2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/37209We present a preliminary measurement of the branching fraction for the decay of the neutral $B$ meson into the final state J/$ψπ^+π^-$. The data set contains approximately 56 million $B\bar{B}$ pairs produced at the $Υ(4S)$ resonance and recorded by the BaBar detector at the PEP-II $e^+e^-$ collider. The result of this analysis is $\mathcal{B}(B^0\to J/ψπ^+π^-$) = (5.0 $\pm$ 0.7 $\pm$ 0.6)$\times 10^{-5}$, where the first error is statistical and the second is systematic.16 pages, 1 postscript figure, presented at the XXXVIIth Rencontres de Moriond on QCD and Hadronic InteractionsHigh Energy Physics - Experimenttext