Underlying Event measurements in pp collisions at $\sqrt{s}$ = 0.9 and 7 TeV with the ALICE experiment at the LHC

TL;DR

This paper measures Underlying Event activity in proton-proton collisions at 0.9 and 7 TeV using the ALICE detector, revealing increased activity at higher energies and comparing results to various models which underestimate the observed multiplicities.

Contribution

First detailed measurement of Underlying Event observables in pp collisions at multiple energies with the ALICE experiment, providing data for model validation.

Findings

Underlying Event activity increases with collision energy.

Models underestimate multiplicity and summed pT by 10-30%.

Transverse region is most sensitive to Underlying Event activity.

Abstract

We present measurements of Underlying Event observables in pp collisions at $\sqrt{s}$ = 0.9 and 7 TeV. The analysis is performed as a function of the highest charged-particle transverse momentum $p_{\rm T, LT}$ in the event. Different regions are defined with respect to the azimuthal direction of the leading (highest transverse momentum) track: Toward, Transverse and Away. The Toward and Away regions collect the fragmentation products of the hardest partonic interaction. The Transverse region is expected to be most sensitive to the Underlying Event activity. The study is performed with charged particles above three different $p_{\rm T}$ thresholds: 0.15, 0.5 and 1.0 GeV/$c$. In the Transverse region we observe an increase in the multiplicity of a factor 2-3 between the lower and higher collision energies, depending on the track $p){\rm T}$ threshold considered. Data are compared to Pythia 6.4, Pythia 8.1 and Phojet. On average, all models considered underestimate the multiplicity and summed $p_{\rm T}$ in the Transverse region by about 10-30%.