We report the first observation of dijet production by Double Pomeron Exchange
(DPE) in pbar-p collisions at sqrt(s)=1.8 TeV using the Collider Detector at 
Fermilab (CDF). 

In DPE events, large rapidity gaps, defined as a region of rapidity or 
pseudorapidity devoid of particles, are left in both forward directions. In a 
sample of events containing a leading antiproton in diffractive pbar-p 
interactions at sqrt(s)=1.8 TeV, we select events with at least 2 jets 
with transverse energy E_T>7 GeV in the kinematical region 0.035<xi_pbar<0.095
and |t_pbar|<1.0 GeV^2 where xi_pbar is the fractional momentum loss of the 
pbar and t_pbar is the four momentum transfer squared at the pbar vertex. 

In this sample of single diffractive (SD) dijet events, we observe DPE dijet 
events as an excess of events with a rapidity gap in the forward 
pseudorapidity region 2.4<eta<5.9 on the outgoing p direction. Using the 
information on particles detected with the CDF detector, we estimate the 
fractional momentum loss of the p to be 0.01<xi_p<0.03. We measure the cross 
sections for non-diffractive (ND), single diffractive, and DPE dijet 
productions. 

In comparison of DPE dijets with SD and ND dijets, we find that the DPE dijet 
is characterized by the E_T spectrum being similar to that of SD dijet and 
being slightly softer than that of ND dijet. Also, the DPE dijet is 
kinematically produced toward the outgoing pbar direction, and is more 
back-to-back than SD and ND dijets.

The cross section ratio R_DPE/SD of DPE to SD dijet production is compared 
between the data and the predictions of the Monte Carlo simulations based on 
the Ingelman-Schlein model with the Donnachie-Landshoff pomeron flux factor. 
We find the presence of a disagreement of R_DPE/SD ratio between the data and 
the simulations.

The dijet mass fraction, defined as the mass of dijets divided by the total
mass of the central system, is evaluated based on the measurements of xi for 
the p and pbar. From the observed data distribution, we find that the 
contribution of non-factorizable DPE events, in which only the two jets are 
produced in the central system, is small in our kinematical range.