P-P' STRINGS IN M(ATRIX) THEORY

We study the off-diagonal blocks in the M(atrix) model that are suppos ed to correspond to open strings stretched between a Dp-brane and a Dp '-brane. It is shown that the spectrum, including the quantum numbers, of the zero-modes in the off-diagonal blocks can be determined from t he index theorem and unbroken supersymmetry, and indeed reproduces str ing theory predictions for p-p' strings. Previously the matrix descrip tion of a longitudinal fivebrane needed the introduction of extra degr ees of freedom corresponding to 0-4 strings by hand. We show that they are naturally associated with the off-diagonal zero-modes, and the su persymmetry transformation laws and low-energy effective action postul ated for them are now derivable from the M(atrix) theory. (C) 1998 Els evier Science B.V.