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Search for light long-lived neutral particles produced in $pp$ collisions at $\sqrt{s} =$ 13 TeV and decaying into collimated leptons or light hadrons with the ATLAS detector

T0 review · 1 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read No excess of displaced dark-photon jets is observed in 36.1 fb⁻¹ of 13 TeV pp collisions, and Higgs-boson decays to two dark photons are excluded above 4 pb for dark-photon decay lengths between 1.5 mm and 307 mm.

desk verdict A competent, honest ATLAS LLP search whose main weakness is an under-documented and misprinted lifetime-reweighting step; deserves peer review but needs a closure test. read the letter →

arxiv 1909.01246 v2 pith:46H7M3DT submitted 2019-09-03 hep-ex

ATLAS Collaboration This is my paper · ORCID
classification hep-ex
keywords darkphotonlong-livedneutralparticledisplacedvertexexoticHiggsdecayhiddensectorleptonjetsATLASLHC
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

In 36.1 fb⁻¹ of proton–proton collisions at $\sqrt{s} = 13$ TeV, the paper searches for long-lived dark photons produced in decays of a 125 GeV Higgs boson or an 800 GeV heavy scalar, where each dark photon decays into a narrow jet-like cluster of leptons or light hadrons away from the interaction point. The observed events agree with the expected background, so no new particle is claimed. Instead, the paper sets 95% confidence-level upper limits on the production cross section times branching fraction as a function of the dark-photon proper decay length. For a Higgs boson decaying into two dark photons, production above 4 pb is excluded for decay lengths between 1.5 mm and 307 mm. A reader should care because this directly constrains a well-motivated hidden-sector scenario that is otherwise hard to probe with prompt searches.

What carries the argument

The load-bearing object is the displaced dark-photon jet (DPJ): a cone of collimated muons in the muon spectrometer, or a narrow energy cluster in the hadronic calorimeter, with no matching inner-detector track. Two boosted decision trees separate signal DPJs from cosmic-ray muons and from multi-jets, dedicated displaced-object triggers collect the events, and the event-level background is predicted by an ABCD method in the plane of $\max(\Sigma p_T)$ (inner-detector track isolation) versus $|\Delta\phi|$ (azimuthal opening angle between the two DPJs). Signal efficiencies are extrapolated to arbitrary proper lifetimes by reweighting each dark-photon decay time, which turns the measured event counts into limits on $\sigma \times B$ as a function of $c\tau$.

What would settle it

A direct measurement would subdivide the signal-region control sample into smaller bins and compute the linear correlation between $\max(\Sigma p_T)$ and $|\Delta\phi|$ under the final selection; if that correlation is significantly above the few-percent level assumed, the ABCD prediction shifts and the reported exclusion windows would move.

Watch

Extended reading notes

Core claim

The paper's central claim, stated on its own terms, is that the FRVZ hidden-sector benchmark, where a Higgs boson decays to dark fermions that each emit a dark photon and the dark photon decays to Standard Model fermions, produces no observable excess above the multi-jet and cosmic-ray background in the ATLAS detector. Using the full 2015–2016 13 TeV dataset, the analysis excludes at 95% CL a cross section times branching fraction above 4 pb for $H \to 2\gamma_d + X$ when the dark-photon proper decay length lies between 1.5 mm and 307 mm. For two dark photons per Higgs decay, the excluded window is 3.7–178 mm, and for an 800 GeV scalar with $\sigma \times B = 5$ pb, the excluded windows extend to about 1.4 m and beyond. The result is an exclusion, not a discovery, and it is also interpreted as a 90% CL upper limit on the kinetic mixing parameter $\epsilon$ as a function of dark-photon mass.

Load-bearing premise

The ABCD background estimate assumes that the two event-level variables, inner-detector track isolation $\max(\Sigma p_T)$ and opening angle $|\Delta\phi|$, are nearly uncorrelated in the signal region, so that the background there can be extrapolated from three sidebands; the paper measures a linear correlation below 6% in validation-region data and in multi-jet simulation, but the test is limited by the size of the control sample.

Editorial extensions

If this is right

  • If correct, any FRVZ-model dark photon with mass near 0.4 GeV, a Higgs-decay branching fraction around 10%, and proper decay length in the excluded windows cannot be produced through Standard Model Higgs decays at the assumed rate; that parameter region is closed.
  • The purely hadronic channel, exploited here for the first time, means dark photons that decay to pions rather than leptons are now constrained at 13 TeV, not just muonic final states.
  • Because the limits are set as a function of $c\tau$, the same result can be applied to other lifetimes without running a new search, provided the acceptance model holds.
  • For the 800 GeV scalar, the excluded lifetime window reaches up to about 1.4 m in the muonic channel, closing a long-lifetime region that fixed-target and beam-dump experiments do not cover.

Reading between the lines

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

  • A straightforward extension would apply the same displaced-DPJ reconstruction to events with only one DPJ; the present requirement of two back-to-back jets, inherited from the two-body heavy-scalar topology, leaves single-jet or non-collinear hidden-sector signatures unconstrained.
  • The ABCD correlation check is the least protected part of the chain: with more luminosity, the signal-region sidebands could be subdivided further, and a residual correlation at the few-percent level could be measured directly rather than inferred.
  • The efficiency-versus-lifetime tables could be recast by other experiments into limits on any vector-portal model with similar kinematics, not only FRVZ, making the public result a reusable constraint.
  • Scaling the excluded cross section roughly as the inverse of integrated luminosity suggests that the full Run 2 dataset could push the 4 pb threshold down by about a factor of three, if the background remains smooth.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

1 major / 5 minor

Summary. This paper reports a search by the ATLAS Collaboration for long-lived dark photons produced in 13 TeV pp collisions, using 36.1 fb^-1 of 2015-2016 data. The search targets the FRVZ benchmark model in which a 125 GeV Higgs boson (or an 800 GeV heavy scalar) decays to two dark fermions, each yielding one or two dark photons that decay displaced into collimated muons or light hadrons, reconstructed as dark-photon jets (DPJs). Three final states are considered: muonic-muonic, muonic-hadronic, and hadronic-hadronic DPJ pairs. Dedicated triggers and BDT-based selections are used, and the dominant multi-jet background is estimated with an ABCD method using two nearly uncorrelated variables (max track isolation and |Delta phi|). Observed event yields agree with the predicted backgrounds in all validation and signal regions (e.g., 113 observed vs 128 +/- 26 expected in the muonic-muonic signal region). No excess is found, and 95% CL upper limits on sigma x B as a function of the dark-photon proper decay length c tau are derived. The headline result is that sigma x B above 4 pb is excluded for H to 2 gamma_d + X with m_H = 125 GeV for c tau between 1.5 mm and 307 mm; further exclusions are given for four-dark-photon decays and for the 800 GeV scalar. The results are also interpreted as 90% CL limits on the kinetic mixing parameter versus dark-photon mass.

Significance. If the c-tau-dependent limits are correct, the search provides the first 13 TeV ATLAS constraints in this channel using the fully hadronic signature, extends Run-1 displaced-lepton-jet results, and tests the FRVZ model in a previously allowed region of low kinetic mixing. The analysis is internally consistent: the ABCD background method is validated in dedicated validation regions and in a mock signal region, the linear correlation of the ABCD variables is checked (<6%) and found to have negligible impact, cosmic-ray and beam-induced backgrounds are estimated from dedicated datasets, and the dominant systematic uncertainties are evaluated with data-driven tag-and-probe and data/MC closure methods. The manuscript is careful and the presentation is generally clear. However, the central c-tau-dependent limits rely on a lifetime-reweighting extrapolation that is both misprinted and not validated by a closure test; this is the main reason the paper does not merit immediate acceptance.

major comments (1)
  1. [Section 9 (Results and interpretation), weight formula and Figure 4] As printed, the per-dark-photon weight w_i(t_i) = tau_ref e^{-t_i/tau_ref} * e^{-t_i/tau_new} tau_new is dimensionally inconsistent and does not equal the standard decay-time reweighting ratio (tau_ref/tau_new) exp[t_i(1/tau_ref - 1/tau_new)]; if taken literally, the extrapolated efficiencies in Figure 4 and the excluded c-tau intervals in Table 6 would be incorrect. Even if this is a typesetting artifact, the paper provides no closure test demonstrating that reweighting reproduces the signal efficiency of an independently generated sample at a different c-tau. Given that the extrapolation factors are large (about 33x toward 1.5 mm and about 6x toward 307 mm) and that the acceptance is strongly nonlinear in the decay position (pixel layers, hadronic calorimeter, muon trigger chambers), the c-tau-dependent limits rest on an unvalidated extrapolation. The authors should correct the formula, add a closure test comparing reweighted efficiencies at two or three test lifetimes against dedicated MC samples, and either assign a systematic uncertainty to the reweighting procedure or demonstrate that it is negligible.
minor comments (5)
  1. [Section 8 (Systematic uncertainties), muon reconstruction paragraph] The text refers to 'J/phi -> mu mu' but should read 'J/psi -> mu mu'; the same acronym is written correctly as J/psi elsewhere in the paper.
  2. [Section 7 (Multi-jet background estimation)] The symbol for the track isolation scalar sum appears as 'Í pT' (an apparent OCR artifact); it should be written consistently as Sigma(pT) throughout.
  3. [Section 5.1 (Dark-photon jet classification)] The sentence 'The search is limited to |eta| < 2.5, corresponding to the ID coverage, to ensure that selected muons are isolated from ID tracks' is unclear; the muons are required to be unmatched to ID tracks, not 'isolated from ID tracks' in the usual isolation sense.
  4. [Section 9 (Results and interpretation), weight notation] The notation 'tau ref' and 'tau new' is typeset inconsistently without subscripts; using tau_ref and tau_new as subscripts would improve readability and avoid confusion with the product structure of the weight.
  5. [Table 2 and Section 7] The validation-region BDT windows (e.g., '-0.75 < muBDT < 0.35' and 'muBDT > -0.7') are asymmetric but the reasons for these specific choices are not explained; a brief note on how the windows were chosen to avoid signal leakage would help the reader.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: limits follow from external FRVZ signal model, data-driven ABCD background, and standard per-event lifetime reweighting.

full rationale

The paper's central claims (background-only observation and 95% CL sigma x B limits as a function of ctau) are not derived from the quantities they purport to predict. The background in signal region A is estimated from data in regions B, C, D via the ABCD product NA = NB*ND/NC, with data in A excluded from the estimate; this is an extrapolation across a nearly uncorrelated plane, validated in VR data and by a mock-signal-region closure test (e.g., 231 +/- 58 expected vs 184 observed in the muDPJ-muDPJ channel). The signal acceptance and efficiency come from FRVZ-model MC generated with external model parameters, and the sigma x B limits are obtained by the standard CLs method. The lifetime extrapolation uses per-DPJ weights based on the exponential decay-density ratio; while the printed formula in Sec. 9 is garbled and no closure test against independently generated samples is shown, that is a correctness/validation concern, not a circular one, because the weights do not feed fitted data back into the prediction. Normal ATLAS self-citations (e.g., for trigger-efficiency systematics from Refs. [11] and [97]) are used only as external performance measurements and do not carry the load of the physics claim. No step in the derivation reduces by construction to an input fitted to the target result.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The analysis rests on standard statistical methods and on data-driven background estimation; the FRVZ model is used as a benchmark and is not the subject of the claim. No invented entities are introduced by this paper.

free parameters (2)
  • BDT discriminant thresholds = muBDT > 0.21, hBDT > 0.91
    Chosen by maximizing S/sqrt(S+B) on simulated signal and background; these fixed selection cuts affect signal efficiency and background rejection but are not physical parameters.
  • ABCD region boundaries = max(Sum pT) = 4.5 GeV, |Delta phi| = 0.625
    Hand-picked boundary definitions for the ABCD plane; the final limit depends on them but they are not fitted to data and are standard choices for such data-driven estimates.
assumptions (5)
  • domain assumption The ABCD background estimate is valid in the signal region, i.e., max(Sum pT) and |Delta phi| are sufficiently uncorrelated and the product formula NA = NB * ND / NC holds.
    Section 7; validated in VR data with correlation below 6% and in a mock signal-region test, but the test is statistics-limited.
  • domain assumption Geant4-based detector simulation accurately reproduces signal and background efficiencies, including the unusual hadronic calorimeter jets.
    Section 4; standard HEP assumption; data-driven corrections are applied for muon trigger and reconstruction, BDT shapes, and jet energy scale, but the fully hadronic DPJ efficiency carries large uncertainties.
  • domain assumption The signal efficiency extrapolation to different ctau via exponential lifetime weights is exact, i.e., acceptance and reconstruction efficiency do not depend on lifetime beyond the decay position distribution.
    Section 9; standard weighting method; relies on per-dark-photon factorization of the decay probability, which is correct for exponential decays in the FRVZ kinematics.
  • domain assumption The FRVZ benchmark model with the chosen masses (m_gamma_d = 0.4 GeV, m_fd2 = 5 GeV, m_HLSP = 2 GeV, m_sd = 2 GeV) and alpha_d below 0.01 is representative for the searched signature.
    Section 3; the limits are quoted for this model and are not directly generalizable to other dark-photon production modes.
  • standard math The CLs prescription yields valid 95% confidence intervals for the limits.
    Section 9; standard statistical method cited to Read, accepted by the field.

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Cite this review

Pith. "Pith review of Search for light long-lived neutral particles produced in $pp$ collisions at $\sqrt{s} =$ 13 TeV and decaying into collimated leptons or light hadrons with the ATLAS detector." pith.science (2026). https://pith.science/paper/46H7M3DT

@misc{pith2026190901246,
  author       = {Pith},
  title        = {Pith review of: Search for light long-lived neutral particles produced in $pp$ collisions at $\sqrts =$ 13 TeV and decaying into collimated leptons or light hadrons with the ATLAS detector},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/46H7M3DT}},
  note         = {Machine review of arXiv:1909.01246}
}
abstract

Several models of physics beyond the Standard Model predict the existence of dark photons, light neutral particles decaying into collimated leptons or light hadrons. This paper presents a search for long-lived dark photons produced from the decay of a Higgs boson or a heavy scalar boson and decaying into displaced collimated Standard Model fermions. The search uses data corresponding to an integrated luminosity of 36.1 fb$^{-1}$ collected in proton-proton collisions at $\sqrt{s} =$ 13 TeV recorded in 2015-2016 with the ATLAS detector at the Large Hadron Collider. The observed number of events is consistent with the expected background, and limits on the production cross section times branching fraction as a function of the proper decay length of the dark photon are reported. A cross section times branching fraction above 4 pb is excluded for a Higgs boson decaying into two dark photons for dark-photon decay lengths between 1.5 mm and 307 mm.

Discussion (0). Continue with ORCID to comment.

Forward citations

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