Improved limits on $B^0$ decays to invisible $(+\gamma )$ final states

We establish improved upper limits on branching fractions for $B^0$ decays to final states where the decay products are purely invisible (i.e., no observable final state particles) and for final states where the only visible product is a photon. Within the Standard Model, these decays have branching fractions that are below the current experimental sensitivity, but various models of physics beyond the Standard Model predict significant contributions for these channels. Using $471\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance by the BABAR experiment at the PEP-II $e^{+}{e}^{-}$ storage ring at the SLAC National Accelerator Laboratory, we establish upper limits at the 90% confidence level of $2.4\times {10}^{-5}$ for the branching fraction of $B^0\to \mathrm{\text{invisible}}$ and $1.7\times {10}^{-5}$ for the branching fraction of $B^0\to \mathrm{\text{invisible}}+\gamma$.

Figure 1

Distributions of the NN output for simulated $B^0\to \mathrm{\text{invisible}}$ events with a $D$ meson on the tag side. The black solid line is the signal while the red dashed line is the background. The solid gray vertical line defines the NN output signal region.

Figure 2

Results of the maximum likelihood fit of $E_{\mathrm{extra}}$ for $B^0\to \mathrm{\text{invisible}}$ (left) and $B^0\to \mathrm{\text{invisible}}+\gamma$ (right).