The Underlying Event and the Total Cross Section from Tevatron to the LHC

TL;DR

The paper investigates whether a simple eikonal framework with soft and hard multiparton interactions can simultaneously describe the total proton-proton cross section, jet production cross sections, and the underlying event from Tevatron to LHC. It builds a model with an impact-parameter dependent eikonal chi(b,s), uses an overlap function A(b), and defines sigma_inc_hard and sigma_inc_soft to predict various cross sections and a de-eikonal cross section to enforce consistency with sigma_tot. By scanning parameter space (μ^2, p_t^{min}) under different sigma_tot extrapolations and including saturation effects, it finds that while Tevatron data allow a small region, plausible LHC sigma_tot values leave no allowed region in the simple model, implying the need for extensions such as energy-dependent parameters or different matter distributions for hard vs soft scatters. The authors discuss Tevatron double-parton scattering as supporting evidence for a denser hard-scatter matter distribution and emphasize that upcoming LHC measurements of sigma_tot will be critical to test these ideas and constrain any proposed extensions.

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, we derive constraints on its parameters at the LHC. 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.