Measurement of the Underlying Event Activity at the LHC with sqrt(s)= 7 TeV and Comparison with sqrt(s) = 0.9 TeV

TL;DR

This study measures the underlying event activity in proton-proton collisions at 0.9 and 7 TeV using CMS data, revealing increased activity with energy and comparing results with QCD-inspired models.

Contribution

It provides the first detailed comparison of underlying event activity at different energies using CMS data and evaluates the accuracy of PYTHIA models.

Findings

Average multiplicity and scalar-pT sum increase with leading track-jet pT.

Activity roughly doubles when energy increases from 0.9 to 7 TeV for high pT.

QCD-inspired models show varying agreement with the measured data.

Abstract

A measurement of the underlying activity in scattering processes with a hard scale in the several GeV region is performed in proton-proton collisions at sqrt(s) = 0.9 and 7 TeV, using data collected by the CMS experiment at the LHC. The production of charged particles with pseudorapidity |eta| < 2 and transverse momentum pT > 0.5 GeV/c is studied in the azimuthal region transverse to that of the leading set of charged particles forming a track-jet. A significant growth of the average multiplicity and scalar-pT sum of the particles in the transverse region is observed with increasing pT of the leading track-jet, followed by a much slower rise above a few GeV/c. For track-jet pT larger than a few GeV/c, the activity in the transverse region is approximately doubled with a centre-of-mass energy increase from 0.9 to 7 TeV. Predictions of several QCD-inspired models as implemented in PYTHIA are compared to the data.