Central Pseudorapidity Gaps in Events with a Leading Antiproton at the Fermilab Tevatron pbar-p Collider

We report a measurement of the fraction of events with a large pseudorapidity gap $\Delta\eta$ within the pseudorapidity region available to the proton dissociation products $X$ in $\bar p+p\to \bar p+X$. For a final state $\bar p$ of fractional momentum loss $\xi_{\bar p}$ and 4-momentum transfer squared $t_{\bar p}$ within $0.06<\xi_{\bar p}<0.09$ and $|t_{\bar p}|<1.0$ [0.2] GeV$^2$ at $\sqrt s=1800$ [630] GeV, the fraction of events with $\Delta \eta>3$is found to be $0.246\pm 0.001 {\rm (stat)}\pm 0.042 {\rm (syst)}$ [$0.184\pm 0.001 {\rm (stat)}\pm 0.043 {\rm (syst)}$]. Our results are compared with gap fractions measured in minimum bias $\bar pp$ collisions and with theoretical expectations.