  We develop an action formalism to calculate probabilities of rare events in
cluster-cluster aggregation for arbitrary collision kernels and establish a
pathwise large deviation principle with total mass being the rate. As an
application, the rate function for the number of surviving particles as well as
the optimal evolution trajectory are calculated exactly for the constant, sum
and product kernels. For the product kernel, we argue that the second
derivative of the rate function has a discontinuity. The theoretical results
agree with simulations tailored to the calculation of rare events.
