TASI Lectures on Non-BPS D-Brane Systems

In this set of lectures various properties of D-branes are discussed. After re-viewing the basics, we discuss unstable D-brane/anti-D-brane systems, a subject pioneered by Sen. Following him, we discuss the construction of the non-BPS D0-brane in type I theory. This state is stable since it carries a conserved Z 2 charge. The general classification of D-brane charges using K-theory is discussed. The results for the type I theory, and the T-dual type I ′ theory, are emphasized. Compactification of type I on a circle or torus gives a theory with 16 supersymmetries in 9d or 8d. In each case the moduli space has three branches. The spectrum of non-BPS D-branes are different for each of these branches. We conclude by pointing out some problems with the type I D7-brane and D8-brane predicted by K-theory.