Preliminary study on the Cosmic-Ray Electron spectrum measurement using a new event-level analysis of Fermi-LAT

The study of Galactic Cosmic Ray Electrons (CREs) saw important developments in recent years, with the assumption of e+ production only in interaction of hadronic Cosmic rays with interstellar matter challenged by new measurements of CRE spectrum and related quantities. Indeed, all recent experiments seem to confirm an hardening in the e+, a feature that is totally in contrast with the all-secondaries hypothesis, even if significant disagreements are present about the CRE spectral behavior and the possible presence of spectral features. These disagreements, together with insufficient precision of current measurements, prevent the identification of the primary e+ sources, with models involving Dark matter or astrophysical sources like Supernova Remnants (SNRs) and Pulsar Wind Nebulae (PWNe) all able to explain current data. The Fermi-LAT contribution to the CRE study was fundamental, with the 2009 measurement of the e+e- spectrum extended to the 7 MeV – 1 TeV range with a statistics already exceeding previous results by many order of magnitude; however, LAT results could be further and significantly refined exploiting the full LAT statistics (at present, almost 6 yr) and the improvements made in LAT event reconstruction since 2009. The aim of this Thesis is to proceed in this direction. This Thesis will first present an extensive review of the CRE models and of current results, followed by the update of CRE measurement using 4 years data reprocessed with the improved event reconstruction introduced in 2011. The results obtained, even if not in complete agreement with the previous measurement, substantially confirm its main results; in addition, the data analysis performed allows to identify the effects of the main changes introduced in the event reconstruction. The LAT collaboration has developed a new event reconstruction process, expected to be released at the end of 2014, expected to lead to substantial improvements in LAT performances. In view of an improvement in the e+ e- result using this new event reconstruction, this Thesis will present a description of the main changes introduced, together with a study of the effect of the update of the GEANT4 software used for particle simulation. Finally, I have performed an extensive comparison of flight and simulated data; the results obtained show the high level of data-simulation agreement reached, which is crucial for the reduction of systematic uncertainties that dominated previous LAT measurements.