CS shower crashes with "Invalid color flow" in single-top setup
This has been a bit hard to reproduce and to debug (thanks to @shoeche for help analyzing logs), but I'm pretty sure now that this is due to a logic error in the lines preceding the return false;
in CSSHOWER++/Showers/Shower.C:559. The problem is that at that point we have already changed the stored winning splitting information (e.g. m_flavA = m_sudakov.GetFlavourA();
etc.), overwriting any previous stored winning splitting information, and then actually rejecting the split eventually. The shower will then proceed to make the kinematics, but acting on the wrong splitter! I have confirmed this in the log, i.e. that the forced splitter is not actually affected (other than it's ktstart
being set to zero in line 558), instead the actually winning splitter is split, but with wrong information, leading to the colour partners not being set correctly (we get (none),(none)
in the log) etc., eventually leading to the crash.