Search for QCD-instanton induced events in deep inelastic ep scattering at HERA

TL;DR

This paper reports a search for QCD-instanton-induced events in deep inelastic ep scattering at HERA, setting an upper limit on the instanton cross section that exceeds theoretical expectations.

Contribution

First experimental search for QCD-instanton events in ep scattering at HERA, establishing a new upper limit on the instanton cross section.

Findings

Upper limit on instanton cross section: 26 pb

Data analyzed with QCDINS Monte Carlo model

No significant excess observed over background

Abstract

A search for QCD-instanton-induced events in deep inelastic ep scattering has been performed with the ZEUS detector at the HERA collider, using data corresponding to an integrated luminosity of 38 pb^{-1}. A kinematic range defined by cuts on the photon virtuality, Q^2 > 120 GeV^2, and on the Bjorken scaling variable, x > 10^{-3}, has been investigated. The QCD-instanton induced events were modelled by the Monte Carlo generator QCDINS. A background-independent, conservative 95% confidence level upper limit for the instanton cross section of 26 pb is obtained, to be compared with the theoretically expected value of 8.9 pb.