  Distributions sensitive to the underlying event are studied in events
containing one or more charged-particle jets produced in pp collisions at
sqrt(s) = 7 TeV with the ATLAS detector at the Large Hadron Collider (LHC).
These measurements reflect 800 inverse microbarns of data taken during 2010.
Jets are reconstructed using the antikt 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 pT^JET in
regions transverse to and opposite the leading jet for 4 GeV < pT^JET < 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 pT^JET 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.
