QCD Instantons at HERA.pdf

a r X i v : h e p - p h / 9 7 0 9 2 0 6 v 1 1 S e p 1 9 9 7 DESY 97-165 ISSN 0418-9833 MPI-PhE/97-21 hep-ph/9709206 August 1997 QCD INSTANTONS AT HERA? MICHAEL KUHLEN Max-Planck-Institut fu?r Physik, Werner-Heisenberg-Institut Fo?hringer Ring 6, D-80805 Mu?nchen, Germany E-mail: kuhlen@desy.de ABSTRACT Phenomenological aspects of chirality violating processes induced by QCD in- stantons in deep inelastic scattering are discussed. First instanton searches and the prospects for their experimental discovery at HERA are presented. 1. Introduction Since a long time it has been recognized that the standard model contains pro- cesses which cannot be described by perturbation theory, and which violate classical conservation laws like baryon number (B) and lepton number (L) in the case of the electroweak interaction, and chirality (Q5) in the case of the strong interaction 1. Such anomalous processes are induced by so-called instantons2. The name indicates that these are non-perturbative fluctuations that are confined to “an instant” in space- time, with no corresponding free particle solutions for t → ±∞. The interest in instantons remained somewhat academic, as observable effects were predicted to ex- ist only at extremely high energies of O(105 TeV), until it was discovered that their exponential suppression is much reduced by the emission of gauge bosons3. In elec- troweak theory with massive gauge bosons still rather high energies of O( 10 TeV) would be required, but not so in QCD with massless gluons and strong coupling. Instanton effects could play a ro?le in QCD reactions already at present day collid- ers. Deep inelastic ep scattering at HERA is particularly interesting, because the virtuality of the photon probe Q2 provides a hard scale for the instanton subprocess, which is needed for theoretically sound predictions4,5,6. Instanton effects have not yet been observed in nature. Their experimental discovery would be of fundamental significance for particle phys