Abstract
We discuss the possibility of performing precise tests of mu /e universality in B rightarrow pi ell ^+ell ^- decays. We show that in wide regions of the dilepton invariant mass spectrum the ratio between muonic and electronic decay widths can be predicted with high accuracy, both within and beyond the Standard Model. We present numerical expressions which can be used to extract precise information on short-distance dynamics if a deviation from universality is observed in the data.
Highlights
The experimental measurements of the μ/e universality ratios RK and RK ∗ in B → K (∗) + − decays [1,2,3,4] indicate a violation of Lepton Flavor Universality (LFU) of about 20% in the decay rates, well above the Standard Model (SM) expectation [5,6]
We show that in wide regions of the dilepton invariant mass spectrum the ratio between muonic and electronic decay widths can be predicted with high accuracy, both within and beyond the Standard Model
The connection between b → s and b → d flavor-changing neutral-current (FCNC) amplitudes is a firm prediction of all models based on a minimally broken U (2)5 flavor symmetry [29,30,31], such as the ones proposed in Refs. [27,28], and the wider class models discussed in Refs. ***[32,33,34,35]
Summary
The experimental measurements of the μ/e universality ratios RK and RK ∗ in B → K (∗) + − decays [1,2,3,4] indicate a violation of Lepton Flavor Universality (LFU) of about 20% in the decay rates, well above the Standard Model (SM) expectation [5,6]. The b → s data points towards a non-standard phenomenon of short-distance origin, with a statistical significance exceeding 4σ Our main goal is to identify the regions of the dilepton invariant mass spectrum where the decay rate is dominated by short-distance dynamics and to estimate the sensitivity to LFU-violating amplitudes in those regions. The first attempt to use e+e− → qqdata to describe long-distance effects in rare B decays dates back to Ref. [43,46,48], the analytic structure of the amplitude is independent of the factorization hypothesis and this method can be generalized, via appropriate dispersion relations, beyond naïve factorization Using this method we will show that in a wide range of the dileptonspectrum Rπ can allow us to extract precise information on LFU-violating dynamics.
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