Prospects for precision YN and n¯ N measurements with an LH 2 /LD 2 target in the BESIII detector

Tagged hyperon and antineutron beams from $J/ψ$ and $ψ(3686)$ decays allow BESIII to study (anti)hyperon--nucleon ($YN$) and antineutron--nucleon (n̄N) scattering, but the present measurements use the beam pipe as the target, whose only free protons are in a thin cooling-oil layer. We present a conceptual feasibility study of a liquid hydrogen (LH₂) or liquid deuterium (LD₂) target between the beam pipe and the Cylindrical Gas Electron Multiplier Inner Tracker. Relative to the beam pipe, a 30~mm LH₂ layer increases the effective free-proton areal density by factors of 7--22 for $Λ$, Σ⁺, $Ξ$, and n beams (3.4 for Ω⁻). A 30~mm LD₂ layer gives 0.9--2.9 times the effective neutron areal density for the same beams (0.4 for Ω⁻) and replaces the composite Be, C, and Au nuclei with the deuteron. These ratios characterize the target itself and do not include channel-dependent efficiencies, backgrounds, or deuteron corrections. In detector simulations without localized support material, the target affects tracking mainly below 0.5~GeV/c; above this momentum, proton, kaon, and pion efficiencies stay within 1.6 percentage points of the baseline. In a signal-only full-event simulation of np→2π⁺π⁻, the LH₂-to-beam-pipe selection-efficiency ratio is 0.86, and the vertex and energy-closure resolutions are comparable to those for beam-pipe interactions. An internal LH₂/LD₂ target is thus a promising concept for higher-statistics and less model-dependent $YN$ and nN measurements at BESIII, requiring further engineering and channel-level validation.