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arxiv: 1906.08302 · v1 · pith:MUWJKZIQnew · submitted 2019-06-19 · ✦ hep-ex

Measurements of γ from tree-level decays

A. Bertolin This is my paper

Pith reviewed 2026-05-25 19:34 UTC · model grok-4.3

classification ✦ hep-ex
keywords CKM angle gammatree-level decaysLHCbbeauty mesonscharm mesonsCP violationcombination of measurements
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The pith

LHCb reaches best precision on the CKM angle γ by combining up to 16 tree-level decay measurements.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents the LHCb strategy for extracting the CKM angle γ from tree-level decays of beauty and charm mesons. Up to 16 independent determinations from different modes are available, each with its own sensitivity. The highest accuracy is obtained by combining all results rather than relying on any single channel. A sympathetic reader would care because γ fixes one vertex of the unitarity triangle and thereby tests the Standard Model account of CP violation in the quark sector.

Core claim

The best accuracy is reached by combining all of the available results from up to 16 independent determinations using a variety of beauty and charm mesons decay modes.

What carries the argument

The weighted combination of multiple independent γ measurements from distinct decay modes, each contributing according to its sensitivity.

If this is right

  • The combined value supplies the tightest current constraint on γ from tree-level processes.
  • Each additional mode with good sensitivity further reduces the total uncertainty when added to the average.
  • The approach directly tests consistency among different beauty and charm final states under the assumption of a single CKM phase.
  • The result can be compared with determinations from loop-mediated processes to search for discrepancies.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Future LHCb data-taking periods will increase the weight of modes that currently have lower statistics.
  • The same combination method could be applied to other CKM angles once sufficient independent tree-level channels become available.
  • Any unaccounted systematic correlation that grows with integrated luminosity would eventually limit further improvement.

Load-bearing premise

The determinations from different decay modes have comparable or properly weighted sensitivities and minimal unaccounted correlations in the combination procedure.

What would settle it

A large inconsistency appearing between the combined result and a new high-statistics measurement from a single mode that was already included in the average would indicate that correlations or sensitivities were misestimated.

Figures

Figures reproduced from arXiv: 1906.08302 by A. Bertolin.

Figure 1
Figure 1. Figure 1: FIG. 1: Invariant mass distributions of (left) [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: Observed invariant mass distribution for (right) [PITH_FULL_IMAGE:figures/full_fig_p002_4.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Graphical representation of the [PITH_FULL_IMAGE:figures/full_fig_p002_3.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6 [PITH_FULL_IMAGE:figures/full_fig_p003_6.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p003_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: Fit to the [PITH_FULL_IMAGE:figures/full_fig_p003_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8: Folded time asymmetry for (left) [PITH_FULL_IMAGE:figures/full_fig_p004_8.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10: 1-CL plots for combinations split by the initial [PITH_FULL_IMAGE:figures/full_fig_p004_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11: Comparison between the present LHCb [PITH_FULL_IMAGE:figures/full_fig_p005_11.png] view at source ↗
read the original abstract

The LHCb approach for a precise determination of the CKM angle $\gamma$ from tree--level decays is presented. Up to 16 independent determinations, using a variety of beauty and charm mesons decay modes, are available at LHCb. Not all of them have the same sensitivity to $\gamma$. The best accuracy is reached by combining all of the available results. These proceedings review some of the independent determinations used in the combination and present the latest combined result available, as of Summer 2018.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The manuscript presents the LHCb approach for determining the CKM angle γ from tree-level B and D meson decays. It states that up to 16 independent measurements from a variety of decay modes are available, notes that not all have equal sensitivity to γ, and asserts that the best precision is achieved by combining all results. Some individual determinations are reviewed and the latest combined result (as of Summer 2018) is given.

Significance. A properly validated combination of multiple tree-level γ measurements would supply the most precise LHCb determination of this angle from direct decays, providing an important input to CKM unitarity tests and new-physics searches. The experimental nature of the inputs (LHCb data) is a strength when the weighting and correlation treatment can be shown to be sound.

major comments (1)
  1. [Abstract] Abstract: The claim that 'the best accuracy is reached by combining all of the available results' is load-bearing for the paper's central message, yet the text supplies no information on how the measurements are weighted by sensitivity or on the construction and validation of the covariance matrix that accounts for shared experimental or theoretical systematics across modes. Without this, the optimality of the quoted combined uncertainty cannot be assessed.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive comment on the manuscript. We address the point raised below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The claim that 'the best accuracy is reached by combining all of the available results' is load-bearing for the paper's central message, yet the text supplies no information on how the measurements are weighted by sensitivity or on the construction and validation of the covariance matrix that accounts for shared experimental or theoretical systematics across modes. Without this, the optimality of the quoted combined uncertainty cannot be assessed.

    Authors: This manuscript is a short proceedings contribution that provides an overview of the LHCb tree-level γ measurements and states the combined result. The detailed combination procedure—including the weighting of the up to 16 individual measurements according to their sensitivity to γ, the construction of the covariance matrix for shared experimental and theoretical systematics, and its validation—is described in a dedicated LHCb combination paper. We will revise the text to include an explicit reference to that work together with a brief clarifying sentence on the combination method. revision: yes

Circularity Check

0 steps flagged

No significant circularity; experimental combination of independent measurements

full rationale

The paper reports LHCb experimental measurements of the CKM angle γ from tree-level decays across up to 16 modes. The central claim is that the best accuracy comes from combining these results, which is a statistical aggregation of data-driven determinations rather than a mathematical derivation. No equations or steps reduce by construction to their own inputs, no fitted parameters are relabeled as predictions, and no self-citation chains serve as load-bearing justifications for uniqueness or ansatzes. The work is self-contained against external experimental benchmarks (detector data), with the combination procedure relying on measured sensitivities rather than internal redefinitions.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The claim rests on the validity of the CKM framework, the assumption that tree-level decays minimize theoretical uncertainties, and the statistical independence of the decay mode measurements.

free parameters (1)
  • combined γ
    The central value of γ is obtained by fitting the combination of measurements.
axioms (1)
  • domain assumption Tree-level decays have negligible theoretical uncertainties from higher-order effects.
    The paper focuses on tree-level decays to allow precise extraction of γ.

pith-pipeline@v0.9.0 · 5593 in / 1019 out tokens · 30707 ms · 2026-05-25T19:34:16.312655+00:00 · methodology

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Reference graph

Works this paper leans on

11 extracted references · 11 canonical work pages · 1 internal anchor

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