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arxiv: 2604.14735 · v2 · pith:VY4YLAU4new · submitted 2026-04-16 · ✦ hep-ex

Dalitz decay of K^*(892) rightarrow K ell^+ell^-: A New Probe for Hadronic Structure and Dark Photon Searches

Benhou Xiang , Wanling Chang , Shuangshi Fang , Jingqing Zhang This is my paper

Pith reviewed 2026-05-21 01:30 UTC · model grok-4.3

classification ✦ hep-ex
keywords Dalitz decayK* mesontransition form factordark photonbranching fractionBESIIIhadronic structure
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The pith

The branching fraction for the rare Dalitz decay of the K* meson into a kaon and lepton pair is calculated for the first time with a single-pole form factor.

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

This paper delivers the first calculation of the branching fraction and dilepton mass spectrum for the decay K*(892) to K lepton pair. It models the virtual photon emission through a single-pole approximation of the transition form factor and shows how the resulting spectrum could reveal a light dark photon as a narrow peak. A sympathetic reader cares because the work turns a previously uncalculated process into a practical tool for testing meson internal structure and for hunting dark-sector particles at existing facilities.

Core claim

Using a single pole approximation for the form factor, we present the calculation of the branching fraction for this rare decay channel for the first time. Furthermore, we also investigate the potential to search for a light A' boson (dark photon) appearing as a narrow resonance in the dilepton spectrum, and discuss the experimental sensitivity and new physics opportunities at the dedicated BESIII experiment.

What carries the argument

Single-pole approximation of the transition form factor F_{K^*K}(q^2), which parametrizes the virtual-photon emission and is inserted into the decay rate formula to obtain numerical predictions.

If this is right

  • Numerical values for the branching fractions of both the electron and muon modes are obtained for the first time.
  • The dilepton invariant-mass distribution is predicted and can be compared directly with data.
  • A light dark photon would appear as a narrow peak superimposed on the smooth spectrum.
  • Dedicated searches at BESIII become feasible with this channel for both hadronic structure and dark-sector physics.

Where Pith is reading between the lines

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

  • The same single-pole method could be applied to analogous Dalitz decays of other light vector mesons to build a consistent picture of transition form factors.
  • Future precision data on this decay would test whether the pole mass extracted here matches values from other processes involving the same mesons.
  • If confirmed, the channel adds an independent handle on electromagnetic transitions between vector and pseudoscalar mesons below the phi resonance.

Load-bearing premise

The single-pole approximation accurately describes the transition form factor over the full kinematic range relevant to the decay.

What would settle it

An experimental measurement of the branching fraction or the shape of the dilepton mass spectrum at BESIII that deviates clearly from the predicted value would show the approximation fails.

Figures

Figures reproduced from arXiv: 2604.14735 by Benhou Xiang, Jingqing Zhang, Shuangshi Fang, Wanling Chang.

Figure 1
Figure 1. Figure 1: FIG. 1. The differential decay rate for [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. The sensitivity of the coupling constant [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
read the original abstract

We present the first comprehensive study of the rare Dalitz decay $K^*(892) \rightarrow K \ell^+ \ell^- (\ell = e, \mu)$, providing a prediction for the branching fraction and the dilepton mass spectrum. This decay involves the emission of a virtual photon which converts into a lepton pair, offering a probe of the transition form factor $F_{K^*K}(q^2)$ and underlying meson structure. Using a single pole approximation for the form factor, we present the calculation of the branching fraction for this rare decay channel for the first time. Furthermore, we also investigate the potential to search for a light $A^\prime$ boson (dark photon) appearing as a narrow resonance in the dilepton spectrum, and discuss the experimental sensitivity and new physics opportunities at the dedicated BESIII experiment. Our results establish $K^*(892) \rightarrow K \ell^+ \ell^- (l = e, \mu)$ as a new laboratory for hadronic structure and dark-sector searches.

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

2 major / 1 minor

Summary. The manuscript presents the first calculation of the branching fraction for the rare Dalitz decay K^*(892) → K ℓ⁺ ℓ⁻ (ℓ = e, μ) using a single-pole approximation for the transition form factor F_{K^*K}(q²). It also explores the potential for detecting a light dark photon A' as a narrow resonance in the dilepton invariant mass spectrum and discusses experimental sensitivity at the BESIII experiment.

Significance. If the single-pole form factor model holds over the relevant kinematic range, this work would establish a new probe for hadronic structure via the transition form factor and provide concrete predictions for dark-sector searches at BESIII. The calculation fills a gap in rare K* decays, but its utility depends on the model's accuracy without which the branching fraction and sensitivity projections lose reliability.

major comments (2)
  1. Abstract: the claim that the branching fraction is calculated 'for the first time' using the single-pole form factor provides no error analysis, comparison to data, or alternative models, leaving the numerical prediction's robustness unclear.
  2. Transition form factor section: the single-pole parametrization F(q²) = F(0)/(1 - q²/M_V²) is used for the full range up to q²_max = (m_{K^*} - m_K)² without referenced cross-checks (lattice, dispersive, or multi-pole); this is load-bearing for the integrated rate and dark-photon projections, as deviations of O(10-30%) would affect the quoted results.
minor comments (1)
  1. Notation: ensure consistent use of K^*(892) versus K^* throughout the text and figures.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments on our manuscript. We address each major comment below, indicating the revisions made to strengthen the presentation of our results and the limitations of the single-pole approach.

read point-by-point responses

  1. Referee: Abstract: the claim that the branching fraction is calculated 'for the first time' using the single-pole form factor provides no error analysis, comparison to data, or alternative models, leaving the numerical prediction's robustness unclear.

    Authors: We agree that the abstract would benefit from greater context on the robustness of the result. Although this represents the first explicit calculation of the branching fraction for K*(892) → K ℓ⁺ℓ⁻, we have revised the abstract to explicitly note the single-pole approximation and its associated model dependence. In the body of the paper we have added a dedicated paragraph on systematic uncertainties arising from the choice of pole mass, together with a brief comparison to the form-factor treatment used in analogous Dalitz decays (e.g., φ → η e⁺e⁻). These additions make the numerical prediction’s limitations clearer while preserving the claim of novelty for this specific channel. revision: yes

  2. Referee: Transition form factor section: the single-pole parametrization F(q²) = F(0)/(1 - q²/M_V²) is used for the full range up to q²_max = (m_{K^*} - m_K)² without referenced cross-checks (lattice, dispersive, or multi-pole); this is load-bearing for the integrated rate and dark-photon projections, as deviations of O(10-30%) would affect the quoted results.

    Authors: The referee correctly notes that the single-pole form factor is a central assumption. We have added references to existing phenomenological studies that employ the same parametrization for K* transitions and have inserted a new subsection quantifying the effect of varying the vector-meson mass parameter within its phenomenological range. This yields an estimated 15–25 % variation in the branching fraction, which we now propagate to the dark-photon sensitivity projections. A complete lattice-QCD or dispersive analysis lies outside the scope of the present work; we have therefore clarified in the text that the quoted numbers should be regarded as baseline predictions under the single-pole hypothesis, with the associated model uncertainty stated explicitly. revision: partial

Circularity Check

0 steps flagged

No significant circularity: branching fraction derived from external single-pole ansatz

full rationale

The paper adopts a single-pole parametrization of the transition form factor F_{K^*K}(q^2) as an input model and performs the first explicit integration to obtain the branching fraction and dilepton spectrum. This constitutes a standard model-based prediction rather than a self-referential loop. No equations or citations in the provided text reduce the output branching fraction to a fit performed on the same decay data, nor does any load-bearing step rely on a self-citation chain that itself assumes the target result. The derivation remains self-contained once the pole mass and normalization are taken from external sources or prior literature.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 1 invented entities

The prediction depends on the validity of the single-pole form-factor model and on the possible existence of a dark photon; both are introduced without new independent evidence in the abstract.

free parameters (1)
  • pole mass in single-pole form factor
    Parameter of the single-pole approximation for F_{K^*K}(q^2)
axioms (1)
  • domain assumption Single-pole dominance for the K* to K transition form factor
    Invoked to model the q^2 dependence of the virtual-photon emission
invented entities (1)
  • light dark photon A' no independent evidence
    purpose: To produce a narrow resonance in the dilepton mass spectrum
    Hypothetical particle whose signal is discussed as a possible deviation from the standard-model prediction

pith-pipeline@v0.9.0 · 5729 in / 1253 out tokens · 43329 ms · 2026-05-21T01:30:57.583369+00:00 · methodology

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Reference graph

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