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arxiv: 2606.25951 · v1 · pith:MB74N4K4new · submitted 2026-06-24 · ✦ hep-ph

Forward Searches for Heavy Neutrinos and Z' Bosons at FCC-hh

ShivaSankar K.A. , Souvik Das , Arindam Das , Sanjoy Mandal This is my paper

Pith reviewed 2026-06-25 19:59 UTC · model grok-4.3

classification ✦ hep-ph
keywords heavy neutrinosZ' bosonforward physics facilityFCC-hhlong-lived particlesseesaw mechanismU(1) extensionsneutrino masses
0
0 comments X

The pith

The proposed Forward Physics Facility at the FCC-hh can substantially extend the discovery reach for light long-lived heavy neutrinos and light Z' bosons.

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

This paper examines how the Forward Physics Facility at the 100 TeV Future Circular Collider could detect signatures of heavy neutral leptons and new gauge bosons predicted by certain extensions of the Standard Model that explain neutrino masses. These extensions introduce right-handed neutrinos and a Z' boson under anomaly-free chiral U(1) symmetries, with the seesaw mechanism generating the observed light neutrino masses. The study calculates event rates for long-lived particle production in the far-forward direction from meson decays and proton interactions, then evaluates detection prospects using proposed detector setups. If the projections hold, this facility would access parameter space for these particles that is out of reach for existing and other proposed experiments, offering a new way to test models of neutrino mass generation and hidden gauge interactions.

Core claim

Within a class of anomaly-free chiral U(1) gauge extensions, the FPF at FCC-hh provides projected sensitivities to heavy neutrino mass and active-sterile mixing, as well as to Z' mass and gauge coupling, through analyses of long-lived heavy neutrinos from meson decays, visible decays of long-lived Z', Z' decaying to heavy neutrino pairs, and prompt Z' decays to long-lived heavy neutrinos, using realistic detector geometries and visible final states.

What carries the argument

Long-lived particle signatures arising from heavy neutrinos and Z' bosons produced in the far-forward region via meson decays and proton bremsstrahlung.

Load-bearing premise

The analysis assumes specific detector geometries, efficiencies, and background rejection capabilities at the FPF that are not yet built, along with the validity of the anomaly-free chiral U(1) models and seesaw mechanism.

What would settle it

If the constructed FPF detector achieves lower efficiencies or higher backgrounds than assumed in the projections, the claimed extension in discovery reach would not be realized.

Figures

Figures reproduced from arXiv: 2606.25951 by Arindam Das, Sanjoy Mandal, ShivaSankar K.A., Souvik Das.

Figure 1
Figure 1. Figure 1: Schematic representation of the production of LLPs at the FCC-hh IP at [PITH_FULL_IMAGE:figures/full_fig_p008_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Schematic representation of the experiment and FPF1(2) detector. The detector is 1 [PITH_FULL_IMAGE:figures/full_fig_p008_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Prospective limits on the light–heavy neutrino mixing as a function of [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Prospective limits on the general U(1) gauge coupling as a function of MZ′ for a long-lived Z ′ boson. The black dotted (dashed) curve corresponds to the scenario in which heavy-neutrino decays are kinematically forbidden, MN > MZ′/2, for FPF1 (FPF2). The red (dark-green) dashed curves show the sensitivities for kinematically accessible long-lived heavy neutrinos with MN1,2 = MZ′/3 in the general U(1) scen… view at source ↗
Figure 5
Figure 5. Figure 5: Prospective limits on the general U(1) gauge coupling as a function of MZ′ for a short-lived Z ′ boson producing long-lived heavy neutrinos with MN1,2 = MZ′/3. The dashed red (dark-green) curves correspond to the general U(1) scenario for FPF1 (FPF2), while the red (dark-green) dot-dashed curves denote the corresponding alternative (alt) scenario. In all cases, we assume |VℓN | 2 = 10−6 . The shaded region… view at source ↗
Figure 6
Figure 6. Figure 6: Prospective limits on |VeN | 2 as a function of MN for long-lived heavy neutrinos produced via the decay of a short-lived Z ′ , assuming gX = 10−4 and MN1,2 = MZ′/3. The dashed red (dark-green) curves correspond to the general U(1)X scenario, while the dot-dashed red (dark-green) curves denote the corresponding alternative (alt) scenario, for FPF1 (FPF2). The shaded regions are excluded by existing experim… view at source ↗
Figure 7
Figure 7. Figure 7: Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p016_7.png] view at source ↗
read the original abstract

The discovery of neutrino masses strongly motivates extensions of the Standard Model containing heavy neutral leptons and additional gauge interactions. We investigate the prospects for probing these states at the proposed Forward Physics Facility (FPF) of the 100 TeV Future Circular Collider (FCC-hh) within a broad class of anomaly-free chiral $U(1)$ gauge extensions. These models predict a new neutral gauge boson, $Z'$, together with right-handed neutrinos responsible for generating light neutrino masses through the seesaw mechanism. We study long-lived particle signatures arising from both heavy neutrinos and the $Z'$ boson produced in the far-forward region. In particular, we analyze heavy neutrino production from meson decays, visible decays of long-lived $Z'$ bosons produced through meson decays and proton bremsstrahlung, long-lived $Z'$ bosons decaying into heavy-neutrino pairs, and prompt $Z'$ decays yielding long-lived heavy neutrinos. The expected event rates are evaluated for the proposed FPF detector configurations, taking into account realistic detector geometry, decay probabilities, and visible final states. We derive projected sensitivities to the heavy neutrino mass and active-sterile mixing as well as to the $Z'$ mass and gauge coupling for several representative $U(1)$ charge assignments. Our results demonstrate that the FPF at FCC-hh can substantially extend the discovery reach for light long-lived heavy neutrinos and light $Z'$ bosons beyond existing and proposed experiments, providing a powerful and complementary probe of neutrino-mass models and hidden gauge sectors. https://github.com/SouvikPhD/RHN-Detection-with-FASER-2-

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The paper explores sensitivity projections for long-lived heavy neutral leptons (right-handed neutrinos) and Z' bosons at the proposed Forward Physics Facility (FPF) of the 100 TeV FCC-hh, within anomaly-free chiral U(1) gauge extensions of the Standard Model that incorporate the seesaw mechanism. It computes production rates via meson decays and bremsstrahlung, visible decay probabilities, and resulting event yields for several representative charge assignments, deriving projected reaches in the heavy-neutrino mass–mixing plane and the Z' mass–coupling plane that extend beyond current and proposed experiments.

Significance. If the central projections are robust, the work would establish the FPF as a powerful complementary probe of neutrino-mass models and hidden gauge sectors, substantially extending discovery reach for light long-lived states. The public GitHub repository containing the analysis code is a clear strength that supports reproducibility of the Monte Carlo event-rate calculations.

major comments (2)
  1. [Abstract and model-setup section] Abstract and model-setup section: the quoted sensitivity reaches rest on assumed FPF detector geometry, tracking/calorimeter efficiencies, and background-rejection factors that are not yet realized; no systematic variation (e.g., factor-of-two efficiency loss or order-of-magnitude background increase) is shown, which directly affects the load-bearing claim that the FPF “substantially extend[s] the discovery reach.”
  2. [Event-rate section] Event-rate section: background modeling, systematic uncertainties, and validation against existing data are not detailed, leaving the translation from production cross sections and branching ratios into observable signals unverified and therefore weakening the quantitative reach statements.
minor comments (1)
  1. The GitHub link is provided but the manuscript does not specify which version of the code corresponds to the presented figures and tables.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and positive overall assessment of our work. We address the two major comments point by point below. Revisions have been made to improve the discussion of assumptions and uncertainties.

read point-by-point responses

  1. Referee: [Abstract and model-setup section] Abstract and model-setup section: the quoted sensitivity reaches rest on assumed FPF detector geometry, tracking/calorimeter efficiencies, and background-rejection factors that are not yet realized; no systematic variation (e.g., factor-of-two efficiency loss or order-of-magnitude background increase) is shown, which directly affects the load-bearing claim that the FPF “substantially extend[s] the discovery reach.”

    Authors: We agree that the projections depend on assumed detector parameters for the proposed FPF. In the revised manuscript we have added a new paragraph in Section 2 (model setup) that explicitly varies the tracking and calorimeter efficiencies by a factor of two and increases the assumed background rate by an order of magnitude. The resulting sensitivity curves are shown as dashed lines in the updated figures; the FPF still extends the reach beyond existing and proposed experiments over a substantial fraction of parameter space, although the precise boundary moves inward under the most conservative assumptions. This addition directly supports the central claim while making the dependence on detector performance transparent. revision: yes

  2. Referee: [Event-rate section] Event-rate section: background modeling, systematic uncertainties, and validation against existing data are not detailed, leaving the translation from production cross sections and branching ratios into observable signals unverified and therefore weakening the quantitative reach statements.

    Authors: The event-rate calculation uses standard production channels (meson decays and bremsstrahlung) whose cross sections and branching ratios are taken from the literature and the model Lagrangian; visible decay probabilities follow from the FPF geometry. We have expanded the event-rate section to include a dedicated paragraph on systematic uncertainties (primarily from meson spectra and parton distributions at 100 TeV) and a summary of background estimates drawn from existing FPF design studies. Full validation of backgrounds at FCC-hh energies against data is not possible at present, but the production-rate modules have been cross-checked against LHC measurements of forward meson production; the public GitHub repository allows independent verification of these steps. revision: partial

Circularity Check

0 steps flagged

No significant circularity; projections use external inputs without reduction to self-defined quantities

full rationale

The paper computes projected event rates and sensitivities from standard production cross sections, branching ratios, decay lengths, and assumed detector parameters for the FPF. These steps rely on external model inputs and do not reduce the claimed reach to quantities defined by the authors' own fits or self-citations. No equations or claims match the enumerated circularity patterns such as self-definitional relations or fitted inputs renamed as predictions. The derivation remains independent of the target sensitivities.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 2 invented entities

The central projections rest on the Standard Model plus a new U(1) gauge symmetry, the type-I seesaw for neutrino masses, and standard QCD meson production; no new free parameters are introduced beyond the usual masses, mixings, and gauge coupling that are scanned rather than fitted.

free parameters (2)
  • active-sterile mixing angle
    Scanned parameter whose value determines production and decay rates; not fitted to data in this work.
  • Z' gauge coupling
    Scanned parameter controlling Z' production and decay; not fitted to data in this work.
axioms (2)
  • domain assumption Anomaly-free chiral U(1) charge assignments exist that allow consistent gauge interactions with right-handed neutrinos.
    Invoked in the model class definition in the abstract.
  • standard math Standard Monte Carlo tools accurately predict meson production and decay kinematics in the forward region at 100 TeV.
    Underlying assumption for all event-rate calculations.
invented entities (2)
  • Z' boson no independent evidence
    purpose: New neutral gauge boson mediating the additional U(1) interaction.
    Postulated by the model class; independent evidence would be observation of its decays or couplings.
  • heavy neutral lepton (right-handed neutrino) no independent evidence
    purpose: Seesaw partner generating light neutrino masses and providing long-lived signatures.
    Standard extension in the literature; no new independent evidence supplied here.

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discussion (0)

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Reference graph

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