Prompt searches for feebly interacting particles at the LHC

CMS collaborations; Joscha Knolle for the ATLAS

Prompt searches for feebly interacting particles at the LHC

T0 review · 0 major / 3 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read This review of eight ATLAS and CMS searches claims that the 2015-2018 LHC data at 13 TeV now exclude previously unexplored masses and couplings for dark mesons, heavy neutral leptons, dark matter mediators, and dark photons.

desk verdict A competent, faithful proceedings summary of eight ATLAS/CMS prompt searches; no new results but a useful map for non-experts, with one minor typo about the dark photon model. read the letter →

arxiv 2412.06297 v1 pith:74WYNGIX submitted 2024-12-09 hep-ex hep-ph

Joscha Knolle for the ATLAS , CMS Collaborations This is my paper · ORCID

classification hep-exhep-ph

keywords feeblyinteractingparticlespromptsignaturesheavyneutralleptonsdarkmesonsphotonsmatterATLASandCMSLHCRun2

topics Dark Matter

open problems Dark Matter

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

This paper is a summary of eight recent ATLAS and CMS searches for promptly produced feebly interacting particles, using the full 2015-2018 LHC Run 2 dataset of 13 TeV proton-proton collisions with an integrated luminosity of about 140 $fb^{-1}$. It seeks to establish that these searches jointly extend the excluded parameter space for four classes of models: dark mesons decaying to top and bottom quarks, heavy neutral leptons, dark matter in several mediator scenarios, and dark photons. In some cases the paper reports that the searches give the first collider-based constraints for the considered model. If the claims are correct, the low-mass windows that strong-coupling searches cannot reach are now being closed by prompt-signature searches, and the combination of complementary final states is the reason the coverage improves.

What carries the argument

The central mechanism is the 'prompt signature' assumption together with the shared Run 2 dataset: each of the eight analyses selects events where the new particle is produced and decays at the interaction point, so the experimental handle is the visible final-state particles rather than a displaced vertex. The carrying objects are the dedicated trigger and reconstruction strategies - reclustered large-radius jets used as dark pion candidates, same-sign dileptons plus jets for boosted heavy neutral lepton decays, scouting triggers that retain only partial event information and reach muon transverse momenta as low as 4 GeV, and multivariate discriminants for background separation. Limits are derived from fits to reconstructed mass or missing transverse momentum distributions and quoted at 95% CL. These strategies, distributed across complementary final states, are what allow the searches to cover the mass and coupling ranges they claim.

What would settle it

If any one of the eight analyses were re-run with a corrected background model that moved its 95% CL upper limit above the boundary the paper shows as excluded, the claim of extended coverage would be wrong for that model. A concrete place to look is the CMS scouting dimuon search: a narrow resonance with local significance above 5 $\sigma$ anywhere between 1.1 and 7.9 GeV in the same 140 $fb^{-1}$ data would contradict the dark photon exclusion presented in Fig. 4.

Watch

Extended reading notes

Core claim

The central claim, stated in the abstract and summary, is that the eight analyses cited as Refs. [9-16] rule out previously unexcluded masses and couplings with the 140 $fb^{-1}$ Run 2 dataset. ATLAS excludes dark pion masses up to 940 GeV in a stealth dark matter model for dark-pion-to-dark-rho mass ratios of 0.15-0.45, the first collider search for this model. CMS probes heavy neutral leptons with masses between 10 GeV and 1.5 TeV mixed with one neutrino generation, including the first exclusion of exclusive tau-neutrino couplings above the W boson mass, and excludes Z' bosons decaying to HNL pairs for Z' masses from 0.4 to 4.6 TeV. Simplified dark matter models with an s-channel mediator are constrained through top-quark pair plus missing transverse momentum, four-top, mono-photon, and dijet resonance channels, and a dark Higgs model is excluded for dark matter masses of 100-300 GeV with mediator masses up to 2.5 TeV. A 2HDM with a pseudoscalar mediator is constrained by a three-channel combination, the CMS scouting analysis excludes dimuon resonances with masses in 1.1-2.6 and 4.2-7.9 GeV and sets dark photon mixing limits, and ATLAS excludes SM Higgs branching fractions above 1.3% for decays to a photon plus a massless dark photon, with heavy Higgs limits up to 3 TeV.

Load-bearing premise

The whole summary rests on the eight underlying ATLAS and CMS analyses being correct in their background estimates, systematic uncertainties, and statistical procedures; this paper reports those results and does not independently verify them.

Editorial extensions

If this is right

Dark pion masses up to 940 GeV are excluded for the considered mass ratios, so the stealth dark matter model survives only at higher masses or smaller couplings.
Heavy neutral leptons with exclusive tau-neutrino mixing and masses above the W boson are probed for the first time, complementing lower-mass displaced-vertex searches.
The scouting-based dimuon search closes a low-mass window for dark photons, excluding narrow resonances in the 1.1-2.6 GeV and 4.2-7.9 GeV ranges that standard triggers cannot reach.
SM Higgs decays into a photon and a massless dark photon are limited to branching fractions below 1.3%, and heavy Higgs production in that decay channel is excluded for masses up to 3 TeV.
The combined Run 2 programme, not any single analysis, defines the new excluded region; the paper shows this explicitly for heavy neutral leptons by comparing prompt, displaced, and vector-boson-fusion channels.

Reading between the lines

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

The paper's success with scouting at muon $p_T$ of 4 GeV suggests a natural extension it does not discuss: applying the same low-threshold trigger to electron pairs or dijets could close analogous low-mass windows for other dark-sector mediators.
Because only prompt decays are considered, sufficiently small couplings would make the same models evade these limits by becoming long-lived; a combined prompt-plus-displaced reinterpretation of the eight analyses would give the true Run 2 coverage, a step the review takes only for heavy neutral leptons.
If the exclusions hold, dark-sector model builders should shift benchmarks toward heavier masses or smaller couplings; in particular, the 0.4-4.6 TeV excluded $Z'$ range pushes left-right symmetric HNL scenarios toward the multi-TeV scale.
The 13.6 TeV Run 3 dataset, already larger than Run 2, will extend these limits statistically, but the largest discovery potential may come from reinterpreting the same final states in additional simplified models rather than from the energy increase alone.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

You asked about Knolle's LHCP proceedings write-up, arXiv:2412.06297. The short version: it's exactly what it says on the tin—a review of eight published ATLAS and CMS searches for prompt feebly interacting particles, with no new analysis, data, or derivation. The reader's report is right: novelty is zero, but the summary is accurate and the claims are faithful restatements of the cited results.

What it does well: the organization is clear, the model-by-model structure (dark mesons, HNLs, DM mediators, dark photons) is easy to follow, and the figures are well chosen. For someone coming into the field, it gives a quick, reliable map of what ATLAS and CMS have done in this corner of Run 2. The paper also correctly flags where a search is a first collider constraint (e.g., the dark meson search and exclusive tau-coupling HNL search), and it doesn't oversell beyond what the cited papers say.

The main soft spot is the one the stress-test caught: the paragraph on Ref. [15] describes the minimal dark photon model as "resulting in a massless dark photon," which is wrong in context—the search targets dimuon resonances at 1.1–2.6 and 4.2–7.9 GeV, so the dark photon is massive by construction. It reads like a typo (they meant massless mixing or something else) and doesn't affect any of the reported limits, but it should be fixed before publication. Minor.

The other caveat is not a flaw in this genre: the paper takes the statistical procedures and backgrounds of the eight underlying analyses on trust, citing them. That's appropriate for a proceedings review; independent verification belongs in the original papers. The reader's weakest assumption is real but applies to any such summary.

Overall, this is a solid, honest review. It's not a research contribution, and nobody should cite it as a primary source for a physics result. But as an overview for graduate students or theorists wanting the current experimental status in prompt FIP searches, it's useful and accurate. I'd bring it to a reading group for that purpose, not because it changes anything.

Recommendation: if this came through a peer-review route (as some proceedings do), send it to a referee—it's the kind of descriptive paper that a knowledgeable editor can accept after checking the typo. It deserves a light-touch review, not a desk reject.

Referee Report

0 major / 3 minor

Summary. This paper is a conference proceedings contribution by J. Knolle on behalf of the ATLAS and CMS Collaborations, reviewing eight recent prompt searches for feebly interacting particles at the LHC using the 2015–2018 13 TeV dataset of roughly 140 fb–1. The covered searches address dark mesons decaying to top and bottom quarks, heavy neutral leptons in trilepton and Z′-mediated channels, simplified dark matter models with s-channel mediators, a dark Higgs model with associated W-boson-pair production, a 2HDM-plus-pseudoscalar-mediator combination, low-mass dimuon resonances using scouting triggers, and Higgs-boson decays to a photon plus a massless dark photon. For each selected analysis, the paper reports the main exclusion limits, expected and observed 95% CL intervals, and the considered model parameters. The central claim is that the presented results significantly extend the probed parameter space and, in some cases, provide the first collider constraints for the considered models.

Significance. The paper is a compact, well-organized survey of current ATLAS and CMS prompt searches for feebly interacting particles. Its strength is its faithful, concise reporting of the eight cited analyses, with correct attribution to the original papers and appropriate reproduction of their figures. The paper contains no new derivation or independent calculation, so its scientific value rests on the accuracy of the summary and on the trustworthiness of the underlying experimental papers. As a proceedings contribution, it serves the community as a useful reference point for the status of these searches. The claim that the limits extend the parameter space and include first collider constraints is a faithful restatement of the cited results; the paper does not overreach beyond what the original analyses report.

minor comments (3)

[Section 2, paragraph on Ref. [15]] The sentence describing the minimal dark photon model in the scouting-trigger dimuon search says that the model results in a massless dark photon. This is a typo: the search targets dimuon resonances with masses of 1.1–2.6 and 4.2–7.9 GeV, which requires a massive dark photon. Please correct the wording to state that the dark photon is massive, and if appropriate clarify the mass-generation mechanism (e.g., a dark Higgs or Stueckelberg mass term).
[Abstract and introduction] The abstract states that the results are based on an integrated luminosity of “about 140 fb–1”. The individual analyses use 138 or 139 fb–1; while “about 140” is acceptable for a proceedings, adding a brief footnote or parenthetical listing the exact luminosities of the individual analyses would remove potential ambiguity for readers who compare the cited papers.
[Section 2, HNL and Z′ paragraphs] In the heavy neutral lepton and Z′ sections, comparison figures are cited as Ref. [20], which is a CMS review article rather than a primary search. The text already says “various analyses” and “different production channel,” but it would be clearer to state explicitly that the comparison is taken from the review, not from the search under discussion, to avoid any impression that the new limits are being directly compared with one another.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: faithful proceedings summary of eight external ATLAS/CMS analyses with no derivation chain.

full rationale

This is a conference proceedings that summarizes eight published ATLAS and CMS searches. It contains no derivation chain: no equations are derived, no parameters are fitted, and no new limit is computed. The central claim—that the exclusion limits significantly extend the probed parameter space and in some cases provide first collider-based constraints—is a restatement of the conclusions of the cited experimental analyses. Those citations are external evidence, not self-citations importing an unverified premise: each cited paper is an independent experimental result with its own data, background estimation, and limit-setting procedure, and the summary does not use the citations to justify an assumption that then produces the same conclusion. The descriptive statement about exclusion limits is not a prediction derived from an input; it is a characterization of the cited results. There is no fitted input called prediction, no uniqueness theorem imported from the authors' own prior work, and no ansatz smuggled in via citation. The only noticeable issue is a likely typo in the description of Ref. [15], where a minimal dark photon model is described as 'resulting in a massless dark photon' even though the search targets dimuon resonances at masses of 1.1–2.6 and 4.2–7.9 GeV, which requires a massive dark photon; this is an accuracy concern, not a circularity concern, and it does not affect the reported limits or the summary claim.

Assumptions & free parameters 0 free parameters · 1 assumptions · 0 invented entities

The paper's central claim is a summary statement, so the only load-bearing premise is the reliability of the cited experimental publications. No free parameters, derived quantities, or new entities are introduced.

assumptions (1)

domain assumption The cited ATLAS and CMS analyses (Refs. [9-16]) were performed correctly, with valid background estimates, systematic uncertainties, and limit-setting procedures.
The review's summary of exclusion limits is only valid if the underlying experimental results are correct; no independent verification is provided in this paper.

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0 comments

Cite this review

Pith. "Pith review of Prompt searches for feebly interacting particles at the LHC." pith.science (2026). https://pith.science/paper/74WYNGIX

@misc{pith2026241206297,
  author       = {Pith},
  title        = {Pith review of: Prompt searches for feebly interacting particles at the LHC},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/74WYNGIX}},
  note         = {Machine review of arXiv:2412.06297}
}

abstract

Recent results from the ATLAS and CMS experiments in searches for prompt signatures of feebly interacting particles are presented. All presented results are based on the 2015-2018 data set of $13\,\mathrm{TeV}$ proton-proton collisions, corresponding to an integrated luminosity of about $140\,\mathrm{fb}^{-1}$. The discussed models include dark mesons, heavy neutral leptons, dark matter, and dark photons. The obtained exclusion limits significantly extend the probed parameter space and, in some cases, provide the first collider-based constraints for the considered models.

Figures

Figures reproduced from arXiv: 2412.06297 by the authors.

**Figure 1.** Left: Dark meson limits on the dark-pion-to-dark-rho-meson mass ratio as a function of the dark pion mass, obtained by ATLAS in the one-lepton channel [9]. Right: HNL limits on the HNL-to-muonneutrino mixing parameter as a function of the HNL mass, obtained by CMS in various analyses for the Majorana nature [20]. flavour combinations, dedicated strategies are applied for different target HNL mass ranges, including… view at source ↗

**Figure 2.** Left: Left-right symmetry model limits on the HNL mass as a function of the new vector boson mass, obtained by CMS in two analyses in the muon channel [20]. Right: Scalar DM mediator limits on the cross section relative to the predicted value as a function of the scalar mediator mass, obtained by ATLAS in various search channels [12]. production [26] search, and the corresponding exclusion limits are shown in [PITH… view at source ↗

**Figure 3.** Left: Dark Higgs model limits on the dark Higgs boson mass as a function of the new vector boson mass, obtained by CMS in the search for DM produced in association with a W boson pair [13]. Right: Limits on the CP-odd Higgs boson mass as a function of the pseudoscalar mediator mass in a 2HDM model extended with DM and a pseudoscalar mediator, obtained by ATLAS in different search channels [14]. Additionally, the con… view at source ↗

Figures from the paper (2 more)

**Figure 4.** Figure 4 [PITH_FULL_IMAGE:figures/full_fig_p005_4.png]

**Figure 5.** Figure 5: Heavy Higgs boson limits on the production cross section as a function of the heavy Higgs boson mass, obtained by ATLAS in the search for Higgs boson decays to a photon and a dark photon [16]. 3. Summary The large amount of recorded data and the high collision energy o…

Discussion (0). Continue with ORCID to comment.

Reference graph

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Pith tools

Reviewed August 11, 2026 · model on record in the stance chip above.

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