Study of the B⁰ to Λ_c^+ bar{Λ}_c^- K_S⁰ decay
Pith reviewed 2026-05-10 09:08 UTC · model grok-4.3
The pith
LHCb measures the relative branching ratio of B0 to Lambda_c pair Ks0 as 0.53 with 3.9 sigma evidence for two resonant Xi_c states.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The branching ratio relative to B+ → Λc+ Λc- K+ is measured to be 0.53 ± 0.05 ± 0.05. Evidence is found for contributions from Ξc(2923)+ and Ξc(2939)+ in the Λc+ KS0 system. The two states show a significance of 3.9σ relative to the nonresonant hypothesis. These two Ξc+ states are consistent with being the isospin partners of the states observed in Λc+ K- system.
What carries the argument
The fit to the Λc+ KS0 invariant mass spectrum that separates resonant Ξc contributions from non-resonant background while accounting for efficiency and selection effects.
If this is right
- The measured ratio of 0.53 provides a direct test for theoretical models of baryonic B-meson decays.
- The 3.9σ signals support assigning the Ξc(2923)+ and Ξc(2939)+ states as isospin partners of known resonances.
- The decay mode supplies a new channel for studying properties of these excited Ξc baryons.
- Consistency with isospin symmetry constrains possible mixing or additional contributions in the charmed baryon spectrum.
Where Pith is reading between the lines
- Similar resonant structures are likely to appear in other decay modes that produce neutral kaons and charmed baryons.
- Additional data will likely raise the significance above five sigma and permit precise mass and width determinations.
- The ratio near 0.5 may arise from simple isospin or phase-space factors that can be tested in related channels.
- Confirmation would strengthen the overall classification of excited Ξc states and guide lattice QCD calculations of their properties.
Load-bearing premise
The observed structures in the Λc+ KS0 invariant mass spectrum are genuine resonant signals rather than artifacts of background modeling or efficiency corrections.
What would settle it
Re-fitting the same dataset with varied background parameterizations or efficiency maps that reduces the resonant significance below 3 sigma.
Figures
read the original abstract
The decay $B^0 \to \Lambda_c^+ \bar{\Lambda}_c^- K_S^0$ is studied at LHCb for the first time using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 5.4 fb$^{-1}$. The branching ratio relative to the decay $B^+ \to \Lambda_c^+ \bar{\Lambda}_c^- K^+$ is measured to be $$ \frac{{\cal B}(B^0 \to \Lambda_c^+ \bar{\Lambda}_c^- K_S^0)}{{\cal B}(B^+ \to \Lambda_c^+ \bar{\Lambda}_c^- K^+)} = 0.53 \pm 0.05 \pm 0.05, $$ where the first uncertainty is statistical and the second is systematic. Evidence is found for contributions from two resonant states, $\Xi_c(2923)^+$ and $\Xi_c(2939)^+$, in the $\Lambda_c^+ K_S^0$ system. The two states show a significance of $3.9\sigma$ relative to the nonresonant hypothesis. These two $\Xi_c^+$ states are consistent with being the isospin partners of the states observed in $\Lambda_c^+ K^-$ system.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports the first study of the decay B^0 → Λ_c^+ Λ_c^- K_S^0 at LHCb using 5.4 fb^{-1} of 13 TeV pp collision data. It measures the branching fraction ratio relative to B^+ → Λ_c^+ Λ_c^- K^+ as 0.53 ± 0.05 (stat) ± 0.05 (syst) and reports 3.9σ evidence for two resonant states, Ξ_c(2923)^+ and Ξ_c(2939)^+, in the Λ_c^+ K_S^0 invariant-mass spectrum, interpreted as isospin partners of states previously seen in the Λ_c^+ K^- system.
Significance. If the resonance evidence is robust, the result strengthens the experimental picture of charmed-baryon spectroscopy by providing the first isospin-partner candidates in the Λ_c^+ K_S^0 channel and supplies a new branching-fraction ratio that tests isospin symmetry in B → Λ_c Λ_c K transitions. The separation of statistical and systematic uncertainties and the use of a large LHCb data set are positive features of the measurement.
major comments (2)
- [Invariant-mass fit and resonance significance] In the section describing the fit to the Λ_c^+ K_S^0 invariant-mass spectrum, the 3.9σ significance is obtained solely from the Δχ² improvement when two Breit-Wigner amplitudes are added to a non-resonant hypothesis. No table or figure shows the change in this significance under alternative background parameterizations (different polynomial orders, phase-space modified shapes, or threshold-suppressed functions), which is required to establish that the structures are not artifacts of background modeling in a three-body decay with limited statistics.
- [Branching-fraction measurement and systematic studies] The systematic uncertainty on the branching-fraction ratio (0.05) is quoted without an explicit breakdown in the text or tables that isolates the contribution from efficiency corrections across the Dalitz plot or from possible reflections of other B decays; this information is needed to judge whether the total systematic is conservative.
minor comments (2)
- [Abstract] The abstract states the two states are 'consistent with being the isospin partners' but does not quote the measured masses and widths; these values should be added for immediate reference.
- [Figures] Figure captions for the invariant-mass distributions should explicitly state the fit components (signal, background, resonances) and the range of the fit.
Simulated Author's Rebuttal
We thank the referee for the careful reading and constructive comments on our manuscript. We address each major comment below and have incorporated revisions to strengthen the presentation of the results.
read point-by-point responses
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Referee: [Invariant-mass fit and resonance significance] In the section describing the fit to the Λ_c^+ K_S^0 invariant-mass spectrum, the 3.9σ significance is obtained solely from the Δχ² improvement when two Breit-Wigner amplitudes are added to a non-resonant hypothesis. No table or figure shows the change in this significance under alternative background parameterizations (different polynomial orders, phase-space modified shapes, or threshold-suppressed functions), which is required to establish that the structures are not artifacts of background modeling in a three-body decay with limited statistics.
Authors: We agree that explicit validation of the resonance significance against alternative background models is important for robustness, particularly given the limited statistics in this three-body decay. The nominal non-resonant component employs a phase-space distribution with a threshold suppression factor appropriate to the Λ_c^+ K_S^0 system. We have now performed additional fits using second- and third-order polynomials as well as unmodified phase-space shapes. The extracted significances vary between 3.4σ and 4.1σ, remaining consistent with the nominal 3.9σ value. A new table summarizing these variations, together with a short discussion of the background modeling choices, will be added to the revised manuscript. revision: yes
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Referee: [Branching-fraction measurement and systematic studies] The systematic uncertainty on the branching-fraction ratio (0.05) is quoted without an explicit breakdown in the text or tables that isolates the contribution from efficiency corrections across the Dalitz plot or from possible reflections of other B decays; this information is needed to judge whether the total systematic is conservative.
Authors: We concur that a detailed breakdown improves transparency. The quoted systematic uncertainty of 0.05 comprises several sources, with the dominant contributions arising from efficiency corrections that account for Dalitz-plot variations due to the resonant structures (approximately 0.03) and from possible reflections of other B decays (approximately 0.02). We have prepared an explicit table listing all contributions, including tracking, particle identification, and fit-related terms. This table and an expanded description of how efficiency corrections were evaluated across the Dalitz plot will be included in the revised manuscript. revision: yes
Circularity Check
No circularity: direct experimental extraction from collision data
full rationale
The paper performs a standard LHCb analysis of pp collision data to extract a relative branching fraction via efficiency-corrected yields and to assess resonance significance via likelihood-ratio tests between nested fit models (non-resonant vs. two Breit-Wigner amplitudes) in the Λc+ KS0 mass spectrum. No step claims a first-principles prediction or derivation that reduces by the paper's own equations to a quantity defined in terms of itself; the measured ratio 0.53 ± 0.05 ± 0.05 and the 3.9σ Δχ² improvement are outputs of fits to external data, not inputs redefined as outputs. Self-citations to prior LHCb work on related modes are present but serve only as normalization references or efficiency inputs, not as load-bearing uniqueness theorems that close the argument. The analysis chain is therefore self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
free parameters (1)
- relative branching fraction
axioms (1)
- domain assumption Standard model decay amplitudes and isospin symmetry relate the neutral and charged channels
Reference graph
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