It is easy to see that this is also true in other subtraction methods. For example, in the one used in the , the momentum of the collinear counterterm is built by setting the 3-momentum of the $b$ quark parallel to the sum of the 3-momenta of the $b$ and $g$ particles in the partonic CM frame. Furthermore, the momentum of the $d \bar{{\mu}} \nu$ system is boosted along the direction of the merged $b$ quark in order to conserve 3-momentum, and the absolute value of the $b$ quark momentum is chosen in such a way that the final state CM energy is conserved. This procedure is designed to conserve the mass of the final state system, and the mass of the system that recoils with respect to the splitting partons, i.e. the $\bar{{\mu}} \nu d$ system, while the mass of the top resonance is not conserved.

We thus expect that the collinear singularities present in the real and subtraction terms will be exposed in the narrow width limit, spoiling the convergence of the subtraction method. In fact, double logs of the resonance width will arise in different regions of the real cross section, yielding to a failure of convergence 