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Search for resonances decaying into top-quark pairs using fully hadronic decays in pp collisions with ATLAS at sqrt(s) = 7 TeV

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ATLAS Collaboration

High Energy Physics - Experiment · Submitted 9 Nov 2012 · 66 references

Pith's verdict: A careful, well-documented ATLAS fully hadronic ttbar resonance search whose limits are real but whose 'combined' reach is a per-mass-point pick between two correlated samples, not a combination.

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arXiv ID1211.2202 (version 3)
Pith IDRPR5GUJK, Pith's permanent record of this paper and its review
CitationATLAS Collaboration. "Search for resonances decaying into top-quark pairs using fully hadronic decays in pp collisions with ATLAS at sqrt(s) = 7 TeV." arXiv:1211.2202 (2012). https://pith.science/paper/RPR5GUJK
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The paper

The paper claims

ATLAS finds no bump in the two-jet mass spectrum of boosted top pairs, and converts the null result into 95% credibility exclusions of a narrow $Z'$ boson (0.70–1.00 TeV and 1.28–1.32 TeV) and a broad Kaluza–Klein gluon (0.70–1.62 TeV).

The reading

This paper asks whether the LHC has produced a heavy particle that decays into a top quark and its antiparticle in the case where both tops decay hadronically and each one's debris is collimated into a single massive jet. Working with 4.7 fb$^{-1}$ of 7 TeV proton–proton collisions recorded by ATLAS, the collaboration looks for a bump in the invariant mass of the two leading jets after each jet has been required to look like a top and to carry a bottom-quark tag. No bump appears: 953 events survive one selection and 123 the other, in agreement with the predicted Standard Model top-pair plus multijet background of $770^{+220}_{-180} + 130^{+70}_{-70}$ and $59^{+27}_{-26} + 53^{+6}_{-6}$ respectively. The null result is then turned into 95% credibility upper limits on production rate times branching fraction for two benchmark resonances, a narrow leptophobic $Z'$ boson and a wide Kaluza–Klein gluon from a warped extra dimension. The value of doing this in the fully hadronic channel is that no isolated lepton is needed, so acceptance survives for tops boosted hard enough that their decay products merge, which is exactly where the lepton-plus-jets searches that held the previous ATLAS bounds lose sensitivity.

What carries the argument

The load-bearing device is the pair of top taggers plus the requirement that each tagged jet have an associated $b$ decay. The HEPTopTagger works by undoing Cambridge/Aachen clustering on a fat jet until all subjet masses fall below 50 GeV, testing every three-subjet combination for compatibility with $t to Wb to q'bar{q}b$ through invariant-mass ratios, filtering the triplet to remove underlying event and pile-up, and keeping candidates with mass between 140 and 210 GeV, choosing the one closest to 172.3 GeV. The Top Template Tagger instead defines an infrared-safe overlap function $OV3 = max_{tau_n} exp[-sum_{i=1}^{3} (E_i - sum_{Delta R < 0.2} E_{topo})^2 / (2 sigma_i^2)]$ between observed calorimeter energy flow and a library of boosted three-body top decay templates, and pairs it with a pile-up-corrected jet mass window of $pm 50$ GeV around the top mass. The second load-bearing device is the background estimate: because the two tagging probabilities factorise for QCD jets, the multijet yield in the signal region can be written as a product of observed yields in sideband cells, for example $P' = N_J N_O N_F / (N_E N_C)$, which is checked against four other cell combinations and against an independent estimate from the measured light-jet rejection.

Extended reading notes

Core claim

The paper's claim is a set of exclusions rather than a detection. Two independent jet-substructure taggers are applied to the same 4.7 fb$^{-1}$ dataset: the HEPTopTagger, which re-clusters a Cambridge/Aachen fat jet of $R=1.5$ into three subjets and tests their mass ratios against a hadronic $t$ decay, and the Top Template Tagger, which scores an anti-$k_t$ $R=1.0$ jet against roughly 300,000 parton-level templates and requires an overlap $OV3 > 0.7$ together with a jet mass within 50 GeV of the top mass. Each tagged jet must additionally be matched to a $b$-tagged anti-$k_t$ $R=0.4$ jet within $R=1.4$ or $R=1.0$. Multijet background is estimated from data by exploiting the fact that, for QCD events, the probability to fake a top tag and the probability to fake a $b$ tag are uncorrelated, giving a two-dimensional sideband extrapolation into the signal region; the SM $t$-$t$ contribution is normalised from simulation and cross-checked in a sideband fit of the tagger mass distribution. With data consistent with background in both selections, Bayesian limits with Gaussian nuisance parameters exclude, at 95% credibility, $Z'$ bosons in the ranges $0.70 < m_{Z'} < 1.00$ TeV and $1.28 < m_{Z'} < 1.32$ TeV, and Kaluza–Klein gluons in the range $0.70 < m_{g_{KK}} < 1.62$ TeV, with the low-mass end supplied by the HEPTopTagger selection and the high-mass end above about 1.4 TeV by the Top Template Tagger selection.

Load-bearing premise

The signal efficiency at every resonance mass comes entirely from simulation, and the only check of the taggers on real top jets in the data has 25–30% relative precision (0.81 ± 0.25 for the leading jet, 0.62 ± 0.20 for the recoil jet), yet Section 8 states that no additional uncertainty for the modelling of top-tagging variables is added, so that precision enters the limit only as a cross-check rather than as a nuisance parameter.

Editorial extensions

If this is right

  • Any model predicting a top-philic resonance in the excluded windows is ruled out at 95% credibility for the specific benchmark widths considered, $Gamma/m = 1.2%$ for the $Z'$ and $15.3%$ for the Kaluza–Klein gluon.
  • The fully hadronic channel delivers sensitivity where lepton-plus-jets analyses do not, so the combined ATLAS reach on top-pair resonances extends to higher mass than the lepton-based searches alone.
  • The two taggers are shown to be complementary rather than redundant: the HEPTopTagger carries the low-mass reach and the Top Template Tagger the high-mass reach, which sets the template for how a combined tagger strategy should be staged.
  • The data-driven sideband method for multijet background, built on the decorrelation of top-tag and $b$-tag probabilities, is validated and can be reused for other boosted searches with two independent tagging axes.

Reading between the lines

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

  • The exclusion is stated for resonances with the same colour structure as a Kaluza–Klein gluon, and the paper itself notes that a colour-singlet object of the same mass and width would shift sensitivity by roughly 10%, so the quoted mass window should not be read as a universal bound on any di-top resonance.
  • Because the two selections use the same 4.7 fb$^{-1}$ sample and are statistically correlated, the combined exclusion is taken as an envelope of the two rather than a statistical combination; a genuine joint fit would likely tighten the window near 1.0–1.3 TeV where neither tagger alone is strongest.
  • The tagger efficiency is measured in situ only to 25–30% relative precision, so the near-future version of this analysis with more luminosity will be limited by tagger calibration rather than by statistics at low mass, which argues for dedicated boosted-top calibration samples.
  • The absence of an excess in the fully hadronic channel at these masses, taken together with the lepton-plus-jets exclusions quoted from the same collaboration, squeezes the plausible parameter space for a narrow top-philic $Z'$ into the 1.0–1.3 TeV gap that neither channel covers well.
Original abstract

A search for resonances produced in 7 TeV proton-proton collisions and decaying into top-quark pairs is described. In this Letter events where the top-quark decay produces two massive jets with large transverse momenta recorded with the ATLAS detector at the Large Hadron Collider are considered. Two techniques that rely on jet substructure are used to separate top-quark jets from those arising from light quarks and gluons. In addition, each massive jet is required to have evidence of an associated bottom-quark decay. The data are consistent with the Standard Model, and limits can be set on the production cross section times branching fraction of a Z' boson and a Kaluza-Klein gluon resonance. These limits exclude, at the 95% credibility level, Z' bosons with masses 0.70-1.00 TeV as well as 1.28-1.32 TeV and Kaluza-Klein gluons with masses 0.70-1.62 TeV.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 5,050 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.37 · human participants 0.01 · controlled comparison 0.11 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.16 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v3. Results and the main claim are underlined; .

A search for resonances produced in 7 TeV proton-proton collisions and decaying into top-quark pairs is described. In this Letter events where the top-quark decay produces two massive jets with large transverse momenta recorded with the ATLAS detector at the Large Hadron Collider are considered. Two techniques that rely on jet substructure are used to separate top-quark jets from those arising from light quarks and gluons. In addition, each massive jet is required to have evidence of an associated bottom-quark decay. The data are consistent with the Standard Model, and limits can be set on the production cross section times branching fraction of a Z' boson and a Kaluza-Klein gluon resonance. These limits exclude, at the 95% credibility level, Z' bosons with masses 0.70-1.00 TeV as well as 1.28-1.32 TeV and Kaluza-Klein gluons with masses 0.70-1.62 TeV.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 A search for resonances produced in 7 TeV proton-proton collisions and decaying into top-quark pairs is described. contribution 1.00 0.93 0.04 asserted 0.77
s1 In this Letter events where the top-quark decay produces two massive jets with large transverse momenta recorded with the ATLAS detector at the Large Hadron Collider are considered. method 0.90 0.77 0.02 asserted 0.86
s2 Two techniques that rely on jet substructure are used to separate top-quark jets from those arising from light quarks and gluons. method 1.00 0.89 0.02 asserted 1.00
s3 In addition, each massive jet is required to have evidence of an associated bottom-quark decay. method 1.00 0.87 0.02 asserted 0.94
s4 The data are consistent with the Standard Model, and limits can be set on the production cross section times branching fraction of a Z' boson and a Kaluza-Klein gluon resonance. result 0.93 0.93 0.77 hedged possibility 0.67
s5 These limits exclude, at the 95% credibility level, Z' bosons with masses 0.70-1.00 TeV as well as 1.28-1.32 TeV and Kaluza-Klein gluons with masses 0.70-1.62 TeV. result 0.98 0.94 0.81 asserted 1.00

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.82.

What was examined, and what was not

sources read
  • arXiv listing abstract v3
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Distributions of (a) mean HEPTopTagger top-quark candidate mass and (b) mean reconstructed tt¯ mass as a function of the average number of interactions per bunch-crossing, hµi, for data and simulated tt¯ events with the full selection applied. Only statistical uncertainties are shown. – 7 –
Figure 2. Distributions of the reconstructed tt¯mass predicted by MC simulations for (a) Z 0 boson and (b) KK gluon benchmark models with various mass values for the HEPTopTagger analysis with the full selection applied. For each model, σ(pp → Z 0/KK gluon) × BR(Z 0/KK gluon → tt¯) is fixed to 1 pb and an integrated luminosity of 4.7 fb−1 is assumed. – 8 –
Figure 3. The OV3 distributions for the leading jets in the 2 TeV Z 0 → tt¯MC sample, a multijet￾dominated 2011 data sample, and the multijet MC sample. The data and multijet MC distributions are from the samples prior to making any b-tagging or jet mass requirements on either jet, and so are dominated by light quark/gluon jets. The jet mass, mj , defined as the invariant mass of the topoclusters added together as massless four-momenta [51], has been shown to be an effective discriminant between top￾quark jets and light quark/gluon jets, even in the presence of multiple pp interactions [52,…
Figure 4. Pile-up-corrected jet mass distribution in the multijet and tt¯ MC samples for (a) the leading and (b) recoil jets. In both cases, the jet mass requirement has been applied on the opposing jet in the event. The distributions are independently normalised to unit area. – 12 –
Figure 5. The jet mass distributions for the leading (a) and for the recoil (b) jet when all other requirements have been made on the sample except the mass and OV3 requirements on the jet being considered. The fits are described in the text. – 13 –
Figure 6. The distribution of the HEPTopTagger top-quark jet candidate mass in the sideband region Y for data, the templates for multijet background and SM tt¯ production and the fitted sum. To check that the multijet and SM tt¯ background predictions are consistent with the data and to illustrate that the HEPTopTagger identifies top-quark jets effectively, Figs. 7 and 8 show comparisons of predicted and observed distributions in the signal region: of the fat-jet mass (
Figure 7. Signal region distributions of (a) the mass of the leading pT fat jet and (b) the mass of the leading pT top-quark candidate. Also shown are the prediction for SM tt¯ production, the multijet background contribution as estimated from data, and a hypothetical Z 0 boson signal. – 16 –
Figure 8. Signal region distributions of the top-quark candidate substructure variables m23/m123 (a) and arctan(m13/m12) (b). Also shown are the prediction for SM tt¯ production, the multijet background contribution as estimated from data, and a hypothetical Z 0 boson signal. – 17 –
Figure 9. The 16 subsamples into which the Top Template Tagger data are divided, based on whether the leading and recoil jets have a b-quark tag, and on whether they satisfy the Top Template tag requirements of OV3 > 0.7. The jet mass requirement of |mj − mt| < 50 GeV is applied to both jets for all subsamples. The colour coding (in the online version) reflects the anticipated level of expected signal from both SM tt¯ production and possible production of tt¯ states through resonant production: < 0.25% (light green: A,C,E), 0.25 − −10% (shades of yellow: B, D, F–J, O), and > 10% (red: K–N).…
Figure 10. The data-driven prediction of the tt¯ mass distribution for the multijet background in the Top Template Tagger signal region. The points are the average prediction and statistical uncertainties from the five calculations, and the envelope is the range of the predictions in each bin. 8 Systematic uncertainties The following systematic uncertainties are considered and propagated to the predicted mtt¯ distributions for both analyses. These are presented in order of their relative size, with the b-tagging efficiency and the jet energy scale being the two largest sources of systematic…
Figure 11. Transverse momentum distributions for the leading (a) and recoil (b) jets in the Top Template Tagger signal region. Shown are the data distribution, the predicted SM tt¯ contri￾bution and the multijet background contributions as estimated from data. – 21 –
Figure 12. Jet mass distributions (a) for the leading and (b) recoil jets in the Top Template Tagger signal region. Shown are the data distribution, the predicted SM tt¯ contribution and the multijet background contributions as estimated from data. – 22 –
Figure 13. Distributions of the tt¯ invariant mass mtt¯. The HEPTopTagger data, the SM tt¯ background prediction, the multijet background prediction and a hypothetical Z 0 signal with mZ0 = 1 TeV are shown in (a). The Top Template Tagger data, the SM tt¯ background prediction, the multijet background prediction and a hypothetical KK gluon signal with mKKg = 1.6 TeV are shown in (b). Data points show statistical uncertainties only. – 25 –
Figure 14. Expected and observed 95% CL upper limits on the production cross section times branching fraction σ × BR as a function of (a) the Z 0 boson mass and (b) the KK gluon mass for the HEPTopTagger selection. The red bands are the model predictions including theoretical uncertainties. The Z 0 boson leading-order (LO) cross section is multiplied by 1.3 to account for expected higher-order corrections. The KK gluon LO cross section is used. – 29 –
Figure 15. Expected and observed 95% CL upper limits on the production cross section times branching fraction σ × BR as a function of (a) the Z 0 boson mass and (b) the KK gluon mass for the Top Template Tagger selection. The red bands are the model predictions including theoretical uncertainties. The Z 0 boson LO cross section is multiplied by 1.3 to account for expected higher order corrections. The KK gluon LO cross section is used. – 30 –

Figures reproduced from arXiv:1211.2202 by the authors.

References
How the literature uses it

47 arXiv papers cite this in 61 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 40 sentences without a confident label

Uses its method or tool 5 papers
Uses its data or values 5 papers
  • In [ref] , the mis-tag rate for a light quark or gluon fat jet is quoted to be $≈10%$.

    Boosted Top Quarks and Jet Structure · ATLAS analysis · 0.83

  • The latest results from ATLAS analyze $4.7 fb^-1$ of LHC7 data, excluding KK gluon with masses below 1.62 TeV [ref] .

    The Elusive Gluon · Current bounds · 0.94

  • Top tagging has been used at both CMS [ref] and ATLAS [ref] in other types of searches, and from the CMS search we take the efficiency of top tagging to be 50% for tops with $p_T > 500$ GeV.

    Reach in All Hadronic Stop Decays: A Snowmass White Paper · Signal Efficiency · 0.93

Compares with it 1 paper
Cites it as background 9 papers

What it cites · 12 sentences without a confident label

Uses the method or tool of 18 papers
Uses data or values from 2 papers
Compares with 2 papers
Cites as support 2 papers
Cites as background 8 papers

Discussion

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