UNDERLYING EVENTS AND SOFT QCD

Abstract

Recent highlights in various soft Quantum Chromodynamics (QCD) phenomena are discussed, focusing on the measurement of the underlying event at 2.76 and 13 TeV with the CMS Collaboration at the Large Hadron Collider (LHC). Such soft QCD phenomena are of huge phenomenological and experimental interest at the LHC as they dominate most of the proton-proton interaction cross-section and contribute to the background of other rare and interesting physics processes. The measurement of the underlying event quantifies some of the main features of soft QCD processes, allowing for improvements in its phenomenological modelling and description, which in turn improve the precision of measurements of standard model processes and searches of new physics. A steep rise of the underlying event activity as a function of the jet transverse momentum is observed up to about 8 GeV, followed by saturation which depends strongly on the centre-of-mass energy.