Abstract
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.
Highlights
AND OVERVIEWQuantum chromodynamics (QCD) [1,2] predicts the cross sections for the production of objects with large transverse momentum ðpTÞ [3] in hadronic collisions
The high-pT jet production cross section is calculated [4] by convolving the matrix elements for the scattering of two initial-state partons, with the corresponding parton distribution functions (PDF), to produce a partonic final state
This paper extends previous studies of the underlying event (UE) by measuring the charged-particle multiplicity and transverse momentum density in the transverse and away regions with respect to leading charged-particle jets reconstructed using the anti-kt [12] algorithm and varying the radius parameter, R, of that algorithm between 0.2 and 1.0
Summary
Quantum chromodynamics (QCD) [1,2] predicts the cross sections for the production of objects with large transverse momentum ðpTÞ [3] in hadronic collisions Such calculations are performed by factorizing the interaction into a hard scattering process that can be calculated perturbatively and a set of soft processes that must be described phenomenologically. QCD radiation from the initial- and final-state partons occurs, leading to additional jets and to an increase in the ambient energy These effects vary with the momentum transfer of the hard parton scattering. As long as R remains below a characteristic value determined by the momentum transfer (q2) of the hard scatter, the pT of charged-particle jets formed with larger R are better indicators of the hard scatter energy Such charged-particle jets are reconstructed with high efficiency at low jet transverse momentum, pjTet, and can be used to study the behavior of the UE in the transition region between soft-QCD interactions and hard partonic scattering.
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