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arxiv: 2508.07922 · v2 · submitted 2025-08-11 · ✦ hep-ex

Positron Transport System for Muonium-to-Antimuonium Conversion Experiment

Guihao Lu , Shihan Zhao , Siyuan Chen , Jian Tang This is my paper

Pith reviewed 2026-05-18 23:40 UTC · model grok-4.3

classification ✦ hep-ex
keywords positron transport systemmuonium to antimuonium conversioncharged lepton flavor violationelectrostatic acceleratorsolenoid beamlineCOMSOL simulationGeant4 particle trackingtime of flight background rejection
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The pith

A positron transport system using electrostatic acceleration and solenoids can collect 65.81 percent of signal positrons from antimuonium decays while providing nanosecond timing for background rejection.

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

The Muonium-to-Antimuonium Conversion Experiment searches for charged lepton flavor violation by looking for antimuonium atoms that convert to muonium. The positron transport system is built to gather positrons emitted in antimuonium decays and deliver them to a detector with high efficiency. Field and particle simulations show the system reaches 65.81 percent geometric acceptance, micrometer position resolution, and a tight 6.9 nanosecond transit time spread. This timing window supports time-of-flight cuts that suppress internal conversion backgrounds by seven orders of magnitude. The results demonstrate a workable design for rare-signal collection in the MACE setup.

Core claim

Through field simulations in COMSOL and particle transport simulations based on Geant4, the positron transport system achieves a geometric acceptance of the signal at 65.81(4)% along with a position resolution of 88(1) μm × 102(1) μm. The system achieves 322.4(1) ns transit time with a spread of 6.9(1) ns, which allows for a TOF-based rejection of internal conversion backgrounds by a factor of 10^{-7}.

What carries the argument

The positron transport system, built from an electrostatic accelerator followed by a solenoid beamline, whose electromagnetic fields are modeled in COMSOL and whose particle trajectories are tracked in Geant4.

If this is right

  • The 65.81 percent acceptance increases the number of detectable signal events from rare antimuonium decays.
  • The 88 by 102 micrometer position resolution allows precise reconstruction of the decay vertex.
  • The 6.9 nanosecond transit time spread enables time-of-flight cuts that reduce internal conversion backgrounds by a factor of 10^{-7}.
  • These performance figures support new-physics searches in the MACE experiment and suggest a reusable design for similar high-intensity transport lines.

Where Pith is reading between the lines

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

  • Comparable electrostatic-plus-solenoid layouts could be tested in other low-energy positron or muon experiments to improve collection efficiency.
  • Discrepancies between simulated and measured performance would point to unmodeled effects such as surface charging or fringe fields that require hardware adjustments.
  • Combining the reported timing precision with additional detector layers might further suppress backgrounds beyond the factor of 10^{-7} already achieved.

Load-bearing premise

The electromagnetic fields and particle interactions modeled in COMSOL and Geant4 accurately represent the physical positron transport system that would be built and operated.

What would settle it

Building a prototype of the electrostatic accelerator and solenoid beamline and directly measuring its geometric acceptance, position resolution, and transit time spread in a controlled beam test would confirm or contradict the simulation results.

Figures

Figures reproduced from arXiv: 2508.07922 by Guihao Lu, Jian Tang, Shihan Zhao, Siyuan Chen.

Figure 1
Figure 1. Figure 1: FIG. 1: A schematic figure of the positron transport system. The outer grey shell represents the iron yoke, while the [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: Overview of the magnetic field of the PTS [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Overview of the electric field effects in the PTS [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: The relative displacement of the positron. The horizontal and vertical coordinates [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: Geometric acceptance of PTS for particles with [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Curves of background and signal selection [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
read the original abstract

Muonium-to-Antimuonium Conversion Experiment (MACE) aims to find the charged lepton flavor violation (cLFV) process. A key component of MACE is the positron transport system (PTS) to collect and transport atomic positrons from antimuonium decays, which consists of an electrostatic accelerator and a solenoid beamline. Through field simulations in \textsc{COMSOL} and particle transport simulations based on \textsc{Geant4}, the PTS can achieve a geometric acceptance of the signal at 65.81(4)\% along with a position resolution of 88(1)~$\mu$m~$\times$~102(1)~$\mu$m. The system achieves 322.4(1)~ns transit time with a spread of 6.9(1)~ns, which allows for a TOF-based rejection of internal conversion backgrounds by a factor of $10^{-7}$. These promising results pave the way for new-physics signal identifications and background rejections in MACE and offer a novel paradigm for internal transport system in high-intensity frontiers.

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

0 major / 4 minor

Summary. The manuscript describes the design and simulation of a Positron Transport System (PTS) for the Muonium-to-Antimuonium Conversion Experiment (MACE). The system includes an electrostatic accelerator and a solenoid beamline. Simulations using COMSOL for field calculations and Geant4 for particle tracking yield a geometric acceptance of 65.81(4)% for the signal, a position resolution of 88(1) μm × 102(1) μm, a mean transit time of 322.4(1) ns with a spread of 6.9(1) ns, and a TOF-based background rejection factor of 10^{-7} for internal conversion events.

Significance. Should the simulated performance be achieved in the physical PTS, this design would substantially improve the efficiency and background rejection in the MACE experiment's search for charged lepton flavor violation. The study provides a concrete example of using standard simulation tools to optimize transport systems in high-intensity frontier experiments, with the reported metrics serving as benchmarks for future hardware validation.

minor comments (4)
  1. The uncertainties on the reported performance figures (e.g., 65.81(4)%) should be explained in more detail, including how they were derived from the simulation statistics or variations in parameters.
  2. Specify the exact Geant4 version, physics list, and any custom physics processes or cuts used in the particle transport simulations to allow for independent reproduction of the results.
  3. Include a comparison or reference to similar positron or muon transport systems in other experiments to contextualize the achieved acceptance and resolution.
  4. Ensure that all figures showing field maps or particle trajectories have clear labels, scale bars, and legends for easy interpretation.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for the positive assessment of our work on the positron transport system for the MACE experiment and for recommending minor revision. The reported performance metrics align with the goals of improving efficiency and background rejection in searches for charged lepton flavor violation.

Circularity Check

0 steps flagged

No significant circularity

full rationale

The manuscript is a simulation-only design study whose central numerical claims (65.81(4)% acceptance, 88(1) μm × 102(1) μm resolution, 322.4(1) ns transit time with 6.9(1) ns spread) are direct outputs of COMSOL electrostatic field maps and Geant4 particle-transport runs applied to a stated geometry and physics list. No equation defines a result in terms of itself, no parameter is fitted to a subset and then relabeled as a prediction, and no self-citation chain supplies a uniqueness theorem or ansatz that the present work merely renames. The derivation chain therefore remains self-contained and externally falsifiable by independent simulation or hardware measurement.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central performance claims rest on the accuracy of two standard simulation packages and the assumption that the modeled geometry and fields match the future physical apparatus. No new free parameters, invented entities, or ad-hoc axioms are introduced beyond routine domain assumptions of electromagnetic and particle-transport modeling.

axioms (2)
  • domain assumption COMSOL electromagnetic field simulations accurately reproduce the fields produced by the electrostatic accelerator and solenoid beamline.
    All reported acceptance, resolution, and timing results depend on this modeling step.
  • domain assumption Geant4 particle transport simulations correctly predict positron trajectories, timing, and detection efficiency in the computed fields.
    The quoted 65.81% acceptance, position resolution, and 6.9 ns time spread are outputs of this transport step.

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