Measurement of the Underlying Event Activity in Proton-Proton Collisions at 0.9 TeV

TL;DR

This paper measures the underlying event activity in proton-proton collisions at 0.9 TeV using CMS data, comparing results to QCD-inspired models and highlighting discrepancies in charged hadron production predictions.

Contribution

It provides the first detailed measurement of underlying event activity at 0.9 TeV and evaluates the accuracy of PYTHIA models in describing this activity.

Findings

Models underestimate charged hadron production at eta < 2 and p_T > 0.5 GeV/c.

Data shows higher activity than predicted by models in the transverse region.

Results help improve QCD models for better simulation of proton-proton collisions.

Abstract

A measurement of the underlying activity in scattering processes with transverse momentum scale in the GeV region is performed in proton-proton collisions at sqrt(s) = 0.9 TeV, using data collected by the CMS experiment at the LHC. Charged hadron production is studied with reference to the direction of a leading object, either a charged particle or a set of charged particles forming a jet. Predictions of several QCD-inspired models as implemented in PYTHIA are compared, after full detector simulation, to the data. The models generally predict too little production of charged hadrons with pseudorapidity eta < 2, p_T > 0.5 GeV/c, and azimuthal direction transverse to that of the leading object.