Measurement of B→ππlν with full hadronic reconstruction at Belle

This thesis presents an analysis of the exclusive B→π⁺π^-lν_l decay, where l represents an electron or a muon. The analysis uses the complete Υ(4S) data sample collected by the Belle detector, corresponding to 711 fb^-1. The events are selected by fully reconstructing one B meson into hadronic decay modes, consequently the properties of the other B meson, ''the signal B'', can be inferred via conservation laws. The branching ratio is extracted by using an extended binned maximum likelihood method and measured to be B(B⁺→π⁺π^-l⁺ν_l)=(2.24±0.19_stat±0.13_syst) x 10^-4. This result is the first reported measurement of this decay. The dominant resonance in the dipion mass spectrum is the ρ⁰ meson, observed with a branching ratio of B(B⁺→ρ⁰l⁺ν_l)=(1.71±0.16_stat±0.10_syst) x 10^-4. From this result, the magnitude of the CKM matrix element V_ub is obtained as vertical stroke ∣V_ub ∣ =(3.56±0.17_stat±0.10_syst^+0.38_-0.25theo) x 10^-3, in agreement with the most precise measurement of this value from global fits to the B→πlν_l channel. Another resonance is reported with a product of branching ratios, B(B⁺→f₂(1270)l⁺ν) x B(f₂(1270)→π⁺π^-) = (1.8±0.9_stat^+0.2_-0.1syst) x 10^-5, with a 2.2σ statistical significance without taking into account systematic uncertainties.