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Search for QCD Instanton Induced Events in Deep-Inelastic Scattering at HERA |
| Reference |
Stanislav Mikocki, talk presented at DIS 2000; E. de Wolf, talk presented at ICHEP 2000, April 2000 |
| Document |
Conference paper
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| Contact |
Tancrdedi Carli |
| Abstract |
Signals of QCD instanton induced processes are searched for in deep-inelastic scattering (DIS) at HERA in a kinematic region defined by the Bjorken-scaling variables $x > 10^{-3}$, $0.1 < y < 0.6$ and the polar angle of the scattered positron $ heta_{el} > 156^o$. The cross section for QCD instanton induced processes in this domain is predicted to be sizeable, but two to three orders of magnitude below the standard DIS cross section. Several observables characterising hadronic final state properties of QCD instanton induced events are used to discriminate them from the standard DIS background and to identify a possibly instanton enriched domain. In a phase space region where a reduction of the standard DIS background to the percent level is achieved according to Monte Carlo models, more events are found in the data than expected by standard DIS models. The size of the excess is at a level comparable to the difference of the QCD-based standard DIS models, but is - for the discriminating observables - qualitatively similar to the expected instanton signal. For other discriminating observables the data do not particularly favour an interpretation of the excess in terms of instanton induced processes as modelled in the current implementation of the QCD instanton Monte Carlo simulation. Upper limits on the QCD instanton production cross section between $100$ \pb and $1000$ \pb are derived only from the expected instanton hadronic final state topology and the number of observed events in the data. They exclude the steep rise of the instanton cross section towards large instanton sizes as would be obtained from a naive extrapolation of instanton perturbation theory. The absense of this steep rise is in agreement with comparisons of perturbative calculations to lattice simulations. |
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