On the Standard Model Predictions for Rare K and B Decay Branching Ratios: 2022

TL;DR

This paper discusses the latest precise Standard Model predictions for rare K and B decay branching ratios, emphasizing methods that eliminate CKM parameter dependence and avoid assumptions about new physics contributions.

Contribution

It updates and clarifies the methodology for predicting rare decay branching ratios, stressing the importance of using specific observables to eliminate CKM uncertainties and avoid outdated results.

Findings

Updated SM predictions for rare decay branching ratios.

Methodology using $| ext{ε}_K|$, $ ext{Δ}M_s$, $ ext{Δ}M_d$, and $S_{ψK_S}$ for reliable predictions.

Highlights the non-necessity of assuming no new physics in $ ext{Δ}F=2$ transitions.

Abstract

In this decade one expects a very significant progress in measuring the branching ratios for several rare $K$ and $B$ decays, in particular for the decays $K^+\to\pi^+\nu\bar\nu$, $K_L\to\pi^0\nu\bar\nu$, $B_s\to\mu^+\mu^+$ and $B_d\to\mu^+\mu^+$. On the theory side a very significant progress on calculating these branching ratios has been achieved in the last thirty years culminating recently in rather precise SM predictions for them. It is then unfortunate that some papers still cite the results for $K^+\to\pi^+\nu\bar\nu$ and $K_L\to\pi^0\nu\bar\nu$ presented by us in 2015. They are clearly out of date. Similar comments apply to predictions for $B_{s,d}\to\mu^+\mu^-$. In this note I want to stress again that, in view of the tensions between various determinations of $V_{cb}$ in tree-level decays, presently, the only trustable SM predictions for the branching ratios in question can be obtained by eliminating their dependence on the CKM parameters with the help of $|\varepsilon_K|$, $\Delta M_s$, $\Delta M_d$ and $S_{\psi K_S}$, evaluated in the SM. In this context I am astonished by statements made by some computer code practitioners that setting in this strategy these four $\Delta F=2$ observables to their experimental values is an assumption. The goal of this strategy is not to make an overall SM fit but to predict the SM branching ratios. In the SM there are no new physics (NP) contributions to $\Delta F=2$ transitions and no assumption on the absence of NP is needed. Moreover, presently NP is not required to describe simultaneously the very precise data on $|\varepsilon_K|$, $\Delta M_s$, $\Delta M_d$ and $S_{\psi K_S}$. This strategy for obtaining true SM predictions for rare decay branching ratios is moreover not polluted by hadronic uncertainies and observed anomalies in semi-leptonic decays used often in global analyses as stressed recently in (arxiv: 2209.03968).