Strong-Weak Bi-Adjoints, Gluon-W resonances, and new asymmetric LHC production processes

We study a novel production and decay mechanism for a bi-adjoint spin zero particle in the ($8,3,0$) representation of the Standard Model gauge groups. This work is part of a series studying new production processes for exotic particles in higher representations of Standard Model color and weak charge. Here, we study a specific new dimension 5 effective operator model which couples an exotic bi-adjoint scalar field to the SU(3) and SU(2) field strength tensors. The W-gluon resonant decay of the charged component of this new exotic field is explored. We discuss LHC production modes of both the charged and neutral bi-adjoint states and find a new single production mode for the charged state. This is the dominant LHC production mode for TeV-scale masses. We introduce a new HL-LHC search in which a single bi-adjoint is produced in association with a hard forward quark jet from asymmetric W-gluon fusion. This search yields a 5-sigma discovery potential for bi-adjoint masses up to 3 TeV for 10 TeV scalar effective operator cutoffs. We also find 2-sigma sensitivity at the HL-LHC for bi-adjoint masses up to 4 TeV and effective cutoffs in the 15 TeV range for the full 3 inverse atto-barn data set.

💡 Research Summary

The paper introduces a novel phenomenological framework for a scalar bi‑adjoint field ϕ transforming as (8, 3, 0) under the Standard Model gauge group SU(3)×SU(2)×U(1). Unlike the more commonly studied color octet scalars (8, 1, 0) or the Manohar‑Wise doublet octet (8, 2, 1), this particle carries both a color octet and a weak isospin triplet charge, and it has zero hypercharge. At the renormalizable level the field has only gauge‑kinetic and mass terms; direct couplings to quarks are forbidden by hypercharge conservation, and single production via gluon fusion first appears only at dimension‑7 and is therefore highly suppressed.

The authors focus on the lowest‑dimension operator that can connect ϕ to Standard Model fields: a dimension‑5 effective interaction

 L = (1/Λ) ϕ^{ij} W_{μν}^{ij} G^{μν},

where W_{μν} and G^{μν} are the SU(2) and SU(3) field‑strength tensors, i,j are SU(2) adjoint indices, and Λ is the cutoff scale. After electroweak symmetry breaking this operator yields three‑point vertices that couple the charged components ϕ^{±} to a W^{∓} boson and a gluon, and the neutral component ϕ^{0} to a gluon together with either a photon or a Z boson. Consequently the dominant decay modes are

 ϕ^{±} → W^{±} g (100 % if no mass splitting)  ϕ^{0} → γ g, Z g.

The decay width scales as Γ ∼ m_ϕ³/Λ², remaining narrow for Λ≫m_ϕ, which validates the EFT description in the region of interest.

A key insight of the work is that the same dimension‑5 operator enables a new single‑production mechanism that dominates over gluon‑fusion for TeV‑scale masses. The process is an asymmetric W‑gluon fusion:

 q g → q ϕ^{±},

where a quark radiates an off‑shell W boson that fuses with a gluon to produce the charged bi‑adjoint, while the outgoing quark appears as a hard forward jet. This topology is distinct from the usual s‑channel resonant production and benefits from a characteristic forward jet (large rapidity, high p_T) that can be used as a trigger and to suppress Standard Model backgrounds.

The authors implement the model in MadGraph5_aMC@NLO, generate signal and background events (W+jets, tt̄, QCD multijets) with Pythia8 and Delphes, and design a set of cuts: a forward jet with η > 2.5 and p_T > 200 GeV, reconstruction of the W‑gluon invariant mass within ±10 % of the hypothesized m_ϕ, and a lepton from the W decay. After optimization, the signal‑to‑background ratio is sufficient to claim discovery (5σ) for a bi‑adjoint mass of up to 3 TeV when the cutoff Λ≈10 TeV, assuming the full HL‑LHC integrated luminosity of 3 ab⁻¹. Even for higher cutoffs (Λ≈15 TeV) and masses up to 4 TeV, a 2σ sensitivity is achievable.

The paper situates these results in the broader context of exotic color‑octet phenomenology. It emphasizes that the (8, 3, 0) representation opens a “W‑gluon portal” – a sector where new particles couple primarily to pairs of Standard Model gauge bosons rather than to fermions. This portal is protected by hypercharge conservation, making the dimension‑5 operator the leading interaction. The authors also discuss the impact of possible Higgs‑portal couplings (dimension‑4) on loop‑induced Higgs‑gluon and Higgs‑photon couplings, noting that setting these couplings to zero yields a clean benchmark where the bi‑adjoint does not affect existing Higgs measurements.

In conclusion, the study provides a concrete, theoretically motivated scenario where a bi‑adjoint scalar can be produced singly via an asymmetric W‑gluon fusion process, decay into a distinctive W‑gluon resonance, and be discovered at the HL‑LHC up to multi‑TeV masses. The analysis demonstrates that forward‑jet tagging combined with invariant‑mass reconstruction offers a powerful probe of this otherwise elusive sector, and it opens a new direction for searches involving higher‑dimensional gauge‑boson portals in future high‑energy colliders.