Abstract

The track reconstruction in the Fermi LAT for “Pass 8”, the latest iteration of the reconstruction and event selection procedure, has changed substantially from the “finder-follower” in previous versions to one which aims to consolidate the track finding and vertex association into a single step using a global view of the event. This approach, which we call “Tree-based Tracking”, takes advantage of the topology of gamma-Ray conversions to associate the corresponding hits in the Tracker into a tree structure from which the primary electron and positron tracks can be extracted. The method offers several advantages over the current reconstruction: the gamma-ray association is performed in the first step using all of the information available; it returns fewer unassociated "false" tracks made up from stubs in the bottom of a shower; and it has the ability to return tracks with kinks (where the Kalman Filter based finder-follower tracking will stop). We present a brief description of the Tree-based Tracking and then provide a summary of the current performance in comparison to the Pass 7, the previous version.