A Search for Instantons at HERA

TL;DR

This paper reports a search for QCD instanton-induced events in deep-inelastic scattering at HERA, finding more events than expected but with uncertainties preventing a definitive discovery, and discusses future prospects in higher Q^2 regions.

Contribution

It introduces a new method for searching for QCD instantons in DIS data and evaluates the potential for future searches at higher Q^2 where predictions are more reliable.

Findings

More events than expected by standard models are observed.

Systematic uncertainties hinder instanton discovery.

Future searches could be more promising at higher Q^2.

Abstract

A search for QCD instanton (I) induced events in deep-inelastic scattering (DIS) at HERA is presented in the kinematic range of low x and low Q^2. After cutting into three characteristic variables for I-induced events yielding a maximum suppression of standard DIS background to the 0.1% level while still preserving 10% of the I-induced events, 549 data events are found while 363^{+22}_{-26} (CDM) and 435^{+36}_{-22} (MEPS) standard DIS events are expected. More events than expected by the standard DIS Monte Carlo models are found in the data. However, the systematic uncertainty between the two different models is of the order of the expected signal, so that a discovery of instantons can not be claimed. An outlook is given on the prospect to search for QCD instanton events using a discriminant based on range searching in the kinematical region Q^2\gtrsim100\GeV^2 where the I-theory makes safer predictions and the QCD Monte Carlos are expected to better describe the inclusive data.