Instanton-induced Processes in High-Energy Scattering


[Publications | Experimental Searches | Reviews | Transparencies | Monte Carlo Generator]
Over the last decades we have witnessed the remarkable success of the Standard Model of electroweak (Quantum Flavor Dynamics (QFD)) and strong (QCD) interactions. This success is largely based on the possibility to apply ordinary perturbation theory to the calculation of hard, short-distance dominated scattering processes, since the relevant gauge couplings are small.

There are certain processes, however, which fundamentally can not be described by ordinary perturbation theory, no matter how small the gauge coupling is. These processes are associated with axial anomalies and manifest themselves as anomalous violation of baryon plus lepton number (B+L) in QFD and chirality in QCD. They are induced by topological fluctuations of the non-Abelian gauge fields, notably by instantons.

A number of non-perturbative issues in the Standard Model can be understood in terms of such topological fluctuations and the associated anomalous processes. On the one hand, QCD instantons seem to play an important role in various long-distance aspects of QCD, such as providing a possible solution to the axial U(1) problem or being at work in chiral symmetry breaking. In QFD, on the other hand, similar topological fluctuations of the gauge fields and the associated B+L violating processes are very important at high temperatures and have therefore a crucial impact on the evolution of the baryon and lepton asymmetries of the universe.

A related question is whether manifestations of such topological fluctuations may be directly observed in high-energy scattering processes. It has been raised originally in the late eighties in the context of QFD [1]. But, despite considerable theoretical [2,4,7] and phenomenological [3,5,6,8,] efforts, the actual size of the cross-sections in the relevant, tens of TeV energy regime is still not unanimously established. Meanwhile, the focus switched to quite analogous QCD instanton-induced hard scattering processes in deep-inelastic scattering [10], which are calculable from first principles within instanton-perturbation theory [11], yield sizeable rates for observable final state signatures in the fiducial regime of the latter [12,13,14,15,16], and are actively searched for at HERA [21,22]. Moreover, it has been recognized recently that larger-size QCD instantons, beyond the semi-classical, instanton-perturbative regime, may well be responsible for the bulk of inelastic hadronic processes and build up soft diffractive scattering [17]. QCD-instantons may also be responsible for deviations of the angular distribution of lepton pairs at finite momentum transfers in hadron-hadron collisions from perturbative predictions [18].

Baryon plus lepton number violating processes induced by electroweak instantons have a negligible rate at low energies, but may become important at very high energies, in the 10-100 TeV range [1,2,4,7]. While laboratory searches for electroweak instanton-induced processes [3,6] will have to wait for the commissioning of the VLHC [7], the first signs of their existence may be [5] or may have already been [8] observed at present cosmic ray air shower facilities such as the Pierre Auger Observatory or neutrino telescopes such as AMANDA. But at present colliders, it is not possible to verify their existence.

This is different for processes induced by QCD-instantons. The deep-inelastic scattering regime is strongly favoured in this respect [10], since hard processes induced by QCD-instantons are both calculable [11,12,13] within instanton-pertubation theory and have good prospects for experimental detection at HERA [14]. With the help of the Monte Carlo generator QCDINS for QCD instanton-induced events in deep-inelastic scattering [15], the H1 [19,20,21,22] and ZEUS [23] experiments at HERA are actively searching for signatures of instantons in the hadronic final state. The results obtained so far are quite intriguing and encouraging, although far from being conclusive. Yet, they strongly enhance the motivation for looking more closely [16] at some remaining theoretical issues related to our original predictions. An encouraging over-all agreement with our original predictions seems to emerge.

Publications

Electroweak Instanton-induced Processes

  1. A. Ringwald,
    High Energy Breakdown of Perturbation Theory in the Electroweak Instanton Sector,
    Nucl. Phys. B330 (1990) 1

  2. V.V. Khoze and A. Ringwald,
    Total Cross Section for Anomalous Fermion-Number Violation via Dispersion Relation,
    Nucl. Phys. B355 (1991) 351

  3. A. Ringwald, F. Schrempp and C. Wetterich,
    Phenomenology of Geometrical Flavour Interactions at TeV Energies,
    Nucl. Phys. B365 (1991) 3

  4. V.V. Khoze and A. Ringwald,
    Non-Perturbative Contribution to Total Cross-Sections in Non-Abelian Gauge Theories,
    Phys. Lett. B259 (1991) 106

  5. D. Morris and A. Ringwald,
    Cosmic Ray Signatures of Multi-W Processes,
    hep-ph/9308269;   Astroparticle Phys. 2 (1994) 43

  6. M. Gibbs, A. Ringwald, B. Webber and J. Zadrozny,
    Monte Carlo Simulation of Baryon and Lepton Number Violating Processes at High Energies,
    hep-ph/9406266;   Z. Phys. C66 (1995) 285

  7. A. Ringwald,
    Electroweak Instantons/Sphalerons at VLHC?,
    hep-ph/0212099;   Phys. Lett. B555 (2003) 227

  8. Z. Fodor, S.D. Katz, A. Ringwald and H. Tu,
    Electroweak instantons as a solution to the ultrahigh energy cosmic ray puzzle,
    hep-ph/0303080;   Phys. Lett. B561 (2003) 191.

  9. A. Ringwald,
    An upper bound on the total cross-section for electroweak baryon number violation,
    hep-ph/0307034;   JHEP 0310 (2003) 008.

QCD Instanton-induced Processes

  1. A. Ringwald and F. Schrempp,
    Towards the Phenomenology of QCD Instanton Induced Particle Production at HERA,
    hep-ph/9411217,   in: Quarks '94, Vladimir, Russia, May 11-18, 1994, (World Scientific, Singapore 1995), pp. 170-193

  2. S. Moch, A. Ringwald and F. Schrempp,
    Instantons in Deep-Inelastic Scattering - The Simplest Process,
    hep-ph/9609445;   Nucl. Phys. B507 (1997) 134

  3. A. Ringwald and F. Schrempp,
    Instanton-induced Cross-Sections in Deep-Inelastic Scattering,
    hep-ph/9806528;   Phys. Lett. B438 (1998) 217

  4. A. Ringwald and F. Schrempp,
    Confronting Instanton Perturbation Theory with QCD Lattice Results,
    hep-lat/9903039;   Phys. Lett. B459 (1999) 249

  5. T. Carli, J. Gerigk, A. Ringwald and F. Schrempp,
    QCD Instanton-induced Processes in Deep-inelastic Scattering - Search Strategies and Model Dependencies,
    hep-ph/9906441,   in: Monte Carlo Generators for HERA Physics, DESY-PROC-1999-02, pp. 329-347

  6. A. Ringwald and F. Schrempp,
    QCDINS 2.0 - A Monte Carlo generator for instanton-induced processes in deep-inelastic scattering,
    hep-ph/9911516;   Comput. Phys. Commun. 132 (2000) 267

  7. A. Ringwald and F. Schrempp,
    Zooming-in on Instantons at HERA,
    hep-ph/0012241;   Phys. Lett. B503 (2001) 331

  8. F. Schrempp and A. Utermann,
    QCD instantons and high-energy diffractive scattering,
    hep-ph/0207300;   Phys. Lett. B543 (2002) 197

  9. A. Brandenburg, A. Ringwald and A. Utermann,
    Instantons in Lepton Pair Production,
    hep-ph/0605234;   Nucl.Phys. B754 (2006) 107

Experimental Searches

  1. S. Aid et al. [H1 Collaboration],
    Strangeness Production in Deep-Inelastic Positron-Proton Scattering at HERA,
    hep-ex/9607010;   Nucl. Phys. B480 (1996) 3

  2. S. Aid et al. [H1 Collaboration],
    Charged Particle Multiplicities in Deep Inelastic Scattering at HERA,
    hep-ex/9608011;   Z. Phys. C72 (1996) 573

  3. T. Carli and M. Kuhlen,
    Bounds on QCD Instantons from HERA,
    hep-ex/9708008,   Nucl. Phys. B511 (1998) 85

  4. C. Adloff et al. [H1-Collaboration],
    Search for QCD Instanton-induced Processes in Deep-Inelastic Scattering at HERA,
    hep-ex/0205078;   Eur. Phys. J. C 25 (2002) 495

  5. S. Chekanov et al. [ZEUS Collaboration],
    Search for QCD instanton induced events in deep inelastic ep scattering at HERA,
    hep-ex/0312048;   Eur. Phys. J. C 34 (2004) 255

Reviews

A. Ringwald,
From QCD Instantons at HERA to Electroweak B+L Violation at VLHC,
hep-ph/0302112,
in: Proc. 26th Johns Hopkins Workshop on Current Problems in Particle Theory, Heidelberg, Germany, August 2002, PRHEP-jhw2002/008

A. Ringwald,
Vacuum structure and high-energy scattering,
hep-ph/0210209,
in: Proc. QCD `02, Montpellier, France, July 2002; Nucl. Phys. B (Proc. Suppl.) 121 (2003) 145-148.

F. Schrempp,
Tracking QCD Instantons,
hep-ph/0109032;   J. Phys. G 28 (2002) 915
in: New Trends in HERA Physics 2001, Proc. Ringberg Workshop, Tegernsee, Germany, 17 - 22 June 2001

A. Ringwald and F. Schrempp,
Theory and Phenomenology of Instantons at HERA,
hep-ph/9909338,
in: New Trends in HERA Physics 1999, Proc. Ringberg Workshop, Tegernsee, Germany, 30 May - 4 June, 1999, pp. 203-217

Recent Talks and Lectures (Transparencies)

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Last modified: December 02, 2011
Andreas Ringwald (andreas.ringwald@desy.de)