The unfinished picture of low-energy antineutron interactions: open issues and hints for future research possibilities

TL;DR

The paper surveys open questions in low-energy antineutron interactions, emphasizing isospin selection, Coulomb-free dynamics, and the discrepancies between data and models for $ar{n}N$ and $ar{n}$-induced nuclear interactions. It highlights experimental foundations from LEAR/OBELIX, identifies key puzzles such as annihilation cross sections on nuclei and differential CEX measurements, and proposes a path forward via antineutron beams at CERN's AD. The suggested program includes targeted cross-section measurements, elastic channels, and meson-spectroscopy opportunities to probe strangeness dynamics, baryonium searches, and gluon-rich spectroscopy, with ultracold-neutron approaches for oscillation tests. If realized, slow extraction of $ar{p}$ beams and dedicated $ar{n}$ experiments at AD could yield high-statistics datasets to resolve longstanding ambiguities and explore new physics near thresholds.

Abstract

This report examines the open questions that remain unsolved following the measurements with antineutrons ($\bar n$) as probes conducted up to the 1990s at the LEAR facility at CERN. It also presents suggestions for possible new experiments at a future, upgraded AD complex, which can potentially provide access to new areas of physics.

Figures (18)

• Figure 1: Sketch (not to scale) of the experimental $\bar{n}$ beam arrangement of the OBELIX experiment at LEAR. From Ref. re:agnello.
• Figure 2: Experimental momentum spectrum of antineutrons produced via CEX reaction in the OBELIX $\bar{n}$ facility. From Ref. re:physrep. The plot on the left is a raw spectrum, including random coincidences without background subtraction, while on the right the true spectrum is depicted after the corrections for apparatus resolution.
• Figure 3: Comparison of $\overline{\mathcal{N}}\mathcal{N}\;$ total cross section at low momenta. The open triangles are from Ref. re:iazzi, the full points are from Ref. re:armstrong. The solid line, whose extrapolation to lower momenta is shown by the dashed part, is from Ref. re:bugg. Figure from Ref. re:physrep.
• Figure 4: Fit of $\sigma_{ann}(\bar{n}p)$ measured by OBELIX up to 400 MeV/$c$ including $S$- (solid line), $P$- (dotted line) and $D$- (dashed line at the bottom of the picture) wave contribution. From Ref. re:bertin.
• Figure 5: Three-dimensional plot of the measured $\sigma_{ann}(p_{\bar{n}};\; A)$, as a function of the antineutron momentum and the atomic number. From Ref. re:astrua.