Measurement of the underlying event in pp collisions using the ATLAS detector and development of a software suite for Bayesian unfolding

First measurements are made of the underlying event in calorimeter √ jet events at the LHC, using 37 pb$^{−1}$ of pp collisions at $\sqrt{s}$ = 7 TeV, recorded during 2010 by the ATLAS detector. Results are compared for an assumed di-jet topology based on a single identified jet, and an exclusive di-jet requirement. The number of charged particles in the azimuthal region transverse to the jet axis is recorded, as well as their total and average transverse momentum. The total energy carried by all particles — charged and neutral — is also calculated, using the full calorimeter acceptance |η| < 4.8. Distributions are constructed to show the variation of these quantities versus the transverse momentum of the selected jet, over the range 20 − 800 GeV. Additional jets in the transverse region are shown to dramatically influence the measured activity. Software is developed to perform Bayesian iterative unfolding, testing closure of the process and stability with respect to the number of itera- tions performed. Pseudo-experiments are used to propagate systematic errors, and the intrinsic error due to unfolding is estimated. Although the correction relies on a prior probablitity distribution, model-dependence is reduced to an uncertainty comparable to or smaller than experimental systematic errors. The software is used to correct underlying event measurements for effects introduced by the ATLAS detector. Unfolded results are compared to predictions from different Monte Carlo event generators used in LHC analyses, showing general agreement in the range |η| < 2.5, but discrepancies in the forward region. Comparison with other ATLAS results shows compatible behaviour in events defined by any high-momentum charged particle, or by leptonic Z-boson decays.