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Massive Color-Octet Bosons: Bounds on Effects in Top-Quark Pair Production

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Ulrich Haisch, Susanne Westhoff

High Energy Physics - Phenomenology · Submitted 2 Jun 2011 · 83 references

Pith's verdict: A solid, comprehensive survey of color-octet constraints, but the headline 26% 'model-independent upper bound' is the best-fit value at M_G = 1.5 TeV, not a maximum over the paper's own 95% CL allowed region.

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arXiv ID1106.0529 (version 2)
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CitationSusanne Westhoff, Ulrich Haisch. "Massive Color-Octet Bosons: Bounds on Effects in Top-Quark Pair Production." arXiv:1106.0529 (2011). https://pith.science/paper/VSAFNXPR
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The paper

The paper claims

A single massive color-octet particle exchanged in the s-channel cannot, on its own, produce the large top-quark forward-backward asymmetry reported at the Tevatron, and the paper puts a ceiling of roughly 26 percent on how big that effect,

The reading

The paper asks a narrow, quantitative question: given everything else we already know, how large an effect can a single heavy color-octet vector boson — a massive cousin of the gluon, generically called an axigluon — have on the production of top-antitop pairs at hadron colliders? The authors survey the three places such a particle would necessarily leave traces: mixing of neutral mesons (which forbids generic flavor-changing couplings unless the couplings are aligned in flavor space), precision measurements of how the $Z$ boson couples to quarks, and direct searches for resonances and altered angular distributions in two-jet events at the LHC. Each of these gives a lower bound on the axigluon mass, and the dijet searches give the strongest one. The payoff is a model-independent statement: if top-antitop production is corrected mainly by s-channel exchange of one such resonance, the high-invariant-mass forward-backward asymmetry cannot exceed about 26 percent, and once the LHC dijet data are imposed the surviving mass window is so narrow that the ceiling drops to about 19 percent. A reader should care because the Tevatron anomaly in that same observable is around 47 percent, so the paper's conclusion is that the simplest single-resonance explanation does not fit, and that the effect should instead show up as a resonance in the top-pair mass spectrum around 1.5 to 2 TeV at the LHC.

What carries the argument

The load-bearing object is the axigluon: a massive spin-one field in the adjoint representation of $SU(3)_c$, arising whenever the color gauge group is extended to a product of two $SU(3)$ factors that break to the diagonal. Its couplings to quarks are parametrized by a mixing angle $theta$, with light and heavy quark couplings $g^ell_L = g^h_R = tan theta$ and $g^h_L = g^ell_R = -cot theta$, so that $g^ell_A g^h_A = -4 sin^2(2theta) < 0$ for $theta in [0^circ, 45^circ]$ — the sign condition needed for a positive shift in the asymmetry. The machinery that carries the bound is a set of approximate analytic formulas (Appendix C) expressing the symmetric and asymmetric top-pair cross sections as polynomials in $1/M_G$ multiplying three coupling combinations $c^{pm}_{gG}$, $c^{pm}_{GG}$, and $c_{GG}$, which separate gluon–axigluon interference from axigluon self-interference. Because the symmetric and asymmetric observables are positively correlated through the same couplings, the fit cannot push the asymmetry up without also pushing the total and high-mass cross sections out of agreement with data. That correlation, not any single measurement, is what produces the ceiling.

Extended reading notes

Core claim

The central claim is an upper bound on the top-quark forward-backward asymmetry $A^t_{FB}$ in the high-$M_{tbar t}$ bin, derived under the assumption that the dominant new-physics correction to $qbar q to tbar t$ comes from s-channel exchange of one massive color-octet vector. The authors first establish the indirect and direct constraints. Flavor alignment can push the dangerous tree-level flavor-changing neutral currents into the up-quark sector, leaving only the weak bound $M_G > 0.22$ TeV from $D$–$bar D$ mixing (Eq. 58), which any flavor non-universal axigluon must satisfy. The $Z to qbar q$ observables give $M_G > 0.54$ TeV for the non-universal and $M_G > 0.40$ TeV for the universal case (Eqs. 59, 60), and the oblique parameters give $M_G > 0.16$ TeV in the universal case (Eq. 62). The strongest limits come from the LHC: narrow-resonance dijet searches exclude $0.6$–$1.9$ TeV (non-universal) and $0.6$–$2.1$ TeV (universal) for widths below about 15 percent, and the dijet angular distribution excludes $0.17$–$1.7$ TeV for broad resonances as well. The charge-symmetric top-pair observables then exclude $0.2$–$1.3$ TeV independently of flavor universality. Fitting all four top-pair observables, the best point is $M_G = 1.5$ TeV with $theta = 45^circ$, giving $chi^2/text{ndf} = 7.6/4$ against $13.1/4$ for the Standard Model, with $A^t_{FB} = 16.7%$ and $(A^t_{FB}(M_{tbar t}))_> = 25.7%$. Imposing the dijet constraints shrinks the allowed window to $1.9$–$2.0$ TeV, where the same asymmetries fall to $13.1%$ and $19.1%$. From these numbers the authors conclude that a single color-octet s-channel resonance cannot raise $(A^t_{FB}(M_{tbar t}))_>$ above about 26 percent, well short of the measured $47.5 pm 11.2$ percent.

Load-bearing premise

The numerical ceiling depends on converting the ATLAS dijet limits into axigluon bounds through an acceptance rescaling factor that assumes the detector acceptance depends only on the resonance mass and not on its decay width, an assumption the authors flag as unverified.

Editorial extensions

If this is right

  • Any single color-octet s-channel resonance explaining the Tevatron top asymmetry must sit in the window around 1.9 to 2.0 TeV, where the high-mass asymmetry reaches at most about 19 percent.
  • Such a resonance should appear as a visible bump in the top-antitop invariant mass spectrum at the LHC between 1.5 and 2 TeV, enhancing the tail above 1 TeV by a factor of roughly 1.3 to 2.5 relative to QCD.
  • The bound can be loosened by rescaling the couplings so that the light-quark couplings shrink and the top coupling grows; a suppression factor of about 0.85 in the light-quark couplings is enough to revive the 1.5 TeV best-fit point.
  • A flavor-universal axigluon is excluded at 99.5 percent confidence by the top-pair fit unless its mass exceeds about 3 TeV, because its contribution to the asymmetry interferes destructively with QCD.
  • Flavor non-universality in a color-octet model is only viable if the tree-level flavor-changing couplings are aligned away from the down-quark sector; otherwise neutral meson mixing pushes the mass into the multi-TeV range.

Reading between the lines

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

  • The 26 percent ceiling is a statement about a single resonance; it suggests that if the Tevatron asymmetry is real and not a fluctuation, the explanation likely needs either two nearly degenerate color-octet states whose symmetric contributions cancel, or a t-channel exchange rather than an s-channel one.
  • Because the ceiling comes from the positive correlation between the symmetric cross section and the asymmetry, any measurement that tightens the high-mass top-pair cross section at the LHC will tighten the asymmetry bound without needing a dedicated asymmetry analysis.
  • The narrow surviving window near 2 TeV is close to the reach of early LHC dijet and top-pair resonance searches, so the hypothesis was decidable within a few years of the paper's writing rather than requiring a major luminosity upgrade.
  • The paper's treatment of the dijet acceptance rescaling assumes the detector acceptance depends only on the resonance mass and not on its width; if that assumption fails for broad resonances, the strongest exclusion and hence the numerical ceiling would move.
Original abstract

A critical survey of the existing direct and indirect constraints on massive spin-one color octets is presented. Since such new degrees of freedom appear in any extension of the color gauge group to a product of at least two SU(3) factors, we keep our discussion as independent as possible from the underlying theory. In the framework of scenarios that involve flavor non-universal couplings, we show that excessive flavor-changing neutral currents can be avoided by a suitable alignment in flavor space. Constraints from electroweak precision observables and direct production at hadron colliders still leave space for sizable new-physics effects in top-quark pair production, in particular a large forward-backward asymmetry. In this context, we derive a model-independent upper bound on the asymmetry that applies whenever top-antitop production receives the dominant corrections from s-channel exchange of a new single color-octet resonance.

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, 4,366 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.57 · human participants 0.01 · controlled comparison 0.04 · randomized assignment 0.01 · states quantitative result 0.37 · compares with prior results 0.49 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; .

A critical survey of the existing direct and indirect constraints on massive spin-one color octets is presented. Since such new degrees of freedom appear in any extension of the color gauge group to a product of at least two SU(3) factors, we keep our discussion as independent as possible from the underlying theory. In the framework of scenarios that involve flavor non-universal couplings, we show that excessive flavor-changing neutral currents can be avoided by a suitable alignment in flavor space. Constraints from electroweak precision observables and direct production at hadron colliders still leave space for sizable new-physics effects in top-quark pair production, in particular a large forward-backward asymmetry. In this context, we derive a model-independent upper bound on the asymmetry that applies whenever top-antitop production receives the dominant corrections from s-channel exchange of a new single color-octet resonance.

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 critical survey of the existing direct and indirect constraints on massive spin-one color octets is presented. contribution 0.92 0.65 0.05 asserted 0.92
s1 Since such new degrees of freedom appear in any extension of the color gauge group to a product of at least two SU(3) factors, we keep our discussion as independent as possible from the underlying theory. background 0.48 0.38 0.03 asserted 0.95
s2 In the framework of scenarios that involve flavor non-universal couplings, we show that excessive flavor-changing neutral currents can be avoided by a suitable alignment in flavor space. result 0.55 0.85 0.05 asserted 0.99
s3 Constraints from electroweak precision observables and direct production at hadron colliders still leave space for sizable new-physics effects in top-quark pair production, in particular a large forward-backward asymmetry. result 0.40 0.23 0.43 asserted 0.75
s4 In this context, we derive a model-independent upper bound on the asymmetry that applies whenever top-antitop production receives the dominant corrections from s-channel exchange of a new single color-octet resonance. result 0.85 0.95 0.70 asserted 1.00

What kind of paper the classifier reports this is: review 0.53 · evidence from literature 0.53.

What was examined, and what was not

sources read
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not examined
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  • figures and tables
  • references and citing sentences
  • relations to other papers
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  • P abstract
not run
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  • T body sentences
  • C citations
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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. Tree-level axigluon contribution to the mixing of neutral m
Figure 2. One-loop axigluon contribution to the Zqq¯ vertex. for the complex effective couplings G q P is given in Appendix A. From the limiting behavior (A3), we see that non-logarithmic corrections to the formula (28) are not fully negligible for MG = O(1 TeV). We will therefore use the full real parts of G q P in our numerical analysis. In the case of the top quark, on the other hand, the axigluon corrections to the Z-boson vertex are contained in the Wilson coefficients of the effective operators O 1 QP φ = i(Q¯ P γ µ τ iQP )φ † τ iDµφ , O 2 QP φ = i(Q¯ P γ µQP )φ †Dµφ . (29) Evaluating…
Figure 3. Regions of 68%, 95%, and 99% probability in the
Figure 4. Regions of 68%, 95%, and 99% probability in the Γ
Figure 5. Two-loop axigluon contributions to the oblique parameters
Figure 6. Regions of 68%, 95%, and 99% probability in the
Figure 7. Predictions for resonant axigluon production as a functio
Figure 8. Ratio Fχ(Mjj ) as a function of Mjj . The black line and gray band represent the central value of the QCD prediction with its total error. The ATLAS data points with their statistical uncertainties are overlaid as black error bars. The blue, aqua￾marine, and green lines correspond to QCD plus the effects of a flavor non-universal axigluon with MG = 1.5 TeV and θ = 45◦ for three different theoretical treatments. The first two curves are obtained in an effective theory, while the third one corre￾sponds to the exact result. The latter prediction employs an axigluon propagator with a…
Figure 9. Constraints in the MG – θ plane imposed by the measured amount of flavor violation in the ∆F = 2 sector. The left (right) panel corresponds to the scenario of up-type (down-type) flavor alignment in the non-universal axigluon model. The regions of parameter space colored white are disfavored by the existing experimental data. limit reads MG > |g h L − g ℓ L | 2 6.4 TeV = 6.4 TeV sin (2θ) > 6.4 TeV , (57) where in the last step we have inserted the analytic expressions for the left-handed couplings as given in (4). The 95% CL upper limits arising from |ǫK| and ∆mBd are weaker than…
Figure 10. Constraints in the MG – θ plane arising from the precision measurements in the Z → qq¯ sector. The left (right) panel corresponds to the flavor non-universal (uni￾versal) axigluon model. The magenta colored regions of parameter space are preferred by the global χ 2 fit to Rb, Ab, Ab FB, ΓZ, and σhad. Parameter sets below the dashed magenta curve are disfavored by the bottom-quark POs alone. We emphasize that since the latter bound has been obtained by minimizing the constraints from flavor violation in the ∆F = 2 sector, it is a firm (though quite loose) exclusion limit that has to…
Figure 11. Constraints in the MG – θ plane due to a combined fit to the oblique param￾eters S and T. The left panel shows the results for the case of a flavor non-universal axigluon assuming S4 = 0.15 and T4 = 0.19, while the right plot represents the case of a flavor universal axigluon. The parameter space preferred by the fit is colored blue. to zero for θ → 45◦ , which further impairs the restrictive power of the bottom-quark POs. While the restrictions imposed by Rb, Ab, and A b FB can thus be avoided, combining them with ΓZ and σhad, as defined in (34), turns out to lead to non-trivial…
Figure 12. Constraints from the latest ATLAS narrow resonance s
Figure 13. Constraints on MG –θ arising from the angular distribution in dijet produc￾tion in the non-universal (left) and universal (right) axigluon model. The green areas show the parameter region in accordance with the ATLAS data at 95% CL. dijet resonance bounds (63) and (64) are robust. We also remark that the dependence of (63) on the masses of the fourth-generation quarks is rather mild. If instead of mu4 = 311 GeV and md4 = 372 GeV one would use mu4 = md4 = 600 GeV, the former bound would strengthen slightly and read MG > 2.0 TeV. The angular distribution in dijet production allows…
Figure 14. Left: Ratio Fχ(Mjj ) as a function of Mjj for three different axigluons with MG = 0.3 TeV (red line), MG = 0.6 TeV (green line), and MG = 1.2 TeV (blue line). The shown predictions correspond to QCD-like couplings and ΓG/MG = 10%. For comparison the QCD prediction with errors (black line and gray band) and the ATLAS measurement (black error bars) are also shown. Right: Constraints from the latest ATLAS narrow resonance search in dijet production on a color-octet boson with relative width ΓG/MG that only couples to up quarks. The blank region is excluded at 95% CL for g u L = g u R…
Figure 15. Predictions for the observables in tt¯production obtained in the non-universal axigluon model. From the upper left to the lower right σs, (dσs/dMtt¯) >,
Figure 16. Constraints in the MG –θ plane imposed by the charge-symmetric observ￾ables in tt¯ production in the flavor non-universal (left) and universal (right) axigluon model. The regions below the orange area are excluded at 95% CL. sponding MG –θ planes are displayed in the left and right panels of
Figure 17. Confidence regions in the MG –θ plane resulting from a fit to the tt¯ ob￾servables in the flavor non-universal (left) and universal (right) case. For comparison the constraints from the dijet resonance search (angular distribution) is also shown in the left panel as a dashed (solid) black line. The areas to the left of the curves are disfavored at 95% CL. data at 95% CL (90% CL) is only achieved for a flavor non-universal axigluon with a mass in the range of 1.3 TeV . MG . 2.1 TeV (1.4 TeV . MG . 1.5 TeV). In the global fit, the lower bound on the axigluon mass is essentially…
Figure 18. Predictions for the tt¯ observables in the flavor non-universal axigluon model (thick black lines and numbers) corresponding to the best fit before (left) and after (right) including dijet constraints. For comparison the central values of the associ￾ated measurements (dashed black lines) are also shown. All predictions have been normalized to the SM expectations. The light colored bars illustrate the theoretical uncertainties, while the combined experimental and theoretical errors as displayed in bright colors. See text for further details. that have been discussed above. Large…
Figure 19. Left: Prediction for the tt¯ forward-backward asymmetry at the Tevatron as a function of Mtt¯. The red (blue) curve represents the result for a flavor non-universal axigluon with θ = 45◦ and MG = 1.5 TeV (MG = 1.9 TeV). The central value and uncertainty of the QCD expectation are indicated by the black line and the gray band, respectively. Right: Prediction for the tt¯cross section differential in Mtt¯ for pp collisions at √ s = 7 TeV. The red (blue) line corresponds to a flavor non-universal axigluon with θ = 45◦ and MG = 1.5 TeV (MG = 1.9 TeV). For comparison the SM expectation…
Figure 20. Bounds on MG in the flavor non-universal (left) and universal (right) ax￾igluon model. Colored regions are excluded at 95% CL by D–D¯ mixing (purple), Z → qq¯ (magenta), S and T (blue), σs and (dσs/dMtt¯) > (orange), dijet angular distri￾butions (green) and resonances searches (red) at the LHC, and by the global fit to the Tevatron tt¯ data (yellow). The most stringent constraints on the axigluon parameter space stem from the production of massive color-octet bosons decaying to dijets at the LHC. Searches for narrow resonances in the dijet invariant mass spectrum exclude the mass…

Figures reproduced from arXiv:1106.0529 by the authors.

References
How later papers used it

50 later papers cite it

Evidence: substantial. 41 of the citing papers share no author with it, and 6 of those carry a confident label from Pith's citation classifier.

Pith read 19 of the 81 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its method or tool 1 of 2 papers with this label

  • Estimated the impact on R_b caused by Z_C in Z decay according to formulas in this paper, adopting its own best-fit parameters.

    New color-octet axial vector boson revisited (2011) · 0.91 · paraphrased: the licence does not allow quoting

How the literature uses it

50 arXiv papers cite this in 81 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 · 53 sentences without a confident label

Contests it 1 paper
  • The axigluon model, where the couplings of axigluon to light quarks and heavy quarks are both at the strong coupling level [ref] , is strongly constrained by the dijet [ref] measurements; indeed, the axigluon model proposed in [ref] is already excluded by the latest dijet search by ATLAS [ref] .

    New color-octet axial vector boson revisited · Introduction · contest 0.76

Uses its method or tool 2 papers
Uses its data or values 2 papers
  • For example, using more recent values from Ref. [ref] the change in the coefficient is 0.1%.] [equation] where $x_tW = m̅_t^2(μ_t)/M_W^2$, $x_t3 = m̅_t^2(μ_t)/m_3^2$, and we neglect $M_Z$ in the loop calculation.[Full expressions including the $M_Z$ dependence have been given in Refs. [ref] .

    Indirect constraints on the Georgi-Machacek model and implications for Higgs couplings · $R_b$ · 0.85

  • A tree-level exchange of $X_μ$ can be matched to the four-fermion operator [ref] [equation] with the Wilson coefficient [equation] Using the requirement $Λ> 1.1 ×10^3$ TeV from Re $C_K^1$ and $Λ> 2.2 ×10^4$ TeV from Im $C_K^1$ [ref] , we can constrain the maximum size of $λ̃_11$, $λ̃_33$, assuming the other is vanishing: [equation] We see that these constraints are easily satisfied for $𝒪(TeV)$ scale color octets as long as $λ̃ ∼𝒪(0.1)$.

    Collider constraints and new tests of color octet vectors · Non-universal couplings and flavor constraints · 0.89

Compares with it 2 papers
Cites it as background 19 papers
  • The other observables like S, T parameters and the Z decay width effects further constraint the color-octet vector boson model but these studies are beyond the scope of this article, detail studies can be found in [ref] .

    Top quark physics in the Vector Color-Octet Model · Consistency with the $tt̅$ and dijet production at the LHC · 0.95

  • Another kind of widely discussed NP is the s-channel color-octet axial-vector boson [ref] , where the new boson induces a new s-channel $qq̅→tt̅$.

    New color-octet axial vector boson revisited · Introduction · 0.98

  • Before considering a more motivated model, let us review the impact that a massive gluon $G$ may have on $tt̅$ physics in the simplest cases (for a related discussion see [ref] ).

    The Higgs Boson and New Physics at the TeV Scale · Yukawa couplings to fermions · 0.92

What it cites · 60 sentences without a confident label

Uses the method or tool of 4 papers
Uses data or values from 21 papers
Compares with 5 papers
Cites as background 13 papers

Discussion

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