Measurement of the $\frac{J}{\psi }$ and ${\psi }^{\prime }$ resonance parameters in $\overline{p}p$ annihilation

We report new measurements for the mass, width, and branching ratios for the $\frac{J}{\psi }$ and the ${\psi }^{\prime }$. These charmonium states are formed exclusively in $\overline{p}p$ annihilations at the Fermilab Antiproton Accumulator ring, where stochastically cooled antiprotons are brought into collision with the protons of an internal hydrogen gas jet target. The antiproton energy is precisely controlled and measured allowing an accurate measurement of the resonance parameters. From the shape of the excitation curves we find that the widths of $\frac{J}{\psi }$ and ${\psi }^{\prime }$ are $\Gamma \left(\frac{J}{\psi }\right)=99\mathrm{}\pm 12\pm 6$ keV and $\Gamma \left({\psi }^{\prime }\right)=306\mathrm{}\pm 36\pm 16$ keV, and that the mass of the $\frac{J}{\psi }$ is 3096.87 ± 0.03 MeV/${\mathit{c}}^2$. For the $\frac{J}{\psi }$ we obtain $B(\frac{J}{\psi }\to \overline{p}p)B(\frac{J}{\psi }\to e^{+}{e}^{-})=({1.14}_{-0.12\mathrm{}}^{+0.16\mathrm{}}\pm 0.10)\times {10}^{-4\mathrm{}}$; for the ${\psi }^{\prime }$ we obtain $B({\psi }^{\prime }\to \overline{p}p)\left[B\right({\psi }^{\prime }\to e^{+}{e}^{-})+B({\psi }^{\prime }\to \frac{J}{\psi X}\left)B\right(\frac{J}{\psi }\to e^{+}{e}^{-}\left)\right]=({1.17}_{-0.12\mathrm{}}^{+0.14\mathrm{}}\pm 0.08)\times {10}^{-5\mathrm{}}$.