We analyze the classical and quantum vacua of 2d N=(8,8)
supersymmetric Yang-Mills theory with SU(N) and U(N) gauge group,
describing the worldvolume interactions of N parallel D1-branes with flat
transverse directions R8. We claim that the IR limit of the SU(N)
theory in the superselection sector labeled M(modN) --- identified with
the internal dynamics of (M,N)-string bound states of Type IIB string theory
--- is described by the symmetric orbifold N=(8,8) sigma model into
(R8)D−1/SD when D=gcd(M,N)>1, and by a single
massive vacuum when D=1, generalizing the conjectures of E. Witten and
others. The full worldvolume theory of the D1-branes is the U(N) theory with
an additional U(1) 2-form gauge field B coming from the string theory
Kalb-Ramond field. This U(N)+B theory has generalized field configurations,
labeled by the Z-valued generalized electric flux and an independent
ZN-valued 't Hooft flux. We argue that in the quantum mechanical
theory, the (M,N)-string sector with M units of electric flux has a
ZN-valued discrete θ angle specified by M(modN) dual to
the 't Hooft flux. Adding the brane center-of-mass degrees of freedom to the
SU(N) theory, we claim that the IR limit of the U(N)+B theory in the
sector with M bound F-strings is described by the N=(8,8) sigma
model into SymD(R8). We provide strong evidence for
these claims by computing an N=(8,8) analog of the elliptic genus
of the UV gauge theories and of their conjectured IR limit sigma models, and
showing they agree. Agreement is established by noting that the elliptic genera
are modular-invariant Abelian (multi-periodic and meromorphic) functions, which
turns out to be very restrictive.Comment: 47 pages. Comments welcome