended almost exclusively on properties of the loosely bound neutron matter at the outside of the nucleus.
Computer simulation of a quantum Paul trap, work done during a summer job with Prof. Anthony Starace at the University of Nebraska, Lincoln. My final product was actually a nice little Motif application for the X-Window system, but it used math libraries specific to the DEC Alpha. I've never managed to port it to another platform (nor to add the multithreading that the interface sometimes desperately needed). But in any case, the quantum mechanics worked great!