Soft Colour Interactions and the Diffractive Structure Function

We discuss our model for soft colour interactions and present results on the diffractive structure function F D 2 (β, x IP , Q 2) and inclusive transverse energy flows, which agree with available HERA data. The observation of rapidity gap events at HERA [1, 2] has led to a renewed interest in diffractive phenomena and how they can be understood within QCD. In [3] we presented a new model for diffractive hard scattering based on a mechanism for soft colour interactions (SCI). The starting point is the normal DIS parton level interactions, with perturbative QCD corrections based on standard first order matrix elements and conventional leading log parton showers for higher order parton emission. This gives a state of colour-ordered partons that will have further non-perturbative interactions to produce the final hadron state. The novel feature in our model is the introduction [3] of random soft colour interactions corresponding to non-perturbative gluon exchange between these partons. This changes the colour structure and thereby affects the hadronic final state when a conventional hadronisation model such as the Lund string model [4] is applied. Rapidity gaps may then arise when a gap at the parton level is not spanned by a string. In particular, when the hard process starts with a gluon from the proton, leaving a colour octet remnant and a colour octet hard scattering system, a soft colour exchange between the two octet systems can give two colour singlets separated in rapidity [3]. Large forward rapidity gaps are here favoured by the large momentum of the remnant, in particular at small-x. Our model [5] is implemented in the Monte Carlo (MC) Lepto 6.4 producing complete events. To compare our model with experimental data on rapidity gap events we consider the diffrac-tive structure function F D 2 (β, x IP , Q 2) defined by [6] dσ D dβdx IP dQ 2 = 4πα 2 βQ 4 1 − y + y 2 2 F D 2 (β, x IP , Q 2) (1) (i.e. assuming single photon exchange and neglecting F L). This inclusive quantity contains the dependence on the main variables β ≃ Q 2 /(Q 2 + M 2 X), x IP ≃ (Q 2 + M 2 X)/(Q 2 + W 2) and the usual DIS momentum transfer squared Q 2. The acceptance corrected data from H1 [2] are compared in Fig. 1 to our model …