We present an analysis of the exclusive B+→π+π-ℓ+νℓ decay, where ℓ represents an electron or a muon, with the assumption of charge-conjugation symmetry and lepton universality. The study exploits the full ϒ(4S) data sample collected by the Belle detector, corresponding to 711 fb-1 of integrated luminosity. Events are selected by fully reconstructing one B meson in hadronic decay modes, subsequently determining the properties of the other B meson. We extract the signal yields using a binned maximum-likelihood fit to the missing-mass squared distribution in bins of the invariant mass of the two pions or the momentum transfer squared. We measure a total branching fraction of B(B+→π+π-ℓ+νℓ)=[22.7-1.6+1.9(stat)±3.5(syst)]×10-5, where the uncertainties are statistical and systematic, respectively. This result is the first reported measurement of this decay.
Beleño, C., Frey, A., Adachi, I., Aihara, H., Asner, D. M., Atmacan, H., Aushev, T., Ayad, R., Behera, P., Bennett, J., Bernlochner, F., Bhardwaj, V., Bilka, T., Biswal, J., Bonvicini, G., Bozek, A., Bračko, M., Browder, T. E., Campajola, M., … Belle Collaboration. (2021). Measurement of the branching fraction of the decay B + → π + π − ℓ + ν ℓ in fully reconstructed events at Belle. Physical Review D, 103(11), 112001. https://doi.org/10.1103/PhysRevD.103.112001