Effective theory approach to new physics with flavour: general framework and a leptoquark example

Abstract

Extending the Standard Model with higher-dimensional operators in an effective-fieldtheory (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 increases 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 U 1 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.

Flavor Constraints from Unitarity and Analyticity

“…Assumptions about the flavor structure of new physics, such as minimal flavor violation (MFV) [63][64][65] or more general structures (e.g., Ref. [66]), are independent of our bounds. While MFV enforces certain flavor structures, it imposes no requirements on the signs of couplings, so MFV theories do not automatically obey our bounds.…”

B anomalies under the lens of electroweak precision

“…Indeed, a combined resolution of R K ( * ) anomalies with the long-standing deviations observed in R D ( * ) ≡ Br(B → D ( * ) τ ν)/Br(B → D ( * ) ν) originally found at Babar [64] and subsequently measured at Belle [65] and LHCb [66], has triggered a lot of interest in the theory community. In particular, in order for NP effects to simultaneously account for a ∼ 20% deviation in tree-level charged-weak decays and in loop-level flavour-changing neutral currents (FCNC), models with a highly non-trivial flavour structure are required [67][68][69][70][71][72][73][74][75][76], often being at the edge of flavour physics constraints [77,78] and collider bounds [79,80]. So far, model building has been mainly put forward in the direction of UV-completing low-energy leptoquark benchmarks identified, for instance, in refs.…”

Constraining flavour patterns of scalar leptoquarks in the effective field theory

“…In a previous paper [10] we laid out the main ideas towards an EFT-based approach to NP with flavour. The key element of that analysis is a bottom-up flavour structure with a flavour power counting set by introducing Froggatt-Nielsen (FN) charges [11] for the different fermion fields (for other studies that use FN charges in connection with leptoquark scenarios see [12][13][14]).…”

“…The flavour spurions are selected based on dynamical input. In [10], for instance, we singled out those associated with the exchange of a hypercharge Y = 2 3 vector leptoquark U 1 . With this selection of flavour spurions, the FN charges were constrained by phenomenological input from low-energy flavour observables.…”