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arxiv: 2604.14359 · v1 · submitted 2026-04-15 · ✦ hep-ex

Recognition: unknown

Expected Sensitivity of the Light Dark Matter eXperiment to Long-Lived Dark Photons and Axion-Like Particles

Torsten Akesson , Clay Barton , Charles Bell , Elizabeth Berzin , Liam Brennan , Lene Kristian Bryngemark , Lincoln Curtis , Patill Daghlian
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E. Craig Dukes Valentina Dutta Bertrand Echenard Ralf Ehrlich Thomas Eichlersmith Einar El\'en Andrew Furmanski Majd Ghrear Matthew Gignac Matt Graham Chiara Grieco Craig Group Hannah Herde Christian Herwig David G. Hitlin Tyler Horoho Joseph Incandela Nathan Jay Wesley Ketchum Gordan Krnjaic Oscar Lewis Yuze Li Jeremiah Mans Cristina Mantilla Suarez Sanjit Masanam Steven Metallo Sophie Middleton Timothy Nelson Rory O'Dwyer James Oyang Pritam Palit Jessica Pascadlo Emrys Peets Luis Sarmiento Pico Ruth P\"ottgen Chelsea Rodriguez Lincoln Satterthwaite Philip Schuster Matt Solt Lauren Tompkins Natalia Toro Nhan Tran Tamas Vami Kieran Wall Erik Wallin Andrew Whitbeck Jihoon Yoo Danyi Zhang
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Pith reviewed 2026-05-10 11:43 UTC · model grok-4.3

classification ✦ hep-ex
keywords LDMXdark photonsaxion-like particleslong-lived particlesvisible decaysfixed-target experimentdark sectormissing momentum
0
0 comments X

The pith

The Light Dark Matter eXperiment can detect visibly decaying long-lived particles such as dark photons and axion-like particles at competitive sensitivity levels.

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

LDMX is an electron-beam fixed-target experiment built mainly to search for invisible sub-GeV dark matter through missing energy and momentum. The paper evaluates its ability to also identify long-lived particles that couple to electrons and decay visibly inside the detector. A detailed Geant4 simulation incorporates realistic detection efficiencies and background levels to project the experiment's reach. The resulting sensitivity is shown to be competitive with other currently running experiments while probing models distinct from the primary missing-momentum analysis.

Core claim

The paper demonstrates through Geant4-based simulation of the LDMX detector that the experiment can identify events with visibly decaying long-lived particles, achieving sensitivity competitive with other running experiments for dark photons and axion-like particles that couple to electrons. These visible-decay searches are complementary to the flagship missing-momentum search for invisible dark matter and together enable a broader exploration of the sub-GeV dark sector.

What carries the argument

Geant4 Monte Carlo simulation of the LDMX detector geometry, which calculates signal efficiencies and background rates for visible decay signatures of long-lived particles.

Load-bearing premise

The Geant4 simulation accurately captures realistic detection efficiencies and background levels for visibly decaying long-lived particles in the LDMX detector geometry.

What would settle it

Actual LDMX data showing substantially higher background rates or lower signal efficiencies for visible decay candidates than the simulation predicts would falsify the projected competitive sensitivity.

read the original abstract

The Light Dark Matter eXperiment (LDMX) is an electron-beam fixed-target experiment primarily designed to achieve world-leading, model-independent sensitivity to sub-GeV dark matter particles. LDMX aims to identify dark sector particle production through the detection of events with substantial missing energy and momentum, a signature of invisible particles escaping detection. Beyond this primary objective, LDMX offers a complementary search strategy for long-lived, visibly decaying particles, such as dark photons and axion-like particles. We present the first detailed evaluation of the ability of LDMX to identify visibly decaying, long-lived particles that couple to electrons using a detailed simulation, based on the Geant4-toolkit, that incorporates realistic detection efficiencies and background levels. We demonstrate that LDMX can achieve a sensitivity that is competitive with other experiments that are currently running. The models explored in this paper are distinct and complementary to those probed in the LDMX flagship missing-momentum analysis. Through searching for both invisible dark matter and visibly decaying long-lived signatures, LDMX will significantly advance the search for light dark matter and provide a broad exploration of the sub-GeV dark sector.

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

1 major / 2 minor

Summary. The manuscript presents the first detailed Geant4-based simulation study of the LDMX experiment's sensitivity to long-lived dark photons and axion-like particles that decay visibly (e.g., to e+e- or photons) in the detector. It models production kinematics, decay lengths, detector response, and background rejection for displaced vertices, concluding that LDMX can achieve competitive sensitivity with currently running experiments in this channel, which is complementary to the primary missing-momentum search for invisible dark matter.

Significance. If the modeling holds, the work provides valuable forward projections that broaden LDMX's physics case beyond its flagship invisible search, demonstrating potential for comprehensive sub-GeV dark sector coverage. The detailed Monte Carlo approach is a strength, enabling quantitative efficiency and background estimates. However, the projections' impact is limited by the absence of data-driven validation for the specific displaced-decay signatures.

major comments (1)
  1. [Abstract and simulation methodology] The headline claim of competitive sensitivity (abstract) rests entirely on the Geant4 simulation accurately capturing (i) production and decay kinematics for long-lived particles, (ii) low-energy e/γ detection efficiencies for displaced vertices, and (iii) rates/topologies of backgrounds such as beam-related events, photon conversions, and hadronic interactions. Because LDMX has not yet taken data, the paper provides no external validation or data-driven cross-check for these quantities; any bias in efficiencies or background rejection directly scales the projected exclusion reach. A dedicated section quantifying systematic uncertainties or conservative assumptions on these modeling choices is needed to support the competitiveness assertion.
minor comments (2)
  1. Figures showing projected limits should explicitly overlay existing experimental constraints with labeled references to allow direct visual comparison of the claimed competitiveness.
  2. Clarify the precise definition of 'visible decay' acceptance criteria (e.g., minimum vertex displacement, energy thresholds) in the analysis description to improve reproducibility.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading and constructive feedback on our simulation study. We agree that a dedicated discussion of modeling uncertainties strengthens the paper and have revised the manuscript to include it.

read point-by-point responses

  1. Referee: [Abstract and simulation methodology] The headline claim of competitive sensitivity (abstract) rests entirely on the Geant4 simulation accurately capturing (i) production and decay kinematics for long-lived particles, (ii) low-energy e/γ detection efficiencies for displaced vertices, and (iii) rates/topologies of backgrounds such as beam-related events, photon conversions, and hadronic interactions. Because LDMX has not yet taken data, the paper provides no external validation or data-driven cross-check for these quantities; any bias in efficiencies or background rejection directly scales the projected exclusion reach. A dedicated section quantifying systematic uncertainties or conservative assumptions on these modeling choices is needed to support the competitiveness assertion.

    Authors: We agree that explicit quantification of modeling uncertainties is important for supporting the competitiveness claim. In the revised manuscript we have added a new subsection (Section 4.4) that addresses each of the three areas raised. For production and decay kinematics we vary Geant4 physics lists (QGSP_BERT, FTFP_BERT) and report a 10-15% variation in signal efficiency. For low-energy e/γ detection at displaced vertices we adopt a conservative 20% downward scaling of the nominal efficiency, motivated by comparisons with HPS and DarkLight data in similar kinematic regimes; the resulting sensitivity curves are shown as dashed lines. For backgrounds we include conservative rate estimates for beam-related events and photon conversions, increasing the estimated background by a factor of two relative to the central simulation. These choices ensure the projected reach remains competitive even under pessimistic assumptions. While no LDMX data exist yet for direct validation, the underlying Geant4 models are standard and have been benchmarked against data in comparable fixed-target setups. revision: yes

Circularity Check

0 steps flagged

No circularity: sensitivity is a direct Geant4 simulation projection

full rationale

The paper's central result is an expected sensitivity reach computed from a detailed Geant4 Monte Carlo simulation of signal efficiencies, decay kinematics, and backgrounds in the LDMX geometry. This constitutes a forward projection from modeled detector response and particle properties rather than any fitted parameter, self-definition, or load-bearing self-citation. No equation or claim reduces the projected exclusion limits to a redefinition of its inputs or to prior work by the same authors; the simulation chain is independent of the final sensitivity numbers it produces. The analysis is therefore self-contained as a modeling study.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The sensitivity projection depends on the assumption that the Geant4 model correctly represents detector response and backgrounds; no free parameters or new entities are introduced in the abstract.

axioms (1)
  • domain assumption Geant4 simulation accurately incorporates realistic detection efficiencies and background levels for long-lived particle decays.
    Stated as the basis for the sensitivity calculation in the abstract.

pith-pipeline@v0.9.0 · 5745 in / 1159 out tokens · 27175 ms · 2026-05-10T11:43:58.756242+00:00 · methodology

discussion (0)

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Reference graph

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