Leptonic B Decays at BaBar

Book excerpt: The authors present results on searches for the rare B decays B° → ll− (l = e or?) and B+ → K+?{bar?} in?(4S) decays. The data used in these analyses were collected with the BABAR detector at the PEP-II e+e− storage ring during the years 2000 and 2001. The B° → l+l− search was performed using an integrated luminosity of 54.4 fb−1 accumulated at the?(4S) resonance (about 60 million B{bar B} pairs) while the B+ → K+?{bar?} search was performed with 50.7 fb−1 (about 56 million B{bar B} pairs). Both analyses use an additional 6.4 fb−1 of data accumulated about 40 MeV below resonance. They see no evidence for a signal in either the B → l+l− or B → K+{nu}{bar {nu}} decay modes and set the following upper limits at the 90% CL:?(B° → e+e−)