Catalog · Beauty · B021

B021 $\Lambda_b^0 \to \Lambda \ell^+\ell^-$

Baryonic b $ \to $ s l+l- decay $\Lambda_b0 \to \Lambda$ l+l-

Status REVIEWED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
CDF observed $\Lambda_b \to \Lambda \mu^+ \mu^-$ signal yield	24 signal events	2011	CDF2011:B021:signal_events	source ↑
CDF $\Lambda_b \to \Lambda \mu^+ \mu^-$ observation significance	5.8 Gaussian standard deviations	2011	CDF2011:B021:significance	source ↑
CDF $\Lambda_b \to \Lambda \mu^+ \mu^-$ dataset integrated luminosity	6.8 fb^-1	2011	CDF2011:B021:integrated_luminosity	source ↑
CDF $\Lambda_b \to \Lambda \mu^+ \mu^-$ dataset center-of-mass energy	1.96 TeV	2011	CDF2011:B021:center_of_mass_energy	source ↑
$B(\Lambda_b \to \Lambda \mu^+ \mu^-$), CDF observation	1.73e-6 branching fraction	2011	CDF2011:B021:branching_fraction	source ↑

Why this constrains the RS scan

This mode probes $\Delta B=1$ semileptonic FCNC operators rather than the $\Delta F=2$ four-quark basis implemented for neutral meson mixing. Warped flavor models can generate flavor-changing $Z$ couplings, KK electroweak exchange, and loop-induced dipoles feeding $C_7$, $C_9$, $C_{10}$, and primed semileptonic coefficients. The baryon final state adds spin and polarization information unavailable in $B\to K^{(*)}\ell^+\ell^-$, so it is a complementary diagnostic of chirality and hadronic systematics.

What's changed since the original paper

The arXiv:0804.1954 RS-flavor baseline predates this channel as an observed rare decay (CsakiFalkowskiWeiler2008:CompositeFlavor). CDF reported the first observation in 2011 with 24 signal events, 5.8 standard-deviation significance, and $\mathcal B=(1.73\pm0.42\pm0.55)\times10^{-6}$ (CDF2011:LambdaBToLambdaMuMu). LHCb later used $3.0\,{\rm fb}^{-1}$ to publish the $q^2$-binned branching-fraction and angular analysis quoted above (LHCb2015:LambdaBToLambdaMuMu). Lattice-QCD work by Detmold, Lin, Meinel, and Wingate supplied post-2008 $\Lambda_b\to\Lambda$ form factors, so this baryonic mode should not be folded into the mesonic $B\to K^{(*)}\ell\ell$ treatment (DetmoldLinMeinelWingate2013:LambdaBToLambdaFormFactors).

Validity and model dependence

This is a useful catalog constraint but not a single-number hard bound. The interpretation depends on $q^2$ binning, charmonium vetoes, normalization to $\Lambda_b^0\to J/\psi\Lambda$, baryonic form factors, and nonlocal charm effects. Its most robust role is as a $\Delta B=1$ semileptonic cross-check with different hadronic systematics from mesonic rare $B$ decays.

Code coverage in this repo

NO. The modern phenomenology surface lists only $\epsilon_K$, $K$, $B_d$, $B_s$, and $D^0$ at quarkConstraints/modern/phenomenology.py:23. The only live lepton-flavor module is the $\mu\to e\gamma$ dipole check at flavorConstraints/muToEGamma.py:75. Required plan greps and targeted searches for $\Lambda_b$, $b\to s$, $C_9$, $C_{10}$, and semileptonic rare decays found no B021 implementation.

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

Implementation difficulty

HIGH. A production constraint would need a new $\Delta B=1$ semileptonic Hamiltonian, RS matching onto $C_7,C_9,C_{10}$ and primed operators, baryonic form factors, $q^2$-bin predictions, angular likelihoods, and experimental covariance/normalization treatment.

Key references

PDG2025:LambdaBToLambdaMuMu, LHCb2015:LambdaBToLambdaMuMu, CDF2011:LambdaBToLambdaMuMu, DetmoldLinMeinelWingate2013:LambdaBToLambdaFormFactors, and CsakiFalkowskiWeiler2008:CompositeFlavor.

fallback_value_uncertainty BR(Lambda_b0 -> Lambda mu+ mu-) = 1.08 +- +/- 0.28 10^-6

RESOLVED

Match snapshot line 12

L9: PDGID S040.26: Lambda_b()0 --> Lambda mu+ mu-
L10: Edition: 2025
L11: PDG summary value from Lambda_b^0 decay-mode table:
L12:   B(Lambda_b^0 --> Lambda mu+ mu-) = (1.08 +- 0.28) x 10^-6
L13:   daughter momentum p = 2695 MeV/c
L14: 
L15: PDG API related data for S040.26:

fallback_value_uncertainty

dBR/dq2(Lambda_b0 -> Lambda mu+ mu-) = 1.18 +- +0.09/-0.08 stat, +/-0.03 syst, +/-0.27 normalization 10^-7 (GeV^2/c^4)^-1

RESOLVED

Match snapshot line 18

L15:   Evidence of signal was observed below the square of the J/psi mass.
L16:   For 15 < q^2 < 20 GeV^2/c^4:
L17:     dB(Lambda_b^0 -> Lambda mu+ mu-) / dq^2
L18:       = (1.18 +0.09 -0.08 +-0.03 +-0.27) x 10^-7 (GeV^2/c^4)^-1
L19:     uncertainties are statistical, systematic, and due to the normalization
L20:     mode Lambda_b^0 -> J/psi Lambda.
L21:   Angular observables for 15 < q^2 < 20 GeV^2/c^4:

fallback_value_uncertainty A_FB^l(Lambda_b0 -> Lambda mu+ mu-) = -0.05 +- +/-0.09 stat, +/-0.03 syst

RESOLVED

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L19:     uncertainties are statistical, systematic, and due to the normalization
L20:     mode Lambda_b^0 -> J/psi Lambda.
L21:   Angular observables for 15 < q^2 < 20 GeV^2/c^4:
L22:     A_FB^l = -0.05 +- 0.09(stat) +- 0.03(syst)
L23:     A_FB^h = -0.29 +- 0.07(stat) +- 0.03(syst)
L24: 
L25: Catalog note:

fallback_value_uncertainty A_FB^h(Lambda_b0 -> Lambda mu+ mu-) = -0.29 +- +/-0.07 stat, +/-0.03 syst

RESOLVED

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L20:     mode Lambda_b^0 -> J/psi Lambda.
L21:   Angular observables for 15 < q^2 < 20 GeV^2/c^4:
L22:     A_FB^l = -0.05 +- 0.09(stat) +- 0.03(syst)
L23:     A_FB^h = -0.29 +- 0.07(stat) +- 0.03(syst)
L24: 
L25: Catalog note:
L26:   This is the primary modern LHCb source for the q^2-binned differential

fallback_value_uncertainty CDF observed Lambda_b -> Lambda mu+ mu- signal yield = 24 signal events

RESOLVED

Match snapshot line 14

L11: 
L12: Key extracted values from the arXiv abstract:
L13:   First observation of Lambda_b -> Lambda mu+ mu-.
L14:   Signal yield: 24 signal events.
L15:   Statistical significance: 5.8 Gaussian standard deviations.
L16:   Dataset: 6.8 fb^-1 of ppbar collisions at sqrt(s) = 1.96 TeV collected by CDF II.
L17:   Branching fraction:

fallback_value_uncertainty CDF Lambda_b -> Lambda mu+ mu- observation significance = 5.8 Gaussian standard deviations

RESOLVED

Match snapshot line 15

L12: Key extracted values from the arXiv abstract:
L13:   First observation of Lambda_b -> Lambda mu+ mu-.
L14:   Signal yield: 24 signal events.
L15:   Statistical significance: 5.8 Gaussian standard deviations.
L16:   Dataset: 6.8 fb^-1 of ppbar collisions at sqrt(s) = 1.96 TeV collected by CDF II.
L17:   Branching fraction:
L18:     B(Lambda_b -> Lambda mu+ mu-) = (1.73 +-0.42(stat) +-0.55(syst)) x 10^-6

fallback_value_uncertainty CDF Lambda_b -> Lambda mu+ mu- dataset integrated luminosity = 6.8 fb^-1

RESOLVED

Match snapshot line 16

L13:   First observation of Lambda_b -> Lambda mu+ mu-.
L14:   Signal yield: 24 signal events.
L15:   Statistical significance: 5.8 Gaussian standard deviations.
L16:   Dataset: 6.8 fb^-1 of ppbar collisions at sqrt(s) = 1.96 TeV collected by CDF II.
L17:   Branching fraction:
L18:     B(Lambda_b -> Lambda mu+ mu-) = (1.73 +-0.42(stat) +-0.55(syst)) x 10^-6
L19:

fallback_value_uncertainty CDF Lambda_b -> Lambda mu+ mu- dataset center-of-mass energy = 1.96 TeV

RESOLVED

Match snapshot line 16

L13:   First observation of Lambda_b -> Lambda mu+ mu-.
L14:   Signal yield: 24 signal events.
L15:   Statistical significance: 5.8 Gaussian standard deviations.
L16:   Dataset: 6.8 fb^-1 of ppbar collisions at sqrt(s) = 1.96 TeV collected by CDF II.
L17:   Branching fraction:
L18:     B(Lambda_b -> Lambda mu+ mu-) = (1.73 +-0.42(stat) +-0.55(syst)) x 10^-6
L19:

fallback_value_uncertainty B(Lambda_b -> Lambda mu+ mu-), CDF observation = (1.73 +- 0.42)e-6

RESOLVED

Match snapshot line 18

L15:   Statistical significance: 5.8 Gaussian standard deviations.
L16:   Dataset: 6.8 fb^-1 of ppbar collisions at sqrt(s) = 1.96 TeV collected by CDF II.
L17:   Branching fraction:
L18:     B(Lambda_b -> Lambda mu+ mu-) = (1.73 +-0.42(stat) +-0.55(syst)) x 10^-6
L19: 
L20: Catalog note:
L21:   This source establishes that the process was unobserved in the arXiv:0804.1954