Abstract
Extending the Standard Model with higher-dimensional operators in an effective field theory (EFT) approach provides a systematic framework to study new physics (NP) effects from a bottom-up perspective, as long as the NP scale is sufficiently large compared to the energies probed in the experimental observables. However, when taking into account the different quark and lepton flavours, the number of free parameters in- creases dramatically, which makes generic studies of the NP flavour structure infeasible. In this paper, we address this issue in view of the recently observed “flavour anomalies” in B-meson decays, which we take as a motivation to develop a general framework that allows us to systematically reduce the number of flavour parameters in the EFT. This framework can be easily used in global fits to flavour observables at Belle II and LHCb as well as in analyses of flavour-dependent collider signatures at the LHC. Our formalism represents an extension of the well-known minimal-flavour-violation approach, and uses Froggatt-Nielsen charges to define the flavour power-counting. As a relevant illustration of the formalism, we apply it to the flavour structures which could be induced by a U1 vector leptoquark, which represents one of the possible explanations for the recent hints of flavour non-universality in semileptonic B-decays. We study the phenomenological viability of this specific framework performing a fit to low-energy flavour observables.
Highlights
The SM Lagrangian as the leading term of an effective field theory (EFT) expansion, with corrections suppressed by inverse powers of the new physics (NP) scale(s) [1, 2]
Extending the Standard Model with higher-dimensional operators in an effective field theory (EFT) approach provides a systematic framework to study new physics (NP) effects from a bottom-up perspective, as long as the NP scale is sufficiently large compared to the energies probed in the experimental observables
When taking into account the different quark and lepton flavours, the number of free parameters increases dramatically, which makes generic studies of the NP flavour structure infeasible. We address this issue in view of the recently observed “flavour anomalies” in B-meson decays, which we take as a motivation to develop a general framework that allows us to systematically reduce the number of flavour parameters in the EFT
Summary
The starting point is the maximal flavour symmetry group of the SM commuting with the gauge symmetries, namely. We will consider new flavour structures which may be associated with the exchange of new, relatively heavy, bosonic particles. This subset of structures covers typical models with new scalars or vector bosons coupling to fermion bilinears. If we identify the flavour structure as originating from the tree-level exchange of heavy particles, the flavour coefficients of the 4-fermion operators in SMEFT would factorise as a product of the corresponding spurions, as explained already in the Introduction. We will concentrate on a scenario where only ∆QL and ∆DE are present, which singles out the first, second, fifth and eighth line in table 3
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