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The first search for B0→K0S τ+τ− finds no signal and bounds the branching fraction below 8.3 × 10⁻⁴ at 90% confidence.

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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2026-08-01 01:57 UTC pith:NLCU47KU

load-bearing objection First search for B0→KS0τ+τ−, a clean null result that adds the neutral pseudoscalar-kaon mode to the b→sττ program; deserves refereeing.

arxiv 2607.25607 v1 pith:NLCU47KU submitted 2026-07-28 hep-ex

Search for the boldsymbol{B⁰ to K⁰_(rm S) τ^+ τ^-} decay

Belle , Belle II Collaborations: M. Abumusabh , I. Adachi , A. Aggarwal , Y. Ahn , H. Aihara , M. Akdag , N. Akopov
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S. Alghamdi M. Alhakami A. Aloisio N. Althubiti K. Amos M. Angelsmark N. Anh Ky C. Antonioli D. M. Asner H. Atmacan T. Aushev V. Aushev R. Ayad V. Babu H. Bae N. K. Baghel S. Bahinipati P. Bambade Sw. Banerjee M. Bartl J. Baudot A. Beaubien F. Becherer J. Becker J. V. Bennett V. Bertacchi M. Bertemes E. Bertholet M. Bessner S. Bettarini V. Bhardwaj B. Bhuyan F. Bianchi T. Bilka D. Biswas A. Bobrov D. Bodrov A. Bondar G. Bonvicini J. Borah A. Boschetti A. Bozek M. Bra\v{c}ko P. Branchini R. A. Briere T. E. Browder A. Budano S. Bussino F. Callet Q. Campagna M. Campajola M. Carminati G. Casarosa C. Cecchi M.-C. Chang P. Cheema L. Chen B. G. Cheon C. Cheshta H. Chetri K. Chilikin K. Chirapatpimol H.-E. Cho K. Cho S.-J. Cho S.-K. Choi S. Choudhury S. Chutia J. Cochran J. A. Colorado-Caicedo I. Consigny L. Corona H. Crotte Ledesma S. Cuccuini J. X. Cui S. Das E. De La Cruz-Burelo S. A. De La Motte G. de Marino G. De Nardo G. De Pietro R. de Sangro M. Destefanis S. Dey R. Dhayal A. Di Canto J. Dingfelder Z. Dole\v{z}al X. Dong M. Dorigo K. Dugic G. Dujany P. Ecker J. Eppelt R. Farkas P. Feichtinger T. Ferber T. Fillinger C. Finck G. Finocchiaro F. Forti A. Frey B. G. Fulsom A. Gabrielli P. Gagneja A. Gale E. Ganiev M. Garcia-Hernandez R. Garg G. Gaudino V. Gaur V. Gautam A. Gaz A. Gellrich G. Ghevondyan D. Ghosh H. Ghumaryan R. Giordano A. Giri P. Gironella Gironell A. Glazov B. Gobbo R. Godang O. Gogota W. Gradl E. Graziani D. Greenwald Y. Guan K. Gudkova I. Haide H. Haigh Y. Han K. Hayasaka H. Hayashii S. Hazra C. Hearty M. T. Hedges G. Heine I. Heredia de la Cruz T. Higuchi M. Hoek M. Hohmann R. Hoppe P. Horak C.-L. Hsu T. Humair T. Iijima K. Inami G. Inguglia N. Ipsita A. Ishikawa R. Itoh M. Iwasaki P. Jackson D. Jacobi W. W. Jacobs E.-J. Jang S. Jia Y. Jin A. Johnson K. K. Joo K. H. Kang G. Karyan F. Keil C. Ketter C. Kiesling C. Kim D. Y. Kim H. Kim J.-Y. Kim K.-H. Kim H. Kindo K. Kinoshita P. Kody\v{s} T. Koga S. Kohani A. Korobov S. Korpar E. Kovalenko R. Kowalewski P. Kri\v{z}an P. Krokovny T. Kuhr Y. Kulii J. Kumar R. Kumar K. Kumara T. Kunigo S. Kurokawa A. Kuzmin Y.-J. Kwon S. Lacaprara T. Lam J. S. Lange T. S. Lau R. Leboucher F. R. Le Diberder H. Lee M. J. Lee C. Lemettais P. Leo C. Li H.-J. Li L. K. Li Q. M. Li S. X. Li W. Z. Li Y. Li Y. B. Li Y. P. Liao J. Libby J. Lin S. Lin Z. Liptak V. Lisovskyi C. Liu M. H. Liu Q. Y. Liu Z. Q. Liu D. Liventsev S. Longo A. Lozar T. Lueck C. Lyu J. L. Ma Y. Ma M. Maggiora S. P. Maharana R. Maiti G. Mancinelli R. Manfredi E. Manoni M. Mantovano D. Marcantonio S. Marcello M. Marfoli C. Marinas C. Martellini A. Martens T. Martinov L. Massaccesi M. Masuda T. Matsuda D. Matvienko S. K. Maurya M. Maushart J. A. McKenna Z. Mediankin Gruberov\'{a} R. Mehta F. Meier D. Meleshko M. Merola C. Miller M. Mirra K. Miyabayashi H. Miyake R. Mizuk G. B. Mohanty S. Moneta A. L. Moreira de Carvalho H.-G. Moser N. Mudgal Th. Muller H. Murakami R. Mussa I. Nakamura K. R. Nakamura M. Nakao H. Nakazawa Y. Nakazawa M. Naruki Z. Natkaniec A. Natochii M. Nayak M. Neu S. Nishida R. Nomaru A. Novosel S. Ogawa R. Okubo H. Ono Y. Onuki G. Pakhlova S. Pardi J. Park K. Park S.-H. Park A. Passeri S. Patra T. K. Pedlar R. Pestotnik M. Piccolo L. E. Piilonen P. L. M. Podesta-Lerma T. Podobnik A. Prakash C. Praz S. Prell E. Prencipe M. T. Prim S. Privalov I. Prudiiev H. Purwar P. Rados S. Raiz K. Ravindran J. U. Rehman M. Reif S. Reiter M. Remnev L. Reuter D. Ricalde Herrmann I. Ripp-Baudot G. Rizzo S. H. Robertson J. M. Roney A. Rostomyan N. Rout S. Saha L. Salutari D. A. Sanders S. Sandilya L. Santelj C. Santos V. Savinov B. Scavino S. Schneider G. Schnell M. Schnepf K. Schoenning C. Schwanda Y. Seino K. Senyo J. Serrano M. E. Sevior C. Sfienti C. P. Shen X. D. Shi T. Shillington T. Shimasaki J.-G. Shiu D. Shtol B. Shwartz A. Sibidanov F. Simon J. B. Singh J. Skorupa R. J. Sobie A. Soffer A. Sokolov E. Solovieva W. Song S. Spataro K. \v{S}penko B. Spruck M. Stari\v{c} P. Stavroulakis S. Stefkova R. Stroili M. Sumihama K. Sumisawa M. Takahashi M. Takizawa U. Tamponi K. Tanida F. Testa A. Thaller D. V. Thanh T. Tien Manh O. Tittel R. Tiwary D. Tonelli E. Torassa K. Trabelsi F. F. Trantou I. Tsaklidis M. Uchida I. Ueda T. Uglov K. Unger Y. Unno K. Uno S. Uno P. Urquijo Y. Ushiroda S. E. Vahsen R. van Tonder K. E. Varvell M. Veronesi A. Vinokurova V. S. Vismaya L. Vitale V. Vobbilisetti R. Volpe M. Wakai S. Wallner M.-Z. Wang X. L. Wang A. Warburton S. Watanuki C. Wessel X. P. Xu B. D. Yabsley S. Yamada W. Yan W. P. Yan J. Yelton K. Yi J. H. Yin K. Yoshihara C. Z. Yuan J. Yuan L. Yuan Y. Yusa L. Zani F. Zeng M. Zeyrek B. Zhang X. Zhao V. Zhilich J. S. Zhou Q. D. Zhou X. Y. Zhou L. Zhu R. \v{Z}leb\v{c}\'{i}k
This is my paper
classification hep-ex
keywords B0→K0S τ+τ− decaytau leptonflavour-changing neutral currentbranching fraction upper limithadronic B taggingboosted decision treeΥ(4S) resonanceb→s τ+τ− transition
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.

This paper reports the first search for the extremely rare decay B0 → K0S τ+τ−, in which a neutral B meson decays to a short-lived kaon and a tau-antitau pair. Using 1.16 billion Υ(4S) events recorded by two e+e− collider experiments and a hadronic tag of the other B meson, the analysis finds no signal. It sets a 90% confidence upper limit of 8.3 × 10⁻⁴ on the branching fraction—about five thousand times the Standard Model prediction of roughly 1.6 × 10⁻⁷. Combining with the charged counterpart B+ → K+ τ+τ− yields a combined limit of 5.4 × 10⁻⁴ for B→K τ+τ−, the strongest constraint yet on this flavor-changing neutral-current transition. A sympathetic reader would care because b→s τ+τ− is a sensitive probe of new particles, and this is the first direct measurement for the pseudoscalar kaon final state.

Core claim

The paper reports the first search for the decay B0 → K0S τ+τ−, where K0S is the short-lived neutral kaon and τ± are tau leptons. Using 1.16 billion Υ(4S) events from two e+e− collider experiments, with one B meson fully reconstructed in a hadronic channel and the τ leptons reconstructed from single-charged-particle decays, the analysis finds no significant signal. It sets a 90% confidence upper limit of 8.3 × 10⁻⁴ on the branching fraction, far above the Standard Model expectation of about 1.6 × 10⁻⁷. Combining with the isospin partner B+ → K+ τ+τ− gives a combined limit of 5.4 × 10⁻⁴ for B→K τ+τ−, the most stringent constraint on b→s τ+τ− transitions with a pseudoscalar kaon.

What carries the argument

The analysis combines a hadronic tag of the non-signal B meson (the Full Event Interpretation) with a boosted decision tree (BDT) trained separately for five τ+τ− final-state categories (ℓℓ, ℓ±h∓, ρℓ, no-ℓ). The BDT output is transformed via the probability integral transform to a signal-uniform observable O′, which is fitted with a binned maximum-likelihood model that includes signal, BBbar, and qqbar templates. The q² reconstruction from missing-energy constraints provides additional discrimination. The fit is performed simultaneously across the five categories and the two data sets.

Load-bearing premise

The load-bearing premise is that the simulated background—chiefly B→D(∗)ℓνℓ and B→D(∗)τντ decays—reproduces the shape and normalization of the fit observable O′ in the high-O′ signal region after calibration; if this tail is mis-modeled in a way sideband checks miss, the fitted signal and the limit shift.

What would settle it

A dedicated measurement of the B→D(∗)ℓνℓ background shape using a high-O′ control sample with the K0S mass sideband would directly probe this assumption; any significant data–simulation discrepancy there would invalidate the limit. Equivalently, a future dataset with several times the luminosity would either reveal a signal above 8.3 × 10⁻⁴ or tighten the limit, settling whether the decay is truly absent at that rate.

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

If this is right

  • If the central claim is correct, B0→K0S τ+τ− has not been observed, and its true branching fraction lies below 8.3 × 10⁻⁴ with 90% confidence.
  • The combined limit B(B→K τ+τ−) < 5.4 × 10⁻⁴ improves on the B+→K+ result alone by about 5%, making it the most stringent constraint on this transition.
  • The upper limit rules out the largest beyond-Standard-Model enhancements of b→s τ+τ− with a pseudoscalar kaon, such as those predicted for leptoquark or Z′ mediators.
  • The measurement establishes a hadronic-tagging and five-category analysis strategy that can be applied to other rare B decays with τ pairs in the final state.
  • The observed central value, consistent with zero, provides a reference point for future searches with larger data sets to push toward the Standard Model rate.

Where Pith is reading between the lines

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

  • Beyond the paper: with the full dataset of the newer collider experiment, the same analysis could push the limit well below 10⁻⁴, potentially approaching within two orders of magnitude of the Standard Model prediction of ~1.6 × 10⁻⁷.
  • Beyond the paper: the five-category decomposition and the signal-uniform O′ transform could be adapted to vector-meson final states (e.g., B0→K∗0 τ+τ−) and to other missing-energy decays, where category-specific BDTs may improve sensitivity.
  • Beyond the paper: the 5% gain from isospin combination suggests that including all B→K ττ channels, including those with K0L, could tighten the constraint further; a dedicated measurement of the background shape using a high-O′ control sample would test the key modelling assumption directly.

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

0 major / 4 minor

Summary. The paper reports the first search for B0→K0S τ+τ− using Belle (711 fb−1) and Belle II (365 fb−1) Υ(4S) data. One B meson is hadronically tagged with the FEI; the signal side is reconstructed as K0S→π+π− plus two τ decays with a single charged particle, divided into five exclusive categories. A per-category BDT is trained; the output is transformed to O′ uniform for signal, and a binned maximum-likelihood fit in O′>0.3 extracts the branching fraction with nuisance parameters for all systematics. Backgrounds are calibrated with off-resonance qq data, K0S and Mbc sidebands, the M(K0S t−)-vetoed control sample, and the 0.15≤O′≤0.30 depleted band. The fit gives B(B0→K0S τ+τ−)=(1.2±3.3±2.9)×10−4 and an observed (expected) 90% CL limit of 8.3 (7.4)×10−4. Under isospin, combination with B+→K+τ+τ− yields B(B→Kτ+τ−)<5.4×10−4.

Significance. If the results hold, the B0→K0S ττ limit is the first for this mode and the isospin-combined limit is the most stringent constraint on b→sττ with a pseudoscalar kaon. The analysis is careful: selections were optimised on simulation and fixed before the signal region was examined; qq rates are anchored to off-resonance data; the BB model is validated in several disjoint sidebands/control regions; the dominant systematic (leading B-decay BFs, 2.0×10−4) is propagated via 140 correlated shape nuisance parameters, and MC-sample-size effects are included per bin. The expected and observed limits are close (7.4 vs 8.3×10−4), giving confidence in the background model. The potential high-O′ tail modelling concern is mitigated by these cross-checks and by the explicit per-bin shape systematics; I do not find it load-bearing.

minor comments (4)
  1. [§7, Ref. [10]] The phrase 'recent measurement of B+→K+τ+τ−' is imprecise; the cited work is a search that reports an upper limit. Suggest rewording to 'recent search' or 'recent upper limit'.
  2. [§6.3] The sentence 'The normalisation factors ... agree within statistical uncertainties with those measured in the K0S sideband, except for the no-ℓ (ℓ−h+) categories in Belle (Belle II), thereby validating the procedure' is slightly contradictory. Please quantify the disagreement for the two exceptions and state explicitly why the 50% uncertainty remains adequate.
  3. [§3, Eq. (3.3)] The explicit powers of c in Eq. (3.3) are correct but may confuse; consider stating that natural units are used elsewhere or simplifying the notation.
  4. [Figures 1–2] The red filled histogram is described as 'not visible'; this is expected at the measured branching fraction, but please confirm that the figure files are included in the final submission and that the overlays are legible when reproduced.

Circularity Check

0 steps flagged

No substantive circularity: the branching fraction is the fit's free parameter, backgrounds are anchored to external averages and control samples, and the only self-referential element is a standard combination with an independently-measured charged channel.

full rationale

The paper's central claim — the null observation and 90% CL upper limit B(B0→K0S τ+τ−) < 8.3×10−4 — is derived without circular reduction. In Eq. 5.2 the branching fraction is the free parameter of a binned maximum-likelihood fit to the transformed BDT observable O′; no input is defined in terms of the output. Signal shapes come from the external EvtGen model [33] and the flavio alternative [54]; background templates are scaled to external PDG/HFLAV world averages [4,39] with corrections from off-resonance data and disjoint control regions (K0S sideband, Mbc sideband, M(K0S t−)-vetoed sample, and the 0.15≤O′≤0.30 band, §6.3), yielding D(∗) yield ratios 1.09±0.11 and 1.04±0.14 consistent with unity. The fit-region selection O′>0.3 was fixed on simulation before examining the signal region, and the observed limit (8.3×10−4) is close to the expected (7.4×10−4), with no excess. The largest systematic (2.0×10−4, leading B branching fractions) is explicitly propagated as shape nuisance parameters. The only self-referential element is the §7 combination with the same collaborations' B+→K+τ+τ− measurement [10]; that is a separate, independently-fitted result in a different decay channel, and the combination is a stated isospin assumption with systematic correlations propagated, not a reduction of this fit's inputs. The 40% D→K0L uncertainty taken from Ref. [2] is a minor conservative systematic (0.5×10−4 of 2.9×10−4) and is not load-bearing. No fitted parameter is renamed as a prediction, and no ansatz or uniqueness theorem is imported via self-citation. Residual risk of high-O′ background tail mismodeling is a modelling concern, not circularity, and is covered by the quoted sideband validations and systematics.

Axiom & Free-Parameter Ledger

6 free parameters · 5 axioms · 0 invented entities

The analysis contributes the fitted branching fraction and two hand-chosen thresholds (O′>0.3, q²_rec>12 GeV²); everything else is imported: PDG/HFLAV branching fractions, f00 and lifetime ratio, the generators (KKMC/EvtGen/PYTHIA/PHOTOS/Geant4) and their decay models, and the CLs framework. No new particles or mediators are introduced; the BSM scenarios mentioned (leptoquarks, Z′) are literature context, not postulates of this paper.

free parameters (6)
  • B(B0→K0Sτ+τ−) — parameter of interest = (1.2 ± 3.3 ± 2.9) × 10^-4
    The branching fraction is the free parameter of the simultaneous fit (Eq. 5.2); its fitted value and the derived CLs limit (8.3×10^-4) are the central result. Listed for completeness — it is the measurement target, not an ad hoc input.
  • q̄q background normalisation corrections = 0.67±0.03, 0.57±0.12 (Belle); 0.71±0.05, 1.02±0.16 (Belle II)
    Off-resonance samples (§6.3) are used to rescale simulated q̄q yields in the no-ℓ category and the combined leptonic categories; these fitted factors propagate directly into background template normalisations.
  • B̄0_tag efficiency correction factors = ≈0.753–0.767 (average), uncertainties 5.2–5.7%
    Calibrated per tag mode using B0→D−ℓ+ν and B0→D−π+ control channels (§6.3); applied to signal and B̄B efficiencies, so the final limit scales inversely with them.
  • Fit-region threshold on transformed BDT output O′ = 0.3
    Chosen by hand, "not explicitly optimised" (§4): retains 70% of signal while rejecting 80–95% of background. The limit depends on this choice, though the authors argue lower thresholds add systematic cost.
  • q²_rec lower requirement = 12 GeV²/c⁴
    Hand-chosen selection (§3) below the kinematic threshold 4m_τ² ≈ 12.6 GeV²/c⁴ to retain signal given resolution; affects which background and signal events enter the fit.
  • Combinatorial B̄0_tag normalisation uncertainty = 50% assigned
    A 50% uncertainty on incorrectly reconstructed tag candidates is assigned per category from sideband comparisons (§6.3); a hand-assigned rather than fitted value, but it shapes the B̄B template.
axioms (5)
  • standard math Υ(4S)→B0B̄0 is a two-body decay, so the signal B four-momentum is fixed by the tag momentum and beam energy
    Used in Eq. 3.3 and in deriving q²_rec from √s/2 and p*_B̄0tag = −p*_B0sig; valid kinematically for on-resonance two-body production.
  • domain assumption External world-average inputs: f00 = 0.4861±0.0080, τB+/τB0 = 1.076±0.004, PDG/HFLAV branching fractions
    Taken from Refs. [4,39] (§6.1); the central claim inherits any error in these averages, though they enter mostly as small normalisation/shape systematics.
  • domain assumption Simulation (KKMC, EvtGen, PYTHIA, PHOTOS, Geant4) models background and signal kinematics and detector response
    All templates are simulated (§2); the analysis corrects for known efficiency differences but assumes the generators' decay models and detector response are accurate within the quoted systematics.
  • domain assumption Isospin symmetry: B(B0→K0τ+τ−) = 2 B(B0→K0Sτ+τ−), and B+→K+ vs B0→K0 rates equal modulo the lifetime ratio
    Underpins the combination (§7): B(B→Kττ) ≡ B(B+→K+τ+τ−) = 2(τB+/τB0) B(B0→K0Sτ+τ−). The factor 2 follows from K0–K̄0 mixing into equal K0S/K0L shares; isospin breaking in the rates is assumed negligible.
  • standard math CLs asymptotic approximation is valid for the limit extraction
    Refs. [48–50]; standard in HEP, used with the binned profile likelihood (Eq. 5.1).

pith-pipeline@v1.3.0-alltime-deepseek · 22760 in / 22822 out tokens · 220709 ms · 2026-08-01T01:57:55.145670+00:00 · methodology

0 comments
read the original abstract

We present the first search for $B^0 \to K^0_{\rm S} \tau^+\tau^-$ decays. We look for signal decays in $B^0\bar B^0$ events produced in asymmetric-energy electron-positron collisions. This work uses samples from the Belle and Belle~II detectors, comprising 1.16 billion $\Upsilon(4S)$ events. In $\Upsilon(4S)\to B^0\bar{B}^0$ decays, the non-signal $\bar{B}^0$ meson is fully reconstructed in a hadronic channel. For the signal $B^0$ meson, $\tau$-lepton decays into final states with a single charged particle are selected. A multivariate classifier is used to combine several discriminating inputs into a single fit observable. We observe no evidence for the signal and set an upper limit on the branching fraction $\mathcal{B}(B^0\to K^0_{\rm S} \tau^+\tau^-) < 8.3 \times 10^{-4}$ at the 90\% confidence level. Combining this with the recent measurement of the isospin-partner decay $B^+\to K^+\tau^+\tau^-$, we determine an upper limit $\mathcal{B}(B\to K\tau^+\tau^-) < 5.4\times10^{-4}$ at the 90\% confidence level.

discussion (0)

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