Measurement of the Branching Fraction of the Decay $\boldsymbol{B^{+}\to\pi^{+}\pi^{-}\ell^{+}\nu_\ell}$ in Fully Reconstructed Events at Belle

TL;DR

This paper reports the first measurement of the branching fraction of the rare decay B+ to pi+ pi- lepton+ neutrino using the full Belle dataset, employing full reconstruction and maximum-likelihood fitting techniques.

Contribution

It provides the first experimental measurement of the B+ to pi+ pi- lepton+ neutrino decay branching fraction using a comprehensive analysis of Belle data.

Findings

Measured branching fraction: (22.7 +1.9/-1.6 (stat) ± 3.5 (syst)) x 10^-5

First reported measurement of this decay mode

Utilized full Belle dataset with advanced reconstruction and fitting methods

Abstract

We present an analysis of the exclusive $B^{+}\to\pi^{+}\pi^{-}\ell^{+}\nu_{\ell}$ decay, where $\ell$ represents an electron or a muon, with the assumption of charge-conjugation symmetry and lepton universality. The analysis uses the full $\Upsilon(4S)$ data sample collected by the Belle detector, corresponding to 711 fb$^{-1}$ of integrated luminosity. We select the events 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 ${{\cal B}(B^{+}\to \pi^{+}\pi^{-}\ell^{+}\nu_{\ell})= [22.7 ^{+1.9}_{-1.6} (\mathrm{stat}) \pm 3.5(\mathrm{syst}) ]\times 10^{-5}}$, where the uncertainties are statistical and systematic, respectively. This result is the first reported measurement of this decay.