title: Enhanced nonperturbative effects through the collinear anomaly.
abstract: We show that nonperturbative effects are logarithmically enhanced for transverse-momentum-dependent observables such as qT spectra of electroweak bosons in hadronic collisions and jet broadening at e+ e- colliders. This enhancement arises from the collinear anomaly, a mechanism characteristic for transverse observables, which induces logarithmic dependence on the hard scale in the product of the soft and collinear matrix elements. Our analysis is based on an operator product expansion and provides, for the first time, a systematic, model-independent way to study nonperturbative effects for this class of observables. For the case of jet broadening, we relate the leading correction to the nonperturbative shift of the thrust distribution.
authors.affiliation: Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Becher
authors.affiliation: Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, United Kingdom.
authors.forename: Guido
authors.initials: G
authors.lastname: Bell