Observation of the charmless purely baryonic decay Λ_b⁰ to Λ p bar{p} at LHCb
Pith reviewed 2026-05-10 16:29 UTC · model grok-4.3
The pith
LHCb reports the first observation of the charmless baryonic decay Lambda_b^0 to Lambda p p-bar.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The authors report the first observation of the decay Lambda_b^0 to Lambda p anti-p at 5.1 sigma significance. A simultaneous fit to the invariant mass distributions in long and downstream track categories, using the full Run 2 dataset, yields a relative branching fraction of (5.13 plus or minus 1.28 statistical plus or minus 0.27 systematic) times 10 to the minus 2 in the region where the mass of the two hadrons is below 2.85 GeV, normalized to the topologically similar Lambda_b^0 to Lambda K+ K- mode.
What carries the argument
Simultaneous extended unbinned maximum-likelihood fit to the Lambda p p-bar invariant mass in two track categories, with signal shape modeled by a double Gaussian and backgrounds by polynomials and exponential functions.
If this is right
- This decay provides a new benchmark for theoretical calculations of charmless baryonic branching fractions in b-hadron decays.
- The measured ratio constrains the size of non-factorizable contributions in weak baryonic transitions.
- The analysis technique of simultaneous fitting across track categories can be applied to searches for other rare purely baryonic modes.
- Further studies of this channel can include measurements of its angular distributions to test conservation laws.
Where Pith is reading between the lines
- The existence of this decay at the observed rate suggests that searches for similar charmless baryonic modes in other b-hadrons may yield additional observations.
- This result supplies an experimental anchor point for models attempting to predict absolute branching fractions in baryonic B decays, which have historically been difficult to calculate.
- With more data, CP violation or polarization studies in this final state could become feasible and test for beyond-Standard-Model effects.
Load-bearing premise
Efficiencies, acceptances, and background shapes for both the signal and normalization modes are modeled correctly in the fit, with no large unaccounted contributions from misidentified particles or unknown sources.
What would settle it
An independent analysis of higher-luminosity data showing no excess at the Lambda_b mass or a fitted significance below 3 sigma after systematics would falsify the observation.
read the original abstract
The first observation of a charmless purely baryonic decay, $\Lambda_b^{0} \to \Lambda p \bar{p}$, is reported using the full Run 2 LHCb dataset, corresponding to an integrated luminosity of $6.0~fb^{-1}$. The branching fraction is measured relative to that of the topologically similar normalisation mode $\Lambda_b^{0} \to \Lambda K^+K^-$. A simultaneous fit to the long- and downstream-track categories yields a signal significance of $5.1\sigma$ after including systematic uncertainties. The relative branching fraction is measured to be $\left(5.13 \pm 1.28_{\rm stat} \pm 0.27_{\rm syst}\right)\times 10^{-2}$ in the region $m(h^+h^-)< 2.85$ GeV.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports the first observation of the charmless purely baryonic decay Λ_b^0 → Λ p p-bar using the full Run 2 LHCb dataset (6.0 fb^{-1}). A simultaneous unbinned fit to the Λp p-bar and ΛK^+K^- invariant-mass distributions in long- and downstream-track categories yields a signal significance of 5.1σ (including systematics) and a relative branching fraction of (5.13 ± 1.28_stat ± 0.27_syst) × 10^{-2} in the region m(h^+ h^-) < 2.85 GeV, normalized to the topologically similar mode Λ_b^0 → Λ K^+ K^-.
Significance. If the central result holds, this constitutes the first observation of a purely baryonic charmless b-hadron decay. Such a measurement provides new experimental input on non-perturbative QCD dynamics in baryonic final states without charm quarks and complements existing studies of baryon decays at LHCb. The relative normalization to a similar-topology mode and the use of the complete Run 2 sample are strengths that reduce certain classes of systematic uncertainty.
major comments (2)
- [Section on the invariant-mass fit and background modeling] The 5.1σ significance and the extracted yield both rest on the simultaneous fit's description of combinatorial, partially reconstructed, and potential peaking backgrounds. The manuscript should supply explicit validation (sideband fits, alternative polynomial/exponential parametrizations, or toy-MC coverage) demonstrating that misidentification backgrounds such as Λ_b^0 → Λ π^+ π^- or feed-down from Λ_c^+ decays do not bias the signal region; without these tests the quoted significance remains sensitive to the background-model choice.
- [Section on efficiency and acceptance determination] Efficiencies and acceptances for signal and normalization modes after all PID, tracking, and vertexing selections are central to the relative branching-fraction result. The paper must detail the data-driven corrections applied and quantify any residual differences between the two modes that could shift the reported ratio by an amount comparable to the quoted 0.27 systematic uncertainty.
minor comments (2)
- [Abstract] The abstract states the phase-space restriction m(h^+ h^-) < 2.85 GeV but does not indicate whether this cut is applied to both signal and normalization or only to the signal; a brief clarification would improve readability.
- [Figures showing invariant-mass distributions] Figure captions for the mass distributions should explicitly list the fitted components (signal, combinatorial background, partially reconstructed background) and the track categories shown.
Simulated Author's Rebuttal
We thank the referee for their careful reading of the manuscript and constructive comments. We address each major comment below and have revised the manuscript to strengthen the presentation of the background modeling and efficiency corrections.
read point-by-point responses
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Referee: [Section on the invariant-mass fit and background modeling] The 5.1σ significance and the extracted yield both rest on the simultaneous fit's description of combinatorial, partially reconstructed, and potential peaking backgrounds. The manuscript should supply explicit validation (sideband fits, alternative polynomial/exponential parametrizations, or toy-MC coverage) demonstrating that misidentification backgrounds such as Λ_b^0 → Λ π^+ π^- or feed-down from Λ_c^+ decays do not bias the signal region; without these tests the quoted significance remains sensitive to the background-model choice.
Authors: We agree that additional explicit validation of the background model strengthens the result. The original manuscript describes the simultaneous unbinned fit, with combinatorial background modeled by an exponential function, partially reconstructed backgrounds by dedicated shapes, and peaking backgrounds from misidentification and feed-down considered via simulation. To address the request, we have added sideband fits (excluding the signal region), tests of alternative parametrizations (including higher-order polynomials and different exponential slopes), and toy-MC coverage studies in the revised version. These checks demonstrate that potential biases from Λ_b^0 → Λ π^+ π^- misidentification and Λ_c^+ feed-down are negligible within the quoted uncertainties, and the signal yield and 5.1σ significance (including systematics) remain stable. A new subsection and supplementary figures have been included to document these validations. revision: yes
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Referee: [Section on efficiency and acceptance determination] Efficiencies and acceptances for signal and normalization modes after all PID, tracking, and vertexing selections are central to the relative branching-fraction result. The paper must detail the data-driven corrections applied and quantify any residual differences between the two modes that could shift the reported ratio by an amount comparable to the quoted 0.27 systematic uncertainty.
Authors: We concur that a more detailed exposition of the efficiency corrections is warranted. The manuscript already employs simulation for geometric acceptance and reconstruction efficiencies, supplemented by data-driven corrections: PID efficiencies calibrated using dedicated control samples, tracking efficiencies determined via tag-and-probe methods on data, and vertexing efficiencies validated with control decays. In the revision, we have expanded the relevant section to explicitly tabulate the corrections applied to both the signal (Λ p p-bar) and normalization (Λ K^+ K^-) modes, and we quantify the residual differences after all corrections. These residuals are small (contributing less than 30% of the 0.27 systematic uncertainty on the ratio) and do not introduce a shift comparable to the quoted uncertainty. The efficiency ratio and its uncertainty are now presented with greater transparency. revision: yes
Circularity Check
Direct data-driven measurement with no circular derivation
full rationale
The paper reports an experimental observation of a rare decay mode using LHCb Run 2 data. The central result—the 5.1σ significance and relative branching fraction—is obtained from a simultaneous unbinned maximum-likelihood fit to the invariant-mass distributions of the signal and normalization channels in two track categories. No equations, ansätze, or self-citations reduce the reported yield or branching fraction to a quantity defined by the authors' prior work; the fit parameters are determined directly from the present dataset, with efficiencies and backgrounds modeled using standard, externally validated LHCb procedures. The analysis is therefore self-contained against external benchmarks and exhibits no load-bearing circularity.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Standard assumptions in LHCb data analysis such as accurate Monte Carlo simulation for efficiencies and background shapes.
Reference graph
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