Underlying event characteristics and their dependence on jet size of charged-particle jet events in pp collisions at sqrt[(]s)=7  TeV with the ATLAS detector

Distributions sensitive to the underlying event are studied in events containing one or more chargedparticle
jets produced in pp collisions at
ffiffiffi s p ¼ 7 TeV with the ATLAS detector at the Large Hadron
Collider (LHC). These measurements reflect 800 b1 of data taken during 2010. Jets are reconstructed
using the anti-kt algorithm with radius parameter R varying between 0.2 and 1.0. Distributions of the
charged-particle multiplicity, the scalar sum of the transverse momentum of charged particles, and the
average charged-particle pT are measured as functions of pjet
T in regions transverse to and opposite
the leading jet for 4 GeVT < 100 GeV. In addition, the R dependence of the mean values of these
observables is studied. In the transverse region, both the multiplicity and the scalar sum of the transverse
momentum at fixed pjet
T vary significantly with R, while the average charged-particle transverse
momentum has a minimal dependence on R. Predictions from several Monte Carlo tunes have been
compared to the data; the predictions from Pythia 6, based on tunes that have been determined using LHC
data, show reasonable agreement with the data, including the dependence on R. Comparisons with other
generators indicate that additional tuning of soft-QCD parameters is necessary for these generators.
The measurements presented here provide a testing ground for further development of the Monte Carlo
models.