Observation of Diffractive $\mathit{J}/\mathit{\psi }$ Production at the Fermilab Tevatron

We report the first observation of diffractive $J/\psi (\to {\mu }^{+}{\mu }^{-})$ production in $\overline{p}p$ collisions at $\sqrt{s}=1.8\mathrm{}\mathrm{TeV}$. Diffractive events are identified by their rapidity gap signature. In a sample of events with two muons of transverse momentum $p_T^{\mu }>2\mathrm{}\mathrm{GeV}/c$ within the pseudorapidity region $|\eta |<1.0\mathrm{}$, the ratio of diffractive to total $J/\psi$ production rates is found to be $R_{J/\psi }=[1.45\pm 0.25]\%$. The ratio $R_{J/\psi }\left(x\right)$ is presented as a function of $x$-Bjorken. By combining it with our previously measured corresponding ratio $R_{\mathrm{jj}}\left(x\right)$ for diffractive dijet production, we extract a value of $0.59\pm 0.15$ for the gluon fraction of the diffractive structure function of the proton.