Diffractive dissociation in deep inelastic scattering at HERA

Abstract

The diffraction dissociation of virtual photons is considered in the framework of conventional Regge theory. It is shown that the recent HERA data on large rapidity gap events can be successfully described in terms of the Pomeron structure function. Using Regge factorization, the latter can be related to the deuteron structure function. The parameters which relate these two structure functions are determined from soft hadronic diffraction data. The size of the shadowing corrections at low x and large Q 2 is also obtained. LPTHE Orsay 94-42May 1994 + Permanent address : ITEP, B. Cheremushkinskaya 25, Moscow, Russia. * Laboratoire associé au Centre National de la Recherche Scientifique -URA 63. 2Experiments at HERA provide new possibilities to study diffractive processes and the properties of the Pomeron. Recently, spectacular large rapidity gap events in deep inelastic scattering at very small x have been observed by Zeus [1] and H1 [2] collaborations at HERA. On the other hand, diffractive production has been studied for many years in high energy hadronic interactions and a large amount of information on these reactions has been obtained (see for example reviews [3−5] ). A new unexplored region of diffraction production by highly virtual photons is now accessible at HERA.Here we shall consider the process of diffractive dissociation of highly virtual photons in the framework of Reggeon theory, which is the basis for the description of diffractive processes in hadronic interactions. We shall demonstrate that the characteristic features of the data on large rapidity gaps events can be understood using the Regge based description of structure functions, introduced in a previous work [6] , together with our knowledge of diffractive production in high-energy soft hadronic collisions.The amplitude of the single diffractive production of a hadronic state X is described by the Pomeron exchange diagram shown in Fig. 1. The initial particle i in this figure can be either a hadron or a photon (real or virtual). This diagram corresponds to single diffractive production without dissociation of the initial proton, while double diffraction corresponds to dissociation of both projectile and target. At high energies s >> m 2 , when the total rapidity interval ξ = ℓn s m 2 is large, these diagrams describe kinematical configurations of final particles in which large rapidity gaps (∆y) are present.Consider first the single diffraction dissociation of a real particle i (Fig. 1). In the pole approximation for Pomeron exchange the cross section of this process is given by (see e.g. [3] )where s 1 ≡ M 2 is the square of the invariant mass of the hadronic system X, t = (p ′ − p) 2 3 is the invariant momentum transfer to the final proton and g P pp (t) is the vertex describing the Pomeron coupling to a proton. The Feynman-x of the Pomeron is defined by x P = s 1 s , and ξ ′ = ℓn s/s 1 ≈ ∆y. The last term in eq. (1) is the total Pomeron-particle crosssection [7] . It is obtained by summation over all final states X and over thei...

Charm production in deep inelastic and diffractive scattering

“…The scale Q 2 ef f is a priori not known, but it was argued in Ref. [4] that it should be low, since a hard scale from the top part of Fig. 5b does not …”

“…It was emphasized in Ref. [4] that other definitions of the "flux factor" are possible, e.g. differing from the one above by a constant.…”

“…The Reggeon exchange in the upper part of the diagram is dominated by the Pomeron and f Regge poles. Since each term factorizes into a Reggeon propagator and a vertex, it is possible to obtain the structure function of the Pomeron from that of the deuteron [4].…”

“…The values of the parameters mentioned above have been all taken from Ref. [4]. We again use the parametrization at an initial scale of Q 2 0 = 2 GeV 2 as starting point for QCD evolution to the desired Q 2 value.…”

“…The gluonic content of the Pomeron is poorly known. Studies of the Q 2 -dependence of the Pomeron structure function [4][5][6][7][8] lead to the conclusion that the distribution of gluons in the Pomeron is hard and that they carry the main part of the Pomeron momentum. However there are big differences between gluon distribution functions g P (x, Q 2 ) in different models.…”