Simulation of dark photon sensitivity in $\eta \rightarrow \gamma e^+e^-$ at HIAF

Hao Qiu; Rong Wang; Xiaolin Kang; Xiong He; Xurong Chen; Yang Liu; Ye Tian; Zaiba Mushtaq

REVIEW 3 major objections 4 minor 58 references

Simulation of dark photon sensitivity in $\eta \rightarrow \gamma e^+e^-$ at HIAF

T0 review · 3 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read The paper projects that a one-month run at a proposed eta factory at Huizhou could reach a dark-photon kinetic mixing sensitivity of $\epsilon^2 \sim 10^{-7}$ at 99% CL by searching for a narrow $e^+e^-$ resonance in $\eta \to \gamma…

desk verdict A credible feasibility study with a standard technique and a new numerical projection; the central sensitivity depends on an unvalidated eta yield, so the headline number is plausible but not yet solid. read the letter →

arxiv 2506.11733 v1 pith:L5KEZ37P submitted 2025-06-13 hep-ex

Zaiba Mushtaq , Rong Wang , Xiaolin Kang , Yang Liu , Ye Tian , Xiong He , Hao Qiu , Xurong Chen This is my paper

classification hep-ex

keywords darkphotonkineticmixingetamesonraredecayfixed-targetexperimentsiliconpixeltrackerinvariantmassbumphuntprotonbeamsector

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper projects the dark-photon discovery reach of a proposed eta-meson factory, where a 1.8 GeV proton beam hits a multi-layer lithium target and produces about $5.9\times10^{11}$ $\eta$ mesons in one month. It argues that the decay $\eta \to \gamma e^+e^-$ can be used as a dark-photon search by looking for a narrow bump in the $e^+e^-$ invariant-mass spectrum, and that a compact silicon-pixel spectrometer can reconstruct this channel with about 60% efficiency and roughly 2 MeV mass resolution. The central result is a projected 99% CL upper limit on $\mathrm{Br}(\eta\to\gamma A'\to\gamma e^+e^-)$ of $10^{-6}$ to $10^{-7}$ for $m_{A'} < 0.4$ GeV, corresponding to a kinetic-mixing sensitivity of $\epsilon^2 \sim 10^{-7}$. If correct, this would open a largely unexplored mass region around 50 MeV for visible dark-photon decays, extending the reach of previous beam-dump and collider searches.

What carries the argument

The load-bearing object is the invariant-mass distribution of the $e^+e^-$ pair in $\eta\to\gamma e^+e^-$, where a dark-photon signal would appear as a narrow resonance at the dark-photon mass $m_{A'}$ (simulated here at 50 MeV) above a smooth background. The background is dominated by the Standard Model decay $\eta\to\gamma e^+e^-$, whose branching fraction is 0.69%, plus neutrons misidentified as photons; it is generated with a semiclassical transport event generator for proton–lithium collisions. Sensitivity is carried by the counting formula $S(\mathrm{Br}) = 3\sqrt{N_{\rm bkg}}/(N_\eta\,\epsilon_{\rm reco})$, with $N_\eta = 5.9\times10^{11}$, and by the conversion $S(\epsilon^2) = S(\mathrm{Br})/[2|F(m_A^2)|^2(1 - m_A^2/m_\eta^2)^3]$, where $F$ is the $\eta$ transition form factor encoding the off-shell coupling to two virtual photons. The detector machinery is a compact all-silicon pixel tracker with a lead-glass or lead-glass-plus-scintillator calorimeter, simulated to give about 60% reconstruction efficiency and an $e^+e^-$ mass resolution $\sigma \approx 1.94$ MeV.

What would settle it

A short calibration run at the proposed facility that counts reconstructed $\eta\to\gamma\gamma$ or $\eta\to\gamma e^+e^-$ events would measure the actual eta yield; if the per-month yield falls below roughly $6\times10^{10}$, the projected $\epsilon^2 \sim 10^{-7}$ limit is not reachable. A second check would measure the rate of neutron-induced calorimeter hits passing the 50 MeV photon threshold; if that rate exceeds the transport-model prediction, the background floor rises and the projected limit degrades.

Watch

Extended reading notes

Core claim

The paper argues that the rare decay $\eta\to\gamma A'\to\gamma e^+e^-$ provides a clean dark-photon search channel at a proposed proton-beam eta factory, and that a one-month run would be enough to make it sensitive. Its central projection is a 99% CL upper limit on the branching ratio of $10^{-6}$–$10^{-7}$ for dark-photon masses below 0.4 GeV, equivalent to kinetic-mixing sensitivity $\epsilon^2 \sim 10^{-7}$, with a simulated reconstruction efficiency of about 60% and an $e^+e^-$ mass resolution near 2 MeV. The analysis is carried out as a bump hunt in the $e^+e^-$ invariant-mass spectrum, with the irreducible $\eta\to\gamma e^+e^-$ decay (branching ratio 0.69%) and neutron-induced backgrounds estimated by simulating proton–lithium collisions and scaling the result to the assumed $5.9\times10^{11}$ eta yield. The paper also reports that an upgraded dual-readout calorimeter improves the reconstructed $\eta$ mass resolution from 27.3 MeV to 15.4 MeV and gives a marginal sensitivity gain.

Load-bearing premise

Everything depends on the assumed one-month production of about $5.9\times10^{11}$ eta mesons, which is estimated from a proton–lithium cross-section scaled linearly from proton–proton data at 1.8 GeV; if the real rate is ten times lower, the quoted sensitivity to the mixing parameter weakens by about a factor of three.

Editorial extensions

If this is right

A one-month null result at the proposed facility would exclude dark photons with masses below 0.4 GeV and kinetic mixing above $\epsilon^2 \sim 10^{-7}$ at 99% CL, a part of the few-loop parameter region not currently covered by existing searches.
The 50 MeV dark-photon mass is a single assumption; the same analysis chain can be rerun for any $m_{A'}$ in the accessible range to produce a full exclusion contour.
The 100 MHz-capable all-silicon tracker and the roughly 60%-efficient reconstruction would apply to other rare $\eta$ decays as well, not only to the dark-photon channel.
Under the paper's ideal scenario of a one-year run at higher rate and full duty factor, the projected $\epsilon^2$ sensitivity becomes still stronger than the one-month value.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The yield estimate assumes $\sigma(pA) \approx A\,\sigma(pp)$, a linear scaling that the paper does not test with data; measuring $\eta$ production on at least two target nuclei would validate or correct this assumption directly.
The simulation is anchored at a single dark-photon mass of 50 MeV, leaving open how smooth the projected limit is across the full $0 < m_A < 0.4$ GeV range; a scan at several masses would test the interpolation shown in the sensitivity curve.
Treating neutrons as misidentified photons is a deliberately conservative choice; improved neutron/photon discrimination beyond what the simulation assumes would push the achievable $\epsilon^2$ limit below $10^{-7}$, while a neutron background larger than predicted would degrade it.
Because the background sample is generated without the dark-photon signal and then scaled by a factor of about $10^5$, systematic uncertainties in the transport model's background shape, not just counting statistics, will set the real floor of the search; a data-driven sideband estimate would strengthen the projection.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Read this one if you care about dark photon searches at low-energy fixed-target facilities. The paper is exactly what it says: a simulation-driven sensitivity projection for a proposed eta factory at HIAF. The technique is not new — look for a bump in the e+e- invariant mass in eta -> gamma e+e- — but the specific detector concept, a compact silicon-pixel spectrometer with a lead-glass or dual-readout calorimeter, is new, and the numerical projection (epsilon^2 ~ 1e-7 at 99% CL for mA up to 0.4 GeV from one month of running) would improve on existing limits from KLOE-2 and NA48/2 by roughly an order of magnitude in a region that is sparsely explored. That is a meaningful target for a future experiment.

What the paper does well: the background treatment is sensible. They use GiBUU to generate p-7Li inelastic events, apply mass and photon-energy cuts, and conservatively treat neutrons as misidentified photons when they pass the EMC threshold. The mass resolution of ~2 MeV for the e+e- pair is credible, and they compare two calorimeter designs rather than picking one arbitrarily. The paper is also honest about being a conceptual design; nothing is oversold.

The soft spots are real, and the stress-test note gets them right. The headline sensitivity is directly proportional to 1/sqrt(N_eta), and N_eta = 5.9e11 rests on three unvalidated numbers: the 100 MHz event rate the silicon tracker must handle, the 0.76% eta production probability from GiBUU, and the linear-A scaling of the pp cross section. The probability is not benchmarked against any published p+A eta production data near 1.8 GeV, and the linear scaling ignores absorption, Fermi motion, and final-state interactions, any of which could change the yield by a factor of 2-3. A factor-of-10 overestimate of the yield would move epsilon^2 from ~1e-7 to ~3e-7, which is much less impressive, and worse would erase the advantage over existing limits. The paper gives no systematic uncertainty on N_eta and no cross-check.

That said, this is a feasibility study, not a measurement claim. The method is standard, the simulation pipeline is reasonable, and the issues are exactly what a referee should ask the authors to fix: benchmark the eta yield against data, or at least show how the projection degrades under conservative yield assumptions. The single-mass resolution (50 MeV) applied to the whole range is a lesser issue, as is the assumption of identical efficiencies for signal and background.

Bottom line: this deserves a serious referee. It is a well-scoped, useful simulation study for a proposed facility, and the new projection is worth having on record even if the absolute number has real uncertainty. Send it to review, with a request for yield validation and systematic variations. I'd bring it to a reading group focused on future dark-sector facilities.

Referee Report

3 major / 4 minor

Summary. The paper presents a Monte Carlo simulation study of the sensitivity to a dark photon A' in the decay η → γA'(→ e+e−) at the proposed HIAF η factory. The authors assume a one-month fixed-target run with a 1.8 GeV proton beam on a 7Li target, yielding 5.9×10^11 η mesons, and model both signal and background with the GiBUU event generator together with a ChnsRoot detector simulation of a compact silicon-pixel spectrometer. They report a reconstruction efficiency of about 60%, an e+e− invariant mass resolution of about 1.94 MeV at mA = 50 MeV, projected branching-ratio upper limits of 10^-6 to 10^-7 for dark photon masses up to 0.4 GeV, and a corresponding kinetic-mixing sensitivity of ε² ~ 10^-7 at 99% CL for one month of data. The central projection is summarized in Eqs. (2) and (3) and displayed in Figs. 11 and 12.

Significance. If the assumed η yield and the single-mass simulation inputs are valid, the proposed experiment would probe a dark photon mass region around 10–400 MeV where current constraints leave room for new physics, and the study provides a concrete, falsifiable projection with an explicit detector concept. The paper's strengths include the use of a dedicated physics generator (GiBUU) for background, a detector-level simulation chain (ChnsRoot/FairRoot), explicit formulas for the branching-ratio limit and ε² sensitivity, and a comparison plot against existing experimental limits. However, the headline sensitivity is directly proportional to the square root of the assumed η yield and inherits all uncertainties in that yield; the simulation also uses a single dark photon mass for the efficiency and resolution that are then extrapolated over the whole mass range shown in Fig. 12. These issues are load-bearing for the paper's central claim, so the projection should be treated as preliminary until the yield estimate is benchmarked and the mass dependence of the efficiency and resolution is demonstrated.

major comments (3)

[Section III and Eq. (2)] The central sensitivity projection scales as S ∝ Nη^{-1/2}, and Nη = 5.9×10^11 is obtained from a chain of unvalidated assumptions: a GiBUU p-7Li η production probability of 0.76%, a linear scaling σ(pA) = 0.1 × A mb, a 100 MHz inelastic event rate, a 30% duty factor, and a one-month running period. No benchmark against published p+A η-production data near 1.8 GeV is provided, no nuclear effects such as absorption or Fermi motion are discussed, and no systematic uncertainty is assigned to Nη. Since a factor-of-10 reduction in the yield would worsen the ε² sensitivity from about 10^-7 to about 3×10^-7, near the boundary of existing constraints, the headline reach is not yet established without an independent check of the η yield.
[Section V.D and Fig. 12] The sensitivity curve in Fig. 12 spans dark photon masses from about 10^-2 GeV to 0.4 GeV, but the simulation is performed for a single mass mA = 50 MeV (Section II and Section V.B). The paper does not demonstrate that the 60% efficiency, the 1.94 MeV e+e− resolution, the binning choice, or the scaled GiBUU background remain valid over that entire mass range. Near kinematic endpoints and at low mA, acceptance and resolution are expected to vary with the opening angle and energy of the e+e− pair. The claim of branching-ratio upper limits of 10^-6 to 10^-7 for 0 < mA < 0.4 GeV therefore rests on an unverified extrapolation from one point in mass.
[Sections V.A and V.D, Eq. (2)] The reconstruction efficiency ε_reco is evaluated for the SM decay η → γe+e−, and the same efficiency is applied to the dark-photon signal η → γA'(→ e+e−). These two processes have different e+e− invariant mass distributions and angular correlations, so the signal acceptance is not guaranteed to equal the SM three-body acceptance. In addition, no systematic uncertainty is assigned to ε_reco or to the scaled background counts, even though the branching-ratio limit in Eq. (2) is inversely proportional to ε_reco and directly proportional to sqrt(N_bkg). Since the projected limit and the derived ε² sensitivity depend on these quantities, the absence of any systematic error estimate is a load-bearing omission.

minor comments (4)

[Section IV] The text contains several typos, including 'facilating' for 'facilitating' and 'GIBUU' for 'GiBUU'; figure captions also contain 'Reconstruced' and 'Laed Glass'. These should be corrected in a revision.
[Section V.D.2] The sentence 'The TFF of η meson meson describes...' contains a duplicated word. Also, the definition of Lmass in Eq. (1) is never given explicitly, which makes the Lagrangian incomplete as written.
[Figure 9] The label 'π0η' in the upper-left corner of Fig. 9 appears to be a stray annotation and should either be explained or removed.
[Eq. (2) and Section V.D.1] The statistical interpretation of Eq. (2) should be stated explicitly: the factor of 3 is described as corresponding to a 3σ effect, but the text claims a 99% CL limit. Please clarify whether this is a Gaussian 3σ upper limit, a 90% CL Poisson limit, or a 99% CL Bayesian limit, and specify how the background count N_bkg enters the limit calculation.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the sensitivity projection is a conditional MC extrapolation, not a fitted or self-referential derivation.

full rationale

The paper's derivation chain is a standard simulation projection. The eta yield N_eta = 5.9e11 is estimated from stated assumptions (1.8 GeV protons on lithium, 100 MHz inelastic rate, 30% duty factor, GiBUU eta probability of 0.76%, and sigma_pA ~ 0.1*A mb), and this input is then propagated through Eq. (2) into a branching-ratio upper limit and through Eq. (3) into a mixing-parameter sensitivity. No parameter is fitted to the claimed sensitivity, and the background distributions are generated independently with GiBUU without injecting a dark photon signal. The signal efficiency is obtained from Monte Carlo reconstruction of eta -> gamma e+e- and is applied as a correction, not as a fitted quantity. Citations [31] and [42] identify the proposed facility and detector concept from the same collaboration, but the physics sensitivity does not reduce to those citations; the load-bearing numbers are the paper's own explicit assumptions. Existing experimental limits are used only as external comparisons. The main vulnerability is the unvalidated yield estimate, but that is a correctness or robustness risk, not circularity. The paper therefore contains no self-definitional step, no fitted input renamed as a prediction, and no self-citation chain that forces the result.

Assumptions & free parameters 5 free parameters · 5 assumptions · 0 invented entities

The sensitivity projection is a chain of modeling assumptions: the eta yield (from cross-section scaling and assumed luminosity), the GiBUU background, the equivalence of signal and background efficiencies, and the literature form factor. None of these are validated against measured data in the paper.

free parameters (5)

dark_photon_mass_mA = 50 MeV
Single dark photon mass used in the signal simulation; the sensitivity for other masses is extrapolated using the background shape.
fixed_target_luminosity = 1e35 cm^-2 s^-1
Assumed luminosity used to derive the eta yield; not measured or justified in detail.
inelastic_event_rate = 100 MHz
Assumed event rate for the spectrometer, used in the yield estimate.
duty_factor = 30%
Assumed duty cycle for the one-month run.
running_time = 1 month
Assumed data-taking period, chosen for the projection.

assumptions (5)

domain assumption The eta production cross section in p-p collisions at 1.8 GeV is approximately 0.1 mb, and scales linearly with atomic number A in p-A collisions.
Used to estimate the eta yield of 5.9e11 in one month (Section III).
domain assumption GiBUU accurately models the background in p-7Li collisions at 1.8 GeV.
All background distributions are produced with GiBUU and scaled up by about 1e5 (Section IV).
ad hoc to paper The efficiency of the SM eta -> gamma e+ e- decay is representative of the dark photon signal efficiency.
The efficiency curve in Fig. 5 is applied to both signal and background (Section V.A).
domain assumption The eta transition form factor F(m_A) is known and used in Eq. (3) to convert branching ratio limits to epsilon^2 limits.
The TFF is taken from literature (ref [47]), but its functional form and values are not given in this paper.
domain assumption The fast detector simulation in ChnsRoot reproduces the performance of the conceptual spectrometer.
The mass resolutions and efficiencies are computed with this package (Section IV).

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Cite this review

Pith. "Pith review of Simulation of dark photon sensitivity in $\eta \rightarrow \gamma e^+e^-$ at HIAF." pith.science (2026). https://pith.science/paper/L5KEZ37P

@misc{pith2026250611733,
  author       = {Pith},
  title        = {Pith review of: Simulation of dark photon sensitivity in $\eta \rightarrow \gamma e^+e^-$ at HIAF},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/L5KEZ37P}},
  note         = {Machine review of arXiv:2506.11733}
}

abstract

We present a simulation study of dark photon sensitivity in a suggested $\eta$ factory experiment at Huizhou. The vast number of $\eta$ mesons are produced by bombarding the high-intensity HIAF proton beam on a multi-layer target of light nucleus. The kinematic energy of the beam is at 1.8 GeV. For a prior experiment of one-month running, about $5.9 \times 10^{11}$ $\eta$ samples would be collected, providing substantial data for a sound statistical analysis. A compact spectrometer based on the full silicon-pixel tracker is conceptually designed for the detection of the final-state particles. The GiBUU event generator is utilized for the background estimation without the dark photon signal. A spectrometer simulation package ChnsRoot is constructed for evaluating the spectrometer performances in searching the dark photon in $\eta$ rare decay. The efficiency and resolutions of $\eta \rightarrow \gamma e^+e^-$ decay channel are studied in detail. The branching-ratio upper limit of dark photon in $\eta \rightarrow \gamma e^+e^-$ decay and the sensitivity to the model parameter are given from the simulations.

Figures

Figures reproduced from arXiv: 2506.11733 by the authors.

**Figure 2.** FIG. 2: The conceptual design of compact spectrometer [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** FIG. 3: The distribution of momentum to angle for the final state particles . The upper half of a figure represents [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

Figures from the paper (7 more)

**Figure 5.** Figure 5: FIG. 5: A graph displaying the efficiency of the channel [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]

**Figure 6.** Figure 6: FIG. 6: The fit to the [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]

**Figure 7.** Figure 7: FIG. 7: The fit to the [PITH_FULL_IMAGE:figures/full_fig_p006_7.png]

**Figure 9.** Figure 9: FIG. 9: The projected invariant mass distribution of [PITH_FULL_IMAGE:figures/full_fig_p006_9.png]

**Figure 10.** Figure 10: FIG. 10: The projected invariant mass distribution of [PITH_FULL_IMAGE:figures/full_fig_p007_10.png]

**Figure 11.** Figure 11: FIG. 11: The projected branching ratio upper limit in [PITH_FULL_IMAGE:figures/full_fig_p007_11.png]

**Figure 12.** Figure 12: indicates the projected sensitivity to ϵ 2 as a function of mass of dark photon, alongside comparisons to existing constraints from other experiments. For our proposed setup, the sensitivity reaches ϵ 2 ∼ 10−7 at 99% confidence level (CL) with one month of data. An ex…

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Reference graph

Works this paper leans on

58 extracted references · 19 canonical work pages

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Branching ratio upper limit In the search of dark photons, sensitivity studies are es- sential to establish limits on their possible existence and interaction strengths [35]. By analyzing the invariant mass distribution of e+e−, we set an upper bound on the branching ratio for the dark photon decay channel, pro- viding critical insights into the experimen...

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[GeV/c-e+M(e1 −1010 3 105 107 10Counts Lead Glass - input Lead Glass - Reconstruced 0 0.2 0.4 ]

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4: The event distributions as a function of dilepton mass spectrum for the channel η → e+e−γ are presented

[GeV/c-e+M(e1 −1010 3 105 107 10Counts Plastic scintillator + Lead Glass - Input Plastic scintillator + Lead Glass - Reconstruced FIG. 4: The event distributions as a function of dilepton mass spectrum for the channel η → e+e−γ are presented. The green histogram represents the input Monte Carlo events generated by the event generator, while the blue histo...

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5: A graph displaying the efficiency of the channel η → e+e−γ as a function of mass of e+e−, illustrating the efficiency stability across different mass ranges

[GeV/c-e+M(e0 20 40 60 80 100 Efficiency (%) Version 1 (Lead Glass Calorimeter) Version 2 (Plastic scintillator + Lead Glass) FIG. 5: A graph displaying the efficiency of the channel η → e+e−γ as a function of mass of e+e−, illustrating the efficiency stability across different mass ranges. To fix this, we adjust the properties of neutrons to make them ap...

work page
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7: The fit to the M (e+e−γ) distribution for η → e+e−γ with version 1 (lead glass)

[GeV/cγ-e+Mass(e0 100 200 300 3 10×Events Measured Data Fit Function 2 = 0.547 GeV/cηMass of Lead Glass = 27.3 MeV σ FIG. 7: The fit to the M (e+e−γ) distribution for η → e+e−γ with version 1 (lead glass). The blue line represents data and the red line is the total fit at the mass of η 0.547 GeV. ure 10 depict the simulated invariant mass distributions fo...

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8: The fit to the M (e+e−γ) distribution for η → e+e−γwith version 2 (lead glass + plastic scintillator)

[GeV/cγ-e+Mass(e0 200 400 600 3 10×Events Measured Data Fit Function 2 = 0.547 GeV/cηMass of Plastic Scintillator + Lead Glass = 15.4 MeV σ FIG. 8: The fit to the M (e+e−γ) distribution for η → e+e−γwith version 2 (lead glass + plastic scintillator). The blue line represents data and the red line is the total fit at the mass of η 0.547 GeV. The Figure 10 ...

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9: The projected invariant mass distribution of γe+e− with the background from GiBUU based on the proposed one-month yield of η

[GeV/cγ-e+M(e6 107 108 109 1010 10Counts Plastic Scintillator + Lead Glass Lead Glass 0 πη FIG. 9: The projected invariant mass distribution of γe+e− with the background from GiBUU based on the proposed one-month yield of η. 7 0 0.2 0.4 ]

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10: The projected invariant mass distribution of e+e− in the channel η → e+e−γ is based on the proposed one-month run time of the experiment

[GeV/c-e+M(e4 105 106 107 108 109 10Counts Plastic Scintillator + Lead Glass Lead Glass FIG. 10: The projected invariant mass distribution of e+e− in the channel η → e+e−γ is based on the proposed one-month run time of the experiment. 0 0.2 0.4 ]

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11: The projected branching ratio upper limit in the decay channel η → γA → e+e−γ based on the proposed one-month run time of the experiment

[GeV/c-e+Mass(e9 −108 −107 −106 −105 −10Branching Ratio Upper Limit Lead Glass Plastic Scintilator + Laed Glass FIG. 11: The projected branching ratio upper limit in the decay channel η → γA → e+e−γ based on the proposed one-month run time of the experiment. D. Sensitivity Stu...
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The sensitivity to the mixing parameter ϵ2 [27] is computed based on the upper limit of the branching ratio for the dark photon decay channel

Sensitivity to mixing parameter (ϵ2) In the context of rare decay channel η → e+e−γ, sensi- tivity studies are performed to constrain the mixing pa- rameter ϵ2 that governs the interaction strength between dark photon and the SM particles. The sensitivity to the mixing paramet...
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Ideal sensitivity FIG

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.

[1] [1]

Branching ratio upper limit In the search of dark photons, sensitivity studies are es- sential to establish limits on their possible existence and interaction strengths [35]. By analyzing the invariant mass distribution of e+e−, we set an upper bound on the branching ratio for the dark photon decay channel, pro- viding critical insights into the experimen...

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[2] [2]

[GeV/c-e+M(e1 −1010 3 105 107 10Counts Lead Glass - input Lead Glass - Reconstruced 0 0.2 0.4 ]

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[3] [3]

4: The event distributions as a function of dilepton mass spectrum for the channel η → e+e−γ are presented

[GeV/c-e+M(e1 −1010 3 105 107 10Counts Plastic scintillator + Lead Glass - Input Plastic scintillator + Lead Glass - Reconstruced FIG. 4: The event distributions as a function of dilepton mass spectrum for the channel η → e+e−γ are presented. The green histogram represents the input Monte Carlo events generated by the event generator, while the blue histo...

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[4] [4]

5: A graph displaying the efficiency of the channel η → e+e−γ as a function of mass of e+e−, illustrating the efficiency stability across different mass ranges

[GeV/c-e+M(e0 20 40 60 80 100 Efficiency (%) Version 1 (Lead Glass Calorimeter) Version 2 (Plastic scintillator + Lead Glass) FIG. 5: A graph displaying the efficiency of the channel η → e+e−γ as a function of mass of e+e−, illustrating the efficiency stability across different mass ranges. To fix this, we adjust the properties of neutrons to make them ap...

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[5] [5]

7: The fit to the M (e+e−γ) distribution for η → e+e−γ with version 1 (lead glass)

[GeV/cγ-e+Mass(e0 100 200 300 3 10×Events Measured Data Fit Function 2 = 0.547 GeV/cηMass of Lead Glass = 27.3 MeV σ FIG. 7: The fit to the M (e+e−γ) distribution for η → e+e−γ with version 1 (lead glass). The blue line represents data and the red line is the total fit at the mass of η 0.547 GeV. ure 10 depict the simulated invariant mass distributions fo...

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[6] [6]

8: The fit to the M (e+e−γ) distribution for η → e+e−γwith version 2 (lead glass + plastic scintillator)

[GeV/cγ-e+Mass(e0 200 400 600 3 10×Events Measured Data Fit Function 2 = 0.547 GeV/cηMass of Plastic Scintillator + Lead Glass = 15.4 MeV σ FIG. 8: The fit to the M (e+e−γ) distribution for η → e+e−γwith version 2 (lead glass + plastic scintillator). The blue line represents data and the red line is the total fit at the mass of η 0.547 GeV. The Figure 10 ...

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[7] [7]

9: The projected invariant mass distribution of γe+e− with the background from GiBUU based on the proposed one-month yield of η

[GeV/cγ-e+M(e6 107 108 109 1010 10Counts Plastic Scintillator + Lead Glass Lead Glass 0 πη FIG. 9: The projected invariant mass distribution of γe+e− with the background from GiBUU based on the proposed one-month yield of η. 7 0 0.2 0.4 ]

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[8] [8]

10: The projected invariant mass distribution of e+e− in the channel η → e+e−γ is based on the proposed one-month run time of the experiment

[GeV/c-e+M(e4 105 106 107 108 109 10Counts Plastic Scintillator + Lead Glass Lead Glass FIG. 10: The projected invariant mass distribution of e+e− in the channel η → e+e−γ is based on the proposed one-month run time of the experiment. 0 0.2 0.4 ]

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