D-branes and KK-theory in Type I String Theory

We analyse unstable D-brane systems in type I string theory. Generalizing theproposal in hep-th/0108085, we give a physical interpretation for realKK-theory and claim that the D-branes embedded in a product space X x Y whichare made from the unstable Dp-brane system wrapped on Y are classified by areal KK-theory group KKO^{p-1}(X,Y). The field contents of the unstable D-branesystems are systematically described by a hidden Clifford algebra structure. We also investigate the matrix theory based on non-BPS D-instantons and showthat the spectrum of D-branes in the theory is exactly what we expect in type Istring theory, including stable non-BPS D-branes with Z_2 charge. We explicitlyconstruct the D-brane solutions in the framework of BSFT and analyse thephysical property making use of the Clifford algebra.Comment: 31 pages, corrected typos, added reference