Proton-antiproton annihilation and meson spectroscopy with the Crystal Barrel

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 $\phi$ mesons is larger than predicted in several annihilation channels. Important contributions to the spectroscopy of light mesons have been made. The exotic $\rho ̂\mathrm{}\left(1405\right)\mathrm{}$ meson with quantum numbers $J^{\mathrm{PC}}{=1\mathrm{}}^{-+}$ has been observed in its $\eta \pi$ decay mode. Two $2^{-+}$ isoscalar mesons ${\eta }_2\mathrm{}\left(1645\right)\mathrm{}$ and ${\eta }_2\mathrm{}\left(1870\right),$ and the $0^{-+}$ isoscalar meson $\eta \mathrm{}\left(1410\right)\mathrm{}$ have been observed in the $\eta \pi \pi$ decay channel. From three-body annihilations three $0^{++}$ mesons, $a_0\mathrm{}\left(1450\right),$ $f_0\mathrm{}\left(1370\right),$ and $f_0\mathrm{}\left(1500\right)\mathrm{}$ have been established in various decay modes. One of them, $f_0\mathrm{}\left(1500\right),$ may be identified with the expected ground-state scalar glueball.