STUDY OF DEEP INELASTIC INCLUSIVE AND DIFFRACTIVE SCATTERING WITH THE ZEUS FORWARD PLUG CALORIMETER

Deep inelastic scattering and its diffractive component, ep→e′γ∗p→e′XNep→e′γ∗p→e′XN, have been studied at HERA with the ZEUS detector using an integrated luminosity of 4.2 pb−14.2 pb−1. The measurement covers a wide range in the γ∗pγ∗p c.m. energy W (37–245 GeV), photon virtuality Q 2 (2.2–80 GeV2) and mass MXMX (0.28–35 GeV). The diffractive cross section for MX>2 GeVMX>2 GeV rises strongly with W ; the rise is steeper with increasing Q 2. The latter observation excludes the description of diffractive deep inelastic scattering in terms of the exchange of a single pomeron. The ratio of diffractive to total cross section is constant as a function of W , in contradiction to the expectation of Regge phenomenology combined with a naive extension of the optical theorem to γ∗pγ∗p scattering. Above MXMX of 8 GeV, the ratio is flat with Q 2, indicating a leading-twist behaviour of the diffractive cross section. The data are also presented in terms of the diffractive structure function, View the MathML sourceF2D(3)(β,xP,Q2), of the proton. For fixed β , the Q 2 dependence of View the MathML sourcexPF2D(3) changes with xPxP in violation of Regge factorisation. For fixed xPxP, View the MathML sourcexPF2D(3) rises as β→0β→0, the rise accelerating with increasing Q2. These positive scaling violations suggest substantial contributions of perturbative effects in the diffractive DIS cross section.