Measurement of the $B^0\to J/\psi {\pi }^{+}{\pi }^{-}$ Branching Fraction

We present a measurement of the branching fraction for the decay of the neutral $B$ meson into the final state $J/\psi {\pi }^{+}{\pi }^{-}$. The data set contains approximately $56\times {10}^6$ $B\overline{B}$ pairs produced at the ${\rm Y}(4S)$ resonance and recorded with the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ storage ring. The result of this analysis is $\mathcal{B}(B^0\to J/\psi {\pi }^{+}{\pi }^{-})=(4.6\pm 0.7\pm 0.6)\times {10}^{-5}$, where the first error is statistical and the second is systematic. In addition, we measure $\mathcal{B}(B^0\to J/\psi {\rho }^0)=(1.6\pm 0.6\pm 0.4)\times {10}^{-5}$.

Figure 1

Signal for $B^0\to J/\psi {\pi }^{+}{\pi }^{-}$. The upper plot shows the distribution of events in the $\Delta E\mathrm{\text{−}}{m}_{\mathrm{E}\mathrm{S}}$ plane, where the box represents the final selection criteria. The lower plot shows the distribution in $m_{\mathrm{E}\mathrm{S}}$ of events with $|\Delta E|<39\mathrm{M}\mathrm{e}\mathrm{V}$, where the dashed (solid) line corresponds to events in the $K_S^0$ (non-$K_S^0$) region in $M({\pi }^{+}{\pi }^{-})$ ($0.45–0.55\mathrm{G}\mathrm{e}\mathrm{V}/c^2$). The vertical lines represent the final selection.

Figure 2

Distribution of the invariant mass $M({\pi }^{+}{\pi }^{-})$ for events passing all selection criteria. The solid line is the result of the unbinned likelihood fit. The dashed line represents the sum of background and non-${\rho }^0$ signal components. The dotted (dot-dashed) line shows the total (inclusive-$J/\psi$) background. The spike corresponds to $B^0\to J/\psi K_S^0$ events.