Explaining b→ sℓ+ℓ- with CKM Unitarity Violation? A No-Go Theorem
Abstract
The deficits w.r.t.~the Standard Model predictions in B→ K(*)μ+μ- and Bs→ 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 Vcb, extracted from B→ D(*)ℓν, were larger due to an NP contamination, this would lead, via the assumed CKM unitarity within the SM, to a smaller value of Vts and thus to reduced theory predictions for b→ sℓ+ℓ- 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 CHq(1)=−CHq(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→ sℓ+ℓ- data. The only realistic simple SM extensions to achieve this setup are vector-like quarks. However, we show that they cannot explain the b→ sℓ+ℓ- anomalies without violating bounds for electroweak precision observables as well as K0−K̅0 and Bs−B̅s mixing.
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BibTeX
@misc{crivellin2026explaining,
title = {{Explaining $b\to s\ell^+\ell^-$ with CKM Unitarity Violation? A No-Go Theorem}},
author = {Andreas Crivellin and Matthew Kirk},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.22490},
eprint = {2610.22490},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.22490},
}
RIS
TY - UNPB
TI - Explaining b→ sℓ^+ℓ^- with CKM Unitarity Violation? A No-Go Theorem
AU - Andreas Crivellin
AU - Matthew Kirk
PY - 2026
DA - 2026/10/15
PB - arXiv
DO - 10.48550/arxiv.2610.22490
UR - https://arxiv.org/abs/2610.22490
AB - The deficits w.r.t.~the Standard Model predictions in B→ K^(*)μ^+μ^- and B_s→ 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→ D^(*)ℓν, 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→ sℓ^+ℓ^- 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→ sℓ^+ℓ^- data. The only realistic simple SM extensions to achieve this setup are vector-like quarks. However, we show that they cannot explain the b→ sℓ^+ℓ^- anomalies without violating bounds for electroweak precision observables as well as K^0−K̅^0 and B_s−B̅_s mixing.
ER -