Charged Lepton Flavor Violating Experiments with Muons

Dylan Palo

REVIEW 2 major objections 4 minor 18 references

Charged Lepton Flavor Violating Experiments with Muons

T0 review · 2 major / 4 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read The near-term muon experiments aim to push charged-lepton-flavor-violation searches up to four orders of magnitude beyond current limits, with MEG II already reporting results on track.

desk verdict Useful status report, but the conclusion overstates the 10^4 sensitivity claims for Mu3e and COMET Phase I. read the letter →

arxiv 2505.04764 v1 pith:EH7IGUJ3 submitted 2025-05-07 hep-ex

Dylan Palo This is my paper · ORCID

classification hep-ex

keywords chargedleptonflavorviolationmuondecayMEGIIMu3eMu2eCOMETmuon-to-electronconversionbeyondStandardModel

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 proceedings is a status report on the three "golden" charged-lepton flavor-violating (CLFV) channels with muons: $\mu^+ \to e^+ \gamma$, $\mu^+ \to e^+ e^- e^+$, and $\mu^- N \to e^- N$. Its central claim is that the current generation of experiments will improve sensitivity by up to a factor of $10^{4}$ over previous searches: MEG II is in its fourth year of data-taking and has published a first-year limit of $7.5\times10^{-13}$; Mu3e plans physics runs starting in 2025 aiming for $10^{-15}$; and Mu2e and COMET target conversion sensitivities below $10^{-16}$ starting in 2027 and 2026. Because Standard Model neutrino mixing predicts CLFV rates around $10^{-54}$, an observation in any channel would be unambiguous evidence of new physics, and the experiments are designed either to make that observation or to rule out large parts of the parameter space.

What carries the argument

The argument is carried less by a single new result than by a common experimental strategy: each experiment identifies a distinctive signal signature and suppresses Standard Model processes that mimic it through precision kinematics and event timing. For $\mu^+ \to e^+ \gamma$, MEG II rejects accidental positron-photon coincidences using a liquid-xenon calorimeter, an ultra-light drift chamber, and timing counters. For $\mu^+ \to e^+e^-e^+$, Mu3e reconstructs a momentum-conserving, time- and vertex-coincident triplet with thin monolithic pixel sensors and scintillating timing. For $\mu^-N \to e^-N$, Mu2e and COMET use a pulsed proton beam with $10^{-10}$ extinction to suppress radiative pion capture, a straw tracker with $\sim150$ keV momentum resolution to beat the decay-in-orbit tail, and a cosmic-ray veto to remove cosmogenic electrons. The paper's numbers—$10^{4}$ sensitivity gains, $0.41\pm0.03$ expected background events, and the $6\times10^{-14}$, $10^{-15}$, and $<10^{-16}$ goals—all hang on these suppression mechanisms working at design level.

What would settle it

The decisive check is Mu2e's early data: if the between-pulse beam-extinction measurement is worse than $10^{-10}$, or if the sideband-derived background in the signal region is far above $0.41\pm0.03$ events, the claimed $<10^{-16}$ sensitivity does not hold; a blind signal-region count of several events where fewer than one is predicted would falsify the projection.

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Extended reading notes

Core claim

The paper's central claim is a projection about the near-term experimental frontier: charged lepton flavor violation has never been observed, and the Standard Model with massive neutrinos predicts $\mu^+ \to e^+ \gamma$ at a branching fraction of order $10^{-54}$, far below any conceivable measurement. The report therefore treats any detected CLFV signal as a discovery of physics beyond the Standard Model and argues that the upcoming experiments are capable of making that discovery. Specifically, MEG II has already published $7.5\times10^{-13}$ at 90% CL from its 2021 data and projects $6\times10^{-14}$; Mu3e is constructed for $10^{-15}$ in phase I and $2\times10^{-16}$ in phase II; and Mu2e and COMET are designed for $\mu^- \to e^-$ conversion below $10^{-16}$, a $10^{4}$ improvement over the SINDRUM II limit of $7\times10^{-13}$. The paper further claims that the required background suppression is understood and engineered, with Mu2e expecting only $0.41\pm0.03$ background events in its full dataset.

Load-bearing premise

The projected gains rest on each accelerator and detector performing exactly as simulated, especially on Mu2e's proton beam having essentially no stray protons between pulses (the $10^{-10}$ extinction requirement) and on background predictions such as the $0.41$ expected events being accurate.

Editorial extensions

If this is right

If MEG II completes its planned dataset, the $\mu^+ \to e^+ \gamma$ limit should improve from $4.2\times10^{-13}$ to about $6\times10^{-14}$, a factor of ten beyond MEG.
If Mu3e phase I runs as planned, the $\mu^+ \to e^+e^-e^+$ limit improves from $1.0\times10^{-12}$ to $10^{-15}$, and phase II reaches $2\times10^{-16}$.
If Mu2e and COMET reach design sensitivity, the $\mu^- N \to e^- N$ conversion limit improves from $7\times10^{-13}$ to below $10^{-16}$, a four-order-of-magnitude gain over SINDRUM II.
A null result in all three channels at these sensitivities would rule out large classes of new physics that produce CLFV at observable rates, and an observation in any channel would be a sign of physics beyond the Standard Model.
The published MEG II 2021 limit of $7.5\times10^{-13}$ is consistent with the collaboration's sensitivity projection, so the "on track" claim is already backed by data.

Reading between the lines

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

Beyond the paper: because the three channels are sensitive to different new-physics operators, a signal in only $\mu^+ \to e^+ e^- e^+$ or only $\mu^- N \to e^- N$ would already discriminate among models even if $\mu^+ \to e^+ \gamma$ stays null.
Beyond the paper: the accidental-background scaling $N_{\mathrm{ACC}} \propto R_\mu^2$ implies that simply raising the beam rate will not extend a MEG-II-style search; the paper's hints at photon conversion and pixelated trackers point to a testable design path for a next-generation experiment.
Beyond the paper: the $0.41\pm0.03$ Mu2e background expectation is a quantitative prediction that can be checked with early sideband data, so the sensitivity projection can be validated or refuted years before the full physics result.

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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

Colleague,

This is a conference proceedings on muon CLFV experiments, written by a Mu2e member. It has no new physics, but as a status summary it is mostly accurate and clearly organized. The body gives the right numbers: MEG II's 7.5e-13, SINDRUM's 1.0e-12, SINDRUM II's 7e-13, Mu3e Phase I targeting 1e-15, COMET Phase I improving by two orders of magnitude, and Mu2e's 10^4 goal. Detector descriptions and background discussions (DIO, RPC, cosmic rays) are coherent and match what I know from the cited papers. The author is honest about the MEG II result being a first-year limit, not yet the goal sensitivity.

The soft spot is the conclusion. The abstract says 'up to 10^4' for the collection of experiments, which is defensible as a collective maximum. But the conclusion assigns 10^4 to Mu3e and COMET individually with their near-term data-taking windows. That is inconsistent with the body: Mu3e Phase I, the phase starting in 2025/26, targets 1e-15, which is 10^3 beyond SINDRUM, not 10^4; the 2e-16 target belongs to Phase II. COMET Phase I is a 10^2 improvement. Only Mu2e plausibly approaches 10^4 in the 2027 window, and even that is a projected sensitivity, not a demonstrated limit. This is a genuine conflation of phases and timelines, and it inflates the central status claim. A careful reader who goes through the body will spot it, but the conclusion is the takeaway for many readers.

Other soft spots are minor and inherent to the genre: the projected sensitivities are self-reported collaboration goals, not independent measurements, and the timeline statements are from collaboration documents. The author being a Mu2e member does not bother me; the quoted Mu2e numbers match Mu2e's own published documents, and there is no sign of cherry-picking.

Who is this for? Someone outside the field who wants a quick, accurate snapshot of where the muon CLFV program stands as of early 2025. It delivers that, modulo the conclusion issue. I would send it to a referee mainly to fix the conclusion; a competent referee would catch the discrepancy and the paper would be fine after a small revision. It is not a must-read for experts, but it is a fair overview for students or colleagues moving into the area.

Referee Report

2 major / 4 minor

Summary. This proceedings paper reports the status of the three "golden" charged-lepton-flavor-violating (CLFV) searches with muons: MEG II (mu+ -> e+ gamma), Mu3e (mu+ -> e+ e- e+), and Mu2e/COMET (mu- N -> e- N). For each channel, it summarizes the signal signature, dominant backgrounds, detector design, projected resolutions, and timelines. The central status claim is that the next-generation experiments will improve on the previous best limits by up to four orders of magnitude, with Mu3e beginning physics data-taking in 2025/2026 and Mu2e/COMET in 2027/2026.

Significance. If the projected sensitivities are achieved, the experiments would probe branching ratios down to roughly 10^-14 to 10^-16 and would provide a substantial test of beyond-standard-model physics. The paper is a compact and mostly accurate status report with detailed references to primary documents, and it is transparent about the Monte Carlo basis of sensitivity projections and about remaining engineering milestones such as the 10^-10 proton-beam extinction requirement. Its value is as a review-style summary rather than as a new physics result; the sensitivity numbers are collaboration-reported goals rather than independent measurements, which is normal for this genre but should be kept in mind when citing the projected improvements.

major comments (2)

[Section 5] Section 5 states that Mu3e "plans to improve upon the current sensitivity limit by 10^4 with data-taking planned to start in 1-2 years." This is inconsistent with Sections 3.1 and 3.4: the phase planned for 2025/2026 is Phase I, which targets 10^-15, i.e., a factor of 10^3 beyond the SINDRUM limit of 1.0 x 10^-12, while the 2 x 10^-16 target cited in Section 3.4 belongs to Phase II, which is not described as starting in the 1-2 year window. Even the Phase II goal corresponds to a factor of about 5 x 10^3, not 10^4. The conclusion should attribute the improvement factor to the specific phase and avoid presenting the 10^4 figure as a near-term Mu3e goal.
[Section 5] Section 5 similarly groups COMET with Mu2e as achieving a 10^4 improvement "with data-taking starting in 2027 and 2026 respectively." Section 4.1 states that the phase planned to be physics-ready in 2026 is COMET Phase I, which improves on SINDRUM II by two orders of magnitude; the 10^4-scale improvement is associated with Mu2e and with COMET Phase II, not with the 2026 COMET phase. The conclusion should distinguish Phase I and Phase II sensitivities and dates for COMET, and should state explicitly that the 10^4 figure is a program-wide maximum rather than a per-experiment near-term claim.

minor comments (4)

[Section 2.4] The text refers to the "Michele + distribution"; this should be the "Michel" distribution.
[Section 1.2] The phrase "There is no true Standard Model background" is potentially confusing because the next sentences describe Standard Model processes that mimic the signal; clarifying that this refers to the absence of an irreducible CLFV decay, rather than to the absence of all backgrounds, would improve readability.
[Section 4.3] The phrase "pulsed proton beam (1700 ns)" is ambiguous: it should state whether 1700 ns is the repetition period, the pulse width, or the extinction gap.
[Sections 3.1 and 4.1] A small summary table listing each experiment phase, its start date, its target sensitivity, and its improvement factor over the prior limit would help prevent the phase conflation that currently appears in the conclusion.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: this is a status report whose sensitivity claims are quoted from external experimental documents, not derived from the paper's own inputs.

full rationale

This manuscript is a proceedings-style status report, not a derivation. Its central claims are that MEG II is on track to its goal sensitivity, Mu3e plans a 1e-15 (phase I) or 2e-16 (phase II) search, and Mu2e/COMET target conversion sensitivities below 1e-16. These numbers are quoted from the experiments' own documents and from external publications, and the '10^4 improvement' claims are simple ratios of projected sensitivities to previously published limits. No equation in the paper defines one reported quantity in terms of another in a way that forces a predicted result, and no fitted parameter is relabeled as a prediction. The author's Mu2e affiliation means some quoted Mu2e projections are self-reported, but the paper does not use those projections to derive a new result; it reports status, and the projections are externally checkable against future data. The most significant issue is an internal consistency problem, not circularity: the conclusion attributes a 10^4 improvement to Mu3e and COMET with data-taking starting in 2025-2026, while the body says Mu3e phase I aims for 1e-15 (a 10^3 improvement on SINDRUM's 1e-12) and COMET phase I improves on SINDRUM II by two orders of magnitude. That is a correctness/timeline conflation, not a circular derivation. No circular step is present.

Assumptions & free parameters 0 free parameters · 3 assumptions · 0 invented entities

The report introduces no free parameters and no new entities; its content is entirely a review of external experiments. The listed axioms are the background physics claim and the trust placed in the cited collaborations' projections and background models.

assumptions (3)

domain assumption The Standard Model with massive neutrinos implies a negligible CLFV branching fraction, of order 10^-54 for muon to positron photon decays.
Used in Section 1.1 to establish that observing CLFV would imply new physics; this is standard physics, not derived or tested in the paper.
domain assumption The projected sensitivities, background expectations, and timelines quoted from MEG II, Mu3e, Mu2e, and COMET documents are accurate as published.
Sections 2 through 4 rely on these collaboration-provided numbers as facts without independent verification.
domain assumption Accidental backgrounds scale as the square of the muon beam rate and with detector resolutions, as in the formula N_ACC proportional to R_mu^2 times energy, momentum, time, and angle resolutions in Section 2.3.
This model is used to justify why high beam rate experiments need improved resolutions; it is a standard Poisson-type background argument but it is asserted rather than derived here.

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Cite this review

Pith. "Pith review of Charged Lepton Flavor Violating Experiments with Muons." pith.science (2026). https://pith.science/paper/EH7IGUJ3

@misc{pith2026250504764,
  author       = {Pith},
  title        = {Pith review of: Charged Lepton Flavor Violating Experiments with Muons},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/EH7IGUJ3}},
  note         = {Machine review of arXiv:2505.04764}
}

abstract

We report on the status of charged lepton flavor violating (CLFV) experiments with muons. We focus on the three "golden channels": $\mu^{+} \rightarrow e^{+} \gamma$, $\mu^{+} \rightarrow e^{+} e^{-} e^{+}$ and $\mu^{-} N \rightarrow e^{-} N$. The collection of upcoming experiments aim for sensitivity improvements up to $10^{4}$ with respect to previous searches. The MEG II experiment, searching for $\mu^{+} \rightarrow e^{+} \gamma$, is currently in its 4th year of physics data-taking with a published result from its first year of data. The Mu3e experiment is an upcoming experiment searching for $\mu^{+} \rightarrow e^{+} e^{-} e^{+}$ with plans of physics data-taking as soon as 2025. The Mu2e and COMET experiments are upcoming searches for $\mu^{-} N \rightarrow e^{-} N$ with the goal of physics data-taking starting in 2027 and 2026 respectively. This proceeding summarizes the signal signature, expected background, resolutions, and timelines for the mentioned searches.

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

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Pith tools

Reviewed August 15, 2026 · model on record in the stance chip above.

[1] [1]

A. M. Baldiniet al.[MEG], Eur. Phys. J. C76, no.8, 434 (2016) doi:10.1140/epjc/s10052- 016-4271-x [arXiv:1605.05081 [hep-ex]]

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A. M. Baldiniet al.[MEG II], Eur. Phys. J. C78, no.5, 380 (2018) doi:10.1140/epjc/s10052-018-5845-6 [arXiv:1801.04688 [physics.ins-det]]

arXiv 2018

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Afanacievet al.[MEG II], Eur

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Bellgardtet al.[SINDRUM], Nucl

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