at
HERA ?
Instantons, represent a basic
non-perturbative aspect of non-abelian gauge theories like
Quantum
Chromodynamics (QCD), the very successful theory of strong
interactions within the
Standard
Model. About 25 years ago, they were theoretically
discovered and first studied by Belavin et al. and 't Hooft,
respectively. As topologically non-trivial
fluctuations of the gluon fields,
with a typical size of 0.3-0.5 fermi, QCD-instantons are expected to
play an important
role in the transition region between a partonic and a hadronic
description of strong interactions. Yet, despite substantial
theoretical evidence for the importance of instantons in chiral
symmetry breaking and hadron spectroscopy, their direct experimental
verification is lacking until now.
It turns out, however, that a characteristic short distance
manifestation of instantons can be exploited [1]
for an experimental search: Instantons induce certain (hard) processes
that are forbidden
in usual perturbative QCD. These involve all (light) quark flavours
democratically along with a violation of the quark-'handedness'
(chirality), in accord with the general Adler-Bell-Jackiw chiral
anomaly relation.
Deep-inelastic scattering
at HERA
offers a unique opportunity [1] to discover
these hard processes
induced by QCD-instantons through a sizable rate, calculable
within "instanton-perturbation theory" [2,3,4], along with a characteristic final-state signature
[1, 5, 6].
Among the most important features are a ``fireball''-like final state
with a very high number of hadrons, including K-mesons and
Lambda-hyperons, as well as a high total transerse energy.
With the help of the Monte Carlo generator QCDINS for QCD-instanton induced
events in deep-inelastic scattering [6], the
experiments at HERA are actively searching for signatures of instantons
in the hadronic final state [8,9,10,11,12,13,14,15,16].
Clearly, an experimental verification of such a novel,
non-perturbative manifestation of QCD would be of
basic significance...
Recently, the H1 collaboration has reported
preliminary results of a first
dedicated search for instanton-induced events at HERA [11,12,13]. In a phase space region, where a reduction of
the normal deep inelastic
background to the percent level is achieved according to standard
Monte Carlo models, a statistically
significant excess of events was found in the H1 data. While its size
is at a level still comparable to the
differences among standard deep inelastic scattering event generators,
it is qualitatively similar to the
expected instanton signal. The results presented are quite intriguing
and encouraging, although far from
being conclusive. Yet, they strongly enhance the motivation for
looking more closely [7] at some
remaining theoretical issues related to our original predictions. An
encouraging over-all agreement with our original predictions seems to
emerge in Ref. [7].
