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The "Away" Region
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The above figure shows the data on the average number of "away" region charged particles compared with the QCD Monte-Carlo model predictions of HERWIG, ISAJET, and PYTHIA.
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The above figure shows the data the average scalar PTsum of charged particles in the "away" region compared with the QCD Monte-Carlo model predictions of HERWIG, ISAJET, and PYTHIA.
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The "away" region should be a mixture of the "underlying event" and the "away-side" outgoing hard scattering jet. This can be seen in the above figure, where the predictions of ISAJET for the "away" region are divided into three categories: charged particles that arise from the break-up of the beam particles ("beam-beam remnants"), charged particles that arise from initial-state radiation, and charged particles that result from the outgoing jets plus final-state radiation. For ISAJET the "underlying event" plays a more important role in the "away" region than in the "toward" region since the "away-side" outgoing hard scattering jet is sometimes outside the region |eta| < 1. For the "toward" region ISAJET predicts that the contribution from the outgoing jets plus final state-radiation dominates for PTJ1 values above about 5 GeV/c, whereas for the "away" region this does not occur until around 20 GeV/c. Experimentally one cannot say where a given particle comes from. However, we do know the origins of particles generated by the QCD Monte-Carlo models and this figure shows the composition of the "away" region as modelled by ISAJET.
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