REVIEW 3 major objections 5 minor 21 references
Experimental Study of Rare Kaon Decays at J-PARC with KOTO and KOTO II
T0 review · 3 major / 5 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read KOTO II aims to catch the rarest kaon decay at 5 sigma
desk verdict A readable advocacy summary for KOTO II, but the 5-sigma projection is asserted without the underlying simulation or background systematics, and the 'only future kaon experiment' claim ignores HIKE. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the decay signature: two photons from the $\pi^0$ in a hermetic veto detector, with the decay vertex assumed on the beam axis and candidates defined in the plane of reconstructed vertex position versus $\pi^0$ transverse momentum. The argument is carried by the KOTO II detector concept—a 5-degree neutral beamline, 12-m decay volume, and 3-m calorimeter—together with the background-suppression system that keeps the expected background at 40 events for the full run. This geometry converts an estimated $K_L$ flux into the quoted 35-signal, 40-background counting experiment that underpins the discovery claim.
What would settle it
If KOTO II accumulates $3\times10^7$ seconds of data and sees no excess over the expected 40 background events—or if the observed background in the signal region exceeds the projected level by a significant factor—the central discovery claim is refuted. The concrete check is to count events in the $K_L\to\pi^0\nu\bar{\nu}$ signal box after the full analysis on early data and compare with the 35-signal, 40-background expectation.
Extended reading notes
Core claim
The paper's central claim is that the Standard Model value of $B(K_L\to\pi^0\nu\bar{\nu})\simeq 3\times10^{-11}$ is reachable in the 2030s with the proposed KOTO II detector. Using a 5-degree production angle that yields 2.6 times the $K_L$ flux of the current KOTO beamline and a harder momentum spectrum peaking near 2.9 GeV/$c$, a 12-m decay volume, and a 3-m calorimeter, the authors expect 35 signal and 40 background events in $3\times10^7$ s, enough for a $5\sigma$ discovery. They further state that a branching ratio differing by 40% from the Standard Model value would be distinguishable at 90% confidence, and that a second phase with charged-particle tracking can access $K_L\to\pi^0 e^+e^-$, whose branching ratio is also $\mathcal{O}(10^{-11})$ in the Standard Model.
Load-bearing premise
The projection rests on the assumed background rejection and signal acceptance of a detector that has not yet been built; the paper quotes 35 signal and 40 background events without presenting the Monte Carlo simulation or systematic uncertainties behind those numbers.
Editorial extensions
If this is right
- A $5\sigma$ observation at the Standard Model value would be the first measurement of $B(K_L\to\pi^0\nu\bar{\nu})$, with the 2%-accurate prediction making the comparison a sharp test of new physics.
- Combined with NA62's measured $K^+\to\pi^+\nu\bar{\nu}$ branching ratio, the two kaon channels can discriminate between new-physics models through their correlated predictions.
- If the observed branching ratio differs from the Standard Model by 40%, that is a 90%-confidence indication of new physics in the $s\to d$ transition.
- With tracking added, the same experiment targets $K_L\to\pi^0 e^+e^-$, providing a complementary flavor probe in the second phase.
Reading between the lines
- The authors do not spell out that a KOTO II measurement at the Standard Model rate would effectively close the window for many tree-level new-physics explanations of the $s\to d$ transition at scales up to roughly 1000 TeV, leaving only models whose new physics is aligned with the Standard Model flavor hierarchy.
- A direct early measurement of the background rate in the signal region during commissioning would settle the projection's main uncertainty before the full 35/40 event count is accumulated.
- If Belle II's $B^+\to K^+\nu\bar{\nu}$ measurement matures on a similar timescale, the combined kaon and $B$ channels could constrain the same new-physics parameters far more tightly than the kaon-only correlations shown in the paper.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript presents the physics case and the experimental program of the KOTO experiment and the proposed KOTO II experiment at J-PARC. It reviews the Standard Model prediction for the decay KL→π0νν̄, the current KOTO upper limit, the new-physics motivation from correlations with K+→π+νν̄ and B→K(*)νν̄, and the planned detector and beamline. The central quantitative claim is that KOTO II can achieve a 5σ discovery of KL→π0νν̄ at the SM branching ratio by collecting 35 signal events and 40 background events in 3×10^7 s of running, and that a 90% indication of new physics would be possible if the observed branching ratio differs by 40% from the SM value. The paper also describes plans for measuring KL→π0e+e− and searches for hidden-sector particles, along with detector-developement and European contributions.
Significance. If the projected sensitivity is realized, KOTO II would be the first experiment able to observe the golden kaon decay KL→π0νν̄ at the Standard Model rate, providing a uniquely precise probe of the s→d flavor-changing neutral current. The manuscript accurately cites the SM branching-ratio predictions and the current KOTO bound, and it gives a useful survey of new-physics scenarios and their correlations. The paper also reports concrete detector R&D, a cost estimate, and a collaboration structure, which are strengths for a proposal-style article. However, the central feasibility claim is not derived in the paper: the 35-signal/40-background numbers are quoted without Monte Carlo details, systematic uncertainties, or a statistical interpretation, and the paper is not self-contained on this point.
major comments (3)
- [Section 4, 'The KOTO II experiment'] The central sensitivity claim, 'Discovery of the decay with 5σ significance ... with the collection of 35 signal events and 40 background events expected', is not derived in the paper and, as stated, is not strictly correct. For a counting experiment with S=35 and B=40, the asymptotic likelihood-ratio significance is Z = sqrt(2[(S+B)ln(1+S/B)-S]) ≈ 4.9σ, which is below 5σ even before any systematic uncertainties; the naive S/sqrt(B) = 5.5σ overstates the sensitivity. The paper should state the statistical procedure, quote the exact significance, and explain how the 5σ claim is obtained, for example with a profile-likelihood treatment that includes nuisance parameters.
- [Section 4, 'The KOTO II experiment'] The expected background of 40 events is quoted as a single number with no decomposition and no uncertainty. Because the signal-to-background ratio is only about 0.9, the discovery significance is very sensitive to the background level: a fractional background systematic uncertainty of 10% reduces a simple Z ≈ 4.9 estimate to about 4.7σ, and 15% reduces it to about 4.0σ, so maintaining a genuine 5σ discovery requires the background systematic to be below roughly 8%. The paper should present the dominant background sources (for example KL→π0γγ, neutron-induced π0 production, and accidental overlaps), the assumed signal acceptance and KL flux, and a validation of the simulation against KOTO data, or it should explicitly refer to the corresponding sections of the KOTO II proposal [3] where these are given.
- [Section 4, 'Timeline and Costs'] The 3×10^7 s running projection and the early-2030s start of KOTO II assume the extension of the J-PARC Hadron Experimental Facility, but the manuscript only states that the extension 'is needed' and that its realization 'is being discussed'. Since the central discovery claim depends on this facility upgrade, the paper should explicitly identify this as a conditional assumption and, if possible, state the current approval status or timeline; without the extension, the projected running time is not achievable.
minor comments (5)
- [Abstract and Section 5] The statement that KOTO II is 'the only future rare kaon decay project currently proposed' is contradicted by the paper's own mention of the HIKE project in Section 4; please qualify the claim, for example by saying 'the only dedicated future experiment at J-PARC' or by explicitly distinguishing HIKE.
- [Section 4, 'The KOTO II experiment'] There is a typo: 'sensitive prove' should be 'sensitive probe'.
- [Figure 8 caption] The caption reads 'data takin in 2021'; it should be 'data taken in 2021'.
- [Section 5] The word 'currenlty' should be 'currently'.
- [Figure 3 caption] The figure appears to contain residual text from a Belle II/CKMfitter source, including '18 Global analyses' and a page number '619/688'; the figure should be cleaned or redrawn with a proper caption.
Circularity Check
No significant circularity; the sensitivity projection rests on an external proposal and Monte Carlo, not on a fitted parameter or self-referential definition.
full rationale
The paper is a proposal-style letter for KOTO/KOTO II. Its physics case uses the SM branching ratio of K_L→π0νν from external theory references [4,5] and the current KOTO upper limit from a published experimental result [2], neither of which is derived from the paper's own claims. The central sensitivity statement — 35 signal and 40 background events in 3×10^7 s giving 5σ discovery — is an expected-yield projection taken from the KOTO II proposal [3] and from detector simulations; it is not obtained by fitting a parameter to a subset of data and then predicting the same quantity. There is no equation in the paper that reduces to its own input: the SM branching ratio is an input, the event counts are independent simulation-based inputs, and the significance follows from a standard counting calculation. The self-citation to the KOTO II proposal (Ref. [3], same collaboration) is normal and is not load-bearing in a circular sense; the proposal is an external document with its own stated assumptions and simulations, and the present paper does not use it to define away an alternative. Claims about KOTO being the only dedicated rare-kaon experiment after 2026 and KOTO II being the only proposed future project are factual assertions, not mathematical consequences; even if contestable in light of HIKE, that is a correctness risk, not circularity. The absence of a detailed systematic budget for the 40 background events is a legitimate concern about the strength of the feasibility claim, but it is a question of evidence and validation, not of circular derivation. Overall, no circular step meeting the specified evidentiary standard is present.
Assumptions & free parameters
assumptions (3)
- domain assumption SM prediction BR(KL to pi0 nu nu) is about 3e-11 with 2% uncertainty
- ad hoc to paper KOTO II will achieve the assumed signal acceptance and background level of 40 events
- ad hoc to paper The J-PARC hadron facility extension will be realized in time for KOTO II
Cite this review
Pith. "Pith review of Experimental Study of Rare Kaon Decays at J-PARC with KOTO and KOTO II." pith.science (2026). https://pith.science/paper/4IJBIPB2
@misc{pith2026250502568,
author = {Pith},
title = {Pith review of: Experimental Study of Rare Kaon Decays at J-PARC with KOTO and KOTO II},
year = {2026},
howpublished = {\url{https://pith.science/paper/4IJBIPB2}},
note = {Machine review of arXiv:2505.02568}
}
abstract
The rare kaon decay $K_L\to\pi^0\nu\bar{\nu}$ is extremely sensitive to new physics, because the contribution to this decay in the Standard Model (SM) is highly suppressed and known very accurately; the branching ratio is $3\times 10^{-11}$ in the SM with a theoretical uncertainty of just 2%. The measurement of this branching ratio could provide essential new information about the flavor structure of the quark sector from the $s\to d$ transition. The decay is being searched for in the KOTO experiment at J-PARC, which has obtained the current best upper limit on the branching ratio of $2.2\times 10^{-9}$; a sensitivity to branching ratios below $10^{-10}$ is achievable by the end of the decade. A next-generation experiment at J-PARC, KOTO II, was proposed in 2024 with 82 members worldwide, including significant contributions from European members. The goal of KOTO II is to measure the $K_L\to\pi^0\nu\bar{\nu}$ branching ratio with sensitivity below $10^{-12}$ in the 2030s. Discovery of the decay with $5\sigma$ significance is achievable at the SM value of the branching ratio. An indication of new physics with a significance of 90% is possible if the observed branching ratio differs by 40% from the SM value. Another important goal of KOTO II is to measure the branching ratio of the unobserved $K_L\to \pi^0e^+e^-$ decay, which can give an input to flavor structures of new physics. Other rare $K_L$ decays and hidden-sector particles are also in the scope of the study. After 2026, KOTO will be the only dedicated rare kaon decay experiment in the world, and KOTO II is the only future rare kaon decay project currently proposed. We would like to lead a global initiative for the experimental study of rare kaon decays, with significant contributions and support from the European community.
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
-
[4]
Workshop summary: Kaons@CERN 2023,
G. Anzivinoet al., “Workshop summary: Kaons@CERN 2023,”Eur. Phys. J. C84 no. 4, (2024) 377,arXiv:2311.02923 [hep-ph]
arXiv 2024
-
[5]
Standard Model predictions for rare K and B decays without new physics infection,
A. J. Buras, “Standard Model predictions for rare K and B decays without new physics infection,”Eur. Phys. J. C83no. 1, (2023) 66,arXiv:2209.03968 [hep-ph]. [6]NA62Collaboration, E. Cortina Gilet al., “Observation of theK +→π +ν¯νdecay and measurement of its branching ratio,”JHEP02(2025) 191,arXiv:2412.12015 [hep-ex]
arXiv 2023
-
[7]
K L→π 0ννbeyond the standard model,
Y. Grossman and Y. Nir, “K L→π 0ννbeyond the standard model,”Phys. Lett. B 398(1997) 163–168,arXiv:hep-ph/9701313
arXiv 1997
-
[8]
K→πν νandε’/εin simplified new physics models,
A. J. Buras, D. Buttazzo, and R. Knegjens, “K→πν νandε’/εin simplified new physics models,”JHEP11(2015) 166,arXiv:1507.08672 [hep-ph]
arXiv 2015
-
[9]
Gluino-mediated electroweak penguin with flavor-violating trilinear couplings,
M. Endo, T. Goto, T. Kitahara, S. Mishima, D. Ueda, and K. Yamamoto, “Gluino-mediated electroweak penguin with flavor-violating trilinear couplings,” JHEP04(2018) 019,arXiv:1712.04959 [hep-ph]
arXiv 2018
-
[10]
K→πν νin the MSSM in light of theϵ ′ K/ϵK anomaly,
A. Crivellin, G. D’Ambrosio, T. Kitahara, and U. Nierste, “K→πν νin the MSSM in light of theϵ ′ K/ϵK anomaly,”Phys. Rev. D96no. 1, (2017) 015023, arXiv:1703.05786 [hep-ph]
arXiv 2017
-
[11]
Probing SUSY with 10 TeV stop mass in rare decays and CP violation of kaon,
M. Tanimoto and K. Yamamoto, “Probing SUSY with 10 TeV stop mass in rare decays and CP violation of kaon,”PTEP2016no. 12, (2016) 123B02, arXiv:1603.07960 [hep-ph]. 11
arXiv 2016
-
[12]
Re(ϵ ′ K/ϵK) andK→πν¯νin a two-Higgs doublet model,
C.-H. Chen and T. Nomura, “Re(ϵ ′ K/ϵK) andK→πν¯νin a two-Higgs doublet model,”JHEP08(2018) 145,arXiv:1804.06017 [hep-ph]
arXiv 2018
Show all 21 references
-
[13]
Footprints of leptoquarks: fromR K(∗) to K→πν¯ν,
S. Fajfer, N. Koˇ snik, and L. Vale Silva, “Footprints of leptoquarks: fromR K(∗) to K→πν¯ν,”Eur. Phys. J. C78no. 4, (2018) 275,arXiv:1802.00786 [hep-ph]
2018 arXiv
-
[14]
Constraints on new physics from K→πν¯ν,
X.-G. He, G. Valencia, and K. Wong, “Constraints on new physics from K→πν¯ν,”Eur. Phys. J. C78no. 6, (2018) 472,arXiv:1804.07449 [hep-ph]. [Erratum: Eur.Phys.J.C 80, 738 (2020)]
2018 arXiv
-
[15]
Can we reach the Zeptouniverse with rareKandB s,d decays?,
A. J. Buras, D. Buttazzo, J. Girrbach-Noe, and R. Knegjens, “Can we reach the Zeptouniverse with rareKandB s,d decays?,”JHEP11(2014) 121, arXiv:1408.0728 [hep-ph]
2014 arXiv
-
[16]
Probing third-generation New Physics withK→πν¯νandB→K (∗)ν¯ν,
L. Allwicher, M. Bordone, G. Isidori, G. Piazza, and A. Stanzione, “Probing third-generation New Physics withK→πν¯νandB→K (∗)ν¯ν,” arXiv:2410.21444 [hep-ph]
-
[17]
Electroweak baryogenesis via top-charm mixing,
S. Kanemura and Y. Mura, “Electroweak baryogenesis via top-charm mixing,” JHEP09(2023) 153,arXiv:2303.11252 [hep-ph]
2023 arXiv
-
[18]
Anatomy of kaon decays and prospects for lepton flavour universality violation,
G. D’Ambrosio, A. M. Iyer, F. Mahmoudi, and S. Neshatpour, “Anatomy of kaon decays and prospects for lepton flavour universality violation,”JHEP09(2022) 148,arXiv:2206.14748 [hep-ph]
2022 arXiv
-
[19]
Theoretical implications for a new measurement ofK L→π 0ℓℓ,
G. D’Ambrosio, A. M. Iyer, F. Mahmoudi, and S. Neshatpour, “Theoretical implications for a new measurement ofK L→π 0ℓℓ,”Phys. Rev. D111no. 1, (2025) L011701,arXiv:2409.06545 [hep-ph]
2025 arXiv
-
[20]
Implications of Rare Kaon Decays on Lepton Number Violating Interactions,
F. F. Deppisch, K. Fridell, and J. Harz, “Implications of Rare Kaon Decays on Lepton Number Violating Interactions,”PoSICHEP2020(2021) 130, arXiv:2012.14825 [hep-ph]
2021 arXiv
-
[21]
Loophole inK→πν¯νSearch and New Weak Leptonic Forces,
K. Fuyuto, W.-S. Hou, and M. Kohda, “Loophole inK→πν¯νSearch and New Weak Leptonic Forces,”Phys. Rev. Lett.114(2015) 171802,arXiv:1412.4397 [hep-ph]
2015 arXiv
-
[22]
Z’ -induced FCNC decays of top, beauty, and strange quarks,
K. Fuyuto, W.-S. Hou, and M. Kohda, “Z’ -induced FCNC decays of top, beauty, and strange quarks,”Phys. Rev. D93no. 5, (2016) 054021,arXiv:1512.09026 [hep-ph]
2016 arXiv
-
[23]
New physics implications of recent search forK L→π 0ν¯νat KOTO,
T. Kitahara, T. Okui, G. Perez, Y. Soreq, and K. Tobioka, “New physics implications of recent search forK L→π 0ν¯νat KOTO,”Phys. Rev. Lett.124 no. 7, (2020) 071801,arXiv:1909.11111 [hep-ph]
2020 arXiv
-
[24]
Light Scalars and the Koto Anomaly,
D. Egana-Ugrinovic, S. Homiller, and P. Meade, “Light Scalars and the Koto Anomaly,”Phys. Rev. Lett.124no. 19, (2020) 191801,arXiv:1911.10203 [hep-ph]
2020 arXiv
-
[25]
Probing long-lived axions at the KOTO experiment,
Y. Afik, B. D¨ obrich, J. Jerhot, Y. Soreq, and K. Tobioka, “Probing long-lived axions at the KOTO experiment,”Phys. Rev. D108no. 5, (2023) 055007, arXiv:2303.01521 [hep-ph]. 12 [26]Belle-IICollaboration, W. Altmannshoferet al., “The Belle II Physics Book,” PTEP2019no. 12, (20...
2023 arXiv
Reviewed August 16, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.