SciPost Submission Page
Octet scalars shaping LHC distributions in 4-jet final states
by Bogdan A. Dobrescu and Max H. Fieg
Submission summary
| Authors (as registered SciPost users): | Max Fieg |
| Submission information | |
|---|---|
| Preprint Link: | scipost_202511_00048v1 (pdf) |
| Date submitted: | Nov. 20, 2025, 11:39 p.m. |
| Submitted by: | Max Fieg |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approaches: | Theoretical, Phenomenological |
Abstract
We study properties of a hypothetical scalar particle, $\Theta$, which is a color octet and an electroweak singlet. At hadron colliders, $\Theta$ is pair produced through its QCD coupling to gluons, so that its mass determines the cross section. It decays at tree level into $q\bar q$ through dimension-5 operators, and at one loop into gluons. Thus, the main LHC signature of $\Theta$ is a pair of dijets of equal invariant mass. The CMS search in this channel shows a $3.6\sigma$ excess over the QCD background for a dijet mass $M_{jj} \approx 0.95$ TeV, which can be due to $\Theta$: its production cross section (65 fb for a real scalar) and the acceptance of the CMS event selection applied to $p p \to \Theta \Theta \to \! (q \bar q)(q \bar q)$ yield a rate consistent with the excess. Furthermore, the shape of the $d\sigma/d M_{jj}$ signal is in agreement with the CMS result. Given the data-driven background fit performed by CMS, we find that a complex scalar (whose production rate is twice as large) fits better the data than a real scalar. Besides the pair of dijets, testable LHC signals include a trijet-dijet topology, a $t\bar t$ pair plus a dijet resonance, as well as final states involving a Higgs, $W$ or $Z$ boson plus jets.
Author indications on fulfilling journal expectations
- Provide a novel and synergetic link between different research areas.
- Open a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work
- Detail a groundbreaking theoretical/experimental/computational discovery
- Present a breakthrough on a previously-identified and long-standing research stumbling block
Current status:
Reports on this Submission
Report
I can also see that another referee has already provided feedback on the article. I will not comment on the points raised there, but I seriously wonder why SciPost feel the need to scrutinise at this level, when other established journals treat the time of authors and referees with more respect when it comes to article formats like the one under discussion here.
Clearly, some relevant discussions are brushed over, e.g. the flavour constraints could be an interesting avenue to explore for collider-flavour complementarity, especially for the UV-completed dimension-5 couplings. But I understand that the authors do not attempt a complete phenomenological investigation here (yet).
Overall, I find the analysis interesting, compelling, technically sound, and informative for an ongoing discussion and the general hunt for new physics beyond the SM. It provides strong encouragement to ATLAS to update their search. I am therefore happy to recommend the article for publication in its current form.
I did spot two typos ("progresses" -> "progress", "index associate" -> "associated") that the authors might want to address.
Recommendation
Publish (easily meets expectations and criteria for this Journal; among top 50%)
Strengths
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Detailed treatment of phenomenological basis for colour-octet scalar viability.
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Justification of 100% gluon or quark decay-BR approximations, and resulting implication for BSM pheno through lineshape.
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Highlighting of potential need for more detailed experimental studies of this type.
Weaknesses
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Some lack of clarity in the structuring of the background theory, e.g. which aspects differ from the gluon due to mass and which due to spin.
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Despite the good coverage of underpinning theory, the paper is overall motivated by "cherry picking" an excess and fitting to that distribution only; given the availability of broad-scan reinterpretation tools, it seems important to empirically demonstrate that this model would not also be seen in some of the myriad LHC jet distributions.
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Overall statistical compatibility improves only slightly, with the addition of a semi-tuned signal model into a known excess. A better statistical treatment, ideally including the signal model uncertainties, would help to understand the extent to which this is really significant.
Report
I found it interesting that such a simple model, with a strong coupling and no free parameters other than the scalar's mass, could apparently evade constraints through fortuitously "geometrically" suppressed decays. This seemed so remarkable that I would have liked to see a clearer explanation of whether it is the non-zero mass or the lack of spin -- both of which might be expected to enhance rather than suppress effects -- that drives this effect.
However, I did find myself wanting to see a _proof_ that the model with mass set to match the 3.6 (or 2.5 local) sigma CMS significance really cannot be seen in any of the other jet studies available for reinterpretation. In particular, with the FeynRules/MG5 chain already established, it would be a simple and quick due-diligence process to run the model through the public Contur and MadAnalysis tools to verify that there really is no already known excess in preserved jet and BSM measurements.
Without comparison to a more global context, there is a circular "cherry picking" risk in identifying an analysis known to have an excess, and showing it to be compatible with this (admittedly only 1-parameter) model. Even with a quick global comparison of this sort, the assumptions in the model such as suppression of couplings to top quarks should be made clear. I would also like to see referenced publication of the UFO model along with this paper, so others can also investigate.
Finally, the statistical treatment is rather thin at present. It is only presented in terms of changes absolute chi2 values, with an extra degree of freedom in the choice of a mass to more or less fit the excess (this could have been templated and included in the fit for a more precise treatment, that could perhaps explain the oscillating residuals). It is hard from this to draw conclusions about the statistical meaningfulness of the study, given that _some_ fit improvement was more or less guaranteed: even accounting for the approximate model fit and omissions of signal-model uncertainties, the resulting chi2's do not look particularly good, and the resulting residuals in Figs 7 and 10 still look more like fluctuations or oscillations around the background model shape than a convincing fit to the signal model.
Requested changes
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Run the model/generated events through public recast tools as a sanity check to ensure that the identified distribution really is the only one sensitive to this model, and it is not also constrained or contradicted by other existing measurements.
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Clarify mass/spin origins of phenomenological differences from SM gluon properties in production and e.g. decay to qqbar, and the assumptions made in the 4q decay mode and why they were made (again, is this cherry picking?)
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Provide a more informative statistical interpretation, specifically to understand the degree of meaningful improvement in fit given the choices made in model design, and the remaining uncertainties.
Recommendation
Ask for major revision
Anonymous on 2026-01-16 [id 6236]
Response to the Referee We would like to thank the Referee for their constructive comments about our manuscript. Let us respond in turn to each of the three main issues raised by the Referee. At the end of this document we have written the changes made to the manuscript to address these issues.
1) Comparison with the gluon: The Referee raised a profound question: what is the origin of the differences between the scalar studied here and the gluon. After all they are both color-octet electroweak-singlet bosons. The answer is the following. For a spin-1 particle, like the gluon, the coupling to left-handed quarks is both gauge and Lorentz invariant (the same is true for right-handed quarks), so it is natural to be of order one.
By contrast, for a spin-0 electroweak-singlet particle like $\Theta$, Lorentz invariance forces its coupling to fermions to include one left-handed quark and one right-handed quark, which is not allowed by electroweak gauge invariance. Consequently, couplings of $\Theta$ to quarks may arise only through higher-dimensional operators that include a Higgs field, and thus are suppressed, leading to only weak constraints on single-$\Theta$ production. We emphasize that this is a generic mechanism, which relies on the interplay between the gauge and Lorentz symmetries, and not on ``cherry-picking" assumptions.
To clarify this issue, we have now expanded the first paragraph of Section 2.2, and also included a new paragraph in Section 2.2.2 about the constraint from dijet resonance searches on single-$\Theta$ production in the case of coupling to the up quark.
Mass also plays a role, as all experimental constraints go away in the decoupling limit where the mass is very large. The fact that the strongest current experimental constraint on the mass is near the TeV scale and is provided by an LHC search makes it particularly interesting to continue studying this particle. We have now included a statement about the decoupling limit for large mass, at the beginning of Section 2.2.2. We have also included a couple of clarifying sentences above Eq.(3.3) on the $\Theta$ mass bound derived from the CMS search for pairs of dijet resonances.
2) Constraints from other processes: The Referee raises the important question of whether there are other signatures of a TeV-scale color-octet scalar that may have been searched for in other LHC analyses. The most promising channel other than the 4-jet final state is the dijet final state corresponding to single production of $\Theta$. If there are no additional particles, then the cross section for single production is well below the current dijet bounds, as specified in Section 2.1. With the UV completions specified in Section 2.2, effective couplings of $\Theta$ to quarks allow for larger single production of $\Theta$ from an initial quark-antiquark states. However, the higher-dimensional origin of those couplings implies a suppressed cross section for single-$\Theta$ production, which we now discuss in more detail in Section 2.2.
Searches for final states with six jets are clearly less constraining for $\Theta$-pair production than the ones with four jets, and there are no other existing searches that can potentially be sensitive to the $\Theta$ scalar. For this reason we prefer not to use advanced tools such as Contur, which are very useful for models (unlike ours) with several new particles and many complex final states. Nevertheless, we appreciate the issue raised by the referee, and as a result we lengthened and improved the discussion (Section 2.2.2) of additional possible signals, where we also propose a new LHC search for a trijet-dijet topology.
3) Statistical interpretation: The referee asks for a more meaningful statistical interpretation to quantify the significance of the color-octet scalar hypothesis over the background-only hypothesis. To address this, we construct a test statistic as the log likelihood ratio between the two hypotheses and perform a hypothesis test. We find a $p$-value of $1.4\times 10^{-3}$ for the complex scalar and have added text in Section 4 to specify this. Note that our signal model contains minimal assumptions, which we now spell out more explicitly in Section 4. It is worth emphasizing that our estimate of the $p$-value should be checked by the experimental collaborations, which have a much deeper understanding of the various uncertainties. More importantly, our paper may be used as a motivation for future searches in the 4-jet final state.
Our UFO files are now publicly available at the link in Ref.[60]. The changes to the manuscript are listed in the attached PDF file. We believe that with the above changes and additions, prompted by the Referee report, the paper has been satisfactorily improved.
Attachment:
ManuscriptChanges.pdf