Single diffraction (SD) in pbar-p collisions is characterized by a leading nucleon, which remains intact, and/or a ``rapidity gap'', defined as a region of pseudo-rapidity devoid of particles. In this process, a colorless object with vacuum quantum numbers, generally referred to as ``Pomeron'' (IP), is emitted by one nucleon and interacts with the other producing a cluster of particles, X:

 pbar + p  --->  [pbar' + IP] + p  --->  pbar' + X

In Double Pomeron Exchange (DPE), a Pomeron is emitted from each nucleon and a IP-IP collision occurs producing a system X in the central rapidity region:
 pbar + p  --->  [pbar' + IP] + [p' + IP]  --->  pbar' + X + p' 
The characteristic signature of this this type of an event is a leading nucleon and/or a rapidity gap on both forward regions. 

In Run I, we studied dijet production in DPE events using a ``Roman Pot'' (RP) inclusive trigger that tagged events with a leading antiproton in the RP detectors located along the pbar direction about 57 m downstream from the interaction region. We found that the DPE dijets have similar kinematic properties to those of the SD dijets, but the number of observed DPE events (~100) was too small for detailed studies (Run I PRL). In Run II we have obtained so far about two orders of magnitude more data. These data allow us to study DPE dijet events in detail, and in particularly to study ``exclusive dijet production in DPE'', 

 pbar + p  --->  [pbar' + IP] + [p' + IP]  --->  pbar' + jet + jet + p'

which has been a subject of great interest in the diffractive physics community. This interest stems from the desire to use the cross section for exclusive dijet events to calibrate calculations for diffractive Higgs production. 

Top:
Corrected (left) and uncorrected (right) E_T distributions of the leading jet for events with at least one jet in the RP inclusive trigger sample. The dashed histograms show the distributions for events with the RP+Jet5 trigger bit fired. 

Bottom:
Trigger efficiencies obtained by dividing the dashed histogram by the solid histogram. The arrows point to the curves at efficiencies of 50% and 90%.

All 4 plots combined 
[EPS][GIF]

MP hit multiplicity versus BSC hit counter multiplicity on the pbar (p) side in the RP+Jet5 data sample is shown on the left (right). 

Events in the SD (ND) dominated region 0.01<xi_pbar^X<0.1 (0.3<xi_pbar^X<3.2) are shown on the top (bottom).

All 4 plots combined 
[EPS][GIF]

Left:
Leading (top) and second leading (bottom) jet E_T distributions in DPE, SD, and ND events. 

Right:
Mean pseudo-rapidity (top) and azimuthal angle difference (bottom) of the leading two jets in DPE, SD, and ND events. 

E_T of second to leading jet is required to be >10 GeV in all plots.

Invariant mass of the two leading jets with jet energy corrections in DPE, SD and ND events.

E_T of second leading jet is required to be >10 GeV.

[EPS][GIF]

Number of events as a function of dijet mass fraction in DPE (open circle: 5.5<eta_gap<7.5, filled circle: 3.6<eta_gap<7.5) and SD (triangle) events. 

Dijet mass fraction is defined as dijet mass measured in the cones of R=0.7 divided by the system mass.

The number of events in vertical axis is corrected for effective prescale used in the triggers. Both DPE and SD data samples correspond to 26 pb-1 of data. 

Error bars on the points are statistical. Systematic uncertainty shown on the distribution of DPE events with a gap of 3.6<eta_gap<7.5 is due to calorimeteter energy scale.

E_T of second leading jet is required to be >10 GeV.

[EPS][GIF]

Number of events as a function of dijet mass fraction in DPE events with a gap of 3.6<eta_gap<7.5.

The filled (open) points show the distribution for events with leading jet E_T greater than 10 (25) GeV. 

E_T cut of second to leading jet is not used.

[EPS][GIF]

Calorimeter-Tower E_T in eta-phi space (LEGO) and COT-Track with calorimeter hit information.

Top: 
Run 151920 Event 78696
Et_jet1(2) = 33.4(31.5) GeV (after energy corrected), 
NO hits in BSC or MP on both sides,
xi_pbar(p) = 0.030(0.037), 
Mjj(uncorrected) = 52.9 GeV, Rjj = 0.81

Bottom: 
Run 152581 Event 899811
Et_jet1(2) = 36.2(33.3) GeV (after energy corrected), 
NO hits in BSC or MP on both sides,
xi_pbar(p) = 0.053(0.087), 
Mjj(uncorrected) = 47.4 GeV, Rjj = 0.36