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REVIEW 2 major objections 6 minor 84 references

Search for heavy Majorana neutrinos in vector boson scattering with $\tau$-lepton final states with the ATLAS detector

T0 review · 2 major / 6 minor · reviewed 2026-08-01 · deepseek-v4-flash

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Pith's one-line read This paper searches for heavy Majorana neutrinos coupling to tau leptons in vector boson scattering at the LHC and finds no excess, setting new upper limits on the tau mixing element for masses from 92 GeV to 6.5 TeV.

desk verdict First VBS search with same-sign tau final states, well executed, but the dominant jet-to-tau fake background is not validated in the signal-region phase space, so trust the null result but treat the absolute limits as provisional. read the letter →

arxiv 2607.27307 v1 pith:JLYLQWYA submitted 2026-07-29 hep-ex

ATLAS Collaboration This is my paper · ORCID
classification hep-ex
keywords heavyMajorananeutrinotype-Iseesawvectorbosonscatteringsame-signtaupairsleptonLHCATLASdetectormixingmatrix
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

Using 140 fb^-1 of 13 TeV proton-proton collisions recorded by the ATLAS detector, the paper searches for heavy Majorana neutrinos (hypothetical particles that are their own antiparticles, proposed in seesaw models to explain tiny neutrino masses) produced in vector boson scattering and decaying through same-sign tau pairs, including e-tau and mu-tau final states. No significant deviation from Standard Model backgrounds is observed. From this absence, the paper derives upper limits on the squared mixing element |V_tauN|^2 for masses between 92 GeV and 6.5 TeV, with the strongest exclusion at 900 GeV of 0.30 observed (0.31 expected). These are the first direct constraints for masses above 1.5 TeV and improve existing limits for tau-coupled heavy neutrinos above 1.0 TeV. A sympathetic reader would care because this probes a previously unexplored mass range for a third-generation seesaw mechanism.

What carries the argument

The search exploits the same-sign W-boson scattering topology mediated by a heavy Majorana neutrino. Events are selected with two forward jets with dijet mass above 300 GeV and large rapidity separation, plus a same-sign tau-tau, e-tau, or mu-tau pair with the leading hadronic tau pT above 55 GeV. The dominant background, jets misidentified as hadronic taus, is estimated with a data-driven fake-factor method derived in a W+jets control region; a profile likelihood fit to the leading-tau pT distribution then yields the limits. The tau-hadron tau-hadron channel provides the best sensitivity.

What would settle it

A concrete test would be to reconstruct the invariant mass of the same-sign tau-pair system in the selected events: the paper fits only the leading tau pT, so a narrow resonance peak at any m_N where |V_tauN|^2 is claimed to be excluded would directly falsify the limit claim.

Watch

Extended reading notes

Core claim

The central claim is that if a heavy Majorana neutrino couples to tau leptons, it would be produced via same-sign W±W± vector boson scattering and yield same-sign tau-tau, e-tau, or mu-tau events; comparing the data to simulation finds the data consistent with background. Assuming the type-I seesaw model, in which the cross section scales as sigma ∝ |V_ellN|^2 |V_ell'N|^2, the paper sets 95% CL upper limits on |V_tauN|^2. The limits lie below 1 for m_N from 92 GeV to 6.5 TeV, with the strongest exclusion at m_N = 900 GeV: an observed (expected) limit of 0.30 (0.31). Two-dimensional exclusions in (|V_eN|^2, |V_tauN|^2) and (|V_muN|^2, |V_tauN|^2) are also derived at m_N = 1 and 5 TeV.

Load-bearing premise

The limit interpretation assumes a single heavy Majorana neutrino whose production cross section scales exactly as |V_lN|^2 |V_l'N|^2, and also assumes that jets mimicking tau decays have the same composition in the control region as in the signal region; if either assumption fails, the extracted bounds would not directly apply.

Editorial extensions

If this is right

  • For a type-I seesaw neutrino with m_N = 900 GeV, mixing |V_tauN|^2 ≥ 0.30 is now excluded at 95% CL.
  • The search covers an unexplored mass range for third-generation couplings, with first direct constraints above 1.5 TeV and sensitivity extending to 6.5 TeV.
  • The two-dimensional exclusions in (|V_eN|^2, |V_tauN|^2) and (|V_muN|^2, |V_tauN|^2) allow flavor-dependent seesaw models to be tested.
  • These results complement low-energy searches for lepton-number violation by directly probing the third-generation sector at collider energies.
  • The tau-hadron tau-hadron final state proves to be a powerful probe, motivating continued use of tau-rich signatures in future searches.

Reading between the lines

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

  • If the heavy neutrino is not Majorana, or if more than one heavy state contributes, the quoted |V_tauN|^2 bounds do not directly apply because the production rate scaling would change.
  • Combining this search with existing e-e and mu-mu vector-boson-scattering searches could yield a global fit to all three |V_lN|^2 simultaneously, going beyond the pairwise exclusions presented here.
  • A dedicated measurement of the jet-composition dependence of the fake factor, for instance using gluon-enriched control samples, would test the main background-systematic assumption and could sharpen the limits.
  • At higher luminosity or higher collision energy, the same same-sign-tau signature could push the excluded mixing values down by roughly the square root of the integrated-luminosity gain.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 6 minor

Summary. This paper reports a search for heavy Majorana neutrinos coupling to third-generation leptons in vector boson scattering (VBS) with same-sign ττ, eτ, and μτ final states, using 140 fb^{-1} of 13 TeV pp collisions recorded by ATLAS. Three signal regions are defined with VBS topology (m_jj>300 GeV, |Δη_jj|>4), a b-jet veto, and a same-sign lepton–τhad or τhad–τhad pair. The dominant jet→τhad fake background is estimated with a data-driven fake-factor method derived from a W+jets control region, with smaller contributions from lepton fakes and prompt-τ processes taken from MC. A simultaneous profile-likelihood fit to the leading-τ pT distributions finds no significant excess: observed yields of 887, 579, and 270 events agree with predicted backgrounds of 886±29, 579±23, and 264±14 in the eτ, μτ, and ττ channels, respectively. Upper limits at 95% CL are set on |V_τN|^2 for heavy Majorana neutrino masses between 92 GeV and 6.5 TeV, with the strongest exclusion at m_N=900 GeV (observed/expected 0.30/0.31), and two-dimensional limits are presented for m_N=1 and 5 TeV.

Significance. If the result holds, this is the first search for tau-flavored heavy Majorana neutrinos in vector boson scattering and extends the excluded mass range above the reach of previous same-sign dilepton searches. The analysis uses the full Run 2 dataset, a data-driven fake-factor estimate for the dominant background, two validation regions, a simultaneous fit, and a detailed systematic model (Table 1). These are genuine strengths. The main caveat is that the absolute background normalization, and therefore the numerical values of the derived limits, rests on an extrapolation of the jet→τhad fake factor from a non-VBS W+jets control region into the VBS signal phase space; the closure of this extrapolation is not demonstrated in the signal-region topology. This concern is load-bearing because jet fakes dominate all three signal regions.

major comments (2)
  1. [Background estimation (fake-factor method and validation regions)] The dominant Jet→τhad fake background is estimated with a fake factor derived in a W+jets control region that omits the VBS requirements (no m_jj>300 GeV, no |Δη_jj|>4), requires an opposite-sign μτ pair, and applies EmissT and mT cuts. The only systematic for the WCR→SR jet-composition difference is evaluated by inverting EmissT and mT, which changes the W+jets/QCD mixture but does not reproduce the same-sign, two-forward-jet, high-m_jj topology of the signal regions; in the τhadτhad channel the WCR also requires a muon that is not present in that SR. The AntiVBS validation region covers m_jj<300 GeV and |Δη_jj|<4, not the VBS phase space. Since jet fakes dominate all three SRs, a normalization or shape misestimate of this background propagates directly into the fitted yields and therefore into the derived |V_τN|^2 limits; Table 1 assigns only 1.3/1.2/6.4% fake-factor systematics. The o
  2. [Systematic uncertainties and cross-section interpretation] The conversion from cross-section limits to |V_ℓN|^2 uses σ_ℓℓ' ∝ |V_ℓN|^2|V_ℓ'N|^2, but the text lists only signal uncertainties that affect the acceptance (renormalization/factorization scales, ISR/FSR, multiple parton interactions). It is not stated whether the uncertainty on the predicted total signal cross-section (PDF and scale) is included when translating the cross-section limit into a bound on |V|^2. If it is omitted, the quoted limits on a model parameter may be over-constrained. Please state explicitly the treatment of the signal cross-section normalization uncertainty and, if it is not included, provide its effect on the observed limits.
minor comments (6)
  1. [Abstract] The phrase 'same-sign ττ, eτ or μτ pair' is imprecise: in the eτ and μτ channels the e/μ is a prompt lepton rather than a tau decay. Suggest rewording to 'same-sign lepton–τhad pair'.
  2. [Results paragraph] The sentence 'The feature around 900 GeV is consistent with a statistical fluctuation found in the reconstruction efficiency of the signal events' is vague. Please specify the nature of the efficiency fluctuation and its statistical significance, or remove the statement.
  3. [Figure 4] The two-dimensional exclusion contours are shown only for m_N=1 and 5 TeV. If feasible, include a few more mass points or state that additional masses are available in auxiliary material.
  4. [Limits presentation] A table of observed and expected limits as a function of m_N would improve reproducibility and make the numerical claims easier to verify than relying on the figure alone.
  5. [ℓ→τhad fake correction] The data-driven correction for the ℓ→τhad fake background from Z→ee events is mentioned, but no uncertainty for the correction is specified. Please clarify whether it is included in the quoted systematic uncertainties.
  6. [References] Reference [33] appears as 'Refs. [33]' (plural); it should be singular.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: limits follow from a standard profile-likelihood fit to data with externally normalized signal and data-driven backgrounds.

full rationale

The paper's derivation chain is self-contained in the relevant sense. Signal samples are generated with MadGraph5/Pythia8 using the type-I seesaw UFO model following Ref. [32], normalized to the model cross-section, with |V|^2 treated as a free scaling factor via sigma_ll' proportional to |V_lN|^2 |V_l'N|^2. No signal parameter is fitted to the signal-region data and then re-reported as a prediction: the profile likelihood fit leaves the signal normalization free, observes agreement with the background-only expectation within uncertainties, and converts the resulting cross-section upper limits into |V_tauN|^2 using the same model relation. The dominant jet-to-tau_had background is estimated from a WCR via the fake-factor method with prompt-lepton subtraction; this is a data-driven extrapolation with assigned systematic uncertainties, and the two validation regions (OS and AntiVBS) are checked for closure. Any concern about the WCR-to-SR phase-space extrapolation is a systematic/correctness risk, not a circularity, because the fake factor is not derived from the signal hypothesis or from the fitted parameter. Citations to prior ATLAS performance and search papers (e.g., Refs. [15,71]) are standard tooling and external benchmarks, not self-referential uniqueness arguments. No step reduces by construction to its own input.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The paper introduces no new particles, forces, or parameters. The heavy Majorana neutrino is pre-existing in the literature and is the target of the search, not an entity invented for this analysis. The only free parameters in the likelihood are nuisance parameters for systematic uncertainties, which are standard in collider searches and not 'free' in the sense of being adjusted to force the result.

assumptions (4)
  • domain assumption Type-I seesaw model with a single heavy Majorana neutrino N that couples to leptons via mixing elements V_ℓN.
    The search targets a specific phenomenological model (Refs [9–12,32]) and the limits are interpreted under this model. If the BSM sector differs, the limits may not directly constrain it.
  • domain assumption Cross-section for same-sign lepton pair production scales as σ(ℓℓ') ∝ |V_ℓN|^2 |V_ℓ'N|^2.
    Used in the text (page 6) to convert cross-section limits to mixing limits. This scaling is model-dependent and is imported from the phenomenological prescription of Ref. [32].
  • domain assumption Monte Carlo generators (MadGraph5_aMC@NLO, Pythia8, Sherpa, Powheg) accurately simulate signal and background processes.
    The entire background model for 'Others' and the signal acceptance rely on MC. This is a standard assumption in collider physics, validated in control regions.
  • domain assumption The data-driven fake factor method correctly estimates the jet→τ fake background, with the jet composition in the WCR representative of the SR after corrections.
    Section on Jet→τ fake describes the method; if the extrapolation fails, the background estimate and the derived limits would be biased. This is partially validated in the AntiVBS VR.

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Pith. "Pith review of Search for heavy Majorana neutrinos in vector boson scattering with $\tau$-lepton final states with the ATLAS detector." pith.science (2026). https://pith.science/paper/JLYLQWYA

@misc{pith2026260727307,
  author       = {Pith},
  title        = {Pith review of: Search for heavy Majorana neutrinos in vector boson scattering with $\tau$-lepton final states with the ATLAS detector},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JLYLQWYA}},
  note         = {Machine review of arXiv:2607.27307}
}
abstract

A search for heavy Majorana neutrinos coupling to third-generation leptons in vector boson scattering at the LHC is presented. The search is performed in the final state with a same-sign $\tau\tau$, $e\tau$ or $\mu\tau$ pair using data of proton--proton collisions at $\sqrt{s}$ = 13 TeV, recorded with the ATLAS detector and corresponding to an integrated luminosity of 140 $\text{fb}^{-1}$. No significant deviation from the Standard Model background is observed. Constraints on the squared mixing matrix element between the $\tau$-lepton and the heavy Majorana neutrino ($|V_{\tau N}|^2$) are derived for heavy Majorana neutrino masses between 92 GeV and 6.5 TeV. Additionally, two-dimensional constraints on ($|V_{eN}|^{2}, |V_{\tau N}|^{2}$) and ($|V_{\mu N}|^{2}, |V_{\tau N}|^{2}$) are set. These results improve the limits on heavy Majorana neutrinos coupling to third-generation leptons for heavy Majorana neutrino masses above 1.0 TeV, and extend the exclusion limits to an unexplored mass range above 1.5 TeV.

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