Year: 2026
Type: preprint
Abstract: The deficits w.r.t.~the Standard Model predictions in $B\to K^{(*)}μ^+μ^-$ and $B_s\to ϕμ^+μ^-$ have recently been substantiated by new analyses from LHCb and CMS, as well as improved theoretical predictions, resulting in strong statistical evidence for lepton flavour universal new physics effects. In this letter, we investigate the possibility of generating such an effect through CKM unitarity violation: If the true value of $V_{cb}$, extracted from $B\to D^{(*)}\ellν$, were larger due to an NP contamination, this would lead, via the assumed CKM unitarity within the SM, to a smaller value of $V_{ts}$ and thus to reduced theory predictions for $b\to s\ell^+\ell^-$ branching ratios. A sizable correction to the $W$-$c$-$b$ vertex is only possible close to the custodial-protected limit in which $Z$-$b$-$s$ couplings are absent. However, in a model-independent SMEFT analysis, we show that even if $C_{Hq}^{(1)}=-C_{Hq}^{(3)}$ is realized at a high scale, additional custodial-breaking effects appear due to RGE. Therefore, only a fine-tuned region in the parameter space of modified gauge boson couplings, displaying an overcompensation of the CKM unitarity-violating effect, gives a good fit to $b\to s\ell^+\ell^-$ data. The only realistic simple SM extensions to achieve this setup are vector-like quarks. However, we show that they cannot explain the $b\to s\ell^+\ell^-$ anomalies without violating bounds for electroweak precision observables as well as $K^0-\bar K^0$ and $B_s-\bar B_s$ mixing.
Source: arXiv (Cornell University)
Authors: Crivellin, Andreas, Kirk, Matthew
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7222927141