Theory Seminars

Reconciling the contour-improved and fixed-order series for hadronic tau decays with a new renormalon-free gluon-condensate scheme

by Prof. Diogo Boito

IFAE Seminar Room (Face-to-face Only)

IFAE Seminar Room

Face-to-face Only


The determination of the strong coupling, alpha_s, from hadronic tau decays is one of the most precise extractions based on experimental data. For a long time, different treatments of the renormalization scale in the perturbative series, with contour improved (CIPT) or a strict fixed-order (FOPT) expansion, remained an important source of theoretical uncertainty. Recently, Hoang and Regner have shown that CIPT and FOPT are two intrinsically different perturbative series and that their numerical discrepancy is dominated by the contribution of the first infrared renormalon, associated with the gluon condensate contribution. We will present the construction of a new scheme for a renormalon-free (RF) gluon condensate (analogous to short-distance heavy-quark mass schemes). With this new RF scheme, the discrepancy between CIPT and FOPT is resolved, and any remaining difference can be almost entirely attributed to missing higher orders. Practical applications to alpha_s extractions depend on external knowledge about the norm of the gluon condensate renormalon. We will show that it is possible to constrain this norm with sufficient precision and that the RF gluon condensate is viable in practice and can contribute to an important reduction in the strong coupling uncertainty.

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