First measurement of the B → πℓν and B → ρ(ω)ℓν branching fractions

CLEO has studied B decays to the nal states``, ``, and !``, wherè = e or. We fully reconstruct these modes using a measurement of the missing energy and momentum in each event to infer the neutrino momentum. With the B 0 and B + modes combined according to isospin predictions for the relative partial widths, we obtain B 0:5)10 ?4 , where the errors are statistical, systematic and the estimated model-dependence. We also estimate jV ub j 3 CLEO 1] and ARGUS 2] have demonstrated a non-zero value for jV ub j by observing leptons from B-decays at the (4S) with momenta greater than 2.4 GeV/c. This momentum range is accessible to b ! u`` decays, but is rarely reached in the dominant b ! c`` process. The values for jV ub =V cb j of 5 to 10% obtained from these measurements have large theoretical uncertainties because the details of hadronization signiicantly aaect the lepton spectrum near the endpoint. Study of exclusive b ! u`` channels provides an alternate route to jV ub j. Here, the theoretical challenge is to calculate the form factors. For B ! `` and B ! ``, this is an active eld, encompassing relativistic and nonrelativistic quark models, lattice studies, QCD sum rules and dispersion relation studies. Experimentally, there is the CLEO upper limit 3] in the combined modes ? ` + , 0 ` + , and !` +. This Letter presents a study of the decays B 0 ! ? conjugate modes, wherè = e or. The study is based on an (4S) data sample of 2.66 fb ?1 (2:84 10 6 B B pairs) accumulated by the CLEO experiment at the Cornell Electron Storage Ring (CESR). The CLEO detector 4] contains three concentric wire chambers that detect charged particles over 95% of the solid angle. A 1.5 T superconducting solenoid provides the magnetic eld. The total momentum resolution is p=p = 0:6% for a 2 GeV/c particle. A CsI(Tl) electromagnetic calorimeter situated inside the solenoid detects electrons and photons over 98% of 4. A reconstructed 0 mass resolution of 6 MeV is typical. The undetected neutrino complicates the analysis of semileptonic decays. Using the hermeticity of the CLEO detector, we reconstruct the neutrino by inferring its four{