A QCD analysis of diffractive deep-inelastic scattering data

We perform a novel type of analysis of diffractive deep-inelastic scattering data, in which the input parton distributions of the Pomeron are parameterised using the perturbative QCD expressions. In particular, we treat individually the components of the Pomeron of different size. We are able to describe simultaneously both the recent ZEUS and H1 diffractive data. In addition to the usual two-gluon model for the perturbative Pomeron, we allow for the possibility that it may be made from two sea quarks. A notable feature of deep-inelastic scattering is the existence of diffractive events, γp → Xp, in which the slightly deflected proton and the cluster X of outgoing hadrons are well-separated in rapidity. The large rapidity gap is believed to be associated with Pomeron exchange. The diffractive events make up an appreciable fraction of all (inclusive) deep-inelastic events, γp → X. We will refer to the diffractive and inclusive processes as DDIS and DIS respectively. Here we perform a perturbative QCD analysis of the new high precision DDIS data, recently obtained by the ZEUS [1, 2] and H1 [3] Collaborations at HERA. The analysis is novel in that it treats individually the components of the Pomeron of different transverse size. The description of the DDIS data is based on a purely perturbative QCD framework. We take input forms of the parton distributions of the Pomeron given by the calculation of the lowest-order QCD diagrams for γp → Xp [4]. In previous analyses, the Pomeron was treated as a hadron-like object of more or less fixed size. However, the microscopic structure of the Pomeron is different to that of a hadron. In perturbative QCD, it is known that the Pomeron singularity is not an isolated pole, but a branch cut, in the complex angular momentum plane [5]. The pole singularity corresponds to a single particle, whereas a branch cut may be regarded as a continuum series of poles. That is, the Pomeron wave function consists of a continuous number of components. Each component i has its own size, 1/μi. The QCD DGLAP evolution of a component should start from its own scale μi, provided that μi is large enough for the perturbative evolution to be valid. Therefore, the expression for the diffractive structure function F 2 contains an integral