https://doi.org/10.1140/epjc/s10052-021-08941-z
Regular Article – Theoretical Physics
C-parameter hadronisation in the symmetric 3-jet limit and impact on 
fits
1
CERN, Theoretical Physics Department, 1211, Geneva 23, Switzerland
2
Rudolf Peierls Centre for Theoretical Physics, Clarendon Laboratory, University of Oxford, Parks Road, OX1 3PU, Oxford, UK
3
All Souls College, OX1 4AL, Oxford, UK
4
MTU Aero Engines AG, Dachauer Str. 665, 80995, Munich, Germany
Received:
22
December
2020
Accepted:
1
February
2021
Published online:
16
February
2021
Hadronisation corrections are crucial in extractions of the strong coupling constant () from event-shape distributions at lepton colliders. Although their dynamics cannot be understood rigorously using perturbative methods, their dominant effect on physical observables can be estimated in singular configurations sensitive to the emission of soft radiation. The differential distributions of some event-shape variables, notably the C parameter, feature two such singular points. We analytically compute the leading non-perturbative correction in the symmetric three-jet limit for the C parameter, and find that it differs by more than a factor of two from the known result in the two-jet limit. We estimate the impact of this result on strong coupling extractions, considering a range of functions to interpolate the hadronisation correction in the region between the 2 and 3-jet limits. Fitting data from ALEPH and JADE, we find that most interpolation choices increase the extracted 
, with effects of up to 
relative to standard fits. This brings a new perspective on the long-standing discrepancy between certain event-shape fits and the world average.
© The Author(s) 2021
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