in [@Gieseke:2003rz], in which the starting values of the shower evolution variables for the top and bottom quarks are chosen such that the volumes of phase space accessible to emissions from each quark are approximately equal.

As expected, in both plots we see a high density of points in the limit $x_g \rightarrow 0$, corresponding to soft gluon emission. The POWHEG corrected distribution also has a concentration of points along the upper physical phase space boundary where $x_W$ is maximal and emissions are collinear to the bottom quark. The density of points along the upper boundary is reduced in the parton shower distribution and points are instead concentrated along the lower boundary of the bottom quark emission region. As discussed in [@Gieseke:2003rz], the parton shower approximation agrees with the exact matrix element in the case of collinear radiation from the bottom quark but overestimates it elsewhere in the bottom quark emission region. The factor by which the parton shower approximation exceeds the exact matrix element, increases towards the lower boundary of the region and therefore we see an excess of points near the boundary. The parton shower distribution also has a high density of points in the top quark emission 