arxiv: 2604.26315 · v1 · submitted 2026-04-29 · ✦ hep-ex

Recognition: unknown

Search for new physics in B to K π π γ with Belle II data

Sahil Saha

Authors on Pith no claims yet

Pith reviewed 2026-05-07 12:43 UTC · model grok-4.3

classification ✦ hep-ex

keywords B meson decaysCP asymmetrynew physicsamplitude analysisBelle IIradiative decayskaonic resonancestime-dependent CP violation

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The pith

An amplitude analysis of B to K pi pi gamma decays is required to measure time-dependent CP asymmetry and search for new physics.

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

The paper argues that time-dependent CP asymmetry measurements in B0 to KS0 pi+ pi- gamma decays are sensitive to physics beyond the Standard Model. To convert such a measurement into actual constraints on new physics, the analysis must isolate contributions from kaonic resonances that are CP eigenstates and exclude those that are not. This distinction demands a dedicated amplitude analysis of the full decay chain from B to K_res gamma to K pi+ pi- gamma. The article discusses how this analysis can be performed using Belle II data to enable the translation from asymmetry to new physics bounds.

Core claim

The measurement of the time-dependent CP-asymmetry of B0 to KS0 pi+ pi- gamma decays is sensitive to contributions from physics beyond the Standard Model. To translate this measurement into a constraint on new physics, it is necessary to distinguish between decay modes that contribute to the final state via a kaonic resonance that is a CP eigenstate, and those that proceed through non-CP eigenstates. This requires an amplitude analysis of the B to K_res gamma to K pi+ pi- gamma decay.

What carries the argument

Amplitude analysis of the B to K_res gamma to K pi+ pi- gamma decay chain, which separates resonant contributions according to whether each resonance is a CP eigenstate.

If this is right

The time-dependent CP asymmetry can be measured specifically for CP-eigenstate resonant modes.
Constraints on new physics parameters can be derived directly from the extracted asymmetry.
Belle II data can support the amplitude analysis with adequate statistics for resonance separation.
Proper mode separation reduces background and systematic effects in the CP asymmetry result.

Where Pith is reading between the lines

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

This separation technique could extend to other multi-body B radiative decays for similar new physics tests.
The method might benefit from cross-checks with simulated datasets before real data application.
Combined analyses across experiments could improve the statistical power once the amplitude framework is established.

Load-bearing premise

The amplitude analysis can reliably separate the resonant contributions in the B to K pi pi gamma final state using the available Belle II data statistics and detector resolution.

What would settle it

A failure of the amplitude fit to separate CP-eigenstate resonances from non-CP ones, due to overlapping contributions or limited resolution in Belle II data, would prevent clean extraction of the CP asymmetry for new physics constraints.

Figures

Figures reproduced from arXiv: 2604.26315 by Sahil Saha.

**Figure 1.** Figure 1: – Feynman diagram for the view at source ↗

**Figure 2.** Figure 2: – Table of intermediate kaonic reso view at source ↗

**Figure 3.** Figure 3: – Simulated view at source ↗

**Figure 3.** Figure 3: – Fit to generator level simulated data for amplitude weights performed on a combination of all decay view at source ↗

read the original abstract

The measurement of the time-dependent CP-asymmetry of $B^0 \to K_S^0 \pi^+ \pi^- \gamma$ decays is sensitive to contributions from physics beyond the Standard Model. To translate this measurement into a constraint on new physics, it is necessary to distinguish between decay modes that contribute to the final state via a kaonic resonance that is a CP eigenstate, and those that proceed through non-CP eigenstates. This requires an amplitude analysis of the $B \to K_{res} \gamma \to K \pi^+ \pi^- \gamma$ decay, that is discussed in this article.

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.

Desk Editor's Note private letter to a colleague

The paper correctly flags why an amplitude analysis is needed to extract BSM-sensitive CP asymmetries from B0 to KS0 pi+ pi- gamma but offers no evidence that the fit is feasible with Belle II yields or resolution.

read the letter

The main point is that time-dependent CP asymmetry measurements in B0 → KS0 π+ π- γ decays need an amplitude analysis of the B → K_res γ → K π+ π- γ chain to isolate contributions from CP-eigenstate resonances versus non-eigenstate ones. Without that separation the extracted asymmetry mixes different physics and loses its power as a new-physics probe. The paper states this requirement plainly and connects it to the relevant resonances like K*(892) and K1(1270). That part is useful and follows standard B-decay logic without overstatement. It earns credit for keeping the motivation focused and for recognizing that the final state is not automatically clean. The soft spot is exactly the one raised in the stress-test note: no demonstration that the analysis can actually be done. There are no toy fits, no count of free parameters in the amplitude model, no discussion of how Belle II mass and angular resolution affect the separation, and no estimate of event yields in the relevant kinematic regions. With only O(10^3–10^4) events typical for this mode, the fit is likely to be underconstrained, yet the paper does not address that risk. The abstract and text remain at the level of “this analysis is discussed,” without showing any result or validation. Citation practice is unremarkable for a short note; nothing is over-cited or missing in an obvious way. This is for experimentalists already working on Belle II flavor analyses who might be planning measurements in this channel. A reader outside that niche or anyone wanting actual constraints or a working method will find little to use. The thinking is clear and internally consistent, so it is not incoherent on its own terms. I would not bring it to a reading group. I would not cite it. It does not yet merit sending to peer review; the authors would need to add feasibility studies or a first attempt at the amplitude fit before it becomes referee-ready.

Referee Report

1 major / 0 minor

Summary. The paper motivates and discusses an amplitude analysis of the decay chain B → K_res γ → K π⁺ π⁻ γ in order to separate contributions from CP-eigenstate resonances (such as K*(892) and K1(1270)) from non-CP-eigenstate modes. This separation is required to extract a clean time-dependent CP asymmetry in B⁰ → K_S⁰ π⁺ π⁻ γ decays, which the authors state is sensitive to BSM contributions in the b → sγ transition, using Belle II data.

Significance. If the amplitude analysis can be shown to be robust, the approach would enable more reliable constraints on new physics in radiative B decays by reducing model-dependent uncertainties in the CP fractions. The motivation is logically sound and aligns with standard techniques in B-physics analyses, but the paper's impact hinges on whether the method is demonstrated to be feasible rather than merely motivated.

major comments (1)

[Amplitude analysis discussion] The central claim that the amplitude analysis enables a clean separation of CP eigenstates rests on the ability to constrain multiple interfering complex amplitudes in the Kππ system. The manuscript provides no quantitative study (e.g., toy Monte Carlo or fit stability tests) demonstrating that Belle II statistics (typically O(10³–10⁴) events for this mode) and detector resolution on dipion mass and angular variables suffice to avoid underconstrained fits or large model-dependent ambiguities. This directly affects the reliability of the extracted CP fractions and the subsequent time-dependent CP asymmetry measurement.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the thoughtful review and constructive feedback on our manuscript. The paper is intended to motivate and outline an amplitude analysis approach for separating CP-eigenstate contributions in B → K_res γ → K π⁺ π⁻ γ decays to enable cleaner time-dependent CP asymmetry measurements in B⁰ → K_S⁰ π⁺ π⁻ γ. We address the major comment below.

read point-by-point responses

Referee: The central claim that the amplitude analysis enables a clean separation of CP eigenstates rests on the ability to constrain multiple interfering complex amplitudes in the Kππ system. The manuscript provides no quantitative study (e.g., toy Monte Carlo or fit stability tests) demonstrating that Belle II statistics (typically O(10³–10⁴) events for this mode) and detector resolution on dipion mass and angular variables suffice to avoid underconstrained fits or large model-dependent ambiguities. This directly affects the reliability of the extracted CP fractions and the subsequent time-dependent CP asymmetry measurement.

Authors: We acknowledge that the manuscript does not contain a quantitative feasibility study such as toy Monte Carlo simulations or explicit fit stability tests. This paper focuses on motivating the need for the amplitude analysis and describing its general structure, following established techniques from prior B-physics analyses (e.g., those performed by Belle and BaBar on related Kππ systems). Detailed quantitative assessments of fit constraints, model ambiguities, and sensitivity with Belle II event yields and resolution will be presented in a forthcoming publication that executes the full analysis on data. We will revise the manuscript to include a brief discussion of expected event yields (O(10³–10⁴) reconstructed events) and note that the angular and mass distributions provide sufficient observables to constrain the dominant amplitudes, consistent with successful prior amplitude analyses of similar final states. revision: partial

Circularity Check

0 steps flagged

No circularity: derivation chain self-contained with no reductions to inputs

full rationale

The paper describes the sensitivity of time-dependent CP asymmetry in B0 to KS0 pi+ pi- gamma to BSM contributions and states that an amplitude analysis of B to K_res gamma to K pi+ pi- gamma is needed to separate CP-eigenstate resonances from non-eigenstates. No equations, fitted parameters, self-citations, or ansatze are shown that would make any claimed result equivalent to its inputs by construction. The amplitude analysis is presented as a methodological requirement for the measurement rather than a derived prediction that loops back on itself. This matches the default case of an experimental search paper with independent content.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review provides no explicit free parameters, axioms, or invented entities; the work rests on standard assumptions of resonance modeling and CP properties in B decays that are not detailed here.

pith-pipeline@v0.9.0 · 5398 in / 1087 out tokens · 47934 ms · 2026-05-07T12:43:19.978037+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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