Illustration of a DD event in which a Pomeron is
exchanged in a pbar-p collision at center-of-mass energy
sqrt(s), producing diffractive masses M_1 and M_2
separated by a rapidity gap of width eta_max-eta_min.
The double-diffractive cross section is measured for pbar p interactions which produce a central rapidity gap (overlapping eta=0) with width Delta eta > 3 at sqrt(s)=1800 and 630 GeV. Comparisons are made to predictions from Regge theory based on the triple Pomeron amplitude and factorization and to previous measurements.
Double diffraction (DD) occurs when some object with the quantum numbers of the
vacuum is exchanged between the proton and antiproton, causing the incident
particles to break up (dissociate) into diffractive mass clusters separated
by a region empty of particles, a rapidity gap.
Hard double diffraction, so-called jet-gap-jet events, has been the subject
of studies at the Tevatron and HERA.
In this analysis, we look for events with a central rapidity gap (overlapping
eta=0). We do this as opposed to looking for the largest rapidity gap
in an event because the latter method would be more likely to be biased by
detector effects such as cracks or inefficiencies in the calorimeter.
In the method we choose, we look for the first particle (track or hit
calorimeter tower) on each side of eta=0, and define
(eta_max) eta_min as the
eta of the first particle in the (anti)proton direction. The width of the
rapidity gap is then Delta eta = eta_max - eta_min.
Note that we require the Beam-Beam Counters (BBC's) (3.2<|eta|<5.9)
to be hit on both sides
of the detector. If there are no particles found in the detector with
|eta|<3.2, then |eta_max(min)| is defined to be 3.3,
an overflow bin.
Soft double diffraction has been studied previously at the ISR and by
UA5,
and recently in photoproduction events by H1 at HERA.
Data, non-diffractive (ND), single- (SD) and
double-diffractive (DD) Monte-Carlo generated events.
The MC distributions are normalized as described in the next plot.
Note that the ND and SD events alone cannot account for the events with
large |eta_max(min)| observed in the data. Clearly, there is a DD
signal.
eta_max vs -eta_min at 1800 GeV
eta_max vs -eta_min at 630 GeV
The number of events as a function of
Delta eta^0 = eta_max - eta_min
(for events with a gap overlapping eta=0, this is essentially the gap width;
note however that included in the bins for Delta eta^0 > 3.2
are events in which only the BBC is hit on one side of the detector and
|eta_max(min)| is defined to be 3.3),
for data, and for double-diffractive (DD) + non-DD and only non-DD
Monte Carlo (MC) generated events. The normalization of the non-DD and DD MC
distributions were fit given a fixed single-diffractive contribution
(from known SD cross sections and MC trigger efficiencies) of ~3%.
Delta eta^0 at 1800 GeV
Delta eta^0 at 630 GeV
Preliminary DD cross sections for central rapidity gaps (overlapping eta=0) of width Delta eta > 3 are:
sigma_DD(1800 GeV, Delta eta^0 > 3) = 5.45 +/- 0.02(stat) ^{+1.10}_{-1.17}(syst) mb
sigma_DD( 630 GeV, Delta eta^0 > 3) = 4.57 +/- 0.02(stat) ^{+0.99}_{-1.19}(syst) mb
and for all gaps of width Delta eta > 3 are:
sigma_DD(1800 GeV, Delta eta > 3) = 7.78 +/- 0.03(stat) ^{+1.57}_{-1.67}(syst) mb
sigma_DD( 630 GeV, Delta eta > 3) = 6.12 +/- 0.02(stat) ^{+1.33}_{-1.59}(syst) mb
We extrapolate from central to all gaps using the differential cross section
from Regge theory based on the triple Pomeron amplitude and factorization
in terms of gap width Delta_eta,
center of gap eta_0 and four-momentum transfer squared t of
(d^3sigma_DD / dDelta_eta deta_0 dt) ~ exp(2 epsilon Delta_eta + b t)
where b=2 alpha' Delta_eta and epsilon=0.104, alpha'=0.25 GeV^-2.
In the figure below, we adjust the UA5 cross sections to use this expression
for b rather than b=7 GeV^-2.
DD cross sections for all gaps > 3 vs sqrt(s)
Mary Convery,
Dino Goulianos
The Rockefeller University
Date blessed: 6/2/99, 9/24/99, 7/14/00
last updated 7/31/00
convery@rock16.rockefeller.edu