Study of deep inelastic inclusive and diffractive scattering with the ZEUS forward plug calorimeter

Deep inelastic scattering and its diffractive component, ep -> e'y*p -> e'XN, have been studied at HERA with the ZEUS detector using an integrated luminosity of 4.2 pb(-1). The measurement covers a wide range in the y*p c.m. energy W (37-245 GeV), photon virtuality Q(2) (2.2-80 GeV2) and mass M-X (0.28-35 GeV). The diffractive cross section for M-X > 2 GeV rises strongly with W; the rise is steeper with increasing Q(2). The latter observation excludes the description of diffraclive 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 y*p scattering. Above M-X 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, F-2(D(3)) (beta, x(P), Q(2)) of the proton. For fixed beta, the Q(2) dependence of x(P)F2(D)((3)) changes with x(P) in violation of Regge factorisation. For fixed xp, x(P,) x(P)F(2)(D(3)) rises as beta -> 0, the rise accelerating with increasing Q(2). These positive scaling violations suggest substantial contributions of perturbative effects in the diffractive DIS cross section. (c) 2005 Elsevier B.V. All rights reserved.