Study of $ \bar{p} $ and $ \bar{n} $ annihilations at LEAR with OBELIX, a large acceptance and high resolution detector based on the Open Axial Field Spectrometer

% PS201 Study of $\bar{p}$ and $\bar{n}$ annihilations at LEAR with OBELIX, a large acceptance and high resolution detector based on the Open Axial Field Spectrometer \\ \\OBELIX is designed to study exclusive final states of antiproton and antineutron annihilations at low energies with protons and nuclei. \\ \\The physics motivations of the experiment are:\\ \\\begin{itemize} \item (gg, ggg), hybrids ($ q \bar{q} g $), multiquarks ($ q q \bar{q} \bar{q} $) and light mesons ($ q \bar{q} $) produced in $ N \bar{N} $ annihilations and study of their spectroscopy and decays. Also broad structures will be searched for by comparing identical decay modes in exclusive final states of the same type occuring from initial states with different angular momentum or isospin. \item Study of the dynamics of $ N \bar{N} $ interactions and of the dependence of the final and intermediate resonant states of annihilation upon the quantum numbers of the initial $ N \bar{N} $ state (angular momentum: S and P-wave in $\bar{p}p $ at rest, isospin, energy). \item Search for $\bar{p}$ annihilations onto more than one nucleon (Pontecorvo reactions). \end{itemize} The experiment studies $\bar{p}$ and $ \bar{n} $ annihilations on H$_{2} $, D$_2 $ and heavier gas targets at rest and in small energy steps near threshold. \\ \\Beams of $\bar{p}{^\prime}s $ from the LEAR slow extraction and of $ \bar{n}'$s produced by charge exchange in a liquid H$_2 $ target upstream of the detector are used. \\ \\A Spiral Projection Chamber (SPC) is used as vertex detector. The magnet and the jet drift chamber of the Open Axial Field Spectrometer (AFS) are used for charged particles momentum and dE/dx measurements. Two concentric arrays of plastic scintillators identify and trigger on charged kaons. Gamma showers are reconstructed in three dimensions with gas sampling calorimeter moduli. \\ \\The detector features : 4$\pi$ acceptance and high segmentation for charged and neutral particles; identification of charged kaons up to 1 GeV/c; good momentum resolution \linebreak [1] ($\sigma _{p}$/p \approx 2\% $ at 1 GeV/c) for charged particles excellent angular resolution ($\Delta