CMS measures the ‘underlying event’ in pp collisions

Ever since the earliest experiments with hadron beams, and subsequently during the era of the hadron colliders heralded by CERN’s Intersecting Storage Rings, it has been clear that hadron collisions are highly complicated processes. Indeed, initially it was far from obvious whether it would be possible to do any detailed studies of elementary particle physics with hadron collisions at all.

The question was whether the physics of “interesting” particle production could be distinguished from that of the “background” contribution in hadron collisions. While the former is typically a single parton–parton scattering process at very high transverse momentum (p_T), the latter consists of the remnants of the two protons that did not participate in the hard scatter, including the products of any additional soft, multiple-parton interactions. Present in every proton–proton (pp) collision, this soft-physics component is referred to as the “underlying event”, and its understanding is a crucial factor in increasing the precision of physics measurements at high p_T. Now, the CMS collaboration has released its latest analysis of the underlying event data at 2.76 TeV at the LHC.

The measurement builds on experimental techniques that have been developed at Fermilab’s Tevatron and previously at the LHC to perform measurements that are sensitive to the physics of the underlying event. The main idea is to measure particle production in the region of phase space orthogonal to the high-p_T process – that is, in the transverse plane. In its latest analysis of the underlying event data at 2.76 TeV, CMS has measured both the average charged-particle multiplicity as well as the p_T sum for the charged particles. The scale of the hard parton–parton scattering is defined by the p_T of the most energetic jet of the event.

The measurements are expected to result in more accurate simulations of pp collisions at the LHC. Because the properties of the underlying event cannot be derived from first principles in QCD, Monte Carlo generators employ phenomenological models with several free parameters that need to be “tuned” to reproduce experimental measurements such as the current one from CMS.

An important part of the studies concerns the evolution of the underlying-event properties with collision energy. CMS has therefore presented measurements at centre-of-mass energies of 0.9, 2.76 and 7 TeV. Soon, there will be new data from Run 2 at the LHC. The centre-of-mass energy of 13 TeV will necessitate further measurements, and provide an opportunity to probe the ever-present underlying event in uncharted territory.