Hi Hella,
I must admit that I haven't read the paper in any detail, but I have a question about what they do with the 'DTF chi2' in line 72. I went back to the analysis note, and I am not sure that the way the compute this chi2 make sense.
The only proper way to this with DTF is to create an intermediate 'mother' particle for the Z0 and D. The vertex chi2 of this mother particle tells you something about whether the Z0 and D come from the same vertex. Once you add a PV constraint to this mother particle, the chi2 also tells you something about the consistency with a PV. (I do wonder if you really want to require a PV if you actually do not need it.)
What is certainly 'wrong' is to make PV constrained fits for the D and the Z and then add up the chi2. The result is not a good chi2, except in the limit that the PV errors are infinitely small.
Cheers, Wouter