String Theory

The role of solitonic extended objects known as branes has been pivotal in obtaining detailed knowledge of strongly coupled dynamics in string theory. In an effort to uncover non-perturbative, non-supersymmetric contributions to the string spectrum, I got involved in dicussions with Roberto Emparan (Bilbao/Cern) which led to the construction , together with one of my graduate students, Auttakit Chattaraputi (Durham) , of new four-dimensional solutions of General Relativity (called composite diholes) which can be embedded in 10 or 11-dimensional supergravities, interpreted as systems of intersecting Dirichlet branes (D-branes) and intesecting anti-D-branes, generically called `non BPS states'  (hep-th/9911007)

Dirichlet branes can also be studied as objects described by the conformal field theory of strings that end on them, and this picture has amazing consequences in our understanding of the emergence of all ten-dimensional string theories from a 26-dimensional parent bosonic string theory.  Together with a long-term collaborator, Francois Englert (Brussels) , and Laurent Houart (Brussels) ,  we extend to the open string sector of the bosonic string the universal truncation procedure which allows one to encode all ten-dimensional closed fermionic string theories in the 26-dimensional bosonic string theory compactified, in the left and/or right sector, on the Lie algebra lattice of E8 x SO(16). The study of the open string sector revealed that crucial features of the fermionic strings can be explained from purely bosonic considerations. First of all, through a brane fusion mechanism , the imposition of a tadpole condition in the bosonic string explains the cancellation of anomalies in Type I theory by the Chan-Paton group SO(32) and yields the Chan-Paton groups of the Type O theories. In addition, the tensions of the fermionic space-filling D-branes are computed using properties of the bosonic string only. These results follow from specific properties of the E8 x SO(16) lattice, singling it out of all possible toroidal compactifications. They also point towards a dynamical origin of the truncation, but such dynamics cannot be handled in the context of the perturbative approach given so far. We are now thinking of a non-perturbative approach to the bosonic string (hep-th/0106235) . Work in this area continues with the extra active participation of Auttakit Chattaraputi (Durham).