  In the ground state of the doped two-leg Hubbard ladder there are power-law
decaying d_{x^2-y^2}-type pairing correlations. It is important to know the
strength and the temperature scale of these correlations. For this purpose, we
have performed determinantal Quantum Monte Carlo (QMC) calculations of the
reducible particle-particle interaction in the Hubbard ladder. In this paper,
we report on these calculations and show that, at sufficiently low
temperatures, resonant particle-particle scattering takes place in the
d_{x^2-y^2} pairing channel for certain values of the model parameters. The QMC
data presented here indicate that the d_{x^2-y^2} pairing correlations are
strong in the Hubbard ladder.
