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Transverse Momentum Distribution in the "Transverse" Region


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The above figure shows the data on the transverse momentum distribution of charged particles in the "transverse" region , where PT is measured with respect to the beam axis. The points corresponds to the charged particle density dNchg/dPT and the integral of the distribution gives the average number of charged particles in the "transverse" region, <Nchg(transverse)>. Since these distributions fall off sharply as PT increases, it is essentially only the first few points at low PT that determines <Nchg(transverse)>. The approximately constant plateau seen in the plot of <Nchg(transverse)> versus PTJ1 is a result of the low PT points in the above figure not changing much as PTJ1 changes. However, the high PTJ1 points in the above figure do increase considerably as PTJ1 increases. This effect cannot be seen by simply examining the average number of "transverse" particles. The above figure shows the growth of the "hard scattering" component in the "transverse" region (i.e. three or more hard scattering jets).
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For the QCD Monte-Carlo models, at low values of PTJ1, the PT distribution in the "transverse" region is dominated by the "beam-beam remnant" contribution with very little hard scattering. This can be seen in the above figure which shows both the "beam-beam remnant" component and the total prediction of HERWIG for PTJ1 > 2 GeV/c. The difference between the "beam-beam remnant" contribution and the total corresponds to the "hard scattering" component. For the QCD Monte-Carlo models, the PT distribution in the "transverse" region at low values of PTJ1 measures directly the PT distribution of the "beam-beam remnant" component. This figure shows that HERWIG produces a PT distribution that is too steep. It is, of course, understandable that the Monte-Carlo models might be slightly off on the parameterization of the "beam-beam remnants". This component of the Monte-Carlo models cannot be calculated from perturbation theory and must be determined from data. PYTHIA, which includes multiple parton scattering, does a better job at fitting the PT distribution in the "transverse" region.
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The above figure shows both the "beam-beam remnant" component and the total prediction of HERWIG for PTJ1 > 30 GeV/c. Again, the difference between the "beam-beam remnant" contribution and total corresponds to the "hard scattering" component. Here there is a large "hard scattering" contribution corresponding to the production of more than two large PT jets.
(Return to the "Transverse" Region)