Catalog · Beauty · B017

B017 $B \to X_s \ell^+ \ell^-, B \to K() \ell^+ \ell^-, R_K, R_K$

Inclusive and exclusive b $ \to $ s $\ell^+ \ell^-$ rare B decays

Status SUBTLETY-ADDED VERIFIED High Code: NO Priority Medium

Why this constrains the RS scan

This is a $\Delta B=1$ FCNC probe of the same flavor structure that drives the repo's neutral-meson constraints, but it tests semileptonic operators rather than $\Delta F=2$ four-quark operators. KK electroweak gauge exchange, flavor-changing $Z$ couplings, and loop-induced dipoles can feed $C_9$, $C_{10}$, $C'_9$, and $C'_{10}$, with $C_7$ correlations from the radiative $b\to s\gamma$ lane. Non-universal lepton localization would show up most cleanly in $R_K$ and $R_{K^*}$, while the branching fractions and angular observables probe a broader combination of short-distance coefficients and hadronic inputs.

What's changed since the original paper

Relative to the Csaki--Falkowski--Weiler 2008 RS-flavor baseline, this channel has become data-rich. HFLAV now maintains current rare-$B$ averages through the end of 2025. LHCb's 2023 simultaneous $R_K/R_{K^*}$ update superseded the earlier LHCb LFU results and moved the clean LFU ratios into agreement with the Standard Model. The $B^0\to K^{*0}\mu^+\mu^-$ angular program remains important: LHCb's 2020 CP-averaged analysis states that the earlier tension with SM predictions persists, with significance depending on theory nuisance choices. Post-update global fits therefore no longer look like a simple LFU violation story; they are mainly a correlated $C_9$-type and hadronic-systematics problem.

Validity and model dependence

The LFU ratios are comparatively clean null tests. Branching fractions and angular observables are more model- and theory-limited because they require form factors, nonlocal charm treatment, bin definitions, and correlated experimental likelihoods. A catalog value is useful, but using this channel as a hard RS bound requires a global-fit interface rather than a single-number cut. In custodial RS, reduced $Z b_L b_L$ pressure can make semileptonic $b\to s$ channels relatively more discriminating; quote RS bounds only after specifying custodial protection, fermion embeddings, and brane kinetic terms. Use $R_K/R_{K^*}$ as precision LFU null tests; do not impose the pre-2023 anomaly narrative as a prior.

Code coverage in this repo

NO. The modern phenomenology surface lists only $\epsilon_K$, $K$, $B_d$, $B_s$, and $D^0$ systems at quarkConstraints/modern/phenomenology.py:23. The lepton flavor module implements $\mu\to e\gamma$ at flavorConstraints/muToEGamma.py:75. Required plan greps plus targeted searches for $C_9$, $C_{10}$, $R_K$, $X_s$, $K^*$, and $P'_5$ found no live $b\to s\ell^+\ell^-$ backend.

Linked evidence (opens GitHub blob at flavor-catalog-website/2026q2):

Implementation difficulty

HIGH. A production implementation would need a new $\Delta B=1$ semileptonic Hamiltonian, matching onto $C_7,C_9,C_{10}$ and primed operators, RG conventions, $q^2$-bin-aware SM predictions, form factors, nonlocal charm uncertainties, and experimental covariance handling.

Key references

Process-local snapshots: HFLAV2025Dec:BtoXsll, HFLAV2025Dec:BplusToKplusll, HFLAV2025Dec:B0ToKst0ll, LHCb2023:RKstar, LHCb2020:KstAngular, WenXu2023:BsllGlobalFit, and CsakiFalkowskiWeiler2008:CompositeFlavor.

fallback_value_uncertainty BR(B -> X_s ell+ ell-) = 5.84 +- +/- 0.69 10^-6

AMBIGUOUS

Match 1 of 2 snapshot line 12

L9: Relevant table excerpt:
L10: 
L11: Experiment | Measurement [10^-6] | Delta chi^2 | Reference | Comments
L12: Average | 5.84 +/- 0.69 | 3.22 | p=0.073 (ndf=1) |
L13:   Belle uses m_{l+l-} > 0.2 GeV/c^2, BABAR uses m_{l+l-} > 0.1 GeV/c^2.
L14:   The PDG uncertainty includes a scale factor. Treatment of charmonium
L15:   intermediate components differs between the results.

Match 2 of 2 snapshot line 16

L13:   Belle uses m_{l+l-} > 0.2 GeV/c^2, BABAR uses m_{l+l-} > 0.1 GeV/c^2.
L14:   The PDG uncertainty includes a scale factor. Treatment of charmonium
L15:   intermediate components differs between the results.
L16: PDG | 5.84^{+1.31}_{-1.23} | | pdgLive |
L17: BaBar | 6.73^{+0.70}_{-0.64}^{+0.60}_{-0.56} |
L18:   Phys. Rev. Lett. 112, 211802 (2014) |
L19:   Multiple systematic uncertainties are added in quadrature.

fallback_value_uncertainty BR(B+ -> K+ ell+ ell-) = 5.76 +- +/- 0.40 10^-7

RESOLVED

Match snapshot line 12

L9: Relevant table excerpt:
L10: 
L11: Experiment | Measurement [10^-7] | Delta chi^2 | Reference | Comments
L12: Average | 5.76 +/- 0.40 | 1.37 | p=0.24 (ndf=1) |
L13:   Treatment of charmonium intermediate components differs between the results.
L14: PDG | 4.71 +/- 0.46 | | pdgLive |
L15: BELLE | 5.99^{+0.45}_{-0.43} +/- 0.14 |

fallback_value_uncertainty BR(B0 -> K*(892)0 ell+ ell-) = 9.9 +- +/- 1.2 10^-7

AMBIGUOUS

Match 1 of 2 snapshot line 12

L9: Relevant table excerpt:
L10: 
L11: Experiment | Measurement [10^-7] | Delta chi^2 | Reference | Comments
L12: Average | 9.9 +/- 1.2 | 0.05 | p=0.82 (ndf=1) |
L13:   Treatment of charmonium intermediate components differs between the results.
L14: PDG | 9.9^{+1.2}_{-1.1} | | pdgLive |
L15: Belle | 9.7^{+1.3}_{-1.1} +/- 0.7 |

Match 2 of 2 snapshot line 14

L11: Experiment | Measurement [10^-7] | Delta chi^2 | Reference | Comments
L12: Average | 9.9 +/- 1.2 | 0.05 | p=0.82 (ndf=1) |
L13:   Treatment of charmonium intermediate components differs between the results.
L14: PDG | 9.9^{+1.2}_{-1.1} | | pdgLive |
L15: Belle | 9.7^{+1.3}_{-1.1} +/- 0.7 |
L16:   Phys. Rev. Lett. 103, 171801 (2009) |
L17: BaBar | 10.3^{+2.2}_{-2.1} +/- 0.7 |

fallback_value_uncertainty R_K low-q2 = 0.994 +- +0.090/-0.082 stat, +0.029/-0.027 syst

RESOLVED

Match snapshot line 27

L24: 
L25: Relevant table excerpt from arXiv TeX source results.tex:
L26: Measured values of R_K and R_K* in low-q^2 and central-q^2 regions:
L27: low-q^2 R_K = 0.994^{+0.090}_{-0.082}^{+0.029}_{-0.027}
L28: central-q^2 R_K = 0.949^{+0.042}_{-0.041}^{+0.022}_{-0.022}
L29: low-q^2 R_K* = 0.927^{+0.093}_{-0.087}^{+0.036}_{-0.035}
L30: central-q^2 R_K* = 1.027^{+0.072}_{-0.068}^{+0.027}_{-0.026}

fallback_value_uncertainty R_K central-q2 = 0.949 +- +0.042/-0.041 stat, +/-0.022 syst

RESOLVED

Match snapshot line 28

L25: Relevant table excerpt from arXiv TeX source results.tex:
L26: Measured values of R_K and R_K* in low-q^2 and central-q^2 regions:
L27: low-q^2 R_K = 0.994^{+0.090}_{-0.082}^{+0.029}_{-0.027}
L28: central-q^2 R_K = 0.949^{+0.042}_{-0.041}^{+0.022}_{-0.022}
L29: low-q^2 R_K* = 0.927^{+0.093}_{-0.087}^{+0.036}_{-0.035}
L30: central-q^2 R_K* = 1.027^{+0.072}_{-0.068}^{+0.027}_{-0.026}
L31:

fallback_value_uncertainty R_K* low-q2 = 0.927 +- +0.093/-0.087 stat, +0.036/-0.035 syst

RESOLVED

Match snapshot line 29

L26: Measured values of R_K and R_K* in low-q^2 and central-q^2 regions:
L27: low-q^2 R_K = 0.994^{+0.090}_{-0.082}^{+0.029}_{-0.027}
L28: central-q^2 R_K = 0.949^{+0.042}_{-0.041}^{+0.022}_{-0.022}
L29: low-q^2 R_K* = 0.927^{+0.093}_{-0.087}^{+0.036}_{-0.035}
L30: central-q^2 R_K* = 1.027^{+0.072}_{-0.068}^{+0.027}_{-0.026}
L31: 
L32: Conclusion excerpt from arXiv TeX source:

fallback_value_uncertainty R_K* central-q2 = 1.027 +- +0.072/-0.068 stat, +0.027/-0.026 syst

RESOLVED

Match snapshot line 30

L27: low-q^2 R_K = 0.994^{+0.090}_{-0.082}^{+0.029}_{-0.027}
L28: central-q^2 R_K = 0.949^{+0.042}_{-0.041}^{+0.022}_{-0.022}
L29: low-q^2 R_K* = 0.927^{+0.093}_{-0.087}^{+0.036}_{-0.035}
L30: central-q^2 R_K* = 1.027^{+0.072}_{-0.068}^{+0.027}_{-0.026}
L31: 
L32: Conclusion excerpt from arXiv TeX source:
L33: The first uncertainty in each row is statistical and the second systematic.