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Octet scalars shaping LHC distributions in 4-jet final states

Bogdan A. Dobrescu, Max H. Fieg

TL;DR

The paper investigates TeV-scale color-octet scalars Θ, focusing on their pair production via QCD and decays either to light quark pairs through dimension-5 operators or to gluon pairs via one-loop processes. By analyzing renormalizable Lagrangians, UV completions that generate Θ–quark couplings, and detailed LHC simulations of $pp\to \Theta\Theta\to (jj)(jj)$, the authors assess whether a Θ with $M_\Theta\approx0.95$ TeV can explain the CMS 4-jet excess observed at $\overline{M}_{jj}\approx0.95$ TeV, finding that a real Θ can match the rate and a complex Θ_C can better fit the data due to larger cross sections and distinct jet-mass shapes. They extend the discussion to color-octet complex scalars and triplets, showing how different representations modify production rates and invariant-mass distributions, and they quantify how well these scenarios align with CMS data using detailed χ^2 comparisons in multiple dijet-mair binning schemes. Beyond the CMS excess, the work highlights a rich array of additional LHC signatures such as a trijet-dijet topology, $t\bar t$ plus dijet resonances, and final states with a W, Z, or Higgs boson plus jets, motivating targeted multi-jet searches at current and future LHC runs. The findings illustrate that color-octet scalars constitute compelling multi-jet targets for new physics, with distinctive experimental handles and testable predictions that can be probed in HL-LHC data.

Abstract

We study properties of a hypothetical scalar particle, $Θ$, which is a color octet and an electroweak singlet. At hadron colliders, $Θ$ 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 $Θ$ is a pair of dijets of equal invariant mass. The CMS search in this channel shows a $3.6σ$ excess over the QCD background for a dijet mass $M_{jj} \approx 0.95$ TeV, which can be due to $Θ$: its production cross section (65 fb for a real scalar) and the acceptance of the CMS event selection applied to $p p \to ΘΘ\to \! (q \bar q)(q \bar q)$ yield a rate consistent with the excess. Furthermore, the shape of the $dσ/d M_{jj}$ signal is in agreement with the CMS result. Given the data-driven background fit performed by CMS, we find that a complex scalar 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.

Octet scalars shaping LHC distributions in 4-jet final states

TL;DR

The paper investigates TeV-scale color-octet scalars Θ, focusing on their pair production via QCD and decays either to light quark pairs through dimension-5 operators or to gluon pairs via one-loop processes. By analyzing renormalizable Lagrangians, UV completions that generate Θ–quark couplings, and detailed LHC simulations of , the authors assess whether a Θ with TeV can explain the CMS 4-jet excess observed at TeV, finding that a real Θ can match the rate and a complex Θ_C can better fit the data due to larger cross sections and distinct jet-mass shapes. They extend the discussion to color-octet complex scalars and triplets, showing how different representations modify production rates and invariant-mass distributions, and they quantify how well these scenarios align with CMS data using detailed χ^2 comparisons in multiple dijet-mair binning schemes. Beyond the CMS excess, the work highlights a rich array of additional LHC signatures such as a trijet-dijet topology, plus dijet resonances, and final states with a W, Z, or Higgs boson plus jets, motivating targeted multi-jet searches at current and future LHC runs. The findings illustrate that color-octet scalars constitute compelling multi-jet targets for new physics, with distinctive experimental handles and testable predictions that can be probed in HL-LHC data.

Abstract

We study properties of a hypothetical scalar particle, , which is a color octet and an electroweak singlet. At hadron colliders, is pair produced through its QCD coupling to gluons, so that its mass determines the cross section. It decays at tree level into through dimension-5 operators, and at one loop into gluons. Thus, the main LHC signature of is a pair of dijets of equal invariant mass. The CMS search in this channel shows a excess over the QCD background for a dijet mass TeV, which can be due to : its production cross section (65 fb for a real scalar) and the acceptance of the CMS event selection applied to yield a rate consistent with the excess. Furthermore, the shape of the signal is in agreement with the CMS result. Given the data-driven background fit performed by CMS, we find that a complex scalar fits better the data than a real scalar. Besides the pair of dijets, testable LHC signals include a trijet-dijet topology, a pair plus a dijet resonance, as well as final states involving a Higgs, or boson plus jets.

Paper Structure

This paper contains 9 sections, 21 equations, 10 figures, 1 table.

Table of Contents

  1. Introduction
  2. Interactions of a color-octet real scalar
  3. $\Theta$ scalar and the SM
  4. Renormalizable origins of $\Theta$ couplings to quarks
  5. LHC signals of an octet real scalar
  6. Event generation and overall rate
  7. Invariant mass distributions
  8. Color octets beyond the real scalar
  9. Conclusions

Figures (10)

  • Figure 1: Diagrams responsible for the leading-order $p \, p \to \Theta \, \Theta$ process with initial state gluons. At $\sqrt{s} = 13$ TeV, these account for 91% of the LO production cross section when the mass of the color-octet scalar $\Theta$ is $M_\Theta = 1$ TeV (the remaining 9% is due to $q\bar{q}$ initial states from a diagram similar with the last one shown here).
  • Figure 2: Cross section for $p \, p \to \Theta \, \Theta$ computed with Madgraph Alwall:2014hca at NLO, at center-of-mass energies of 13 TeV (dashed red line), 13.6 TeV (solid blue line) and 20 TeV (gray dotted line). As the process relies on the QCD couplings of the gluons, the cross section depends only on the mass of the color-octet real scalar $\Theta$.
  • Figure 3: Tree-level diagrams that generate the dimension-5 operators (\ref{['eq:eff_ops']}) when the $\Theta_{\rm _D}$ scalar or any of the $\chi$, $\omega$, $\Upsilon$ Vquarks are integrated out.
  • Figure 4: Diagrams for the parton-level 4-jet processes due to production of a $\Theta$ pair. The production part of each diagram represents only one of the four tree-level processes responsible for $p \, p \to \Theta \, \Theta$ (see Figure \ref{['fig:ThetaThetaGluons']}). Left diagram involves only the $\Theta$ decay into gluons through a $\Theta$ loop discussed in Section \ref{['sec:SMT']}, while the right diagram involves only the $\Theta$ decay into quark-antiquark pairs through a higher-dimensional operator (the dark blob contains any of the diagrams in Figure \ref{['fig:UVdiagrams']}), as explained in Section \ref{['sec:SMTc']}.
  • Figure 5: Average dijet (left panel) and 4-jet (right panel) invariant mass distributions for $pp\rightarrow \Theta\Theta\rightarrow (gg) (gg)$, for a color-octet scalar mass $M_\Theta =950$ GeV. Distribution shapes correspond to $\sqrt{s}=13$ TeV, while distribution normalizations are based on 138 fb$^{-1}$ of simulated data and ${\cal B} (\Theta \to gg) = 1$. Solid blue histogram in each panel shows the signal distribution generated by Madgraph Alwall:2014hca at LO, with showering and detector effects simulated with Pythia Sjostrand:2007gs and Delphes deFavereau:2013fsa, respectively, using the CMS event selection CMS:2022usq. For comparison, the dashed red histograms represent the distributions after pairing jets at parton level. The broad left-hand peak in $\overline{M}_{jj}$ is due to mispaired dijets, and includes mainly events with $M_{4j}$ near 2 TeV.
  • ...and 5 more figures