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Add StandAlone muon vertexing to Athena

Makayla Vessella requested to merge mvessell/athena:musavkv into main

Here goes nothing... this MR adds a new bespoke secondary vertexing algorithm (running in DAOD_LLP1) for ultra-displaced LLP searches that utilizes StandAlone muons, documented here:

https://indico.cern.ch/event/1514909/#b-606559-standalone-muon-verte

https://indico.cern.ch/event/1502566/contributions/6325119/

This algorithm is at the center of a new search for ultra-displaced dimuon vertices.

As these objects are extremely rare in SM signatures, this adds a near-minimal amount of CPU, disk, and memory overhead to data and MC background and a very small amount for LLP signal. (~0.01 kb disk/event, <0.01MB max Vmem, ~0.03 ms/event CPU increase for ttbar). The algorithm has been extensively profiled to ensure no memory leaks and introduces a new configurable flag into the central VKalVrt vertex fitting that allows for this ultra-displaced behavior without modifying the implementation for all other existing consumers.

FYI @akvam

Edited by Makayla Vessella

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