Dr Guglielmo De Nardo (Napoli University and INFN (IT))
Within the standard model, the decay B+ --> tau+ nu has the largest branching fraction of any leptonic B decay, yet its reconstruction is made complicated by the multiple final-state neutrinos. The study of this decay is made possible by the ability to reconstruct the other B in the event, unique to the e+e- environment of the B factories. We present the result of an improved analysis using the full BABAR dataset, where the other B is tagged with a large number of hadronic decays. We observe evidence for B+ --> tau+ nu at more than 3 standard deviations, and discuss the implied constraints on models of new physics. The exclusive semileptonic transitions b -->c tau nubar are sensitive to physics beyond the standard model. For example, in type-II two-Higgs-doublet models, the B -->D(*) tau nubar branching fractions are functions of the ratio between tan(beta) and the charged Higgs mass. Using the full BABAR data set, we present measurements of the branching fractions of these decays, normalized relative to those of the decays B -->D(*) l nubar, where l is an electron or a muon. The measurements are performed using a reconstructed D (*) and an electron, muon, or tau recoiling against a fully-reconstructed B meson, taking advantage of the clean B-factory environment and high luminosity. The branching-fraction ratios are extracted from a simultaneous two-dimensional fit to the measured lepton spectrum and the reconstructed missing mass squared. The experimentally challenging background contributions from decays involving D** states are estimated using a control sample. We report the first observation of the semileptonic B --> D tau nubar decay and discuss implications of our results for scenarios of physics beyond the standard model.