Underlying event characteristics and their dependence on jet size of charged-particle jet events in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mi>p</mml:mi></mml:math>collisions at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msqrt><mml:mo mathvariant="bold" stretchy="false">(</mml:mo></mml:msqrt><mml:mi>s</mml:mi><mml:mo mathvariant="bold" stretchy="false">)</mml:mo><mml:mo mathvariant="bold">=</mml:mo><mml:mn>7</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi></mml:math>with the ATLAS detector

Distributions sensitive to the underlying event are studied in events containing one or more charged-particle jets produced in $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$ with the ATLAS detector at the Large Hadron Collider (LHC). These measurements reflect $800\text{ }\text{ }\ensuremath{\mu}{\mathrm{b}}^{\ensuremath{-}1}$ of data taken during 2010. Jets are reconstructed using the anti-${k}_{t}$ 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 ${p}_{\mathrm{T}}$ are measured as functions of ${p}_{\mathrm{T}}^{\mathrm{jet}}$ in regions transverse to and opposite the leading jet for $4\text{ }\text{ }\mathrm{GeV}<{p}_{\mathrm{T}}^{\mathrm{jet}}<100\text{ }\text{ }\mathrm{GeV}$. 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 ${p}_{\mathrm{T}}^{\mathrm{jet}}$ 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.

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