  The $LQ\bar D$ operator in R-parity-violating supersymmetry can lead to meson
decays to light neutralinos and neutralino decays to lighter mesons, with a
long lifetime. Since the high-luminosity LHC is expected to accumulate as much
as 3/ab of data, several detectors proposed to be built at the LHC may probe
unexplored regions in the parameter space, for long-lived neutralinos. We
estimate the sensitivity of the recently proposed detectors, CODEX-b, FASER,
and MATHUSLA, for detecting such light neutralinos singly produced from $D$-
and $B$-meson decays in a list of benchmark scenarios, and discuss the
advantages and disadvantages of the proposed detectors in this context. We also
present our results in a model independent fashion, which can be applied to any
long-lived particle with mass in the GeV regime.
