B0 mixing leads to crash

[Frank Siegert]

We see a crash when enabling mixing of neutral mesons (e.g. B0) by adding the following lines in Decaydata.db/HadronConstants.dat:

x_B = 0.776
y_B = 0.0
Mixing_B = 1

The problem seems to be happening in Hadron_Decay_Handler::FillDecayTree with the removal of the non-mixed decay blob from the bloblist (if (*bit==blob) p_bloblist->erase(bit);). I don't see right away what is going wrong and don't have the time to follow up right now, so am reporting it here so that it is not lost.

A simple run card to reproduce this together with the changes in <prefix>/share/SHERPA-MC/Decaydata.db above is:

(run){
  EVENTS = 10000
  BEAM_1 = 2212; BEAM_ENERGY_1 = 4000;
  BEAM_2 = 2212; BEAM_ENERGY_2 = 4000;

  MI_HANDLER = None
  MASSIVE[5]=1
}(run)

(processes){
  Process 93 93 -> 5 -5 93{0}
  Order (*,2);
  CKKW sqr(30/E_CMS)
  End process;
}(processes)

[Frank Siegert]

PS: ATLAS wants to use B0 mixing in their samples, so this is not only an academic exercise.