What is the scale of new physics behind the B-flavour anomalies?

Luca Di Luzio; Marco Nardecchia

doi:10.1140/epjc/s10052-017-5118-9

EPJ

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2023 Impact factor 4.2

Particles and Fields

Open Access

Eur. Phys. J. C (2017) 77: 536
https://doi.org/10.1140/epjc/s10052-017-5118-9

Regular Article - Theoretical Physics

What is the scale of new physics behind the B-flavour anomalies?

Luca Di Luzio¹^* and Marco Nardecchia²

¹ Department of Physics, Institute for Particle Physics Phenomenology, Durham University, Durham, DH1 3LE, UK
² Theoretical Physics Department, CERN, Geneva, Switzerland

^* e-mail: luca.di-luzio@durham.ac.uk

Received: 14 June 2017
Accepted: 26 July 2017
Published online: 10 August 2017

Abstract

Motivated by the recent hints of lepton flavour non-universality in B-meson semi-leptonic decays, we study the constraints of perturbative unitarity on the new physics interpretation of the anomalies in $b \rightarrow c \ell \bar{\nu }$ and $b \rightarrow s \ell \bar{\ell }$ transitions. Within an effective field theory approach we find that $2 \rightarrow 2$ fermion scattering amplitudes saturate the unitarity bound below 9 and 80 TeV, respectively for $b \rightarrow c \ell \bar{\nu }$ and $b \rightarrow s \ell \bar{\ell }$ transitions. Stronger bounds, up to few TeV, are obtained when the leading effective operators are oriented in the direction of the third generation, as suggested by flavour models. We finally address unitarity constraints on simplified models explaining the anomalies and show that the new physics interpretation is ruled out in a class of perturbative realizations.