Forward jets and large rapidity gaps

Abstract

Hadronic jets are extremely abundant at the LHC, and testing QCD in various corners of phase-space is important to understand backgrounds and some specific signatures of new physics. In this article, various measurements aiming at probing QCD in configurations where the theory modelling becomes challenging are presented. Azimuthal angle de-correlations are sensitive to hard as well as soft QCD emission, and in most of the events jets are produced in a back-to-back configuration. Events where jets have a large rapidity separation are also rare, and those without additional radiation between the jets are exponentially suppressed. The modelling of radiation between very forward and backward jets is complicated, and may require theoretical tools different with respect to those normally used for central, high-[Formula: see text] events. Observables can be constructed that are sensitive to all these effects, such as the study of azimuthal angle de-correlations in events where the two leading jets have large rapidity separations. The two general-purpose detectors of the LHC have measured these observables, and for some of them interesting deviations with respect to the most commonly used theoretical models are observed.