Towards the Theory of Diffractive DIS∗

The large rapidity gap events, observed at HERA, have changed considerably our physical picture of deep inelastic scattering during the past years. We review the present theoretical understanding of diffractive DIS with emphasis on the close relation to inclusive DIS. This includes success and limitations of the leading twist description, the connection between diffractive and inclusive parton distributions in the semiclassical approach, the colour structure of the proton and comparison with data. The progress report concludes with a list of open questions. ∗Presented at New Trends in HERA Physics 1999, Ringberg Workshop, June 1999 1 Inclusive and diffractive DIS The intriguing phenomenon of the frequent appearance of large rapidity gaps in electron proton collisions at HERA [1] has changed our physical picture of deep inelastic scattering (DIS) to a large extent. The large rapidity gap events are very difficult to understand in the parton model where the struck quark is expected to break up the proton leading to a continuous flow of hadrons between the current jet and the proton remnant. To develop a physical picture of diffractive DIS [2] it is convenient to view the scattering process in the proton rest frame. In this frame the virtual photon fluctuates into partonic states qq̄, qq̄g, ... which then scatter off the proton. From the leading twist contributions to inclusive and diffractive structure functions one obtains the parton distribution functions in a frame where the proton moves fast. This connection holds for diffractive as well as non-diffractive processes. In the following we shall review the present status of our theoretical understanding of diffractive DIS with emphasis on the close analogy to inclusive DIS. This is appropriate since diffractive DIS is dominated by the leading twist contribution, which has been one of the most surprising aspects of the large rapidity gap events. The scattering of the partonic fluctuations of the photon off the proton will be treated in the semiclassical approach. After a comparison of theoretical predictions with data we shall conclude with a discussion of some open questions. Inclusive DIS Inclusive deep inelastic scattering [3] is characterized by the kinematic variables Q = −q2 , W 2 = (q + P ) , x = Q 2 Q2 + W 2 , (1) where q and P are the momenta of the virtual photon and the proton, respectively. The cross section is determined by the hadronic tensor,