Abstract
Abstract The underlying event activity in proton-proton collisions at forward pseudo-rapidity (−6.6 < η < −5.2) is studied with the CMS detector at the LHC, using a novel observable: the ratio of the forward energy density, dE/dη, for events with a charged-particle jet produced at central pseudorapidity (|η jet| < 2) to the forward energy density for inclusive events. This forward energy density ratio is measured as a function of the central jet transverse momentum, p T, at three different pp centre-of-mass energies ( $ \sqrt{s}=0.9 $ , 2.76, and 7 TeV). In addition, the $ \sqrt{s} $ evolution of the forward energy density is studied in inclusive events and in events with a central jet. The results are compared to those of Monte Carlo event generators for pp collisions and are discussed in terms of the underlying event. Whereas the dependence of the forward energy density ratio on jet p T at each $ \sqrt{s} $ separately can be well reproduced by some models, all models fail to simultaneously describe the increase of the forward energy density with $ \sqrt{s} $ in both inclusive events and in events with a central jet.
Highlights
The underlying event activity is studied at forward pseudorapidity (−6.6 < η < −5.2) in a novel way by measuring the ratio of the forward energy density per unit of pseudorapidity for events with a charged-particle jet produced at central pseudorapidity (|ηjet| < 2) to the forward energy density for inclusive, dominantly nondiffractive, events
Results are obtained for a sample of events dominated by nondiffractive collisions so that the energy density ratios are not biased by rapidity gaps in the CASTOR pseudorapidity range
The results presented in this paper show that the multiple-parton interaction (MPI) model, as implemented in pythia, and tuned to central inclusive and underlying event data, is capable of describing the pT dependence of the forward energy density
Summary
One theoretical framework used to describe the underlying event is the multiple-parton interaction (MPI) model, which assumes that parton interactions occur in addition to the primary hard scattering. In the MPI model as implemented in pythia 6 [23], this correlation is realised by a suppression factor of low-pT parton interactions at small impact parameter. Such central collisions have a larger overlap of the matter distributions of the colliding hadrons and are more likely to have many parton interactions. The comparison of particle and energy densities between events with hard jets in the final state and inclusive events yields information on underlying events with many parton interactions relative to those with few of them. At this centre-of-mass energy, the energy density in the considered pseudorapidity range is sensitive to the details of remnant fragmentation
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