arxiv: 2605.03427 · v1 · submitted 2026-05-05 · ✦ hep-ph

Recognition: unknown

Search for Long-Lived Dark Photons from Dark Radiation at the LHC

Chuan-Ren Chen , Van Que Tran

Authors on Pith no claims yet

Pith reviewed 2026-05-07 15:58 UTC · model grok-4.3

classification ✦ hep-ph

keywords dark photonslong-lived particlesdark matterLHCdark radiationkinetic mixingFASERMATHUSLA

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The pith

A dark radiation channel from Z decays to dark matter produces more long-lived dark photons than standard sources at the LHC.

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

The paper examines how a fermionic dark matter particle χ can decay to a Z boson and emit dark photons as radiation when the Zχχ coupling is induced by quantum loops involving the top quark and a new colored scalar. This production route becomes the leading source of dark photons when their mixing with ordinary photons is small and their mass sits well above the GeV scale. Because more dark photons are made, detectors built to catch long-lived particles gain reach into regions of parameter space that match the observed dark matter density and that conventional meson-decay or bremsstrahlung channels cannot access. The work therefore maps the improved discovery potential of FASER2, FACET, and MATHUSLA under this mechanism.

Core claim

We investigate a novel production mechanism for long-lived dark photons at the LHC, arising from dark radiation emitted from χ in Z→χ̄χ decays, where χ is a fermionic dark matter candidate. The effective Zχχ coupling is generated radiatively through one-loop diagrams involving the top quark and a new colored scalar. We show that dark photons produced via this dark radiation channel can dominate over the conventional sources—meson decays and proton bremsstrahlung—across wide regions of parameter space, particularly for small kinetic mixing and dark photon masses well above the GeV scale. Using this enhanced production mechanism, we analyze the sensitivity of dedicated long-lived particle det

What carries the argument

The radiatively generated effective Zχχ coupling through one-loop diagrams with the top quark and a new colored scalar, which permits dark photon emission as radiation in Z decays to dark matter.

If this is right

Dark photon production from the dark radiation channel exceeds meson decays and proton bremsstrahlung for small kinetic mixing and masses well above the GeV scale.
FASER2, FACET, and MATHUSLA can reach previously inaccessible regions of dark photon parameter space that remain consistent with the observed dark matter relic abundance.
These detectors gain sensitivity beyond what conventional production mechanisms alone would allow.
The new channel opens parameter space that standard dark photon scenarios cannot probe at the LHC.

Where Pith is reading between the lines

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

Similar loop-induced couplings could enhance production rates for other hidden-sector particles in Z or Higgs decays at colliders.
Non-observation at the projected sensitivities would constrain the mass and couplings of the new colored scalar needed for the dark matter relic density.
The mechanism might generalize to other mediator bosons that couple to both the Standard Model and a dark sector.
Combined analyses of long-lived particle searches and direct dark matter detection could test the same loop diagrams from complementary angles.

Load-bearing premise

The effective Z χ χ coupling is generated radiatively through one-loop diagrams involving the top quark and a new colored scalar, with parameter choices that simultaneously satisfy the observed dark matter relic abundance.

What would settle it

A measured dark photon signal rate in FASER2 or MATHUSLA that falls below the prediction for the loop-induced Zχχ coupling at dark photon masses above 1 GeV and kinetic mixing below 10^-4 would indicate that this dark radiation channel does not dominate.

Figures

Figures reproduced from arXiv: 2605.03427 by Chuan-Ren Chen, Van Que Tran.

**Figure 1.** Figure 1: One-loop triangle diagrams contributing to view at source ↗

**Figure 2.** Figure 2: The exclusion limits from invisible Z decay (solid red), ATLAS monojet (solid black) and DM direct detections (dashed curves) projected on (mχ, g χ Z ) plane. III. EXPERIMENTAL CONSTRAINTS In this section, we discuss the current experimental constraints on the loopinduced Zχχ¯ interaction. The relevant constraints include invisible Z decay, mono7 view at source ↗

**Figure 3.** Figure 3: The projected contours of the correct DM relic density in the view at source ↗

**Figure 4.** Figure 4: Projected 95% exclusion sensitivity for FASER (dashed red), FASER2 (dashed blue), FACET (dashed green), and MATHUSLA (dashed purple) in the plane of BR(Z → χχ¯ ) and the proper lifetime of the dark photon. We fix mχ = 2 GeV and ∆ = 0.3. The horizontal solid black line shows the current constraint from the Z invisible decay width. current Z invisible width bound, FASER2, FACET, and MATHUSLA improve signific… view at source ↗

**Figure 5.** Figure 5: Projected 95% exclusion sensitivity for FASER2, FACET, and MATHUSLA in the (mA′, ϵ) plane. We fix ∆ = 0.3, assume BR(Z → χχ¯ ) = 10−4 , and vary gD to reproduce the observed DM relic abundance. Solid green, blue, and purple contours correspond to the dark-radiation induced dark photon signals at FASER2, FACET, and MATHUSLA, respectively. The dotted green and blue curves show the sensitivities to the conven… view at source ↗

**Figure 6.** Figure 6: The constraints from the Higgs signal strength data projected on view at source ↗

**Figure 7.** Figure 7: Left panel: Form factors as a function of colored scalar mass with fixing view at source ↗

read the original abstract

We investigate a novel production mechanism for long-lived dark photons at the LHC, arising from dark radiation emitted from $\chi$ in $Z\to\bar{\chi}\chi$ decays, where $\chi$ is a fermionic dark matter candidate. The effective $Z\chi\chi$ coupling is generated radiatively through one-loop diagrams involving the top quark and a new colored scalar. We show that dark photons produced via this dark radiation channel can dominate over the conventional sources-meson decays and proton bremsstrahlung-across wide regions of parameter space, particularly for small kinetic mixing and dark photon masses well above the GeV scale. Using this enhanced production mechanism, we analyze the sensitivity of dedicated long-lived particle detectors, including FASER2, FACET, and MATHUSLA. We find that these experiments can significantly surpass existing bounds, probing regions of dark photon parameter space consistent with the observed dark matter relic abundance and inaccessible in conventional dark photon scenarios.

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's new dark radiation channel for long-lived dark photons might extend LHC sensitivity but the loop suppression makes dominance over conventional sources questionable while fitting relic density.

read the letter

The main point to know is that this paper claims a new production channel for long-lived dark photons via dark radiation from fermionic dark matter in Z decays can dominate over standard sources like meson decays, especially at small kinetic mixing. The novel element is the radiatively induced Z chi chi coupling from top and colored scalar loops, which allows chi production at the LHC followed by A' emission. The work does well in calculating the sensitivity reach for dedicated detectors such as FASER2, FACET, and MATHUSLA, and in arguing that this opens up dark matter consistent parameter space not accessible otherwise. The soft spot is in the production rate itself. A one-loop effective coupling is suppressed, and matching the relic density constrains the parameters tightly. It is not clear from the claims whether the required couplings avoid direct search limits on the colored scalar or actually yield higher A' yields than bremsstrahlung in the relevant mass range. The paper makes strong statements about wide regions of dominance, but those need solid numerical support to hold up. This paper is aimed at the long-lived particle and dark sector phenomenology community. Readers looking for new ideas on hidden photon production would find the detector projections useful. It shows enough coherent model building to merit a serious referee who can verify the loop calculations and scans. I recommend putting it through peer review.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes a new production channel for long-lived dark photons (A') at the LHC via dark radiation emitted from fermionic dark matter χ produced in Z → χχ decays. The effective Zχχ coupling arises radiatively from one-loop diagrams involving the top quark and a new colored scalar. The authors claim this mechanism dominates over conventional sources (meson decays, proton bremsstrahlung) for small kinetic mixing ε and m_A' ≫ GeV, enabling dedicated LLP detectors (FASER2, FACET, MATHUSLA) to probe relic-density-consistent parameter space inaccessible via standard dark photon scenarios.

Significance. If the quantitative results on dominance and sensitivity hold, the work would usefully connect dark matter relic density constraints to long-lived particle searches, expanding the testable dark photon parameter space at the LHC. The explicit incorporation of relic abundance consistency is a strength, as it anchors the target region in observation rather than arbitrary choices. The approach relies on standard effective field theory and Monte Carlo tools for projections.

major comments (2)

[§2] §2 (Model and loop-induced coupling): The central claim of dominance requires that the radiatively generated Zχχ coupling (suppressed by 1/(16π²) and Yukawa/mass factors) produces sufficient χ yield for the subsequent A' radiation to exceed meson and bremsstrahlung rates at small ε. Provide the explicit one-loop expression for the effective coupling and benchmark points (colored scalar mass, Yukawas) demonstrating simultaneous satisfaction of Ωh² ≈ 0.12 and BR(Z → χχ) large enough for dominance; without this, the overlap of regimes remains unverified.
[§4–5] §4–5 (Production rates and sensitivity projections): The assertion that the dark radiation channel dominates and allows these experiments to surpass existing bounds in relic-consistent regions must be supported by explicit rate comparisons and exclusion contours. Include a table or figure quantifying event yields from the new channel versus conventional sources across ε and m_A' (with error bands or parameter scans), and overlay the projected reaches on current limits.

minor comments (2)

[§2] Clarify the definition and range of the new colored scalar parameters in the text and any tables to avoid ambiguity with direct search constraints.
[§5] Ensure all figures showing sensitivity have consistent axis labels (ε vs m_A') and explicitly mark the relic-density-consistent bands.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough review and constructive feedback on our manuscript. We appreciate the emphasis on providing explicit derivations and quantitative comparisons to strengthen the central claims. Below we address each major comment in detail, and we will incorporate the requested additions into a revised version of the manuscript.

read point-by-point responses

Referee: [§2] §2 (Model and loop-induced coupling): The central claim of dominance requires that the radiatively generated Zχχ coupling (suppressed by 1/(16π²) and Yukawa/mass factors) produces sufficient χ yield for the subsequent A' radiation to exceed meson and bremsstrahlung rates at small ε. Provide the explicit one-loop expression for the effective coupling and benchmark points (colored scalar mass, Yukawas) demonstrating simultaneous satisfaction of Ωh² ≈ 0.12 and BR(Z → χχ) large enough for dominance; without this, the overlap of regimes remains unverified.

Authors: We agree that an explicit derivation of the loop-induced Zχχ coupling is essential for verifying the dominance claim. In the revised manuscript we will include the complete one-loop expression for the effective coupling, obtained from the diagrams involving the top quark and the new colored scalar, with all suppression factors (1/(16π²), Yukawa couplings, and mass ratios) shown explicitly. We will also add a dedicated subsection with benchmark points specifying the colored scalar mass, Yukawa values, and resulting BR(Z → χχ) that simultaneously satisfy Ωh² ≈ 0.12 and ensure the dark radiation channel exceeds conventional rates for small ε and m_A' ≫ GeV. These benchmarks will be chosen from the viable parameter space already explored in our Monte Carlo simulations. revision: yes
Referee: [§4–5] §4–5 (Production rates and sensitivity projections): The assertion that the dark radiation channel dominates and allows these experiments to surpass existing bounds in relic-consistent regions must be supported by explicit rate comparisons and exclusion contours. Include a table or figure quantifying event yields from the new channel versus conventional sources across ε and m_A' (with error bands or parameter scans), and overlay the projected reaches on current limits.

Authors: We concur that explicit rate comparisons are required to substantiate the dominance and enhanced sensitivity. In the revised manuscript we will add a new figure (or table) that directly compares the expected event yields from the dark radiation channel against meson decays and proton bremsstrahlung as functions of ε and m_A', including scans over the relevant model parameters with uncertainty bands derived from the Monte Carlo statistics. The projected exclusion contours for FASER2, FACET, and MATHUSLA will be overlaid on existing limits, restricted to the relic-density-consistent regions, thereby demonstrating the newly accessible parameter space. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper defines a model with a radiatively generated effective Zχχ coupling via one-loop diagrams and computes the resulting dark photon production rate from Z decays. It then compares this rate to conventional sources (meson decays, bremsstrahlung) across parameter space and projects detector sensitivities. The observed relic abundance is invoked only as an external benchmark to identify interesting regions of parameter space, not as a fitted input that defines the production mechanism or sensitivity by construction. No self-definitional equations, fitted quantities renamed as predictions, or load-bearing self-citations appear in the provided text. The derivation chain remains independent of its own outputs.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 1 invented entities

The central claim rests on the existence of a new colored scalar to generate the effective coupling and on the assumption that parameters can be chosen to match the dark matter relic density while keeping the dark photon long-lived.

free parameters (2)

kinetic mixing parameter
Tuned to small values where the new channel dominates over meson decays and bremsstrahlung.
dark photon mass
Assumed well above the GeV scale for the dominance and long-lived regime.

axioms (2)

domain assumption Existence of fermionic dark matter candidate chi with radiatively generated effective Z chi chi coupling via top quark and new colored scalar loops.
Invoked as the source of the novel dark radiation production mechanism.
domain assumption Dark photons produced this way are long-lived and detectable by FASER2, FACET, and MATHUSLA.
Required for the sensitivity projections to surpass existing bounds.

invented entities (1)

New colored scalar no independent evidence
purpose: To mediate the one-loop diagrams that generate the effective Z chi chi coupling.
Postulated without independent evidence outside the model construction.

pith-pipeline@v0.9.0 · 5458 in / 1562 out tokens · 63769 ms · 2026-05-07T15:58:21.542259+00:00 · methodology

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discussion (0)

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