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GLO VTX: add missing protection against accessing MC labels of background events #2684

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GLO VTX: add missing protection against accessing MC labels of background events #2684

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2 changes: 1 addition & 1 deletion Modules/GLO/src/VertexingQcTask.cxx
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Expand Up @@ -215,7 +215,7 @@ void VertexingQcTask::monitorData(o2::framework::ProcessingContext& ctx)
mTimeUncVsNContrib->Fill(nContr, timeUnc);
mBeamSpot->Fill(x, y);

if (mUseMC && mcLbl[i].isSet()) { // make sure the label was set
if (mUseMC && mcLbl[i].isSet() && mcLbl[i].getSourceID() == 0) { // make sure the label was set and to use only the underlying event

@sawenzel sawenzel Aug 21, 2026

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I have some doubts about this: In an O2DPG simulation when using embedding, we will have both SourceID == 0 (the background) and SourceID == 1 (the signal) ... and both might contribute to the vertexing task.

We could however use mcLbl.isNoise() or mcLbl.isValid() or something. ... or directly exclude QED by sourceID != 99.

@ehellbar ehellbar Aug 21, 2026

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we do the same check also twice earlier in the function, when we loop over the labels. So for simulations with embedding, this never worked properly, then, always skipping signal and only filling background?

@ehellbar ehellbar Aug 21, 2026

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or, is background == simulated events and signal == embedded signal in this case? Then we might not want the embedded signal in those histogram?

@shahor02 shahor02 Aug 21, 2026

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This particular task extracts the MCHeader multiplicity as a very crude alias of the occupancy, relevant only for the PbPb collision. Assuming that the underlying PbPb event is always source 0, a few tracks coming from the overlaid signal with source>0 should be irrelevant.

@sawenzel sawenzel Aug 21, 2026

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If it's only used for PbPb then it's probably fine

auto header = mMCReader.getMCEventHeader(mcLbl[i].getSourceID(), mcLbl[i].getEventID());
auto purity = mcLbl[i].getCorrWeight();
auto mult = header.GetNPrim();
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