Measurement of the Diffractive Structure Function          
          of the Antiproton in Proton-Antiproton Collisions          
                  at $\sqrt{s}=1800$ and $630$ GeV                   

                         Kenichi Hatakeyama                          

Abstract:

A measurement of the effective diffractive structure function
$F^D_{jj}$ of the antiproton obtained from a study of single
diffractive dijet events produced in association with a leading
antiproton in $p\bar p$ collisions at the center-of-mass energy
$\sqrt{s}=1800$ and $630$ GeV is presented. Inclusive samples of
single diffractive events were collected during the Tevatron collider
run of 1995$-$1996 using the Collider Detector at Fermilab (CDF) by
triggering on a leading antiproton detected in a forward Roman Pot
spectrometer. From these samples, single diffractive dijet subsamples
were selected by requiring two or more jets with transverse energy
$E_T\ge7$ GeV in an event.

From the dijet data samples, an effective leading order diffractive
structure function $F^D_{jj}$ of the antiproton is extracted. In the
kinematic region of antiproton fractional momentum loss
$0.035\le\xi\le0.095$, four-momentum transfer squared
$|t|\le1.0$ GeV$^2$ and $\beta=x_{\bar p}/\xi<0.5$, where
$x_{\bar p}$ is the Bjorken scaling variable of the struck parton in
the antiproton, $F^D_{jj}(\beta,\xi)$ is found to have the form
$F^D_{jj}(\beta,\xi) \propto \beta^{-1.0\pm0.1}\xi^{-0.9\pm0.1}$
at $\sqrt{s}=1800$ GeV.

To address the question of QCD factorization in diffraction processes,
i.e. universality of the diffractive structure function, several
comparisons are made on the measured $F^D_{jj}$. In comparisons with
expectations based on results obtained in diffractive deep inelastic
scattering experiments at HERA, $F^D_{jj}$ measured in this analysis
is found to be smaller by approximately an order of magnitude,
indicating a breakdown of QCD factorization in diffraction processes.
In comparisons with results obtained in $p\bar p$ collisions at
$\sqrt{s}=630$ GeV, the ratio in normalization of the measured
$F^D_{jj}$ at the two energies is found to be 
$R_{\frac{630}{1800}}=1.3\pm0.2(\mbox{stat}){}^{+0.4}_{-0.3}(\mbox{syst})$.
This is compatible with the factorization expectation of unity, but is
also in agreement within errors with predictions in the range
1.6$-$1.8 from phenomenological models that explain the suppression of
the diffractive structure function in $p\bar p$ collisions relative to
that in $\gamma^* p$ collisions.

Comparisons with results from a study of dijet events by double
pomeron exchange and single diffractive events containing a $J/\psi$
meson by the CDF collaboration, and with results from a study of
single diffractive dijet events obtained by the UA8 collaboration in
$p\bar p$ collisions at $\sqrt{s}=630$ GeV are also presented.


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