Rare Decays in BaBar: Search for Decays of the Neutral B Meson into Two Leptons and Measurement of CP Asymmetry in Inclusive Radiative Decays of the B Meson into a Strange-quark System

The purpose of this thesis is the study of rare decays with the BABAR detector. In the first part we present the study of the leptonic decays B0 → e+e−, B0 → µ+µ− and B0 →e±µ∓, based on a dataset corresponding to (383.6± 4.2)× 106 B ¯B pairs. We do not find evidence of any of the three decay modes, and obtain upper limit on the branching fractions, at 90% confidence level, of BR(B0 → e+e−) < 11.3 × 10−8, BR(B0 →µ+µ−) <5.2×10−8, and BR(B0 → e±µ∓) < 9.2×10−8. In the second part we present the measurement of CP asymmetry in rare inclusive B → Xsγ decays on the recoil of fully-reconstructed hadronic B decays, using a dataset corresponding to (465.0±5.1)×106 B ¯ B pairs. We measure a value of ACP = −0.12 ±0.11±0.03.