Proton-antiproton annihilation and meson spectroscopy with the Crystal Barrel

Abstract

This report reviews the achievements of the Crystal Barrel experiment at the Low-Energy Antiproton Ring (LEAR) at CERN. During seven years of operation Crystal Barrel has collected very large statistical samples in $\overline{p}p$ annihilation, especially at rest and with emphasis on final states with high neutral multiplicity. The measured rates for annihilation into various two-body channels and for electromagnetic processes have been used to test simple models for the annihilation mechanism based on the internal quark structure of hadrons. The production of $\ensuremath{\varphi}$ mesons is larger than predicted in several annihilation channels. Important contributions to the spectroscopy of light mesons have been made. The exotic $\ensuremath{\rho}\ifmmode \hat{}\else \^{}\fi{}(1405)$ meson with quantum numbers ${J}^{\mathrm{PC}}{=1}^{\ensuremath{-}+}$ has been observed in its $\ensuremath{\eta}\ensuremath{\pi}$ decay mode. Two ${2}^{\ensuremath{-}+}$ isoscalar mesons ${\ensuremath{\eta}}_{2}(1645)$ and ${\ensuremath{\eta}}_{2}(1870),$ and the ${0}^{\ensuremath{-}+}$ isoscalar meson $\ensuremath{\eta}(1410)$ have been observed in the $\ensuremath{\eta}\ensuremath{\pi}\ensuremath{\pi}$ decay channel. From three-body annihilations three ${0}^{++}$ mesons, ${a}_{0}(1450),$ ${f}_{0}(1370),$ and ${f}_{0}(1500)$ have been established in various decay modes. One of them, ${f}_{0}(1500),$ may be identified with the expected ground-state scalar glueball.