Low-Pt Tracking for ATLAS Analyses

Currently, by default, the ATLAS experiment only reconstructs tracks down to 500 MeV in high pileup conditions. However, reconstructing tracks with Pt below 500 MeV can provide useful information to several analysis. A two-pass framework for the reconstruction of low-Pt tracks has been introduced, where default tracking is performed first, and then low-Pt tracks are reconstructed using the hits that were not used by the default tracks. To reduce reconstruction time, the seeds for the low-Pt tracks are required to point within a “region of interest” (RoI): the seeds must be within a user-specified distance of the event’s hard scatter longitudinally along the beamline. The following study was performed on a Powheg+Pythia8 Z boson production sample, where the Z boson was forced to decay to two muons. The invariant mass of the two muons must be between 60 and 110 GeV. The sample had a charged particle filter applied, such that there were fewer than 12 truth particles with Pt > 500 MeV and |eta| < 2.5 originating from the Z boson-producing interaction. The pileup conditions of the sample approximate those of data taking at the Large Hadron Collider (LHC) from 2015-2018. This is the first study on this two-pass framework; it is intended to be applied to a small dedicated subset of events for analysis purposes, rather than a full reprocessing of ATLAS data.