String Theory and M-Theory

6.3 Type I superstring theory 223
Other type I D-branes
The only massless R–R field in the type I spectrum is C2. Therefore, aside
from the D9-branes, the only stable type IIB D-branes that survive the
orientifold projection are the ones that couple to this field. They are the
D1-brane and its magnetic dual, the D5-brane.
The world-volume theories of these D-branes are more complicated than
in the type IIB case. The basic reason is that there are additional massless
modes that arise from open strings that connect the D1-brane or the D5brane
to the 16 D9-branes. Moreover, this is taking place in the presence of
an O9 − plane.
Let us consider first a system of N coincident D1-branes. In the type
IIB theory the world-volume theory would be a maximally supersymmetric
U(N) gauge theory. However, due to the presence of the orientifold plane in
the type I theory, the gauge symmetry is enhanced to SO(2N), and there is
half as much unbroken supersymmetry as in the type IIB case. Moreover,
the world-volume theory contains massless matter supermultiplets that arise
as modes of open strings connecting the D1-branes to the D9-branes. These
transform as (2N, 32) under SO(2N) × SO(32). The SO(32) gauge symmetry
of the ten-dimensional bulk is a global symmetry of the D1-brane
world-volume theory.
The analysis of the world-volume theory of a system of N coincident D5branes
is carried out in a similar manner. The U(N) gauge symmetry that
is present in the type IIB case is enhanced to USp(2N) due to the O9 −
plane, and the amount of unbroken supersymmetry is cut in half. Moreover,
there are massless supermultiplets that arise as modes of open strings connecting
the D5-branes to the D9-branes. They transform as (2N, 32) under
USp(2N) × SO(32).
The K-theory analysis of possible charges of type I D-branes, which is not
presented here, accounts for all of the D-branes listed above. Moreover, it
also predicts the existence of a stable point particle in ¡ 9,1 that carries a 2
charge and is not supersymmetric. Thus this particle is a stable non-BPS
D0-brane. This particle, like all D-branes, is a nonperturbative excitation
of the theory. Moreover, it belongs to a spinor representation of the gauge
group. Its existence implies that, nonperturbatively, the gauge group is
actually Spin(32)/ 2 rather than SO(32). The stability of this particle
is ensured by the fact that it is the lightest state belonging to a spinor
representation. The mod 2 conservation rule is also an obvious consequence
of the group theory: two spinors can combine to give tensor representations.
In Chapter 8 it is argued that type I superstring theory is dual to one of