On Flavourful Easter eggs for New Physics hunger and Lepton Flavour Universality violation

Abstract

Within the standard approach of effective field theory of weak interactions for $\hat{I}”B=1$ transitions, we look for possibly unexpected subtle New Physics effects, here dubbed "flavourful Easter eggs". We perform a Bayesian global fit using the publicly available HEPfit package, taking into account state-of-the-art experimental information concerning these processes, including the suggestive measurements from LHCb of $R_K$ and $R_{K^{\ast }}$, the latter available only very recently. We parametrize New Physics contributions to $b\to s$ transitions in terms of shifts of Wilson coefficients of the electromagnetic dipole and semi-leptonic operators, assuming CP-conserving effects, but allowing in general for violation of lepton flavour universality. We show how optimistic/conservative hadronic estimates can impact quantitatively the size of New Physics extracted from the fit. With a conservative approach to hadronic uncertainties we find nonzero New Physics contributions to Wilson coefficients at the level of $\sim 3\ddot{I}ƒ$, depending on the model chosen. Furthermore, given the interplay between hadronic contributions and New Physics effects in the leptonic vector current, a scenario with nonstandard leptonic axial currents is comparable to the more widely advocated one with New Physics in the leptonic vector current.

27 pages, 6 figures and 5 tables. v2: numerical results and plots replaced with higher statistics MC runs, references added. v3: final version to appear in EPJC