Search for QCD Instanton-Induced Processes in Deep-Inelastic Scattering at HERA

TL;DR

This study searches for QCD instanton-induced processes in deep-inelastic scattering at HERA, finding no significant excess over standard model predictions and setting upper limits on the instanton cross-section.

Contribution

First experimental search for QCD instanton processes in DIS at HERA, providing upper limits and comparing results with theoretical predictions and lattice simulations.

Findings

No significant instanton signal observed.

Upper limits on instanton cross-section between 60pb and 1000pb.

Results consistent with non-perturbative lattice QCD simulations.

Abstract

Signals of QCD instanton-induced processes are searched for in deep-inelastic scattering (DIS) at the electron-proton collider HERA in a kinematic region defined by the Bjorken-scaling variables x > 10^(-3), 0.1 < y < 0.6 and photon virtualities 10 <= Q^2 < 100GeV^2. Several observables characterising hadronic final state properties of QCD instanton-induced events are exploited to identify a potentially instanton-enriched domain. While an excess of events with instanton-like topology over the expectation of the standard DIS background is observed it can not be claimed to be significant given the uncertainty of the simulation. Upper limits on the cross-section for instanton-induced processes of between 60pb and 1000pb are set dependent on the kinematic domain considered. The data do not exclude the cross-section predicted by instanton perturbation theory for small instanton sizes. At large instanton sizes a naive extrapolation of instanton perturbation theory yields a cross-section in the range of sensitivity of this study. Such a cross-section is not observed, in agreement with non-perturbative lattice simulations of the QCD vacuum.