Catalog · Beauty · B005

B005 $\mathcal{B}(B_s^0\to\mu^+\mu^-)$

Rare leptonic decay $B_s0 \to \mu^+ \mu^-$

Status REVIEWED VERIFIED Medium Code: NO Priority High

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
$BR(B_s0 \to \mu^+ \mu^-$)	3.34e-9 branching fraction	-	PDG2026:BsMuMu
$BR(B_s0 \to \mu^+ \mu^-$)	3.45e-9 branching fraction	-	HFLAV2023:BsMuMu
SM $BR(B_s \to \mu^+ \mu^-$)	3.64e-9 branching fraction	-	Czaja and Misiak 2024	source ↑

Why this constrains the RS scan

This mode probes the short-distance $b\to s\mu^+\mu^-$ Hamiltonian in a clean single-number observable. In anarchic RS scenarios, flavor-changing neutral gauge couplings, KK electroweak exchange, Higgs/radion scalar exchange, or leptoquark-like completions can shift axial-vector, scalar, and pseudoscalar Wilson coefficients. The scalar and pseudoscalar terms are especially important because they can remove the Standard Model helicity suppression and are prominent in large-$\tan\beta$ supersymmetric or extended-Higgs interpretations.

What's changed since the original paper

Since the arXiv:0804.1954 RS-flavor baseline, this channel changed from an upper-limit target into a precision observable. LHCb and CMS first established the decay, and the present average combines modern LHCb, CMS, and ATLAS inputs. CMS now reports the most precise single-experiment branching-fraction measurement, while LHCb provides a simultaneous branching-fraction and effective-lifetime analysis. Theory has also improved: modern SM predictions include precise lattice $f_{B_s}$, CKM inputs, electroweak and QCD corrections, and the time-integration correction associated with the $B_s$ width difference.

Validity and model dependence

This is a robust short-distance rare-decay constraint, but it is not covered by the repo's neutral-meson-mixing machinery. The SM comparison is clean relative to angular $b\to s\ell\ell$ observables, yet it still depends on the definition of the time-integrated branching fraction, $f_{B_s}$, CKM inputs, and the treatment of scalar and pseudoscalar operators. It is directly useful for models that predict $C_{10}^{(\prime)}$, $C_S^{(\prime)}$, or $C_P^{(\prime)}$, including scalar leptoquarks, $Z'$ models, and tan-beta-enhanced Higgs sectors.

Code coverage in this repo

NO. The required catalog greps and a focused search for B\_s.*mu, mumu, C10, scalar/pseudoscalar rare-decay terms, and $b\to s\mu\mu$ found no live $B_s^0\to\mu^+\mu^-$ implementation in quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, or tests/. The nearest related code is $B_s$ mixing, e.g. quarkConstraints/deltaf2.py:239 and quarkConstraints/deltaf2.py:922, which is a $\Delta F=2$ observable rather than this $\Delta B=1$ decay.

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

Implementation difficulty

MEDIUM. A catalog-to-code implementation needs a new $\Delta B=1$ leptonic rare-decay observable, Wilson conventions for $C_{10}^{(\prime)}, C_S^{(\prime)}, C_P^{(\prime)}$, and a branching-ratio formula with the $B_s$ lifetime and width-difference convention. It does not need an angular likelihood or exclusive form factors, but a production-grade RS matching path would need carefully documented scalar and electroweak operator normalization.

Key references

Process-local raw reference keys before bibliography consolidation: PDG2026:BsMuMu, HFLAV2023:BsMuMu, CMS2023:BsMuMu, LHCb2022:BsMuMu, ATLAS2019:BsMuMu, CzajaMisiak2024:BsMuMuSM, and CsakiFalkowskiWeiler2008:CompositeFlavor.

fallback_value_uncertainty BR(B_s0 -> mu+ mu-) = (3.34 +- 0.27)e-9

AMBIGUOUS

Match 1 of 6 snapshot line 2

L1: Source key: PDG2026:BsMuMu
L2: URL: https://pdgprod.lbl.gov/pdgprod/pdgLiveJson/BranchingRatio.action?cl=B1&desig=15&init=0&node=S086&parCode=S086
L3: API URL: https://pdgprod.lbl.gov/pdgprod/pdgLiveJson/api/listings/S086.15
L4: Access date: 2026-05-16
L5: Local snapshot purpose: current canonical PDG value and input trace for B_s0 -> mu+ mu-.

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L2: URL: https://pdgprod.lbl.gov/pdgprod/pdgLiveJson/BranchingRatio.action?cl=B1&desig=15&init=0&node=S086&parCode=S086
L3: API URL: https://pdgprod.lbl.gov/pdgprod/pdgLiveJson/api/listings/S086.15
L4: Access date: 2026-05-16
L5: Local snapshot purpose: current canonical PDG value and input trace for B_s0 -> mu+ mu-.
L6: 
L7: PDG citation shown on page:
L8: S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024) and 2025 update.

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L9: 
L10: PDG live page extract:
L11: PDGID: S086.15
L12: Mode: B_s0 -> mu+ mu-
L13: Fraction (Gamma_i / Gamma): (3.34 +/- 0.27) x 10^-9
L14: 
L15: PDG API extract:

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L10: PDG live page extract:
L11: PDGID: S086.15
L12: Mode: B_s0 -> mu+ mu-
L13: Fraction (Gamma_i / Gamma): (3.34 +/- 0.27) x 10^-9
L14: 
L15: PDG API extract:
L16: request_timestamp: 2026-05-16 08:33:24 PST

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L16: request_timestamp: 2026-05-16 08:33:24 PST
L17: edition: 2026
L18: pdgid: S086.15
L19: description: B_s()0 --> mu+ mu-
L20: related_data pdgid: S086R14
L21: related_data description: G(B_s0 --> mu+ mu-)/G(total)
L22: note: Test for DeltaB = 1 weak neutral current.

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L18: pdgid: S086.15
L19: description: B_s()0 --> mu+ mu-
L20: related_data pdgid: S086R14
L21: related_data description: G(B_s0 --> mu+ mu-)/G(total)
L22: note: Test for DeltaB = 1 weak neutral current.
L23: 
L24: Selected API measurement inputs used in average:

fallback_value_uncertainty BR(B_s0 -> mu+ mu-) = (3.45 +- 0.29)e-9

AMBIGUOUS

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L1: Source key: HFLAV2023:BsMuMu
L2: URL: https://hflav-eos.web.cern.ch/hflav-eos/rare/Apr2023/html/Bs/BR_Bs0_mu%2B_mu-.html
L3: Access date: 2026-05-16
L4: Local snapshot purpose: HFLAV rare-decay average and experiment input table.
L5:

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L4: Local snapshot purpose: HFLAV rare-decay average and experiment input table.
L5: 
L6: Page title:
L7: cal B(B_s^0 -> mu+ mu-)
L8: 
L9: Relevant table extract, units 10^-9:
L10: Average: 3.45 +/- 0.29, Delta chi^2 = 3.94, p = 0.27 (ndf=3).

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L7: cal B(B_s^0 -> mu+ mu-)
L8: 
L9: Relevant table extract, units 10^-9:
L10: Average: 3.45 +/- 0.29, Delta chi^2 = 3.94, p = 0.27 (ndf=3).
L11: Comment: The ATLAS measurement is correlated with B(B^0 -> mu+ mu-). This correlation is not taken into account in the HFLAV average.
L12: PDG: 3.01 +0.36 -0.33, with link to pdgLive.
L13: CMS: 3.83 +0.38 -0.36 +0.24 -0.21, reference arXiv:2212.10311.

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L8: 
L9: Relevant table extract, units 10^-9:
L10: Average: 3.45 +/- 0.29, Delta chi^2 = 3.94, p = 0.27 (ndf=3).
L11: Comment: The ATLAS measurement is correlated with B(B^0 -> mu+ mu-). This correlation is not taken into account in the HFLAV average.
L12: PDG: 3.01 +0.36 -0.33, with link to pdgLive.
L13: CMS: 3.83 +0.38 -0.36 +0.24 -0.21, reference arXiv:2212.10311.
L14: LHCb: 3.09 +0.46 -0.43 +0.15 -0.11, reference Phys. Rev. D 105, 012010 (2022).

fallback_value_uncertainty SM BR(B_s -> mu+ mu-) = (3.64 +- 0.12)e-9

RESOLVED

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L13: Abstract excerpt:
L14: The rare decay B_s -> mu+ mu- provides an important constraint on possible deviations from the Standard Model in b-s-l-l interactions.
L15: The present weighted average of its branching ratio measurements amounts to (3.34 +/- 0.27) x 10^-9.
L16: The theoretical prediction is (3.64 +/- 0.12) x 10^-9 within the Standard Model.
L17: The paper reviews calculations that have contributed to this prediction and discusses the associated uncertainties.

fallback_value_uncertainty BR(B_s0 -> mu+ mu-) = (3.83 +- 380000000)e-9

AMBIGUOUS

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L13: Data set: CMS 2016-2018 pp collisions at sqrt(s)=13 TeV, integrated luminosity 140 fb^-1.
L14: 
L15: Extract from arXiv TeX source BPH-21-006_temp.tex:
L16: The B_s0 -> mu+mu- branching fraction is
L17: [3.83 +0.38 -0.36 (stat) +0.19 -0.16 (syst) +0.14 -0.13 (f_s/f_u)] x 10^-9.
L18: The B^0 -> mu+mu- branching fraction central fit is
L19: [0.37 +0.75 -0.67 (stat) +0.08 -0.09 (syst)] x 10^-10.

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L14: 
L15: Extract from arXiv TeX source BPH-21-006_temp.tex:
L16: The B_s0 -> mu+mu- branching fraction is
L17: [3.83 +0.38 -0.36 (stat) +0.19 -0.16 (syst) +0.14 -0.13 (f_s/f_u)] x 10^-9.
L18: The B^0 -> mu+mu- branching fraction central fit is
L19: [0.37 +0.75 -0.67 (stat) +0.08 -0.09 (syst)] x 10^-10.
L20:

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L15: Extract from arXiv TeX source BPH-21-006_temp.tex:
L16: The B_s0 -> mu+mu- branching fraction is
L17: [3.83 +0.38 -0.36 (stat) +0.19 -0.16 (syst) +0.14 -0.13 (f_s/f_u)] x 10^-9.
L18: The B^0 -> mu+mu- branching fraction central fit is
L19: [0.37 +0.75 -0.67 (stat) +0.08 -0.09 (syst)] x 10^-10.
L20: 
L21: Abstract excerpt:

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L19: [0.37 +0.75 -0.67 (stat) +0.08 -0.09 (syst)] x 10^-10.
L20: 
L21: Abstract excerpt:
L22: The branching fraction of the B_s0 -> mu+mu- decay and the effective B_s0 meson lifetime are the most precise single measurements to date. No evidence for the B^0 -> mu+mu- decay has been found.

fallback_value_uncertainty BR(B_s0 -> mu+ mu-) = (3.09 +- 460000000)e-9

AMBIGUOUS

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L14: Data set: pp collisions at sqrt(s)=7, 8, and 13 TeV, corresponding to 1, 2, and 6 fb^-1.
L15: 
L16: Extract from arXiv TeX source:
L17: B_s0 -> mu+mu- branching fraction:
L18: (3.09 +0.46 +0.15 -0.43 -0.11) x 10^-9.
L19: B_s0 -> mu+mu- effective lifetime:
L20: (2.07 +/- 0.29 +/- 0.03) ps.

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L15: 
L16: Extract from arXiv TeX source:
L17: B_s0 -> mu+mu- branching fraction:
L18: (3.09 +0.46 +0.15 -0.43 -0.11) x 10^-9.
L19: B_s0 -> mu+mu- effective lifetime:
L20: (2.07 +/- 0.29 +/- 0.03) ps.
L21: Ratio between B0 -> mu+mu- and B_s0 -> mu+mu- branching fractions:

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L16: Extract from arXiv TeX source:
L17: B_s0 -> mu+mu- branching fraction:
L18: (3.09 +0.46 +0.15 -0.43 -0.11) x 10^-9.
L19: B_s0 -> mu+mu- effective lifetime:
L20: (2.07 +/- 0.29 +/- 0.03) ps.
L21: Ratio between B0 -> mu+mu- and B_s0 -> mu+mu- branching fractions:
L22: R_{mu+mu-} < 0.095 at 95% confidence level.

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L18: (3.09 +0.46 +0.15 -0.43 -0.11) x 10^-9.
L19: B_s0 -> mu+mu- effective lifetime:
L20: (2.07 +/- 0.29 +/- 0.03) ps.
L21: Ratio between B0 -> mu+mu- and B_s0 -> mu+mu- branching fractions:
L22: R_{mu+mu-} < 0.095 at 95% confidence level.

fallback_value_uncertainty BR(B_s0 -> mu+ mu-), ATLAS Run 1 + 2015-2016 combination = (2.8 +- 800000000)e-9

AMBIGUOUS

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L9: Local snapshot purpose: ATLAS primary input listed in HFLAV/PDG averages.
L10: 
L11: ArXiv abstract excerpt:
L12: A study of the decays B_s0 -> mu+mu- and B0 -> mu+mu- has been performed using 26.3 fb^-1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016.
L13: The 2015-2016 data alone give B(B_s0 -> mu+mu-) = (3.2 +1.1 -1.0) x 10^-9 and B(B0 -> mu+mu-) < 4.3 x 10^-10 at 95% confidence level.
L14: The result combined with the Run 1 ATLAS result gives B(B_s0 -> mu+mu-) = (2.8 +0.8 -0.7) x 10^-9 and B(B0 -> mu+mu-) < 2.1 x 10^-10 at 95% confidence level.
L15: The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B0 -> mu+mu-) - B(B_s0 -> mu+mu-) plane.

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L10: 
L11: ArXiv abstract excerpt:
L12: A study of the decays B_s0 -> mu+mu- and B0 -> mu+mu- has been performed using 26.3 fb^-1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016.
L13: The 2015-2016 data alone give B(B_s0 -> mu+mu-) = (3.2 +1.1 -1.0) x 10^-9 and B(B0 -> mu+mu-) < 4.3 x 10^-10 at 95% confidence level.
L14: The result combined with the Run 1 ATLAS result gives B(B_s0 -> mu+mu-) = (2.8 +0.8 -0.7) x 10^-9 and B(B0 -> mu+mu-) < 2.1 x 10^-10 at 95% confidence level.
L15: The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B0 -> mu+mu-) - B(B_s0 -> mu+mu-) plane.

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L11: ArXiv abstract excerpt:
L12: A study of the decays B_s0 -> mu+mu- and B0 -> mu+mu- has been performed using 26.3 fb^-1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016.
L13: The 2015-2016 data alone give B(B_s0 -> mu+mu-) = (3.2 +1.1 -1.0) x 10^-9 and B(B0 -> mu+mu-) < 4.3 x 10^-10 at 95% confidence level.
L14: The result combined with the Run 1 ATLAS result gives B(B_s0 -> mu+mu-) = (2.8 +0.8 -0.7) x 10^-9 and B(B0 -> mu+mu-) < 2.1 x 10^-10 at 95% confidence level.
L15: The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B0 -> mu+mu-) - B(B_s0 -> mu+mu-) plane.

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L12: A study of the decays B_s0 -> mu+mu- and B0 -> mu+mu- has been performed using 26.3 fb^-1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016.
L13: The 2015-2016 data alone give B(B_s0 -> mu+mu-) = (3.2 +1.1 -1.0) x 10^-9 and B(B0 -> mu+mu-) < 4.3 x 10^-10 at 95% confidence level.
L14: The result combined with the Run 1 ATLAS result gives B(B_s0 -> mu+mu-) = (2.8 +0.8 -0.7) x 10^-9 and B(B0 -> mu+mu-) < 2.1 x 10^-10 at 95% confidence level.
L15: The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B0 -> mu+mu-) - B(B_s0 -> mu+mu-) plane.