The Exclusive Vision of Rare $K$ and $B$ Decays and of the Quark Mixing in the Standard Model

Abstract: The most common predictions for rare $K$ and $B$ decay branching ratios in the Standard Model are based on the CKM elements $|V_{cb}|$ and $|V_{ub}|$ resulting from global fits, that are in the ballpark of their inclusive and exclusive determinations, respectively. In the present paper we follow another route. We assume that the future true values of $|V_{cb}|$ and $|V_{ub}|$ will be both from exclusive determinations and set them equal to the most recent ones from FLAG. An unusual pattern of SM predictions results from this study with some existing tensions being dwarfed and new tensions being born. In particular using the HPQCD $B^0_{s,d}-\bar B^0_{s,d}$ hadronic matrix elements a $3.1\sigma$ tension in $\Delta M_s$ independently of $\gamma$ is found. For $60^\circ\le\gamma\le 75^\circ$ the tension in $\Delta M_d$ between $4.0\sigma$ and $1.1\sigma$ is found and in the case of $\epsilon_K$ between $5.2\sigma$ and $2.1\sigma$. Moreover, the room for new physics in $K^+\to\pi^+\nu\bar\nu$, $K_L\to\pi^0\nu\bar\nu$ and $B\to K(K^*)\nu\bar\nu$ decays is significantly increased. We compare the results in this EXCLUSIVE scenario with the HYBRID one in which $|V_{cb}|$ in the former scenario is replaced by the most recent inclusive $|V_{cb}|$ and present the dependence of all observables considered by us in both scenarios as functions of $\gamma$. We also compare the determination of $|V_{cb}|$ from $\Delta M_s$, $\Delta M_d$, $\epsilon_K$ and $S_{\psi K_S}$ using $B^0_{s,d}-\bar B^0_{s,d}$ hadronic matrix elements from LQCD with $2+1+1$ flavours, $2+1$ flavours and their average. Only for the $2+1+1$ case values for $\beta$ and $\gamma$ exist for which the same value of $|V_{cb}|$ is found: $|V_{cb}|=42.6(4)\times 10^{-3}$. This in turn implies a $2.7\sigma$ anomaly in $B_s\to\mu^+\mu^-$.