Search for emerging jets in $pp$ collisions at $\sqrt{s} = 13.6$ TeV with the ATLAS experiment

ATLAS Collaboration

REVIEW 2 major objections 3 minor 2 cited by

Search for emerging jets in $pp$ collisions at $\sqrt{s} = 13.6$ TeV with the ATLAS experiment

T0 review · 2 major / 3 minor · reviewed 2026-08-16 · deepseek-v4-flash

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Pith's one-line read No emerging jets found: Z' dark mediator excluded to 2.6 TeV

desk verdict A clean ATLAS null search that closes two unprobed emerging-jet channels and debuts a reusable transformer tagger, with the caveat that the quoted exclusions are benchmark-specific rather than universal dark-QCD bounds. read the letter →

arxiv 2505.02429 v2 pith:VHD3MX5W submitted 2025-05-05 hep-ex

ATLAS Collaboration This is my paper · ORCID

classification hep-ex

keywords emergingjetsdarkQCDlong-livedparticlesdisplacedverticesZ'mediatort-channelscalartransformerjettaggerLHCsearch

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 search aims to establish whether dark-sector “emerging jets”—jets crowded with displaced vertices from long-lived dark pions decaying back to Standard Model particles—are produced in proton collisions through two channels not previously targeted: an s-channel Z' vector mediator and a t-channel scalar mediator. Using 51.8 inverse femtobarns of 13.6 TeV proton–proton collision data, it finds no significant excess over the Standard Model background and reports the first direct exclusions for both production modes. If correct, Z' mediators between 600 and 2550 GeV are ruled out for quark and dark-quark couplings of 0.01 and 0.1 (for dark pion decay lengths of 5–50 mm), and scalar mediators between 600 and 1375 GeV are ruled out for a quark–dark-quark coupling of 0.1.

What carries the argument

The central object is the emerging-jet topology: a jet containing several displaced vertices produced by dark pions that travel macroscopic distances before decaying into Standard Model quarks. Two selection strategies carry the analysis: a cut-based chain using the prompt-track fraction, the number of displaced vertices, the energy-correlation function ECF2, and the number of subjets, with background estimated by the ABCD method; and a transformer-based jet tagger trained on 12 million jets that classifies each jet with a score p_EJ, with background estimated from a measured per-jet mistag rate. A dedicated trigger selecting jets with a very low prompt-track fraction extends sensitivity to lower dijet masses, while a standard single-jet trigger covers the high-mass region.

What would settle it

Recompute the 95% confidence-level exclusion limits after re-running the same selections on signal samples with a different dark-sector mass hierarchy or a different number of dark quark flavours; if the boundary moves by more than the quoted uncertainties, the reported numbers depend on the unstudied parameter choice.

Watch

Extended reading notes

Core claim

The paper claims that, after a fully data-driven background estimate, the observed event counts agree with the Standard Model prediction in every signal region, so no evidence for emerging jets is found. The strongest result is a set of 95% confidence-level exclusion limits on mediator production: for a dark pion proper decay length between 5 and 50 mm, Z' masses from 600 to 2550 GeV are excluded at couplings g_q = 0.01 and g_qD = 0.1, and for a quark–dark-quark coupling of 0.1, Phi masses from 600 to 1375 GeV are excluded. The paper also claims that at m_Z' = 1500 GeV with a 50 mm dark pion decay length, quark couplings above 0.003 are excluded when the dark coupling exceeds 0.03, a sensitivity more than twenty times stronger than existing dijet resonance searches. These constitute the first direct constraints on emerging-jet pair production through an s-channel Z' mediator and the first search for t-channel scalar-mediated emerging jets.

Load-bearing premise

The quoted mass limits assume the simulated dark sector is configured with seven dark quark flavours, three dark colours, the mass hierarchy m_rhoD = 2 Lambda_D = 2 m_qD = 4 m_piD, and dark pions forced to decay to down quarks, and the paper states that the impact of varying these parameters was not explicitly studied.

Editorial extensions

If this is right

The excluded Z' mass range, up to 2550 GeV, is the first direct bound on this specific emerging-jet production channel, where previously only pair-produced bi-fundamental mediators had been constrained.
The ML-based strategy is more powerful than the cut-based one, excluding Z' masses up to 2550 GeV versus 2150 GeV and giving an order-of-magnitude stronger limits on the t-channel scalar mediator.
The search probes quark–dark-quark couplings down to about 0.003, more than twenty times smaller than what dijet resonance searches reach, opening a previously inaccessible part of dark-sector parameter space.
The limits stay nearly flat for dark pion decay lengths between 1 and 100 mm and only weaken above 100 mm, when a growing fraction of dark pions decay beyond the inner tracker.
The results are expected to remain sensitive to a broader set of dark QCD parameters, since variations in the dark colour and flavour numbers mostly change the dark pion multiplicity rather than the core signature.

Reading between the lines

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

The transformer tagger's auxiliary tasks—classifying track origin and grouping tracks into vertices—could be transferred directly to other long-lived-particle searches at the same experiment, potentially sharpening their sensitivity without retraining from scratch.
The analysis is limited by the 61% systematic uncertainty attached to the mistag-rate parameterisation in the high-mass region; a finer parameterisation in terms of more jet observables could bring a notable gain in the reported limits.
With only a fraction of the full Run 3 dataset used here, simply doubling the integrated luminosity would push the Z' exclusion boundary beyond 2550 GeV, assuming the background estimates scale as expected.
The quoted exclusion numbers rest on one specific dark-sector benchmark (N_f = 7, N_c = 3, and the stated mass hierarchy); a reanalysis using alternative benchmark points would be a natural test of how much of the reported region actually survives.

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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 should know two things about this one. First, it is a well-executed null search: observed yields match data-driven predictions across all four signal regions, with nonclosure uncertainties quantified and validated in control regions. Second, the headline exclusions are explicitly tied to a specific dark-sector benchmark (N_c=3, N_f=7, the m_rhoD=2Lambda_D=2m_qD=4m_piD hierarchy, dark pions forced to down quarks). The stress-test note gets this right: Section 8 admits the impact of varying those parameters was not studied, and the argument that only dark-pion multiplicity changes is qualitative. So read the 2550 GeV Z' and 1375 GeV Phi limits as constraints on that benchmark, not as universal dark-QCD bounds. That does not sink the paper, but it should be stated plainly in any summary.

What is genuinely new: this is the first search for s-channel Z' mediated emerging jets and the first for t-channel scalar mediated emerging jets. The prior CMS searches targeted bi-fundamental mediator pair production, so the coverage is complementary and real. The transformer-based tagger, adapted from GN2 and using large-impact-parameter tracks, is also a legitimate technical contribution; it can be reused in other displaced-signature searches. The analysis is careful about triggers (the dedicated emerging-jet trigger is a nice Run 3 addition), and the background estimation is data-driven with multiple closure tests. The ML-based background has a large systematic uncertainty in the high-mjj region (61%), but they validate with a signal-free region and it does not undermine the null result.

The soft spots beyond the benchmark dependence are minor. The paper does not provide supplementary data or code for reinterpretation, which limits its utility for model-builders, but that is standard for ATLAS. The full text I was sent has some rendering artifacts in the figures and a duplicated section, but those are clearly presentation issues, not physics. The authors cite the relevant prior searches and do not oversell their novelty; they explicitly frame the limits as benchmark-specific, which is honest.

Who is this for? Anyone working on long-lived particles, dark sectors, or jet tagging will find it useful. It deserves a serious referee: the analysis is internally consistent, the new channels are meaningful, and the tagger is a reusable tool. I would support acceptance, and if I were the editor I would send it to review without hesitation.

Referee Report

2 major / 3 minor

Summary. The paper reports a search for emerging jets—jets containing multiple displaced vertices from decays of long-lived dark mesons—using 51.8 fb⁻¹ of pp collisions at √s = 13.6 TeV recorded by ATLAS in 2022–2023. Two production modes are targeted: s-channel production of a Z′ boson decaying to a dark-quark pair, and t-channel exchange of a scalar mediator Φ producing dark quarks plus up to two SM quarks. Two complementary strategies are used: a cut-based selection on jet observables (PTF, ECF2, N_vtx, N_subjet) with a data-driven ABCD background estimate, and an ML-based strategy using a GN2-style transformer tagger with a data-driven mistag-rate background estimate. Both strategies are split into low- and high-m_jj regions with distinct triggers, including a dedicated emerging-jet trigger introduced for Run 3. No significant excess is observed: the yields in the four signal regions (Table 8) agree with the data-driven predictions (e.g., cut-based high-m_jj: 8 observed vs 7.5 ± 1.1 ± 1.1 predicted; ML-based low-m_jj: 24 vs 31.8 ± 0.8 ± 7.5). The background methods are validated in control regions and signal-adjacent validation regions (Tables 5–7). Using profile-likelihood fits and the CLs procedure, 95% CL exclusions are set for the specified benchmark dark sector: Z′ masses 600–2550 GeV for g_q = 0.01, g_qD = 0.1 with cτ between 5 and 50 mm, and Φ masses 600–1375 GeV for κ = 0.1.

Significance. The core null result is robust and model-independent: the observed yields agree with fully data-driven background predictions in all four signal regions, and the background uncertainties are quantified through ABCD closure tests, ML-inverted and VR-tag validation regions, and alternative mistag-rate parameterizations. The search is methodologically significant: it is the first application of a GN2-type transformer tagger to a beyond-the-Standard-Model signature in ATLAS, the first ATLAS ML tagger to exploit the large-impact-parameter tracking pass, and the first ML-based identification of displaced vertices in an ATLAS LLP search; the tagger demonstrably generalizes to the t-channel signal, which is absent from its training. The statistical model (profile likelihood, CLs, pyhf) is standard, and the asymptotic approximation is checked against pseudo-experiments. If the results stand, they provide the first direct constraints on emerging-jet pair production via an s-channel Z′ mediator and the first search for t-channel scalar-mediated emerging jets.

major comments (2)

[§3.1, §8, Table 1, Abstract/Conclusions] The headline exclusions (Z′ up to 2550 GeV, Φ up to 1375 GeV) are computed for a single dark-sector benchmark with N_c = 3, N_f = 7, the mass hierarchy m_rhoD = 2Λ_D = 2m_qD = 4m_piD, dark pions forced to decay to down quarks, and no dark baryons, yet the Abstract and Conclusions quote these ranges as conditions only on the mediator couplings and on cτ. Section 8 states that the impact of varying N_c, N_f, and the mass hierarchy “has not been explicitly studied” and that these parameters are “primarily expected to affect the dark pion multiplicity” (Ref. [26]), which is a qualitative argument. Since the dark-pion multiplicity and pT spectrum directly determine N_vtx, displaced-track counts, PTF, ECF2, N_subjet, and the transformer tagger's track-level features, alternative dark-sector parameter points could shift the signal acceptance and hence the quoted mass ranges by an amount the paper does not quantify. I do not regard this as an internal inconsistency, because the benchmark is fully specified in Table 1 and Section 3.1, but the manuscript should carry the benchmark condition explicitly whenever the 2550 GeV/1375 GeV numbers are quoted in the Abstract and Conclusions, and should reference the m_piD scan of Figure 12 as partial robustness evidence or soften the final claim of Section 8 that the results “are therefore expected to remain sensitive across a broader range of dark QCD parameter choices.”
[§6.2 and §8] In the ML-based strategy, the n_tag < 2 control region used to measure the mistag rate is not included as Poisson terms in the profile likelihood of Section 8; the robustness of the background extraction therefore rests on the <10% signal-contamination check and on the 23–61% parameterization systematics rather than on a simultaneous CR fit. Given the small SR yields this is adequate, but explicitly stating why the CR was not incorporated into the fit would strengthen the statistical presentation.

minor comments (3)

[§1] A passage of heavily corrupted text beginning “figure QZ a diagram illustrati¯g…” appears in Section 1 near Figures 1–2 and is repeated, including duplicated paragraphs on the trigger regions and paper structure; this makes part of the introduction unreadable and must be corrected in the published version.
[§9] In the first paragraph of the Conclusions, “Φ masses up to 1350 GeV are excluding” should read “are excluded.”
[§5.1.1] The performance statement for the ECF2/pT > 40 GeV requirement (approximately 40% background rejection at over 90% signal efficiency) is not tied to a displayed distribution; a small table or figure of the signal and background efficiencies as a function of the ECF2/pT threshold would aid the reader.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the exclusions compare fixed signal benchmarks against data-driven backgrounds and validated control regions.

full rationale

The paper's derivation chain is self-contained: signal cross sections, widths, and dark-sector parameters are fixed input benchmarks (Section 3.1), and no output limit is obtained by fitting those inputs to data. Backgrounds are estimated data-driven via the ABCD and mistag-rate methods (Sections 6.1 and 6.2) and are validated in signal-adjacent control regions, so the CLs limits in Section 8 are genuine comparisons of simulated signal plus fitted background against observed yields rather than predicted quantities that reduce to their inputs. The transformer tagger (Section 5.2) is trained on simulated MC with disjoint folds and its threshold is fixed before evaluating the signal regions, so it is not a fitted parameter renamed as a prediction. Citations to prior phenomenological work (Refs. [26] and [37]) supply external benchmark model parameters, not a self-citation chain bearing the central claim. The admitted lack of explicit variation of N_c, N_f, and the mass hierarchy (Section 8) is a model-dependence caveat affecting the generality of the quoted exclusions, not a circular argument, because the analysis does not use those parameters as free outputs and no prediction is equivalent to an input by construction.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

All free parameters are fixed benchmark inputs from prior literature, not fitted to the observed data. The analysis does not introduce new physical entities; it searches for pre-existing dark sector proposals. The main unmodeled freedom is the dark-sector parameter choice, which the paper acknowledges in Section 8.

free parameters (4)

Dark sector benchmark parameters (Nc, Nf, mass hierarchy) = Nc=3, Nf=7, m_rhoD = 2 Lambda_D = 2 m_qD = 4 m_piD; m_piD in {5, 10, 20} GeV
Set by hand following Refs [22,26]; signal acceptance and hence the exclusion limits depend on these choices.
Z' couplings g_q and g_qD = g_q = 0.01, g_qD = 0.1
Benchmark couplings used for the quoted Z' exclusion; cross-section and limits scale with these values.
Phi coupling kappa_1j = kappa_1j = 0.1
Benchmark for the t-channel model, chosen per Ref [37]; used for the quoted Phi exclusion.
Dark pion proper decay length c_tau_piD = 5-50 mm for quoted limits; samples generated at 1-1000 mm
The quoted exclusions are conditional on this lifetime range, as stated in the abstract and Section 8.

assumptions (5)

domain assumption A dark sector with a confining SU(N_c) gauge group and N_f dark quark flavours exists and couples to the Standard Model via a mediator.
Motivated by Refs [19-24] and previously searched by ATLAS and CMS; not independently verified.
domain assumption Dark pions are long-lived and decay back to Standard Model quarks with proper decay lengths in the 1-1000 mm range.
Core of the emerging jet signature, following Ref [26]; the quoted exclusions assume 5-50 mm.
domain assumption The QCD multijet background is the dominant background and can be modelled data-driven via the ABCD or mistag-rate methods.
Validated with closure tests in Sections 6.1 and 6.2; requires small correlations between the ABCD variables, which is tested in control regions.
standard math Monte Carlo generators (Pythia, MadGraph, Powheg, Sherpa) accurately model signal and control signatures.
Standard high-energy physics practice, used for signal acceptance and background validation, with systematic variations applied.
domain assumption The GN2-derived transformer tagger trained on simulated multijet and signal samples generalizes to data.
Validated with the VR tag selection (Table 7) and a qualitative comparison of p_EJ distributions in data and MC (Figure 6).

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Cite this review

Pith. "Pith review of Search for emerging jets in $pp$ collisions at $\sqrt{s} = 13.6$ TeV with the ATLAS experiment." pith.science (2026). https://pith.science/paper/VHD3MX5W

@misc{pith2026250502429,
  author       = {Pith},
  title        = {Pith review of: Search for emerging jets in $pp$ collisions at $\sqrts = 13.6$ TeV with the ATLAS experiment},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/VHD3MX5W}},
  note         = {Machine review of arXiv:2505.02429}
}

abstract

A search for emerging jets is presented using 51.8 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 13.6$ TeV, collected by the ATLAS experiment during 2022 and 2023. The search explores a hypothetical dark sector featuring 'dark quarks', which are charged under a confining gauge group and couple to the Standard Model via a new mediator particle. These dark quarks undergo showering and hadronization within the dark sector, forming long-lived dark mesons that decay back into Standard Model particles. This results in jets which contain multiple displaced vertices known as emerging jets. The analysis targets events with pairs of emerging jets, produced either through a vector mediator, $Z'$, in the $s$-channel, or a scalar mediator, $\Phi$, in the $t$-channel. No significant excess over the Standard Model background is observed. Assuming a dark pion proper decay length between 5 and 50 mm, $Z'$ mediator masses between 600 and 2550 GeV are excluded for quark and dark quark coupling values of 0.01 and 0.1, respectively. For a quark-dark quark coupling of $0.1$, $\Phi$ mediator masses between 600 and 1375 GeV are excluded. These results provide the first direct constraints on emerging jet pair production via a $Z'$ mediator, and represent the first search to investigate emerging jet production via $t$-channel exchange of a scalar mediator.

Figures

Figures reproduced from arXiv: 2505.02429 by the authors.

**Figure 2.** Feynman diagrams for @⇡@¯⇡ production with up to two additional SM jets via a scalar . regions based on the invariant mass of the two leading jets (< 99), each with distinct trigger strategies. The low-< 99 region (< 99 < 1 TeV) utilises a dedicated jet trigger, introduced for Run 3, specifically designed to select emerging jets. The high-< 99 region (< 99 > 1 TeV) relies on a standard single-jet trigger. Combining… view at source ↗

Discussion (0). Continue with ORCID to comment.

Forward citations

Cited by 2 Pith papers

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

Search for resonant production of lepton-enriched semivisible jets in proton-proton collisions at $\sqrt{s}$ = 13 TeV
hep-ex 2026-08 conditional novelty 7.0 of 10

A CMS search with 138 fb^-1 of 13 TeV data finds no lepton-enriched semivisible jet resonance and excludes Z' masses up to 4.7 TeV (SVJ l) and 1.8-3.5 TeV (SVJ tau) at 95% CL.
Search for emerging jets in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment
hep-ex 2025-10 accept novelty 5.0 of 10

No excess of emerging-jet events appears in ATLAS Run 2 data, excluding pair-produced dark scalar mediators up to about 2 TeV for 20 GeV dark pions with 20 mm decay length.

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2025

Pith tools

Reviewed August 16, 2026 · model on record in the stance chip above.

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