them depends on the absence of a soft interaction between the spectator parts of the proton and antiproton, the so-called gap survival probability\[36\]. This lack of factorization makes it difficult to make a precise comparison between theory and experiment.

There is new data from DØ\[9,10\] and an interesting new analysis comparing the 1800 GeV data with that at 630 GeV. If $f_{gap}$ is the fraction of all two-jet events (separated by a given rapidity) with a gap between them then DØfinds that

$${f_{gap}(630)\over f_{gap}(1800)} = 2.6 \pm 0.6_{stat.}\eqno(4.2)$$

for $\Delta Y \geq 3.8$. Thus the gap fraction $decreases$ with increasing energy. While this number cannot be directly compared to BFKL dynamics because $\Delta Y$ has been taken to be the same at the two energies, while a BFKL test should have $\Delta Y(1800)-\Delta Y(630) = \ell n {1800\over
630},$ it does suggest that the survival probability has a rather strong energy dependence making BFKL tests more difficult in rapidity gap events. It will be interesting to see whether models of the gap survival probability can easily accomodate the energy dependence in Eq.(4.2)\[38\].

At Fermilab the gap fraction is typically 0.01 while at HERA more like 0.07. The gap survival probability 