Long-Lived HNLs via ALP Portal at the LHC

Ana Mart\'in-Gal\'an; Chandan Hati; Martin Hirsch; Rebeca Beltr\'an

arxiv: 2510.26946 · v3 · submitted 2025-10-30 · ✦ hep-ph

Long-Lived HNLs via ALP Portal at the LHC

Rebeca Beltr\'an , Chandan Hati , Martin Hirsch , Ana Mart\'in-Gal\'an This is my paper

Pith reviewed 2026-05-18 02:29 UTC · model grok-4.3

classification ✦ hep-ph

keywords Heavy Neutral LeptonsAxion-Like ParticlesALP PortalLong-Lived ParticlesLHC SearchesNeutrino Mixing

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The pith

Long-lived heavy neutral leptons can be produced at the LHC via an axion-like particle portal, offering improved sensitivity to their parameters.

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

This paper investigates the production of heavy neutral leptons through axion-like particles that couple to gluons at the LHC. Such ALPs would be produced in large numbers and could then yield long-lived HNLs observable in dedicated far detectors or the ATLAS experiment during the high-luminosity phase. The resulting sensitivities to HNL mixing angles and masses could surpass those from standard production modes. The work also addresses the case of very heavy ALPs by using an effective field theory description with dimension-eight operators.

Core claim

If ALPs with sizable couplings to gluons exist, HNLs produced via the ALP portal may provide unprecedented sensitivities to HNL parameters at the high-luminosity LHC. Prospects are studied for long-lived HNLs using future far detectors as well as ATLAS. For ALP masses above the TeV scale, this reduces to production via a dimension-8 operator connecting HNL pairs to gluons, with corresponding sensitivities calculated.

What carries the argument

The ALP portal, through which gluon-coupled ALPs produce pairs of heavy neutral leptons.

If this is right

Improved sensitivity to HNL-neutrino mixing angles at the HL-LHC.
Detection prospects for long-lived HNLs in far detectors and ATLAS.
Constraints on effective dimension-8 HNL-gluon operators in the heavy ALP limit.

Where Pith is reading between the lines

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

This production mechanism could link searches for ALPs and HNLs in a joint analysis.
Future detector designs might prioritize locations or technologies optimized for ALP-decay signatures.
The approach extends to other portals or production modes for long-lived particles.

Load-bearing premise

ALPs with sizable couplings to gluons exist and will be abundantly produced at the LHC.

What would settle it

No observation of the predicted long-lived HNL signals in high-luminosity LHC data from far detectors or ATLAS, which would indicate either absent ALPs or HNL parameters outside the sensitive range.

read the original abstract

Heavy neutral leptons (HNLs) and axion-like particles (ALPs) are both considered well-motivated candidates for beyond the standard model (BSM) physics. If ALPs with sizable couplings to gluons exist, they will be abundantly produced at the LHC. Therefore, HNLs produced via the ALP portal may provide unprecedented sensitivities to HNL parameters. Here, we study the prospects for the high-luminosity LHC to search for long-lived HNLs. We consider future far detectors as well as ATLAS in our simulations. In the limit where the ALP mass is above the TeV scale, HNLs are effectively produced by a dimension-8 operator connecting HNL pairs to gluons. For completeness, we therefore also calculate future LHC sensitivities for HNLs produced via $N_R$SMEFT operators with gluons.

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.

Desk Editor's Note private letter to a colleague

This is a standard sensitivity projection for long-lived HNLs via the ALP portal at HL-LHC, with far-detector simulations and an effective operator fallback, but the gains stay conditional on ALP gluon couplings.

read the letter

The main thing to know is that this paper runs a sensitivity study for long-lived heavy neutral leptons produced through an axion-like particle portal. They simulate prospects at the high-luminosity LHC using both future far detectors and ATLAS, and they also treat the heavy-ALP limit as a dimension-8 gluon operator for completeness. The abstract states the improved reach explicitly as conditional on ALPs with sizable gluon couplings being produced abundantly at the LHC. That framing is honest and keeps the claim from overreaching. What the work does reasonably well is lay out concrete search strategies for this production mode and include both the light-ALP and effective-operator cases, which gives a fuller picture than some narrower studies. The approach follows established collider-phenomenology methods for long-lived particles, so the setup itself looks standard rather than inventive. The soft spots are modest but real. Without the full text I cannot check the Monte Carlo details, background modeling, or efficiency assumptions behind the projected sensitivities, which makes the quantitative numbers hard to assess. The headline claim of unprecedented reach rests on the ALP premise, which the authors flag as an if rather than a given. This is not a logical flaw, just a limit on how much weight the projections can carry on their own. The paper is aimed at phenomenologists and experimental groups working on long-lived BSM searches at the LHC. Readers already following HNL or ALP literature would pick up useful targeted strategies for planning. It deserves a serious referee because the topic is relevant, the methods are grounded, and the conditional framing is clear, even if the referee will want to see the full simulation validation.

Referee Report

1 major / 0 minor

Summary. The manuscript studies prospects for detecting long-lived heavy neutral leptons (HNLs) produced via an axion-like particle (ALP) portal at the high-luminosity LHC. It posits that ALPs with sizable gluon couplings, if they exist, would be abundantly produced and could yield HNLs with unprecedented parameter sensitivities. Simulations are performed for future far detectors and ATLAS; the heavy-ALP limit is treated via an effective dimension-8 gluon operator, with additional results shown for HNL production through N_R SMEFT gluon operators.

Significance. If the projected sensitivities are borne out by the full calculations, the work would provide a concrete new search channel for HNLs that complements existing direct-production strategies. The conditional framing on ALP existence and the dual treatment of the portal versus SMEFT operators are appropriately cautious. Credit is due for explicitly separating the ALP-portal scenario from the effective-operator limit and for including both far-detector and ATLAS projections.

major comments (1)

The abstract states that simulations for HL-LHC far detectors and ATLAS are performed, yet no quantitative details (cross sections, efficiencies, background estimates, or error bars) are supplied. Because these elements are load-bearing for the central claim of 'unprecedented sensitivities,' the absence of the full text prevents verification of whether the projected reach is actually supported.

Simulated Author's Rebuttal

1 responses · 1 unresolved

We thank the referee for their careful review and for highlighting the need for verifiable details in support of our sensitivity claims. We address the major comment below.

read point-by-point responses

Referee: The abstract states that simulations for HL-LHC far detectors and ATLAS are performed, yet no quantitative details (cross sections, efficiencies, background estimates, or error bars) are supplied. Because these elements are load-bearing for the central claim of 'unprecedented sensitivities,' the absence of the full text prevents verification of whether the projected reach is actually supported.

Authors: The provided manuscript excerpt consists solely of the abstract, which is a concise summary and does not contain the requested quantitative details. The complete paper includes dedicated sections on the Monte Carlo simulation framework, production cross sections for the ALP-gluon portal (and the heavy-ALP dimension-8 limit), detector acceptances and efficiencies for the far detectors and ATLAS, background estimates, and the resulting sensitivity projections including statistical and systematic uncertainties. These calculations are what support the projected reach. Since the full text was not accessible for this review, we cannot reproduce the specific numerical values here. We are prepared to add a short summary of key results (e.g., order-of-magnitude improvements in HNL mixing angles) to the abstract in a revised version if the referee considers it helpful for immediate verification. revision: partial

standing simulated objections not resolved

Specific numerical values for cross sections, efficiencies, background estimates, and error bars cannot be supplied because only the abstract (not the full manuscript) is available in the current context.

Circularity Check

0 steps flagged

No significant circularity

full rationale

The abstract outlines a conditional phenomenological sensitivity study for long-lived HNLs produced via an ALP portal (or effective dimension-8 operator) at the HL-LHC, using simulations for far detectors and ATLAS. No derivation chain, equations, fitted parameters, or self-referential claims appear; the analysis is presented as a forward projection under an explicit 'if' premise about ALP existence, remaining fully self-contained without any reduction of outputs to inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review prevents full identification of free parameters or invented entities; relies on standard BSM assumptions.

axioms (1)

domain assumption ALPs with sizable couplings to gluons exist and are abundantly produced at the LHC
Explicit premise stated in the abstract to motivate the HNL production channel.

pith-pipeline@v0.9.0 · 5657 in / 997 out tokens · 32339 ms · 2026-05-18T02:29:41.774733+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

ALPs with couplings to gluons will be abundantly produced at the LHC... HNLs produced via the ALP portal... effective operators... d=7 GN and d=8 gluon-HNL
IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

sensitivity estimates... far detectors... MATHUSLA... ANUBIS... ATLAS

What do these tags mean?

matches: The paper's claim is directly supported by a theorem in the formal canon.
supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses: The paper appears to rely on the theorem as machinery.
contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
unclear: Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.

Forward citations

Cited by 2 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

Long-lived sterile neutrinos from axionlike particles at the Super Tau-Charm Facility
hep-ph 2026-05 unverdicted novelty 4.0

STCF can reach |V_eN|^2 values one to two orders of magnitude below current bounds for heavy neutral leptons via displaced-vertex searches from ALP decays in D-meson production.
A search for heavy axion-like particles in light-by-light scattering at the FCC-hh
hep-ph 2026-03 unverdicted novelty 3.0

Computes production cross sections and sensitivity limits for heavy ALPs in light-by-light scattering at FCC-hh across pp, pPb, and PbPb modes.