Abstract

The software developed before launch to simulate and reconstruct Fermi LAT events has performed beyond expectations, allowing for extensive scientific analysis. One unexpected feature of the LAT data was the presence of particle pile-up, remnant detector response to particles that are nearly coincident with the triggered event. These pile-up (or "ghost") particles affect all LAT subsystems, leading to an overall degradation in signal efficiency and background rejection power. In the LAT anticoincidence detector (ACD), ghosts are manifested as out-of-time depositions of energy in the scintillating fiber tiles. As part of the Pass-8 effort, we have developed a technique to utilize information from the ACD fast hardware trigger, rather than from the slower analog electronics, to identify and mitigate ghost effects in the ACD and improve gamma-ray efficiency.