fix: allow more secondary vertices #4470
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I think we need to discuss widely how to approach this. I've also observed issues with the numbering. |
Hi @paulgessinger, I see it also breaks some tests. I'll keep it as a draft for now. |
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allow more secondary vertices
--- END COMMIT MESSAGE ---
In Fatras, a barcode enumerates primary and secondary vertices. As of now, there are reserved 12 bits for each, i.e. maximum number of vertices is 4096.
In pp collisions, we may have 200 primary vertices. One vertex may have up to around 100 secondary vertices (tested in 1000 events). Thus, there's a large reserve for both.
In PbPb collisions, we have only 1 primary vertex but we may have up to 6000 secondary vertices (again, tested in 1000 events). That exceeds the limit of 12 bits, causing an error. This problem is present in about 10% of all PbPb events.
Moreover, particle ID can go up to cca. 55k which is uncomfortably close to the limit of 65k. See the distributions in the attached file.
This PR takes 8 bits from sub-particle ID and allocates another 4 bits for secondary vertices (allowing more than 65000 of them) and another 4 bits for particle ID (allowing more than 1M of them).
barcodes.pdf