Recognition: unknown
Measurement of γ using B^{pm}rightarrow DK^{pm} and B^{pm}rightarrow Dπ^{pm} decays with Drightarrow K_{rm S}⁰π⁺π⁻ and Drightarrow K_{rm S}⁰K⁺K⁻
Pith reviewed 2026-05-07 04:00 UTC · model grok-4.3
The pith
LHCb measures the CKM angle γ to (68.1 ± 6.7)° from B± → DK± and B± → Dπ± decays with D → KS0 π+π− and D → KS0 K+K−.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using a data sample corresponding to 5.8 fb−1 collected in 2024 by the upgraded LHCb experiment, the collaboration observes CP violation through differences in the Dalitz plot distributions of D → KS0 π+π− and D → KS0 K+K− decays between B+ and B− mesons. The CKM angle γ is determined to be (68.1 ± 6.7)°. Additional parameters describing the B± → DK± and B± → Dπ± amplitudes are measured simultaneously.
What carries the argument
The Dalitz-plot amplitude models for D → KS0 π+π− and D → KS0 K+K−, which map the strong-phase variation across the three-body phase space and thereby convert the observed B+ versus B− asymmetry into a value for the weak phase γ.
If this is right
- The measured γ supplies a new constraint on the apex of the CKM unitarity triangle.
- The fitted values of the amplitude ratios r_B and strong phases δ_B for the DK and Dπ modes can be used in global fits of CKM parameters.
- The result serves as a benchmark for the performance of the upgraded LHCb detector in precision flavor-physics measurements.
- The analysis framework can be applied to future data sets with higher integrated luminosity to reduce the uncertainty on γ.
Where Pith is reading between the lines
- Averaging this result with independent γ determinations from other decay channels would tighten the overall world constraint on the CKM unitarity triangle.
- Improved external constraints on the D decay amplitudes, for example from larger charm-factory data sets, would directly reduce the dominant systematic uncertainty on γ.
- The same interference technique could be extended to related modes such as B → D* K to cross-check the extracted strong phases.
Load-bearing premise
The amplitude models used to describe the D meson Dalitz plots are accurate enough that any mismatch with reality does not shift the extracted value of γ.
What would settle it
An independent analysis of the same B decay modes but with an alternative, data-driven parameterization of the D decay amplitudes that yields a central value for γ outside the interval 55°–81° would indicate that the reported result is biased by the model choice.
Figures
read the original abstract
A measurement of the CKM angle $\gamma$ using the decay channels $B^{\pm}\rightarrow DK^{\pm}$ and $B^{\pm}\rightarrow D\pi^{\pm}$, where the $D$ meson decays to $D\rightarrow K_{\rm S}^{0}\pi^{+}\pi^{-}$ or $D\rightarrow K_{\rm S}^{0}K^{+}K^{-}$, is performed with a data sample corresponding to an integrated luminosity of 5.8 fb$^{-1}$, collected during 2024 by the upgraded LHCb experiment. $C\!P$ violation is observed through a difference in the distributions of the Dalitz plot of the $D$ decay between the $B^{+}$ and $B^{-}$ mesons. The CKM angle $\gamma$ is determined to be $\gamma=(68.1\pm 6.7)^{\circ}$. Other parameters related to the examined $B$ meson decay modes are also measured. This is the first measurement of the CKM angle $\gamma$ using the upgraded LHCb detector.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports a measurement of the CKM angle γ using B±→DK± and B±→Dπ± decays where the D meson decays to KS0π+π− or KS0K+K−. With a data sample of 5.8 fb−1 collected in 2024 by the upgraded LHCb detector, the analysis observes CP violation through differences in the Dalitz-plot distributions between B+ and B− and extracts γ=(68.1±6.7)°. Additional hadronic parameters for the B decays are measured, and the result is presented as the first γ determination with the upgraded detector.
Significance. If the result holds, this provides a competitive, independent determination of γ at the ~10% level that contributes to the world average and to tests of CKM unitarity. The explicit observation of CP violation in these modes and the deployment of the upgraded LHCb data set are clear strengths, demonstrating improved detector performance for Dalitz-plot analyses in flavor physics.
major comments (1)
- [§5 (Results)] §5 (Results): The central value of γ is obtained from a fit that fixes the D-decay amplitude models imported from prior experiments. No dedicated variation study (e.g., alternative resonance parametrizations or strong-phase adjustments) is reported that quantifies the shift in γ; because this model accuracy is the weakest assumption for an unbiased extraction, the quoted ±6.7° uncertainty may be underestimated without such a check.
minor comments (1)
- [Abstract] The abstract would be clearer if the uncertainty on γ were explicitly separated into statistical and systematic components.
Simulated Author's Rebuttal
We thank the referee for the positive evaluation of the significance of our measurement and for the constructive comment on the treatment of the D-decay amplitude model. We address this point in detail below and will revise the manuscript to strengthen the presentation of the associated systematic uncertainty.
read point-by-point responses
-
Referee: The central value of γ is obtained from a fit that fixes the D-decay amplitude models imported from prior experiments. No dedicated variation study (e.g., alternative resonance parametrizations or strong-phase adjustments) is reported that quantifies the shift in γ; because this model accuracy is the weakest assumption for an unbiased extraction, the quoted ±6.7° uncertainty may be underestimated without such a check.
Authors: We agree that the D-decay amplitude model constitutes one of the leading sources of systematic uncertainty in this class of analyses. The model parameters are fixed to the values determined by previous experiments (primarily CLEO and BESIII), which is the standard approach to avoid introducing an unmanageable number of free parameters in the fit. The uncertainty arising from this choice is evaluated by propagating the published uncertainties on the model parameters and is already included in the total ±6.7° uncertainty quoted in the paper. However, we acknowledge that an explicit variation study using alternative resonance parametrizations or adjusted strong phases would provide additional transparency and allow a direct quantification of any shift in the central value of γ. In the revised manuscript we will add such a dedicated study in Section 5, reporting the observed variation in γ as an explicit component of the systematic uncertainty. This will confirm or, if necessary, adjust the robustness of the quoted error. revision: yes
Circularity Check
No significant circularity: data-driven fit to Dalitz plots extracts γ independently
full rationale
The paper performs a maximum-likelihood fit directly to the observed Dalitz-plot distributions in the 2024 LHCb data sample for B± → DK± and B± → Dπ± decays (with D → KS0π+π− and KS0K+K−). The central value γ = (68.1 ± 6.7)° is obtained from the interference parameters (x±, y±) in this fit. D-decay amplitude models are fixed external inputs (from CLEO, BaBar or prior independent analyses), efficiencies come from simulation, and backgrounds are modeled separately; none of these steps reduce the output γ to the input data or to a self-citation by construction. No self-definitional equations, no 'prediction' that is statistically forced by a prior fit, and no load-bearing uniqueness theorem imported from the same authors. The measurement is self-contained and falsifiable against external γ benchmarks.
Axiom & Free-Parameter Ledger
free parameters (2)
- γ
- hadronic parameters
axioms (2)
- domain assumption The Standard Model CKM matrix governs quark flavor mixing and CP violation.
- domain assumption The chosen amplitude models accurately describe the D → KS0π+π− and D → KS0K+K− decays over the Dalitz plot.
Reference graph
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