Prospects for Precision $YN$ and $\bar{n}N$ Measurements with an LH${}_2$/LD${}_2$ Target in the BESIII Detector

Abstract

Tagged hyperon and antineutron beams from $J/\psi$ and $\psi (3686)$ decays allow BESIII to study (anti)hyperon--nucleon ($YN$) and antineutron--nucleon ($\bar{n}N$) 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 (LH${}_2$) or liquid deuterium (LD${}_2$) target between the beam pipe and the Cylindrical Gas Electron Multiplier Inner Tracker. Relative to the beam pipe, a 30~mm LH${}_2$ layer increases the effective free-proton areal density by factors of 7--22 for $\Lambda$, ${\Sigma }^{+}$, $\Xi$, and $\bar{n}$ beams (3.4 for ${\Omega }^{-}$). A 30~mm LD${}_2$ layer gives 0.9--2.9 times the effective neutron areal density for the same beams (0.4 for ${\Omega }^{-}$) 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 $\bar{n}p\to 2{\pi }^{+}{\pi }^{-}$, the LH${}_2$-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 LH${}_2$/LD${}_2$ target is thus a promising concept for higher-statistics and less model-dependent $YN$ and $\bar{n}N$ measurements at BESIII, requiring further engineering and channel-level validation.