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 GeV(2)) 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.