  A new dark sector anti-baryon, denoted $\psi_{\text{D}}$, could be produced
in decays of $B$ mesons. This letter presents a search for $B^{+} \rightarrow
\psi_{\text{D}} + \text{p}$ (and the charge conjugate) decays in $e^{+}e^{-}$
annihilations at 10.58 GeV, using data collected in the ~\babar~ experiment.
Data corresponding to an integrated luminosity of 398 fb$^{-1}$ are analyzed.
No evidence for a signal is observed. Branching fraction upper limits in the
range from $10^{-7} $ -- $ 10^{-5}$ are obtained at 90$\%$ confidence level for
masses of 1.0 $< m_{\psi_{\text{D}}} < $ 4.3 GeV/c$^{2}$. The result is also
reinterpreted to provide the first limits on a supersymmetric model with
R-parity violation and a light neutralino.
