At this workshop, CDF presented preliminary results on the measurement of events where two scattering processes occur in the same event [20]. For distinct processes A and B, the cross section for this ``double parton (DP) scattering" is given by , where is a process independent parameter.
Events were selected with a relatively low transverse energy (ET> 16 GeV) ``photon" trigger in conjunction with three jets with ET> 5 GeV. The separation of DP events from the underlying QCD background is determined by studying variables sensitive to decorrelation effects. In particular, the azimuthal angle between the two best-balancing pairs (``photon"+jet versus dijet) is approximately flat for the DP signal and enables a statistical separation of events. A new feature of this analysis is that events with displaced vertices, where the jets are reconstructed from separate origins, are used to evaluate directly and hence reduce the theoretical uncertainties. This allows the first relatively precise determination of the effective cross section:
No x-dependence is observed, within the uncertainty of 20%. Assuming a uniformly dense ball of partons and using the measured inelastic cross section, one expects 11 mb. The measurement represents a milestone in the study of multiple interactions and provides the first significant experimental constraint on such processes.Multiple interactions, where two or more partons interact in the same event, represent a considerable uncertainty in the analysis of photoproduction events at HERA. In particular, the extraction of the gluon content of the photon at relatively low requires careful modelling of these interactions, since they can contribute up to 50% of the cross section at the relatively low ET values (ET>6 GeV) measured so far. The Tevatron result should aid in a realistic estimate of the uncertainties due to multiple interactions in the extraction of the gluon density of the photon at HERA. Similarly, such measurements improve background estimates to di-boson and boson+jet production at the Tevatron as well as the predictions of jet rates from multiple interactions at the LHC.
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