Four years into the mission, the understanding of the performance of the Fermi Large Area Telescope (LAT) and data analysis have increased enormously since launch. Thanks to a careful analysis of flight data, we were able to trace back some of the mo
st significant sources of systematic uncertainties to using non-optimal calibration constants for some of the detectors. In this paper we report on a major effort within the LAT Collaboration to update these constants, to use them to reprocess the first four years of raw data, and to investigate the improvements observed for low- and high-level analysis. The Pass 7 reprocessed data, also known as P7REP data, are still being validated against the original Pass~7 (P7) data by the LAT Collaboration and should be made public, along with the corresponding instrument response functions, in the spring of 2013.
The event selection developed for the Fermi Large Area Telescope before launch has been periodically updated to reflect the constantly improving knowledge of the detector and the environment in which it operates. Pass 7, released to the public in Aug
ust 2011, represents the most recent major iteration of this incremental process. In parallel, the LAT team has undertaken a coherent long-term effort aimed at a radical revision of the entire event-level analysis, based on the experience gained in the prime phase of the mission. This includes virtually every aspect of the data reduction process, from the simulation of the detector to the event reconstruction and the background rejection. The potential improvements include (but are not limited to) a significant reduction in background contamination coupled with an increased effective area, a better point-spread function, a better understanding of the systematic uncertainties and an extension of the energy reach for the photon analysis below 100 MeV and above a few hundred GeV. We present an overview of the work that has been done or is ongoing and the prospects for the near future.
