Effective Theory Approach to New Physics with Flavour: General Framework and a Leptoquark Example

TL;DR

This paper develops a systematic effective-field-theory framework to analyze new physics effects in flavour physics, reducing parameter complexity and applying it to leptoquark models explaining B-meson decay anomalies.

Contribution

It introduces a novel formalism extending minimal-flavour-violation using Froggatt-Nielsen charges for systematic flavour parameter reduction.

Findings

Framework facilitates global fits to flavour observables.

Applied to a U1 leptoquark model explaining B-decay anomalies.

Performs phenomenological viability analysis with low-energy data.

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 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.