The underlying event and the total cross section from Tevatron to the LHC

Abstract

Multiple partonic interactions are widely used to simulate the hadronic final state in high energy hadronic collisions, and successfully describe many features of the data. It is important to make maximum use of the available physical constraints on such models, particularly given the large extrapolation from current high energy data to LHC energies. In eikonal models, the rate of multiparton interactions is coupled to the energy dependence of the total cross section. Using a Monte Carlo implementation of such a model, we study the connection between the total cross section, the jet cross section, and the underlying event. By imposing internal consistency on the model and comparing to current data we constrain the allowed range of its parameters. We show that measurements of the total proton-proton cross-section at the LHC are likely to break this internal consistency, and thus to require an extension of the model. Likely such extensions are that hard scatters probe a denser matter distribution inside the proton in impact parameter space than soft scatters, a conclusion also supported by Tevatron data on double-parton scattering, and/or that the basic parameters of the model are energy dependent.

Multiplicity dependence of two-particle azimuthal correlations in pp collisions at the LHC

“…This apparent inconsistency can be resolved by aggregating several quasi independent scatterings in the same pp collision [7,8]. If multiple semi-hard scatterings play a dominant role in the production of high multiplicity events, this should lead to distinct experimentally observable effects.…”

A New Model for Soft Interactions in Herwig

“…A similar model for hard multiple parton interactions was implemented into the newer C++ version of Herwig [4,5]. Besides additional hard interactions, soft interactions were introduced within the so-called hot-spot model [6,7].…”

Colour reconnections in Herwig++

“…Before LHC data was available, the MPI model in HERWIG [24] was tuned by The definition of the rescaled angle, Φ rescaled , along with a detailed description of the analysis can be found in Ref. [52] subdividing the two-dimensional parameter space, spanned by the model's main parameters, the inverse proton radius squared μ 2 and the minimum transverse momentum p min ⊥ , into a grid.…”

“…Good agreement with Tevatron data was found with this model. Soft interactions were added to this model in order to improve consistency with more general theoretical input as the total cross section and the elastic slope parameter in high-energy hadronic collisions [24]. The distribution of transverse momenta in the non-perturbative region below p min ⊥ was modelled similarly to the proposal in [25].…”