Transverse Energy Flow in the Underlying Event in the Proton-Proton Collisions at √s = 7 TeV with the ATLAS Calorimeter

Abstract

The European Organization for Nuclear Research (CERN) operates the world's largest and highest energy proton-proton collider at a center of mass energy of √s = 7 TeV, the Large Hadron Collider (LHC). ATLAS is one of the four detectors operating at the LHC. The Underlying Event (UE), which is an unavoidable background at any hadron collider, includes particles from various sources generated in each proton collision. The particle flow in the underlying event is characterized by low transverse energies reflecting the long range character of the individual processes generating them. This regime cannot be described by the usual perturbative models provided by Quantum Chromodynamics (QCD), the theory of the strong force. To model this flow, phenomenological models have to be applied, as provided by Monte Carlo simulations. In this work I define new discriminating variables to constrain these UE models in the new kinematical regime available at LHC. Using calorimeter data from the ATLAS experiment, several Monte Carlo models are tested by comparing the data to these predictions for three different final states (minimum bias, di-jet and direct photon production). The experimental data are fully unfolded to the hadron level within the full acceptance of the ATLAS detector, thus for the first time including the forward direction in hadron collisions. The final results are presented in the context of previous measurements on the characteristics of the strong force in the proton, in deep inelastic lepton-proton scattering.