Run Ii Diffractive Measurements at Cdf

Diffractive events in p̄p collisions are characterized by the presence of a leading proton or antiproton which remains intact, and/or a rapidity gap, defined as a pseudorapidity region devoid of particles. Diffractive events involving hard processes (“hard diffraction”), such as production of high ET jets (see Fig. 1), have been studied extensively to understand the nature of the exchanged object, the Pomeron, which in QCD is a color singlet entity with vacuum quantum numbers. One of the most interesting questions in hard diffractive processes is whether or not they obey QCD factorization, in other words, whether the Pomeron has a universal, process independent, parton distribution function (PDF). Results on diffractive deep inelastic scattering (DDIS) from the ep collider HERA show that QCD factorization holds in DDIS. However, single diffractive (SD) rates of W -boson [1], dijet [2], b-quark [3] and J/ψ [4] production relative to non-diffractive ones measured at CDF are O(10) lower than expectations from PDFs determined at HERA, indicating a severe breakdown of factorization in hard diffraction between Tevatron and HERA. The suppression factor at the Tevatron relative to HERA is approximately equal in magnitude to that measured in soft diffraction cross sections relative to Regge The pseudorapidity η of a particle is defined as η ≡ − ln(tan θ/2), where θ is the polar angle of the particle with respect to the proton beam direction.