  We develop a scaling theory of interaction-induced delocalization of
few-particle states in disordered quantum systems. In the absence of
interactions, all single-particle states are localized in $d<3$, while in $d
\geq 3$ there is a critical disorder below which states are delocalized. We
hypothesize that such a delocalization transition occurs for $n$-particle bound
states in $d$ dimensions when $d+n\geq 4$. Exact calculations of
disorder-averaged $n$-particle Greens functions support our hypothesis. In
particular, we show that $3$-particle states in $d=1$ with nearest-neighbor
repulsion will delocalize with $W_c \approx 1.4t$ and with localization length
critical exponent $\nu = 1.5 \pm 0.3$. The delocalization transition can be
understood by means of a mapping onto a non-interacting problem with symplectic
symmetry. We discuss the importance of this result for many-body
delocalization, and how few-body delocalization can be probed in cold atom
experiments.
