https://doi.org/10.1140/epjc/s10052-019-7581-y
Regular Article - Theoretical Physics
New physics in 
transitions at one loop
1
Laboratoire Charles Coulomb (L2C), University of Montpellier, CNRS, Montpellier, France
2
Departament de Física Quàntica i Astrofísica (FQA), Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), Barcelona, Spain
3
Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Padua, Italy
4
Istituto Nazionale Fisica Nucleare, Sezione di Padova, 35131, Padua, Italy
* e-mail: rupert.coy@ulb.ac.be
Received:
14
October
2019
Accepted:
21
December
2019
Published online:
23
January
2020
We investigate new-physics contributions to 
transitions in the context of an effective field theory extension of the Standard Model, including operator mixing at one loop. We identify the few scenarios where a single Wilson coefficient, 
, induces a substantial shift in the lepton flavour universality ratios 
and 
at one loop, while evading Z-pole precision tests, collider bounds, and other flavour constraints. Novel fits to the present data are achieved by a left-handed current operator with quark-flavour indices (2, 2) or (3, 3). Interestingly, the running of the Standard Model Yukawa matrices gives the dominant effect for these scenarios. We match the favoured effective-theory scenarios to minimal, single-mediator models, which are subject to additional stringent constraints. Notably, we recognise three viable instances of a leptoquark with one coupling to fermions only. If the anomalies were confirmed, it appears that one-loop explanations have good prospects of being directly tested at the LHC.
© The Author(s), 2020
