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
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