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REVIEW 3 major objections 4 minor 55 references

Displaced heavy-neutrino decays at the HL-LHC can reveal mass, lifetime and quantum numbers in two gauge extensions of the Standard Model.

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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2026-07-30 21:59 UTC pith:VZPKW4LX

load-bearing objection Solid incremental LLP benchmarks and AFB/displacement handles for LRSM and U(1)B−L, but the ‘neatly accessed’ claim outruns the parton-level evidence. the 3 major comments →

arxiv 2607.26757 v1 pith:VZPKW4LX submitted 2026-07-29 hep-ph

Characterisation at the HL-LHC of Long-lived Heavy Neutrinos in Gauge Extensions of the Standard Model

Aram Fatah , Stefano Moretti , Antons Nikolajevs This is my paper
classification hep-ph
keywords long-lived particlesheavy neutrinosLeft-Right Symmetric ModelU(1)B−Ldisplaced verticesHL-LHCKeung-Senjanovic processforward-backward asymmetry
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.

Heavy neutrinos that live long enough to decay away from the collision point can leave clean signatures at the High-Luminosity LHC. The paper studies two well-motivated extensions of the Standard Model—the Left-Right Symmetric Model and a U(1)B−L gauge extension—in which new heavy gauge bosons act as portals that produce these neutrinos at usable rates. In regions of parameter space still allowed by existing searches, the neutrinos decay inside the ATLAS and CMS trackers, producing same-sign dileptons plus jets. Because the decays occur far from the interaction point, ordinary Standard Model backgrounds are strongly suppressed. The authors show, at parton level, that the resulting kinematic distributions and angular asymmetries can reconstruct the neutrino mass and lifetime and diagnose the chirality or vector nature of the new gauge couplings. Three benchmark points per model illustrate that the High-Luminosity upgrade should collect enough events for this characterisation.

Core claim

In non-excluded regions of the Left-Right Symmetric Model and the U(1)B−L model where heavy neutrinos are long-lived, High-Luminosity LHC displaced-vertex signals (same-sign dileptons plus jets) allow extraction of the heavy-neutrino mass, width/lifetime and quantum numbers, together with characterisation of the W′ or Z′ portal, because Standard Model backgrounds are strongly reduced away from the interaction point.

What carries the argument

The Keung–Senjanović process (W′ → Nℓ → ℓℓjj) in the Left-Right Symmetric Model and the analogous Z′ → NN → ℓℓjj process in U(1)B−L, combined with laboratory-frame displacement distributions and forward–backward lepton asymmetries measured in the heavy-neutrino rest frame.

Load-bearing premise

A parton-level study without showering, hadronisation or full detector simulation is enough to claim that mass, width and quantum numbers can be cleanly extracted once the decays are displaced.

What would settle it

A full detector-level simulation of the three published benchmarks that either fails to reconstruct the claimed mass peaks and AFB asymmetries above background, or shows that the displaced same-sign dilepton-plus-jets sample is dominated by residual Standard Model processes.

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

If this is right

  • HL-LHC displaced-vertex analyses can target both LRSM and U(1)B−L heavy neutrinos with the same final-state topology.
  • Invariant-mass peaks of lepton–jet systems can measure the heavy-neutrino mass even when the parent W′ or Z′ is off-shell.
  • Forward–backward asymmetries of the secondary leptons can distinguish right-handed W′ couplings from vector-like Z′ couplings.
  • Laboratory-frame displacement spectra directly constrain the proper lifetime (and hence the mixing or Yukawa couplings) of the heavy neutrino.
  • The three benchmark points per model supply concrete targets for dedicated ATLAS/CMS long-lived-particle searches.

Where Pith is reading between the lines

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

  • The same displaced-vertex strategy could be applied to other gauge portals that produce Majorana neutrinos with centimetre-scale lifetimes.
  • Once detector-level efficiencies are known, the AFB measurement may become a model-discrimination tool between left–right and B−L scenarios even with modest event counts.
  • If the off-shell W′ contribution is as large as claimed, existing prompt searches may have underestimated the total rate in the multi-TeV W′ region.

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

3 major / 4 minor

Summary. The paper studies long-lived heavy neutrinos produced via extended gauge portals in the LRSM (W' → N ℓ) and a U(1)_{B−L} model (Z' → NN), with subsequent displaced same-sign dilepton + jets decays. Parameter space is scanned with SARAH/SPheno/MadGraph against published ATLAS/CMS cross-section limits (KS process, dilepton, diboson); three BPs per model are chosen with M(N) ~ 17–280 GeV, cτ ~ 0.04–0.5 cm and rates ~10^{-4} pb. Parton-level invariant-mass peaks, geometric displacements (modified DELPHES) and lepton AFB in the N rest frame are presented to argue that HL-LHC can extract N mass, width/lifetime and quantum numbers, and characterise the W'/Z' portals, because displaced regions are essentially free of SM backgrounds.

Significance. If the characterisation claim holds, the work supplies concrete, non-excluded BPs and a clear set of observables (on-/off-shell mass reconstructions, lab-frame displacements, chiral AFB) that experiments could use to move beyond mere discovery of displaced heavy neutrinos toward property extraction in two well-motivated gauge extensions. The simultaneous treatment of charged- and neutral-current portals, the explicit inclusion of interference where relevant, and the public BPs are useful for the LLP community. The result is a solid proof-of-concept rather than a full experimental projection; its lasting value is the identification of clean kinematic handles once backgrounds and detector effects are under control.

major comments (3)
  1. [Abstract; §IV.C–D; Tabs. I–III; Figs. 8–11] Abstract and §V claim that N mass, width and quantum numbers (and W'/Z' properties) can be ‘neatly accessed’ at HL-LHC. Tabs. I–III give KS rates ~6×10^{-4} pb; after the SR1/SR2 split of §IV.C, charge channels and the observation that off-shell W' dominates and BP2 often decays outside the tracker (Figs. 8–11), no event yields, efficiencies or statistical uncertainties at 3 ab^{-1} are shown. Consequently the AFB distributions (Fig. 10) and mass peaks are not demonstrated to be statistically usable. A minimal estimate of expected reconstructed events per BP/SR is required to support the characterisation claim.
  2. [Abstract; §V] The repeated assertion that displaced same-sign dilepton+jets regions are ‘essentially free from SM backgrounds’ (Abstract, §V) is stated without any estimate of material interactions, heavy-flavour, cosmics or pile-up fakes. Because the entire characterisation argument rests on background-free shapes, even a rough order-of-magnitude assessment (or an explicit statement that none is attempted) is needed; otherwise the claim remains unquantified.
  3. [§I; §IV.C–D; Conclusions] The analysis is purely partonic (§I, Conclusions). While acceptable for a first look, the invariant-mass and AFB observables used for mass and quantum-number extraction are sensitive to jet clustering, lepton isolation and tracker resolution once the N is boosted. The paper should either quantify how these effects smear the peaks/asymmetries or clearly downgrade the language from ‘neatly accessed’ to ‘kinematic handles that survive in a parton-level study’.
minor comments (4)
  1. [Fig. 3; §IV.A] Fig. 3 caption and text refer to ‘observed cross section (see Ref. [29])’ but do not state whether the experimental limits already unfold acceptance; a one-sentence clarification would help the reader compare apples-to-apples.
  2. [§II.A; §IV] Notation switches freely between W' and W_R (and Z'/Z_{B−L}); a consistent choice after the first definition would improve readability.
  3. [Figs. 8–14] Several figures (e.g. 8–11, 13–14) lack explicit luminosity normalisation or unit labels on the vertical axes in the text description; adding them would make the plots self-contained.
  4. [Throughout] Typos: ‘ANAL YSIS’, ‘F eatures’, ‘RESUL TS’, ‘individuate’, and a few missing spaces around equation references.

Circularity Check

0 steps flagged

No circularity: standard external-constraint + MC phenomenology; BPs and distributions are not fitted predictions or definitional identities.

full rationale

The paper follows a conventional BSM collider pipeline. LRSM and U(1)B−L Lagrangians, mass formulae (Eqs. 4–28) and production channels (KS process, Z′→NN) are taken from the established literature. Parameter space is cut by comparing MadGraph cross sections to published ATLAS/CMS limits (dilepton, diboson, KS searches), which are external data. Benchmark points are chosen by hand for large rate and cτ inside the tracker, not obtained by fitting the same observables later plotted. Invariant-mass peaks, displacements and AFB are then computed at parton level from those fixed BPs; they are not statistically forced by a prior fit, nor defined in terms of the quantities they are said to extract. Self-citations (e.g. prior B−L phenomenology by overlapping authors) supply model context only and do not underwrite a uniqueness claim or smuggle an ansatz that forces the HL-LHC characterisation result. The acknowledged parton-level and background-free limitations are correctness/scope issues, not circular reductions. Hence score 0 with no circular steps.

Axiom & Free-Parameter Ledger

4 free parameters · 5 axioms · 0 invented entities

Central claim rests on two standard gauge extensions (LRSM, U(1)B−L), external LHC exclusion contours, the seesaw mass pattern, and the modeling choice that parton-level displaced kinematics plus negligible SM background suffice for HL-LHC characterization. Free parameters are the scanned VEVs, Yukawas, and U(1) couplings used to pick BPs; no new particles beyond those already defined by the models are invented here.

free parameters (4)
  • u_R (LRSM right-handed triplet VEV) = BP-dependent; M_W'≈4.73 TeV in Tab. I
    Scanned in [1.8e3, 1.4e4] GeV to set M_W' and M_N; BP values chosen for large KS rate and target cτ.
  • h_R_11 (LRSM Majorana Yukawa) = Implied by M_Ne = 17–29 GeV in Tab. I
    Scanned in [1e-3, 0.33] to set M_Ne under M_N < M_W'; selects LLP BPs.
  • x (U(1)B−L singlet VEV) and g̃, g̃1 = M_Z'≈2.1–2.6 TeV; M_Ne≈159–277 GeV in Tab. II
    Scanned to set M_Z' and couplings while evading dilepton/diboson and Z–Z' mixing bounds; BPs hand-selected for rate and cτ.
  • Benchmark heavy-neutrino masses / widths = Tabs. I–II (cτ ~ 0.04–0.5 cm)
    Three BPs per model chosen for large production and a spread of lab-frame displacements inside tracker volume—not predicted uniquely by the models.
axioms (5)
  • domain assumption Type-I/II seesaw mass matrices in LRSM and U(1)B−L yield light ν and heavy N with M_N ≈ M_R when M_D ≪ M_R.
    Used throughout §II to identify the LLP as the heavy mostly-sterile state.
  • domain assumption Published ATLAS/CMS KS, dilepton, and diboson limits can be applied by comparing MadGraph parton-level σ to reported observed σ, including interference where stated.
    §IV.A–B; exclusion contours in Figs. 3–7 rest on this comparison.
  • ad hoc to paper Displaced heavy-neutrino decays inside ATLAS/CMS trackers occur in phase space ‘essentially free from SM backgrounds,’ so intrinsic W/Z contamination is subleading.
    Asserted in Abstract, §I, and Conclusions without a quantitative background estimate.
  • ad hoc to paper Parton-level kinematics (invariant masses, AFB in N rest frame, geometric displacements via modified DELPHES) are adequate for a proof-of-concept of mass/width/quantum-number extraction.
    Explicit scope choice in §I and §III; underpins characterization claims in §IV–V.
  • standard math Standard QCD PDFs and dynamical scale in MadGraph at √s=13.6 TeV suffice for the quoted rates.
    §III.A numerical setup.

pith-pipeline@v1.2.0-daily-grok45 · 18731 in / 3550 out tokens · 65502 ms · 2026-07-30T21:59:30.724848+00:00 · methodology

0 comments
read the original abstract

We show how signals of heavy neutrinos with displaced decays can be detected at the Large Hadron Collider in two theoretical setups, both exploiting extended gauge sectors as portals to such new physics, the Left-Right Symmetric Model and $U(1)_{B-L}$. Further, owing to the reduced contamination from backgrounds away from the interaction point, we illustrate how the properties of the heavy neutrinos (mass, width and quantum numbers) can neatly be accessed at the High-Luminosity upgrade of the CERN machine.

Figures

Figures reproduced from arXiv: 2607.26757 by Antons Nikolajevs, Aram Fatah, Stefano Moretti.

Figure 1
Figure 1. Figure 1: FIG. 1. Feynman diagram for the process studied in the [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Feynman diagram for the process studied in [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. (a) Calculated KS process cross section for different [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: As we are interested in heavy neutrino production and displaced (from the interaction point) decays, we show in [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6 [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. (a) Heavy neutrino production cross sections for dif [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
Figure 1
Figure 1. Figure 1: Since, in our chosen BPs, one has M(Ne) < MW′ , the KS process provides a clean signal that would confirm the Majorana nature of the heavy neutrinos as the final state leptons can be any combination of charges. Detecting such a process would show the lepton number violating behaviour of the heavy neutrinos. The choice of BPs for the process pp −→ WR −→ Nee ± −→ e ±e ±jj are displayed in Tab. III, together … view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. (a) The invariant mass distribution (in GeV) for the [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10. (a) The angular distributions between the direc [PITH_FULL_IMAGE:figures/full_fig_p008_10.png] view at source ↗
Figure 12
Figure 12. Figure 12: FIG. 12. The total transverse energy distribution (in GeV) [PITH_FULL_IMAGE:figures/full_fig_p009_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: FIG. 13. (a) The total displacement distributions for the [PITH_FULL_IMAGE:figures/full_fig_p009_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: FIG. 14. (a) The angular distributions between the direc [PITH_FULL_IMAGE:figures/full_fig_p010_14.png] view at source ↗

discussion (0)

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