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F. Schrempp

Publications and source records attributed to F. Schrempp.

At least 19 recordsLinked to original sources
arXiv2008

QCD-Instantons and Conformal Space-Time Inversion Symmetry

In this paper, we explore the appealing possibility that the strong suppression of large-size QCD instantons - as evident from lattice data - is due to a surviving conformal space-time inversion symmetry. This symmetry is both suggested from the striking invariance of high-quality lattice data for the instanton size distribution under inversion of the instanton size rho --> ^2 / rho and from the known validity of space-time inversion symmetry in the classical instanton sector. We project the instanton calculus onto the four-dimensional surface of a five-dimensional sphere via conformal stereographic mapping, before investigating conformal inversion. This projection to a compact, curved geometry is both to avoid the occurence of divergences and to introduce the average instanton size from the lattice data as a new length scale. The average instanton size is identified with the radius b of this 5d-sphere and acts as the conformal inversion radius. For b = , our corresponding results are almost perfectly symmetric under space-time inversion and in good qualitative agreement with the lattice data. For rho / b --> 0, we recover the familiar results of instanton perturbation theory in flat 4d-space. Moreover, we illustrate that a (weakly broken) conformal inversion symmetry would have significant consequences for QCD beyond instantons. As a further successful test for inversion symmetry, we present striking implications for another instanton dominated lattice observable, the chirality-flip ratio in the QCD vacuum.

hep-ph↗
arXiv2005

Instanton-Induced Processes - An Overview

A first part of this review is devoted to a summary of our extensive studies of the discovery potential for instanton(I)-induced, deep-inelastic processes at HERA. Included are some key issues about I-perturbation theory, an exploitation of crucial lattice constraints and a status report about the recent I-search results by the HERA collaborations H1 and ZEUS in relation to our predictions. Next follows a brief outline of an ongoing project concerning a broad exploration of the discovery potential for hard instanton processes at the LHC. I then turn to an overview of our work on high-energy processes, involving larger-sized instantons. I shall mainly focus on the phenomenon of saturation at small Bjorken-x from an instanton perspective. In such a framework, the saturation scale is associated with the conspicuous average instanton size (~0.5 fm), as known from lattice simulations. Another central and intriguing result is the identification of the "Colour Glass Condensate" with the QCD sphaleron state.

hep-ph↗
arXiv2004

Instanton-driven Gluon Saturation

We report on the interesting possibility of instanton-driven gluon saturation in lepton-nucleon scattering at small Bjorken-x. The explicitly known instanton gauge field serves as a concrete realization of an underlying non-perturbative saturation mechanism associated with strong classical fields. The conspicuous, intrinsic instanton size scale known from lattice simulations, turns out to determine the saturation scale. The ``colour glass condensate'' can be identified in our approach with the QCD-sphaleron state, dominating instanton-induced processes in the softer regime.

hep-ph↗
arXiv2004

Instanton-Driven Saturation at Small x

We report on the interesting possibility of instanton-driven gluon saturation in lepton-nucleon scattering at small Bjorken-x. Our results crucially involve non-perturbative information from high-quality lattice simulations. The conspicuous, intrinsic instanton size scale < rho > ~ 0.5 fm, as known from the lattice, turns out to determine the saturation scale. A central result is the identification of the ``colour glass condensate'' with the QCD-sphaleron state.

hep-ph↗
arXiv2003

QCD-Instantons and Saturation at Small x

We briefly report on the contribution of QCD-instantons to the phenomenon of saturation in deep-inelastic scattering (DIS) at small Bjorken-x. The explicitly known instanton gauge field serves as a concrete realization of an underlying non-perturbative saturation mechanism associated with strong classical fields. Two independent strategies were pursued, with consistent results: On the one hand, an approach starting from instanton-perturbation theory was considered and on the other hand, the scattering of a Wilson loop in an instanton gauge field background. In both cases, the conspicuous, intrinsic instanton size scale ~ 0.5 fm, as known from the lattice, turns out to determine the saturation scale.

hep-ph↗
arXiv2002

QCD Instantons and High-Energy Diffractive Scattering

We pursue the intriguing possibility that larger-size instantons build up diffractive scattering, with the marked instanton-size scale approximately 0.5 fm being reflected in the conspicuous ``geometrization'' of soft QCD. As an explicit step in this direction, the known instanton-induced cross sections in deep-inelastic scattering (DIS) are transformed into the familiar colour dipole picture, which represents an intuitive framework for investigating the transition from hard to soft physics in DIS at small x_{Bj}. The simplest instanton (I) process without final-state gluons is studied first. With the help of lattice results, the q bar{q}-dipole size r is carefully increased towards hadronic dimensions. Unlike perturbative QCD, one now observes a competition between two crucial length scales: the dipole size r and the size rho of the background instanton that is sharply localized around approximately 0.5 fm. For r exceeding , the dipole cross section indeed saturates towards a geometrical limit, proportional to the area pi ^2, subtended by the instanton. In case of final-state gluons, lattice data are crucially used to support the emerging picture and to assert the range of validity of the underlying I bar{I}-valley approach. As function of an appropriate energy variable, the resulting dipole cross section turns out to be sharply peaked at the sphaleron mass in the soft regime. The general geometrical features remain like in the case without gluons.

hep-ph↗
arXiv2002

Instantons and Saturation in the Colour Dipole Picture

We pursue the intriguing possibility that larger-size instantons build up diffractive scattering, with the marked instanton-size scale $<ρ> \approx 0.5$ fm being reflected in the conspicuous ``geometrization'' of soft QCD. As an explicit illustration, the known instanton contribution to DIS is transformed into the intuitive colour dipole picture. With the help of lattice results, the $q \bar{q}$-dipole size r is carefully increased towards hadronic dimensions. Unlike pQCD, one now observes a competition between two crucial length scales: the dipole size r and the size $ρ$ of the background instanton that is sharply localized around $<ρ> \approx 0.5$ fm. For r exceeding $<ρ>$, the dipole cross section indeed saturates towards a geometrical limit, proportional to the area $π<ρ>^2$, subtended by the instanton.

hep-ph↗
arXiv2001

Tracking QCD-Instantons

In a first part, I review our detailed investigation of the prospects to discover deep-inelastic processes induced by small QCD-instantons at HERA. This includes the essence of our calculations based on instanton-perturbation theory, crucial lattice constraints and a confrontation of the recent intriguing (preliminary) search results by the H1 collaboration with our predictions. In a second part, I report on two ongoing attempts towards a better understanding of the role of larger-size instantons in the QCD-vacuum and in high-energy (diffractive) scattering processes, respectively: The striking suppression of larger-size instantons in the vacuum is attributed to a residual conformal inversion symmetry, and a precocious lack of ``color transparency'' in the one-instanton contribution to the color-dipole scattering picture is emphasized.

hep-ph↗
arXiv2000

Zooming-in on Instantons at HERA

In view of the intriguing, preliminary search results for instanton-induced events at HERA from the H1 collaboration, some important remaining theoretical issues are discussed. Notably, the question is addressed, to which extent the H1 analysis may be directly compared to our original predictions from instanton-perturbation theory, since certain fiducial cuts are lacking in the H1 data. Various theoretical uncertainties are evaluated and their impact on the observed excess is discussed. An improved understanding of the experimental findings along with an encouraging over-all agreement with our original predictions seems to emerge.

hep-ph↗
arXiv2000

Instantons in Deep-Inelastic Scattering

In view of the new (preliminary) search results for instanton-induced events at HERA from the H1 collaboration, we present a brief discussion of (controllable) theoretical uncertainties, both in the event topology and the calculated rate.

hep-ph↗
arXiv1999

QCDINS 2.0 - A Monte Carlo generator for instanton-induced processes in deep-inelastic scattering

We describe a Monte Carlo event generator for the simulation of QCD-instanton induced processes in deep-inelastic scattering (HERA). The QCDINS package is designed as an ``add-on'' hard process generator interfaced to the general hadronic event simulation package HERWIG. It incorporates the theoretically predicted production rate for instanton-induced events as well as the essential characteristics that have been derived theoretically for the partonic final state of instanton-induced processes: notably, the flavour democratic and isotropic production of the partonic final state, energy weight factors different for gluons and quarks, and a high average multiplicity O(10) of produced partons with a Poisson distribution of the gluon multiplicity. While the subsequent perturbative evolution of the generated partons is always handled by the HERWIG package, the final hadronization step may optionally be performed also by means of the general hadronic event simulation package JETSET.

hep-ph↗
arXiv1999

QCD Instanton-induced Processes in Deep-Inelastic Scattering - Search Strategies and Model Dependencies

We investigate possible search strategies for QCD-instanton induced processes at HERA in the deep-inelastic scattering (DIS) regime. Our study is based on the Monte Carlo generator QCDINS for instanton-induced events and the standard generators for normal DIS events. It appears possible to isolate an instanton enriched data sample via an optimized multi-dimensional cut scenario for a set of six most instanton-sensitive DIS observables. As a further central point, we investigate the stability of our results with respect to a variation of the (hadronization) models available for the simulation of both normal DIS and instanton-induced events. Dependencies on the variation of certain inherent parameters are also studied. Within the ``bandwidth'' of variations considered, we find that the normal DIS background is typically much more sensitive to model variations than the I-induced signal.

hep-ph↗
arXiv1999

Instantons in the QCD Vacuum and in Deep Inelastic Scattering

We give a brief status report on our on-going investigation of the prospects to discover QCD instantons in deep inelastic scattering (DIS) at HERA. A recent high-quality lattice study of the topological structure of the QCD vacuum is exploited to provide crucial support of our predictions for DIS, based on instanton perturbation theory.

hep-ph↗
arXiv1999

Confronting Instanton Perturbation Theory with QCD Lattice Results

We exploit a recent lattice investigation (UKQCD) on the topological structure of the (quenched) QCD vacuum, in order to gain information on crucial building blocks of instanton perturbation theory. A central motivation is to further constrain our previous predictions of instanton-induced hard scattering processes. First, we address the generic problem of extracting quantitative information from cooled lattice data. We find a new scaling variable, interpreted as a "cooling radius", which allows to combine lattice data for a whole range of lattice spacings and cooling sweeps. This variable strongly helps to extract information on the uncooled distributions of interest. After performing the continuum extrapolation of the instanton size and instanton-anti-instanton distance distributions, we find striking agreement with the theoretical predictions from instanton-perturbation theory, for instanton sizes below 0.5 fm and distances above 0.5 fm, respectively. These results imply first direct support for the validity of the known valley interaction between instantons and anti-instantons.

hep-lat↗
arXiv1998

QCD-Instantons at HERA -- An Introduction

We review our ongoing theoretical and phenomenological study of the discovery potential for instanton-induced events in deep-inelastic scattering (DIS) at HERA. Constraints from recent lattice simulations will be exploited and translated into a ``fiducial'' kinematical region for our predictions of the instanton-induced DIS cross-section.

hep-ph↗
arXiv1998

Instanton Searches at HERA

The present status of our ongoing systematic study of the discovery potential of QCD-instanton induced events in deep-inelastic scattering at HERA is briefly reviewed. We emphasize our recent progress in predicting the cross-sections of instanton-induced processes. Our finalized predictions include a dramatic improvement of the residual renormalization-scale dependencies and the specification of a ``fiducial'' kinematical region in the relevant Bjorken variables extracted from recent lattice simulations. Published upper limits on instanton-induced cross-sections based on single observables in the final state such as the flow of strange particles and the multiplicity distribution of charged particles are already of the order of our estimate. Thus, a decisive search for instanton-induced events in deep-inelastic scattering at HERA, based on a multi-observable analysis, seems feasible.

hep-ph↗
arXiv1998

Searching for QCD-Instantons at HERA

We review the present status of our ongoing systematic study of the discovery potential of QCD-instanton induced events in deep-inelastic scattering at HERA.

hep-ph↗