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Search for a resonance in events with four top quarks decaying into two leptons and jets in proton-proton collisions

T0 review · 0 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read Four-top search finds no resonance and sets exclusion limits on a top-philic vector boson.

desk verdict A competent, honest null result that opens the two-lepton channel for four-top resonance searches; the central claim holds, with a disclosed but real caveat on the background normalisation that sets the exact mass reach. read the letter →

arxiv 2608.10148 v1 pith:RBNB5FAN submitted 2026-08-10 hep-ex

classification hep-ex
keywords fourtopquarkstop-philicresonancevectormediatorboostedtaggingvariable-radiusjetsdijetinvariantmass95%confidencelimitsaxion-likeparticle
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 reports a search for a heavy boson that decays to a top-quark pair while being produced together with another top-quark pair, giving events with four top quarks. Using $173\,\text{fb}^{-1}$ of proton-proton collisions at 13 and 13.6 TeV, the CMS Collaboration finds no significant excess over the standard-model background. It sets 95% confidence-level upper limits on the production cross section of vector, scalar, and pseudoscalar mediators with masses between 500 GeV and 4 TeV and relative widths of 4-50%. For the benchmark top-philic vector mediator, resonances with 50% relative width are excluded up to 850 GeV, with an expected exclusion of 1000 GeV. The result is the first dedicated search for these models in the two-lepton channel and the first four-top resonance search to include 13.6 TeV data.

What carries the argument

The mass of the putative resonance is reconstructed as $m_{JJ}$, the invariant mass of the two leading variable-radius jets formed with the HOTVR algorithm, whose distance parameter shrinks as $R_{\text{eff}}=600\,\text{GeV}/p_T$ to follow collimated top-quark decays. A boosted decision tree tags whether each VR jet comes from a hadronically decaying top quark, and the background of misidentified top-tagged jets is estimated from data by transferring events with zero t-tagged VR jets into the two-tagged signal regions using simulation-derived transfer factors, with the overall normalisation corrected in Z-boson control regions. A simultaneous profile-likelihood fit of the $m_{JJ}$ templates in two signal regions and three dilepton channels yields the 95% confidence-level limits via the $CL_s$ criterion.

What would settle it

With the next $100\,\text{fb}^{-1}$ of 13.6 TeV data, count events in SR2b2T near $m_{JJ}\approx500$ GeV: a real 500 GeV resonance would show a growing peak, while a fluctuation would track the transfer-factor background. The transfer assumption itself can be checked by measuring the two-tag correlation in a same-flavour dilepton region outside the Z window and verifying that the control-region correction factor does not change by more than its uncertainty.

Watch

Extended reading notes

Core claim

On its own terms, the paper establishes a null result: the dijet invariant mass spectrum in the two-lepton, two-top-tagged signal regions is consistent with the standard-model background, with a background-only $p$-value of 0.21. The largest local deviation is a 2.1 standard deviation excess for a 500 GeV vector mediator with 4% width, with similar deviations of 2.0 and 2.4 standard deviations for scalar and pseudoscalar hypotheses. The paper therefore reports upper limits rather than evidence: for the $\Gamma_{Z'}/m_{Z'}=50\%$ benchmark, $Z'$ bosons are excluded up to 850 GeV, while the observed limits for 10 and 20% widths are weaker than expected and exclude no mediators. Scalar and pseudoscalar mediators are not excluded. These exclusions apply to the resonant $s$-channel contribution only; non-$s$-channel diagrams and interference with standard-model four-top production are not included, as the paper explicitly states.

Load-bearing premise

The background estimate assumes that the residual mismodelling of the correlation between the two top-tagging probabilities, measured in Z-boson control regions and applied as a normalisation correction, is the same in the signal regions; if that correlation differs there, the background yield could shift by about 20% and change the mass reach.

Editorial extensions

If this is right

  • The benchmark top-philic vector mediator with 50% relative width is excluded below 850 GeV, extending the mass reach of earlier four-top searches.
  • Scalar and pseudoscalar mediators are not excluded, so those interpretations remain viable targets for future searches.
  • Variable-radius jets extend sensitivity below 1 TeV mediator mass, a region where fixed-radius top tagging was inefficient.
  • The analysis provides the first four-top resonance limits that include 13.6 TeV data, combining 138 fb${}^{-1}$ at 13 TeV with 35 fb${}^{-1}$ at 13.6 TeV.
  • The ALP reinterpretation sets new upper limits on $c_t/f_A$ in the mass range around 500 GeV to 4 TeV, complementing tt-resonance and missing-transverse-momentum searches.

Reading between the lines

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

  • If the mild 2.1σ excess near 500 GeV is real, additional 13.6 TeV data should produce a growing narrow peak in $m_{JJ}$; if it is a fluctuation, the spectrum should continue to follow the transfer-factor background.
  • Because the shown exclusions omit interference with standard-model four-top production, a full top-philic model could be more or less constrained than the benchmark limits suggest, particularly for broad mediators.
  • The same two-lepton plus VR-jet topology could be combined with single-lepton four-top searches, and applied to other resonances such as heavy Higgs bosons, to improve overall coverage of top-philic scenarios.
  • A direct measurement of the correlation between the two top-tagging decisions in a dedicated control region would test the background-transfer assumption and could shrink the roughly 20% normalisation uncertainty that currently dominates the background systematics.
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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

0 major / 4 minor

Summary. This paper reports a search for a heavy resonance produced in association with a top-quark pair and decaying to a top-quark pair, in final states with two opposite-sign leptons and jets, using 138 fb^-1 of 13 TeV and 35 fb^-1 of 13.6 TeV CMS data. The resonance mass is reconstructed from the two leading variable-radius jets, and a boosted decision tree is used to identify hadronically decaying top quarks. The dominant background from misidentified top-tagged jets is estimated from data by transferring zero-tagged events in transfer regions to two-tagged signal regions using simulation-derived transfer factors, with normalisation corrections obtained from Z-boson control regions. A simultaneous profile-likelihood fit to the dijet invariant mass finds no significant excess (background-only p-value 0.21), and 95% CL upper limits are set on ttZ', ttphi, and tta production cross sections for mediator masses from 500 GeV to 4 TeV and relative widths from 4% to 50%. For the vector benchmark with 50% relative width, observed exclusion reaches 850 GeV, compared with 1000 GeV expected. Local significances of 2.1-2.4 standard deviations are observed for the 500 GeV signal hypotheses and are attributed to statistical fluctuations.

Significance. If the result holds, this is the first dedicated two-lepton four-top-resonance search and it extends sensitivity below 1 TeV using variable-radius jets; it also provides a new reinterpretation of the results in terms of axion-like particles. The paper's strengths include a data-driven background estimate with dedicated transfer and control regions, a profile-likelihood fitting procedure with CLs limits, a documented treatment of systematic uncertainties including a dedicated 20% flavour-composition uncertainty, and a public HEPData record. The central null result is robust. The main caveat, explicitly acknowledged in Section 5, is that the normalisation corrections measured in the Z+jets-dominated CR2J region are transferred to the b-enriched signal regions under an assumption about the similarity of top-tagging correlation mismodelling; the paper states this assumption and assigns a dedicated 20% uncertainty, so I do not regard this as undermining the central claim that no significant excess is observed. The exact mass reach of the exclusions is conditional on this modelling assumption, but that is a standard feature of searches of this type.

minor comments (4)
  1. [Section 4.4 / Table 1] The region names SR1b2T and SR2b2T are initially easy to misread; although Table 1 defines n_b and n_t, a one-line clarification in the text that 'b2T' denotes the b-jet multiplicity plus the requirement of at least two t-tagged VR jets would improve readability.
  2. [Section 5] The correction factors for the e±mu∓ channel are set to the geometric mean of the e+e- and mu+mu- factors, but no validation of this approximation is shown; a sentence describing the systematic uncertainty assigned to this choice, or a closure check in the e±mu∓ control region, would make the procedure more complete.
  3. [Section 5 / Figure 4] The goodness-of-fit of the constant functions in Figure 4 is reported only as chi2/ndf ≲ 1; quoting the individual chi2/ndf values would be more quantitative and would help readers assess the quality of the normalisation corrections.
  4. [Figure 6] The legend entry 'Theory 95% CL upper limits' in Figure 6 appears to conflate the theoretical cross-section curve with the 95% CL limit curves; please relabel the theory line and the limit curves separately to avoid confusion.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the signal-region background is estimated from orthogonal transfer and Z-boson control regions, not from a fit to the signal region, so the null result and limits are not self-referential.

full rationale

The central claim (no significant excess; 95% CL limits on ttZ', ttphi, and tta cross sections) is derived from a simultaneous maximum-likelihood fit to the SR m_JJ templates. The dominant background is not obtained by fitting the SR itself: Section 5 estimates it from zero-tagged TR data using simulation-derived transfer factors, with genuine-hadronic-top events subtracted and a normalisation correction measured in the Z-mass-window CR2J regions. CR2J and the SRs are disjoint (CR2J selects 80 < m_ll < 101 GeV and does not require b-tagged AK4 jets, while the SRs veto that window and require at least one or two b-tagged jets), so the correction factors are not fit to the same data they are used to predict. The paper's explicit caveat that the correction-factor transfer 'assumes that the residual mismodelling of the correlation between the two t-tagging probabilities is sufficiently similar in the CRs and SRs' is a modelling assumption, not a circular reduction; Section 6 assigns a dedicated about 20% normalisation uncertainty for the b-quark-composition difference. No fitted parameter is renamed as a prediction, no uniqueness claim is imported from author self-citation, and the signal samples and transfer factors use standard external generators and calibrations. The observed excess (84 events vs 61.5 expected pre-fit) is reported and the limits are weakened accordingly, indicating that the analysis does not force agreement by construction.

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

No ad hoc free parameters or new entities are introduced. The Z', scalar, pseudoscalar, and ALP mediators are existing model benchmarks from the cited literature. The analysis depends on standard collider assumptions about simulation, transfer-factor background estimation, and asymptotic statistics; the most fragile is the control-region to signal-region transfer of t-tagging correlation corrections.

assumptions (5)
  • domain assumption Transfer factors computed in simulation can map events with zero t-tagged VR jets to events with two t-tagged VR jets.
    Section 5; the data-driven background estimate is built from this mapping, and the paper finds a residual normalisation mismatch that it corrects with control-region factors.
  • domain assumption The residual mismodelling of the correlation between the two t-tagging probabilities is sufficiently similar in the control regions and the signal regions.
    Section 5; the control-region normalisation corrections are applied to signal regions. The paper assigns a 20% normalisation uncertainty for differences in flavour composition.
  • domain assumption Signal simulation restricted to s-channel tt+X production with no interference with SM four-top production is adequate for the benchmark interpretation.
    Section 3; authors state non-s-channel diagrams and interference are neglected, can be substantial, and limits apply only to this benchmark.
  • standard math Asymptotic formulae for the profile-likelihood test statistic are valid for the CLs limits.
    Section 7; standard LHC practice, but with low event counts per mJJ bin the asymptotic approximation is an approximation rather than an exact coverage guarantee.
  • domain assumption Simulated samples describe the detector response and jet substructure after data-derived scale factors.
    Sections 3 and 4.5; top-tagging BDT scale factors are derived from a tt-enriched region and applied to simulation.

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Cite this review

Pith. "Pith review of Search for a resonance in events with four top quarks decaying into two leptons and jets in proton-proton collisions." pith.science (2026). https://pith.science/paper/RBNB5FAN

@misc{pith2026260810148,
  author       = {Pith},
  title        = {Pith review of: Search for a resonance in events with four top quarks decaying into two leptons and jets in proton-proton collisions},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/RBNB5FAN}},
  note         = {Machine review of arXiv:2608.10148}
}
abstract

A search for a heavy resonance in events containing four top quarks is presented and interpreted in models featuring a new vector, scalar, or pseudoscalar boson that interacts exclusively with top quarks. Proton-proton collision data collected by the CMS experiment at the CERN LHC at centre-of-mass energies of 13 and 13.6 TeV are analysed, corresponding to integrated luminosities of 138 and 35 fb$^{-1}$, respectively. Events with two leptons (electrons or muons) and jets are selected. The mass of the heavy resonance is reconstructed from a pair of jets that are formed using a variable-radius jet clustering algorithm. A machine-learning approach is used to identify whether these jets originate from top quarks that decay fully hadronically. The dominant background from misidentified jets is estimated from data as a function of the dijet invariant mass. No significant excess is observed in data. Upper limits at 95% confidence level are set on the resonance production cross section for resonance masses between 500 GeV and 4 TeV, and relative widths ranging from 4 to 50%. In the vector-mediator benchmark, resonances with 50% relative width are excluded up to masses of 850 GeV, with an expected exclusion limit of 1000 GeV. This is the first dedicated search for these models in the two lepton channel.

Figures

Figures reproduced from arXiv: 2608.10148 by the authors.

Figure 1
Figure 1. The ttϕ and tta processes yield the same topology, with the Z′ boson replaced with a mediator ϕ or a, respectively. We analyse pp collision data at √ s = 13 TeV (2016–2018) and 13.6 TeV (2022), corresponding to an integrated luminosity of 138 fb−1 and 35 fb−1 , respectively. A novelty is the reconstruction of mZ′ from a pair of jets formed using a variable-radius jet clustering algorithm (VR jets) [27, 28], which ex… view at source ↗
Figure 2
Figure 2. Data-to-simulation scale factors for the BDT classifier to identify t jets in bins of VR [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗
Figure 3
Figure 3. Distributions of the BDT classifier score in regions enriched in Z/ [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figures from the paper (7 more)
Figure 4
Figure 4. Figure 4: Observed and estimated mJJ distributions in CR2J2T for e+e − (left) and µ +µ − (right) events for the 2016–2018 data-taking periods (upper row) and the 2022 data-taking period (lower row). The points for the background prediction (red) include the contribution from had…
Figure 5
Figure 5. Figure 5: The background-only post-fit distribution of [PITH_FULL_IMAGE:figures/full_fig_p016_5.png]
Figure 6
Figure 6. Figure 6: Expected and observed upper limits at 95% CL on the product of the t [PITH_FULL_IMAGE:figures/full_fig_p017_6.png]
Figure 9
Figure 9. Figure 9: Also shown are the exclusions from tt resonance searches, taken from the CMS search for pseudoscalar resonances in √ s = 13 TeV data collected in 2016–2018 [22], and a limit on ALPs below tt threshold, taken from the CMS search for tt and single top quark production in…
Figure 7
Figure 7. Figure 7: Expected and observed upper limits at 95% CL on the product of the t [PITH_FULL_IMAGE:figures/full_fig_p019_7.png]
Figure 8
Figure 8. Figure 8: Expected and observed upper limits at 95% CL on the product of the t [PITH_FULL_IMAGE:figures/full_fig_p020_8.png]
Figure 9
Figure 9. Figure 9: Expected (dashed lines) and observed (solid lines, shaded areas) upper limits at [PITH_FULL_IMAGE:figures/full_fig_p021_9.png]

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