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

Abstract

Deep inelastic scattering and its diffractive component, e p → e ′ γ ∗ p → e ′ X N , 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 γ ∗ 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 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 ∗ 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 ) ( β , x P , Q 2 ) , of the proton. For fixed β, the Q 2 dependence of x P F 2 D ( 3 ) changes with x P in violation of Regge factorisation. For fixed x P , x P F 2 D ( 3 ) rises as β → 0 , the rise accelerating with increasing Q 2. These positive scaling violations suggest substantial contributions of perturbative effects in the diffractive DIS cross section.