Diffractive and Low-X Physics at ATLAS

A brief overview of soft di ractive physics from colour singlet exchange at ATLAS is presented. The use of an exclusive single-sided trigger and of searches for large rapidity gaps in the nal state are shown to be e ective methods for studying the di ractive component of the total inelastic cross section in high energy pp collisions. Large rapidity gaps are spoiled by soft gluon emission from multi parton interactions, it is shown that the probability MPI in soft di ractive interactions is very small due to the characteristic large impact parameter in such events. On the scale of pertubative QCD, the study of a jet veto in the ra- pidity interval bounded by a dijet system is presented. Colour singlet exchange is expected to become increasingly important in the joint limit that the rapidity separation of the bounding jets is large and that the mean transverse momentum of the dijet system is large, compared to the jet veto scale in the rapidity interval between them. In this regime the disagreement between data and theory is most pronounced.