Low-$p_{\mathrm{T}}$ tracking for ATLAS in nominal LHC pileup

In the most recent year of data-taking with the ATLAS detector at the Large Hadron Collider (LHC), the minimum transverse momentum ($p_{\mathrm{T}}$) of default track reconstruction was 500 MeV. This bound was set to reduce the amount of combinatorial problem solving required and to save disk space, which are challenges in high pileup environments. However, most proton-proton collisions at the LHC will result in a large number of soft particles, and the reconstruction of these soft particles in high pileup can provide important information for many analyses. This note will explain a method of tracking in high pileup conditions where low-$p_{\mathrm{T}}$ tracks are reconstructed in a second tracking pass after default tracking is performed. In order to prevent a large increase in the per-event reconstruction time, tracks with $p_{\mathrm{T}}$ below 500 MeV are only reconstructed within a ``region of interest'', which is defined event-by-event. Examples of applications for this technique are searches for photon-induced physics at the LHC, charm tagging, and SUSY searches.