Search for $B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu }$ with the BaBar experiment

A search for the rare flavor-changing neutral current process $B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu }$ using data from the BABAR experiment has been performed. A total of $424{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ collision data collected at the center-of-mass energy of the $\mathrm{\Upsilon }(4S)$ resonance is used in this study, corresponding to a sample of $(471\pm 3)\times {10}^6$ $B\overline{B}$ pairs. Signal $B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu }$ candidates are identified by first fully reconstructing a $B^{+}$ decay in one of many possible exclusive decays to hadronic final states, then examining detector activity that is not associated with this reconstructed $B^{+}$ decay for evidence of a signal $B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu }$ decay. The data yield is found to be consistent with the expected background contribution under a null signal hypothesis, resulting in an upper limit of $\mathcal{B}(B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu })<3.0\times {10}^{-5}$ at the 90% confidence level.

Figure 1

Lowest order diagrams of $B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu }$ in the SM. Adapted from Ref. [1].

Figure 2

Output of the $B\overline{B}$ likelihood selector, ${\mathcal{L}}_{B\overline{B}}$, for data (points with error bars) and background MC (stacked, shaded histograms) normalized to the data luminosity, for events with a reconstructed $B_{\mathrm{tag}}$ with $5.27\mathrm{GeV}/c^2<m_{\mathrm{ES}}<5.29\mathrm{GeV}/c^2$. The expected distribution for simulated $B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu }$ events is also shown overlaid for a branching fraction of $0.4\times {10}^{-5}$ (dashed line), with yields per 0.01 given by the $y$ axis on the right-hand side.

Figure 3

The $m_{\mathrm{ES}}$ distribution for data (points with error bars) and background MC (stacked, shaded histograms) normalized to the data luminosity, for events which satisfy the continuum suppression criterion ${\mathcal{L}}_{B\overline{B}}>0.35$. The expected distribution for simulated $B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu }$ events is also shown overlaid for a branching fraction of $0.4\times {10}^{-5}$ (dashed line), with event yields per $2\mathrm{MeV}/c^2$ given by the $y$ axis on the right-hand side.

Figure 4

Distribution of $E_{\text{extra}}$, calculated in the c.m. frame, in data and MC before (top) and after (bottom) application of the MC correction procedure for events with a reconstructed $B_{\mathrm{tag}}$ with $m_{\mathrm{ES}}$ within the signal region. In the upper plot, data are shown as points with error bars, and background MC is shown as stacked, shaded histograms. The expected distribution for simulated $B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu }$ events is shown overlaid for a branching fraction of $0.4\times {10}^{-5}$ (dashed line), with event yields per $0.05\mathrm{GeV}$ given by the $y$ axis on the right-hand side. In the lower plot, the shaded region is the sideband data scaled by $R_{\mathrm{side}}$, and the unshaded region is the $m_{\mathrm{ES}}$ peaking component of the $B^{+}{B}^{-}$ MC scaled by $C_{\mathrm{peak}}$.

Figure 5

The $p{\pi }^{-}$ invariant mass in events with a reconstructed $B_{\mathrm{tag}}$ with $m_{\mathrm{ES}}$ within the signal region, with three charged tracks satisfying the (anti)proton selection and DOCA requirements. Data are shown as points with error bars, the shaded region is the sideband data scaled by $R_{\mathrm{side}}$, and the unshaded region is the $m_{\mathrm{ES}}$ peaking component of the $B^{+}{B}^{-}$ MC scaled by $C_{\mathrm{peak}}$.

Figure 6

The $B_{\mathrm{tag}}{m}_{\mathrm{ES}}$ distribution of events passing all other signal selection requirements for data and for signal MC (inset) scaled to a branching fraction of $\mathcal{B}(B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu })=0.4\times {10}^{-5}$. The signal region is indicated by the vertical dashed lines, and the total background expected in the signal region is $2.3\pm 0.7(\mathrm{stat})\pm 0.6(\mathrm{sys})$ events.

Figure 7

The $p{\pi }^{-}$ invariant mass in events passing all other signal selection requirements. Data are shown as points with error bars, while the background expectation is shown as solid histograms. The negative bin values are a consequence of the background estimation procedure applied to low-statistics histograms. The expected signal distribution from MC is shown in the inset histogram and is scaled to a branching fraction of $\mathcal{B}(B^{-}\to \mathrm{\Lambda }\overline{p}\nu \overline{\nu })=0.4\times {10}^{-5}$.