  Let $T$ be a circle group, and $LT$ be its loop group. We hope to establish
an index theory for infinite-dimensional manifolds which $LT$ acts on,
including Hamiltonian $LT$-spaces, from the viewpoint of $KK$-theory. We have
already constructed several objects in the previous paper \cite{T}, including a
Hilbert space $\mathcal{H}$ consisting of "$L^2$-sections of a Spinor bundle on
the infinite-dimensional manifold", an "$LT$-equivariant Dirac operator
$\mathcal{D}$" acting on $\mathcal{H}$, a "twisted crossed product of the
function algebra by $LT$", and the "twisted group $C^*$-algebra of $LT$",
without the measure on the manifolds, the measure on $LT$ or the function
algebra itself. However, we need more sophisticated constructions. In this
paper, we study the index problem in terms of $KK$-theory.
  Concretely, we focus on the infinite-dimensional version of the latter half
of the assembly map defined by Kasparov. Generally speaking, for a
$\Gamma$-equivariant $K$-homology class $x$, the assembly map is defined by
$\mu^\Gamma(x):=[c]\otimes j^\Gamma(x)$, where $j^\Gamma$ is a $KK$-theoretical
homomorphism, $[c]$ is a $K$-theory class coming from a cut-off function, and
$\otimes$ denotes the Kasparov product with respect to $\Gamma\ltimes C_0(X)$.
We will define neither the $LT$-equivariant $K$-homology nor the cut-off
function, but we will indeed define the $KK$-cycles $j^{LT}_\tau(x)$ and $[c]$
directly, for a virtual $K$-homology class $x=(\mathcal{H},\mathcal{D})$ which
is mentioned above. As a result, we will get the $KK$-theoretical index
$\mu^{LT}_\tau(x)\in KK(\mathbb{C},LT\ltimes_\tau \mathbb{C})$. We will also
compare $\mu^{LT}_\tau(x)$ with the analytic index ${\rm
ind}_{LT\ltimes_\tau\mathbb{C}}(x)$ which will be introduced.
