The Underlying Event in Herwig

Abstract

Multiple partonic interactions (MPI) are vital for a successful description of the underlying event (UE) in hard hadronic collisions and of minimum-bias (MB) data from the Tevatron and the Large Hadron Collider (LHC). A model of independent multiple partonic interactions was first implemented in Pythia [1], where its relevance for a description of hadron collider data was immediately shown. Meanwhile, all major event generators for LHC physics, Herwig [2], Pythia [3, 4] and Sherpa [5], contain MPI models. The core MPI model in Herwig++, which is similar to the Jimmy add-on [6] to the Fortran version of Herwig, was introduced in Ref. [7]. Additional hard parton-parton scatters unitarize the hard jet cross section. Also the jet-like structure of the underlying event is reproduced by this model. With soft components in multiple parton interactions included, which is described in Ref. [8], this model is sufficient to describe the UE data collected at the Tevatron. First MB data from ATLAS [9], however, e.g. the pseudorapidity distribution of charged particles, cannot be reproduced with the core MPI model discussed so far. As shown in Ref. [10], which we summarize in this work, we can significantly improve the description of MB and UE data from the LHC if we include a model for colour reconnections (CR). The idea of CR is based on colour preconfinement [11], which implies that parton jets emerging from different partonic interactions are colour-connected if they overlap in momentum space. As the core MPI model does not take that into account, those colour connections have to be adapted afterwards by means of a CR procedure. The colour connections between partons define colour singlet objects, the clusters. The cluster hadronization model [12], which is implemented in Herwig++, generates hadronic final states based on clusters. Figure 1a shows that in events with multiple parton scatters clusters can be discriminated by the origin of their partonic constituents. We define three classes of clusters. h-type clusters consist of partons generated perturbatively in a single partonic subprocess. The second type of clusters are the subprocesses-interconnecting ones, which