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REVIEW 2 major objections 5 minor 80 references

CEνNS detectors at spallation sources can already constrain heavy neutral leptons through their visible decays, especially muon mixing, with ton-scale upgrades reaching open parameter space.

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

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T0 review · grok-4.5

2026-07-31 03:43 UTC pith:LJD7MPUJ

load-bearing objection Solid, incremental detector-resolved HNL projections for COHERENT and other spallation sources; the “new parameter space” claim only survives under background assumptions the authors themselves show are optimistic. the 2 major comments →

arxiv 2607.25008 v1 pith:LJD7MPUJ submitted 2026-07-27 hep-ph

Searches for heavy neutral lepton decays at spallation neutron sources

Valentina De Romeri , A. Galindo-Uribarri , Ana Mart\'in-Gal\'an , V\'ictor Mart\'in Lozano , G. Sanchez Garcia This is my paper
classification hep-ph
keywords heavy neutral leptonsspallation neutron sourcesCEνNSCOHERENTpion decay at restlong-lived particlesneutrino mixing
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

Spallation neutron sources produce intense MeV-scale neutrinos from stopped pion and muon decays. The same decays can also produce heavy neutral leptons (HNLs) that mix with active neutrinos. This paper shows that detectors built for coherent elastic neutrino–nucleus scattering can catch the subsequent HNL decay to an electron–positron pair inside the detector volume. Existing COHERENT detectors already set meaningful limits on muon-flavor mixing near the kinematic edge, while proposed ton-scale liquid-argon modules can reach previously unexplored regions for both electron and muon mixing. The same strategy extends to planned CEνNS setups at other spallation facilities, turning neutrino monitors into complementary long-lived-particle searches.

Core claim

Existing and near-term COHERENT detectors at the SNS can place competitive 90% CL bounds on HNL–muon mixing in previously open mass windows (roughly 30–100 MeV), while future ton-scale LAr detectors can push both electron- and muon-mixing sensitivities into unexplored territory; analogous CEνNS detectors at ESS, J-PARC and CSNS yield complementary reach, especially in the muon-mixing channel.

What carries the argument

The signal rate is the product of HNL production in pion or muon decay-at-rest (branching ratio ∝ |U_ℓN|²), geometric acceptance, and the in-detector decay probability P_decay = e^{-L/λ}(1 − e^{-s/λ}) into a visible e⁺e⁻ pair (width also ∝ |U_ℓN|²), so the observable scales as |U|⁴.

Load-bearing premise

The headline sensitivity curves assume essentially zero or only a hundred background events; the authors’ own cosmic-ray estimate is tens of thousands of events, which largely erases the new reach.

What would settle it

A real three-year exposure of any listed COHERENT detector that either observes an excess of beam-timed MeV-scale electromagnetic showers consistent with e⁺e⁻ pairs or sets a background-subtracted upper limit weaker than the projected contours under the measured background level.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • Current NaI, LAr and D2O modules can already improve on T2K/MicroBooNE muon-mixing limits near the pion kinematic edge if backgrounds stay low.
  • Ton-scale LAr upgrades become the dominant spallation-source probe of both single-flavor and mixed |U_eN U_μN| scenarios between 30 and 100 MeV.
  • ESS and J-PARC Xe/CsI proposals gain competitive muon-mixing sensitivity in the same mass window once they run.
  • Beam timing, muon vetoes and compact geometries become first-order design drivers for any future HNL-capable CEνNS detector.

Where Pith is reading between the lines

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

  • If cosmic-ray backgrounds cannot be reduced below ~10³ events, the scientific case shifts from existing detectors to purpose-built imaging calorimeters with stronger vetoes.
  • The same π/μ-DAR flux can be re-used for other long-lived states whose visible decays deposit MeV electromagnetic energy, so the analysis template is portable beyond HNLs.
  • A null result under realistic backgrounds would still tighten the laboratory side of the comparison with BBN/CMB bounds in the muon-mixing sector.

Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 5 minor

Summary. The manuscript studies production of MeV-scale heavy neutral leptons in pion and muon decays at rest at spallation neutron sources, followed by visible N -> nu e+ e- decays inside COHERENT-style detectors. It computes projected 90% CL sensitivities for electron, muon, and mixed flavor mixing at current and proposed SNS detectors, and extends the exercise to proposed detectors at the ESS, J-PARC, and CSNS. The main conclusion is that existing SNS detectors could constrain previously open muon-mixing parameter space near the kinematic endpoints, while ton-scale LAr detectors could probe substantially deeper.

Significance. If the assumed background levels and efficiencies can be realized, the study identifies a useful low-cost extension of the CEνNS program, especially for muon-flavor mixing in the roughly 30–100 MeV region, and gives a clear comparison of current SNS detectors, proposed ton-scale upgrades, and other spallation facilities. The phenomenological ingredients are largely standard and consistently implemented: fixed SM+HNL production and decay rates, geometric acceptance, decay-in-volume probability, Asimov-data Poisson limits, and external exclusion overlays. The appendix's explicit variation of efficiency and background level is a strength and makes the sensitivity projections falsifiable with respect to their key assumptions. The paper is a projection rather than a demonstrated constraint, however, and its practical significance depends directly on the background treatment discussed below.

major comments (2)
  1. [Sec. III.C, Eq. (20); Sec. IV, Figs. 1–3; Appendix A, Figs. 4–7] The headline sensitivity claims are much more background-dependent than the main-text presentation suggests. Figures 1–3 use the background-free hypothesis, S_up=2.3, whereas Eq. (20) and the surrounding discussion estimate B~10^4–10^5. The only support offered for the intermediate B=100 case is footnote 3, based on a dedicated D2O simulation with Cherenkov topology, a muon veto, and detector-specific cuts. Those handles do not directly establish B=100 for the NaI, LAr, and CsI calorimeters that carry much of the claimed reach. Since the event yield scales approximately as |U|^4, the vertical position of a |U|^2 contour shifts by sqrt(S_up): about 2.8 between B=0 and B=100, and about 15 between B=0 and B=10^5. Appendix A shows B=100 curves, but the statement in Sec. IV that the results 'hold qualitatively' does not identify quantitatively which presently unconstrained regions survive for
  2. [Sec. III.B, Eq. (14); Sec. III.C] The signal efficiency and background rejection are not treated under a common, detector-specific event selection. Equation (14) assumes a flat efficiency of 50% or 100%, while Sec. III.C notes EνES, charged-current electron production, and Michel electrons but does not estimate their rates under the same visible-energy and timing window. This is particularly important for NaI and CsI—and partly for LAr at the level assumed here—because an unresolved e+e− pair can resemble a single electromagnetic deposition, whereas the D2O background estimate relies on a different event topology. A full Geant4 analysis is not necessarily required, but at least simple rate estimates for these named backgrounds and a consistent definition of the assumed selection efficiency are needed before the B=100 projections can support the stated experimental reach. Alternatively, the figures should be labeled as to
minor comments (5)
  1. [Table I and Sec. IV] Table I lists the current LAr detector as 475 kg, while Sec. IV refers to 'LAr (476 kg)'. Please use one value consistently.
  2. [Table II and Fig. 3] The alignment of Table II is ambiguous, and it does not appear to list transparently all detector configurations shown in Fig. 3, such as the ESS CsI 22 kg and 44 kg entries and the J-PARC CsI 44 kg entry. Please make the mass, size, and baseline assignments for every plotted curve explicit.
  3. [Sec. III.A, Eq. (5)] Equation (5) is described as a 'total decay width,' but it is the partial width for a specific pi+ -> l+ + N channel. Calling it a partial width would avoid confusion with the total pion width.
  4. [Figs. 1–3] The captions and main text describe the cosmological and seesaw overlays somewhat differently: the captions refer to light-gray dashed lines for both BBN and seesaw information, while the text also refers to a purple shaded BBN region. Please harmonize the captions and legends.
  5. [Sec. I] Please proofread the compiled manuscript for missing spaces, for example around 'sterile neutrinos are tightly constrained' and 'the seesaw mechanism' in the introduction.

Circularity Check

0 steps flagged

No circularity: forward sensitivity projections from fixed SM+HNL rates and published detector parameters, not fits rebranded as predictions.

full rationale

The paper computes projected 90% CL exclusion contours for HNL mixing by folding standard two- and three-body production widths (Eqs. 5–8), visible decay widths (Eqs. 9–11), geometric acceptance (Eq. 13), and in-detector decay probability (Eq. 15) into an expected event yield (Eq. 14), then converting that yield to an upper limit via a Bayesian Poisson construction (Eqs. 16–19) under stated background and efficiency assumptions. All inputs are either textbook weak-interaction formulae (cited to external literature such as Ema et al.), tabulated COHERENT/ESS/J-PARC/CSNS detector parameters, or explicit benchmark choices (B = 0, 100, 10^5; ε = 50% or 100%). No parameter is fitted to data and then re-presented as a prediction; the Asimov construction is a pure forecast. Laboratory and cosmological overlays are taken from external compilations. Self-citations concern related CEνNS phenomenology and do not close any logical loop in the HNL rate chain. Background-assumption fragility is a robustness issue, not circularity.

Axiom & Free-Parameter Ledger

3 free parameters · 4 axioms · 0 invented entities

The central sensitivity contours rest on standard electroweak decay formulae, published SNS beam parameters, geometric acceptance approximations, and three benchmark background/efficiency choices that are varied but not derived from first-principles detector simulation.

free parameters (3)
  • flat detection efficiency ε = 0.5 or 1.0
    Set by hand to 50 % (headline) or 100 % (appendix); no energy-dependent simulation is performed.
  • background event count B = 0 / 100 / 1e5
    Three discrete benchmarks (0, 100, 10^5) chosen to span optimistic-to-conservative cosmic-ray estimates; not measured in situ for the HNL signature.
  • N_POT and c_π+ = facility-dependent
    Taken from SNS upgrade projections (1.728e23 /yr, c_π+=0.11) and analogous numbers for other facilities; treated as fixed inputs.
axioms (4)
  • domain assumption Single Dirac HNL mixes with active neutrinos via the effective charged- and neutral-current Lagrangian of Eq. (2).
    Standard minimal sterile-neutrino framework; Majorana case would rescale rates by ~2 but is not explored.
  • domain assumption Pion and muon decays at rest are isotropic and the HNL flux at the detector follows the geometric 1/(4πL²) factor.
    Sec. III.A–B; valid for the short baselines and mild boosts considered.
  • standard math Bayesian Poisson upper limit with flat non-negative prior and Asimov dataset yields the 90 % CL contours.
    Eqs. (16)–(19); textbook counting statistics.
  • ad hoc to paper Cosmic-ray photon flux Φ_γ ≃ 10^{-2} cm^{-2} s^{-1} after duty-cycle suppression gives the order-of-magnitude background ceiling.
    Sec. III.C; deliberately conservative ceiling chosen without full GEANT4 shielding simulation.

pith-pipeline@v1.2.0-grok45-kimik3 · 29015 in / 2737 out tokens · 54873 ms · 2026-07-31T03:43:20.478403+00:00 · methodology

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read the original abstract

Spallation neutron sources provide intense neutrino fluxes from pion and muon decay at rest, with energies in the few tens of MeV range. Experiments such as COHERENT exploit these fluxes to detect neutrinos via coherent elastic neutrino$\unicode{x2013}$nucleus scattering (CE$\nu$NS). However, these facilities also offer a unique opportunity to produce and probe light, secluded, or weakly$\unicode{x2013}$coupled particles. In this work, we investigate the sensitivity of the Spallation Neutron Source at the Oak Ridge National Laboratory to heavy neutral leptons (HNLs) in the MeV$\unicode{x2013}$GeV mass range, as a case study. We consider HNL production in pion and muon decays at rest, followed by their decay into visible Standard Model particles within the detector volume. We analyze a range of current and proposed COHERENT detectors and evaluate their sensitivity to HNL mixing with muon and electron neutrinos, as well as to scenarios with mixed mixing. We find that existing detectors can set meaningful constraints, particularly for muon$\unicode{x2013}$flavor mixing, while future ton$\unicode{x2013}$scale realizations can probe previously unexplored regions of the parameter space. We further discuss prospects at other relevant spallation source facilities and comment on the complementarity of their projected sensitivities. Our results demonstrate that CE$\nu$NS experiments at spallation neutron sources provide a powerful, complementary avenue for new physics searches beyond their primary role as neutrino detectors.

Figures

Figures reproduced from arXiv: 2607.25008 by A. Galindo-Uribarri, Ana Mart\'in-Gal\'an, G. Sanchez Garcia, Valentina De Romeri, V\'ictor Mart\'in Lozano.

Figure 1
Figure 1. Figure 1: FIG. 1: Projected 90% CL exclusion sensitivity of currently operating or soon-to-be-deployed ( [PITH_FULL_IMAGE:figures/full_fig_p008_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: Projected 90% CL exclusion sensitivity in the [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Projected 90% CL exclusion sensitivity of the detectors considered at other spallation source facilities – [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p013_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p014_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p015_7.png] view at source ↗

discussion (0)

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Reference graph

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