**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].

- A. Ringwald and F. Schrempp,

*Towards the Phenomenology of QCD-Instanton Induced Particle Production at HERA,*

*hep-ph/9411217*, in:*Quarks '94*, Proc. 8th Int. Seminar, Vladimir, Russia, May 11-18, 1994,

ed. D. Grigoriev*et al.*(World Scientific, Singapore 1995) pp. 170-193 - S. Moch, A. Ringwald and F. Schrempp,

*Instantons in Deep-Inelastic Scattering - The Simplest Process,*

DESY 96-202;*hep-ph/9609445*;**Nucl.Phys. B507****(1997) 134** - A. Ringwald and F. Schrempp,

*Instanton-Induced Cross Sections in Deep-Inelastic Scattering,*

DESY 98-081;*hep-ph/9806528*;**Phys. Lett. B438 (1998) 217** - A. Ringwald and F. Schrempp,

*Confronting Instanton Perturbation Theory with QCD Lattice Results,*

DESY 98-201;*hep-lat/9903039*;**Phys. Lett. B459 (1999) 249** - T. Carli, J. Gerigk, A. Ringwald and F. Schrempp,

*QCD Instanton-induced Processes in Deep-Inelastic Scattering --*

Search Strategies and Model Dependencies,

DESY 99-067; MPI-PhE/99-02;*hep-ph/9906441*

Proc.*Monte Carlo Generators for HERA Physics*;**DESY-PROC-1999-02**, p.329 - A. Ringwald and F. Schrempp,

*QCDINS 2.0 - A Monte Carlo Generator for Instanton-Induced Processes*

in Deep-Inelastic Scattering,

DESY 99-180;*hep-ph/9911516;***Comput. Phys. Commun. 132 (2000) 267** - A. Ringwald and F. Schrempp,

*Zooming-in on Instantons at HERA,*

*hep-ph/0012241*;**Phys. Lett. B503 (2001) 331**

- S. Aid
*et al.,*H1 Collaboration,

*Strangeness Production in Deep-Inelastic Positron-Proton Scattering at HERA,*

*hep-ex/9607010*;**Nucl. Phys. B480 (1996) 3** - S. Aid
*et al.,*H1 Collaboration,

*Charged Particle Multiplicities in Deep Inelastic Scattering at HERA,*

*hep-ex/9608011*;**Z. Phys. C72 (1996) 573** - T. Carli and M. Kuhlen,

*Bounds on QCD Instantons from HERA,*

DESY 97-151;*hep-ex/9708008*;**Nucl.Phys. B511 (1998) 85** - S. Mikocki for the H1-Collaboration,

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

talk at DIS2000, Liverpool/GB, April 25-30, 2000 - H1-Collaboration,

*Search for QCD Instanton Induced Events in Deep-Inelastic Scattering at HERA*,

submitted to the 30th Int. Conf. on High-Energy Physics, ICHEP200, Osaka, Japan, July 2000 - E.A. De Wolf for the H1-Collaboration,

*Search for QCD-instantons at HERA*,

talk at the 30th Int. Conf. on High-Energy Physics, ICHEP200, Osaka, Japan, July 2000,

to be published in the Proceedings. - B. Koblitz for the H1-Collaboration,

*A Search for Instantons at HERA,*

*hep-ex/0110042*, in:*New Trends in HERA Physics 2001*, Proc. Ringberg Workshop, Tegernsee, Germany, 17 - 22 June 2001 - H1-Collaboration, C. Adloff
*et al.*

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

Eur. Phys. J.**C 25**(2002) 495-509. - The ZEUS-Collaboration,

*Search for QCD-Instanton Induced Events in Deep Inelastic ep Scattering at HERA*,

Eur. Phys. J.**C 34**(2004) 255-265.

- A. Ringwald and F. Schrempp,

*Instanton Phenomenology at HERA*,

DESY 97-115;*hep-ph/9706399*, in: Proc. 5th Int. Workshop on*Deep Inelastic Scattering and QCD*(DIS 97), Chicago, p. 781 (Eds. J. Repond, D. Krakauer) - S. Moch, A. Ringwald and F. Schrempp,

*QCD Instantons in e+- P Scattering,*

DESY 97-114;*hep-ph/9706400*, in: Proc. 5th Int. Workshop on*Deep Inelastic Scattering and QCD*(DIS 97), Chicago, p. 1007 (Eds. J. Repond, D. Krakauer) - A. Ringwald and F. Schrempp,

*QCD Instantons at HERA,*

DESY 98-061;*hep-ph/9805492*, in: Proc. 6th Int. Workshop on*Deep Inelastic Scattering and QCD*(DIS 98), Brussels, p. 537 (Eds. Gh. Coremans, R. Rosen) - A. Ringwald and F. Schrempp,

*Searching for QCD Instantons at HERA,*Talk given at International Euroconference on Quantum Chromodynamics (QCD 98), Montpellier, France, 2-8 Jul 1998,

DESY 98-114;*hep-ph/9808422*,**Nucl. Phys. B (Proc. Suppl.) 74 (1999) 376** - A. Ringwald and F. Schrempp,

*Instanton Searches at HERA,*Talk given at 10th International Seminar on High-Energy Physics (Quarks 98), Suzdal, Russia, 18-24 May 1998,

DESY 98-115;*hep-ph/9808482*, to be published in the Proceedings - A. Ringwald and F. Schrempp,

*QCD-Instantons at HERA -- An Introduction,*Plenary talk given at the 3rd UK Phenomenology Workshop on HERA Physics, Sep 20-25, 1998, Durham/UK,

DESY 98-200;*hep-ph/9812359*,**J. Phys. G: Nucl. Part. Phys.****25**(1999) 1297 - A. Ringwald and F. Schrempp,

*Instantons in the QCD Vacuum and in Deep Inelastic Scattering.*

Talk presented at the 7th International Workshop on Deep Inelastic Scattering and QCD (DIS 99), Zeuthen/Germany, April 19-23, 1999,

DESY 99-062;*hep-ph/9905384*,**Nucl. Phys. B (Proc. Suppl.) 79 (1999) 447** - A. Ringwald and F. Schrempp,

*Theory and Phenomenology of Instantons at HERA,*

DESY 99-136; hep-ph/9909338, in:*New Trends in HERA Physics 1999*, Proc. Ringberg Workshop, Tegernsee, Germany, 30 May - 4 June, 1999, pp. 203-217 - A. Ringwald and F. Schrempp,

*Instantons in Deep-Inelastic Scattering,*

*hep-ph/0006215*, in:*DIS 2000,*Liverpool, UK, April 25-30, 2000 - G.W. Buschhorn,

*Search for QCD-instanton-induced effects in deep inelastic electron proton scattering at HERA,*

*hep-ph/0009064*, in:*New Trends in High Energy Physics*, Crimea, May 27 - June 4, 2000 - G.W. Buschhorn,

*Probing QCD-instanton induced effects in deep inelastic scattering at HERA,*

*hep-ph/0104177*, in:*CICHEP 2001*, Cairo, Egypt - G. Wolf,

*Deep Inelastic Scattering at Large Energy and Momentum Transfers:*

Recent Results from the HERA Collider,

*hep-ex/0105055*, in:*IX Mexican School on Particles and Fields*, Metepec, Puebla, Mexico, 9 - 19 Aug 2000, and:*International School of Subnuclear Physics*, Erice, Italy, 27 Aug - 5 Sep 2000

- F. Schrempp,

*Tracking QCD-Instantons.*

Talk presented at the Ringberg Workshop on*New Trends in HERA Physics 2001*, Ringberg Castle, Tegernsee, Germany, 17-22 June 2001,

*DESY-01-125; hep-ph/0109032*, to be published in the Proceedings (IoP). - T. Carli,

*Small-x QCD Effects in Particle Collisions at High Energies.*

Invited talk at the XX. Int. Sym. on Lepton and Photon Interactions at High Energies, Rome, Italy, July 2001; hep-ph/0111232, to be published in the Proceedings.

Last modified: Jun 3rd, 2004

Fridger Schrempp (fridger.schrempp@desy.de)