  We consider the issue of thermalization in the D1D5 CFT. Thermalization is
expected to correspond to the formation of a black hole in the dual gravity
theory. We start from the orbifold point, where the theory is essentially free,
and does not thermalize. In earlier work it was noted that there was no clear
thermalization effect when the theory was deformed off the orbifold point to
first order in the relevant twist perturbation. In this paper we consider the
deformation to second order in the twist, where we do find effects that can
cause thermalization of an initial perturbation. We consider a 1-loop process
where two untwisted copies of the CFT are twisted to one copy and then again
untwisted to two copies. We start with a single oscillator excitation on the
initial CFT, and compute the effect of the two twists on this state. We find
simple approximate expressions for the Bogoliubov coefficients and the behavior
of the single oscillator excitation in the continuum limit, where the mode
numbers involved are taken to be much larger than unity. We also prove a number
of useful relationships valid for processes with an arbitrary number of twist
insertions.
