arxiv: 2604.17726 · v1 · submitted 2026-04-20 · ✦ hep-ph

Recognition: unknown

Searching for dark photons in J/psi decays

Xiao Liang , Chun-Yuan Li , Bin-Peng Shang , Zong-Guo Si , Hong-Xin Wang , Xing-Hua Yang , Dai-Xing Zhang

Authors on Pith no claims yet

Pith reviewed 2026-05-10 05:17 UTC · model grok-4.3

classification ✦ hep-ph

keywords dark photonJ/ψ decaysNRQCDinvisible decaysBESIIIkinetic mixing

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The pith

J/ψ decays mediated by a light dark photon produce up to 172 events in four-body invisible channels at BESIII for masses below 0.2 GeV.

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

The paper calculates branching ratios and expected event yields for J/ψ two-body and four-body decays that proceed through an intermediate dark photon with mass below 3 GeV. Using the non-relativistic QCD framework, it separates cases where the dark photon decays visibly to Standard Model particles or invisibly to dark-sector particles. For dark photon masses below twice the dark-matter particle mass, four-body final states yield the largest signals while two-body channels remain small. Above that mass threshold, only invisible modes survive with any appreciable rate. These numbers indicate which channels BESIII could use to look for the dark photon.

Core claim

In the NRQCD framework the authors compute the partial widths for J/ψ → γ U, J/ψ → γ U γ, and related channels, where U is the dark photon that mixes kinetically with the photon via parameter ε and couples to a dark fermion χ via g_χ. They report that below m_U = 2 m_χ the two-body channels give 0–37 events with significances 10^{-5}–10^{-6}, the four-body channel gives 94–172 events when m_U < 0.2 GeV, while above threshold the invisible two-body channel yields 0–12 events (significance 10^{-1}–10^{-3}) and the invisible four-body channel yields 0–129 events, with all visible modes strongly suppressed.

What carries the argument

Non-relativistic QCD (NRQCD) factorization applied to the J/ψ → U-mediated amplitudes, with the dark photon propagator and kinetic-mixing parameter ε controlling the coupling to the electromagnetic current.

If this is right

Four-body invisible decays supply the largest expected signal below 0.2 GeV and should be prioritized in analysis.
Two-body invisible decays become the only viable channel once m_U exceeds twice the dark-matter mass.
All visible decay modes are suppressed above the 2 m_χ threshold, narrowing the experimental search strategy.
The p_T distributions and significance estimates provide concrete cuts for background rejection at BESIII.

Where Pith is reading between the lines

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

The same NRQCD machinery could be applied to other charmonium states such as ψ(2S) to enlarge the mass coverage of dark-photon searches.
If the predicted events are observed, the measured rates would directly constrain the product of ε and g_χ at the 10^{-4}–10^{-3} level.
Absence of signal would tighten existing limits on light dark photons in a mass window complementary to beam-dump and collider searches.

Load-bearing premise

The calculations assume that NRQCD accurately describes the charmonium wave function and that the dark photon interacts only through the stated kinetic mixing and g_χ coupling without additional model-dependent vertices.

What would settle it

A null result (zero events above background) in the four-body invisible final state for m_U < 0.2 GeV after the full BESIII J/ψ data sample would contradict the predicted 94–172 events.

Figures

Figures reproduced from arXiv: 2604.17726 by Bin-Peng Shang, Chun-Yuan Li, Dai-Xing Zhang, Hong-Xin Wang, Xiao Liang, Xing-Hua Yang, Zong-Guo Si.

**Figure 2.** Figure 2: FIG. 2: Decay width as a function of the dark photon mass [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗

**Figure 3.** Figure 3: FIG. 3: Decay width as a function of the dark photon mass [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗

**Figure 4.** Figure 4: FIG. 4: The differential cross-section distributions [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗

**Figure 5.** Figure 5: FIG. 5: Decay width ratio Br( [PITH_FULL_IMAGE:figures/full_fig_p014_5.png] view at source ↗

**Figure 6.** Figure 6: FIG. 6: Decay width ratio Br( [PITH_FULL_IMAGE:figures/full_fig_p016_6.png] view at source ↗

read the original abstract

A dark photon is an Abelian gauge boson from a new $U(1)_D$ gauge symmetry, coupled to the Standard Model via kinetic mixing, with $\varepsilon$ inducing an effective coupling to the electromagnetic current and $g_\chi$ to a stable dark matter particle $\chi$. We study $J/\psi$ two-body and four-body decays via a light-mass dark photon ($m_U < 3.0$ GeV) in the framework of non-relativistic QCD (NRQCD), considering both visible and invisible decays of the dark photon into SM fermions or dark sector particles. Numerical results for the decay ratios $\Gamma/\Gamma_{J/\psi}$ and expected event numbers at BESIII are presented, along with the significance $S/\sqrt{B}$ and $p_T$ distributions where applicable. Our results show that, for $m_U < 2m_\chi$, the two-body final state decay channels of the $J/\psi$ mediated by a dark photon have event yields $0\sim 37$ with significances of $10^{-5}\sim10^{-6}$, while the four-body final state channel yields about $94\sim172$ events in the very low mass region $m_U < 0.2\ \text{GeV}$. For $m_U \ge 2m_\chi$, the invisible two-body final state decay channel yields $0\sim12$ events with better significance $10^{-1}\sim10^{-3}$, and the invisible four-body final state decay channel yields $0\sim129$ events, whereas visible decays are all severely suppressed.

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

Gives specific BESIII event yield predictions for dark photons in J/ψ decays via NRQCD, with low significances but some value for experimental planning.

read the letter

This paper gives concrete expected event numbers for dark photon mediated decays of the J/ψ at BESIII, using NRQCD for both two-body and four-body final states in visible and invisible modes. The key numbers are low event yields with tiny significances in most cases, but better prospects for invisible channels at higher dark photon masses. What is new is the numerical results tailored to BESIII for the four-body invisible decays and the split by mass regimes relative to the dark matter threshold. The paper does well by computing the decay ratios and event estimates directly from the kinetic mixing parameter and the dark coupling, plus showing some transverse momentum distributions. The soft spots are the very small significances, often 10 to the minus 5 or 6, which indicate these will be hard to observe without large data samples or better background rejection. There is also limited information on how the results change with variations in the parameters or on the size of theoretical uncertainties from the NRQCD approach. The calculations assume the standard framework without additional effects, which is fine but means the outputs are as good as the inputs. The underlying math and setup appear solid, with no evident problems in the logic or numerics from the available summary. This work is aimed at experimentalists planning or analyzing dark photon searches in charmonium decays at BESIII. A reader who needs specific yield estimates to guide their analysis strategy or to interpret null results would get practical value from it. It deserves a serious referee because it supplies targeted predictions for a real experiment using established techniques. I would recommend sending this to peer review.

Referee Report

0 major / 3 minor

Summary. The manuscript calculates branching ratios and expected BESIII event yields for J/ψ decays mediated by a light dark photon U (m_U < 3 GeV) within NRQCD, for both two-body and four-body final states. It distinguishes visible decays (U → SM fermions) from invisible decays (U → χχ̄), presenting results separately for m_U < 2m_χ and m_U ≥ 2m_χ regimes, including significances S/√B and p_T distributions.

Significance. If the numerical predictions hold, the work supplies concrete, falsifiable targets for dark-photon searches in charmonium at BESIII, with event yields up to ~172 and significances reaching 10^{-1} in selected invisible channels. The use of established NRQCD factorization for J/ψ decays and the provision of p_T spectra constitute clear strengths for experimental follow-up.

minor comments (3)

The abstract quotes event ranges (0∼37, 94∼172, etc.) and significances without stating the specific benchmark values of ε, g_χ, m_U, or the assumed total number of J/ψ events used to generate each interval; this reduces reproducibility and should be clarified in §4 or a dedicated parameter table.
Theoretical uncertainties from NRQCD higher-order corrections, scale choices, or wave-function inputs are not quantified; adding a brief error-band discussion would strengthen the central numerical claims.
Figure captions and legends for p_T distributions should explicitly note the mass and coupling values at which each curve is evaluated.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for the careful reading and positive assessment of our manuscript on searching for dark photons in J/ψ decays. The summary accurately captures our NRQCD-based calculations of branching ratios, event yields at BESIII, significances, and p_T distributions for both visible and invisible channels across the relevant mass regimes. We appreciate the recommendation for minor revision and will incorporate improvements to presentation and clarity in the revised version.

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper performs a standard forward phenomenological calculation of branching ratios and expected BESIII event yields for J/ψ decays mediated by a kinetically mixed dark photon, using the established NRQCD framework with input parameters ε, g_χ, and m_U. All quoted event numbers (0–37, 94–172, etc.) and significances follow directly once those parameters and the total J/ψ sample size are fixed; no step fits parameters to the target search data and then renames the output as a prediction, no self-citation chain bears the central result, and no ansatz or uniqueness claim is smuggled in. The derivation is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

3 free parameters · 1 axioms · 1 invented entities

The central claim rests on the dark photon model with free couplings and the applicability of NRQCD to these processes; these are standard but introduce parameters not derived in the paper.

free parameters (3)

ε (kinetic mixing parameter)
Controls the effective coupling to SM; values are implicitly used to obtain numerical event yields but not derived from first principles.
m_U (dark photon mass)
Scanned in ranges below 3 GeV to produce the quoted results for different regimes.
g_χ (coupling to dark matter)
Determines invisible decay modes; required for the invisible channel predictions.

axioms (1)

domain assumption NRQCD factorization applies to J/ψ decays with dark photon emission
Invoked to compute decay widths in the non-relativistic limit for heavy quarks.

invented entities (1)

dark photon U from U(1)_D no independent evidence
purpose: Abelian gauge boson mediating interactions between SM and dark sector via kinetic mixing
Postulated new particle whose properties are explored but without independent evidence provided in the work.

pith-pipeline@v0.9.0 · 5613 in / 1728 out tokens · 77347 ms · 2026-05-10T05:17:26.686392+00:00 · methodology

discussion (0)

Reference graph

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