[Paper Review] $\boldsymbol{SU(3)_{C}\otimes SU(3)_{L}\otimes U(1)_{X}}$ models in view of the $750$ GeV diphoton signal

R. Martı́nez, F. Ochoa+1 more|arXiv (Cornell University)|Jun 10, 2016

Particle physics theoretical and experimental studies10 references3 citations

TL;DR

This paper investigates $SU(3)_C \otimes SU(3)_L \otimes U(1)_X$ models as a framework to explain the 750 GeV diphoton excess observed by ATLAS and CMS. It analyzes loop-level diphoton decays mediated by exotic charged vector bosons, charged Higgs bosons, and top-/bottom-/electron-like exotic fermions, showing that interference effects significantly alter production cross sections and that models with $\beta = \pm 1/\sqrt{3}$ and $\beta = \pm \sqrt{3}$ can accommodate the signal under specific mass and coupling constraints, particularly when exotic quark masses exceed 900 GeV and Yukawa couplings are small.

ABSTRACT

We analyze the recent diphoton signal reported by ATLAS and CMS collaborations in the context of the $SU(3)_{C}\otimes SU(3)_{L}\otimes U(1)_{X}$ anomaly free models, with a 750 GeV scalar candidate which can decay into two photons. These models may explain the 750GeV signal by means of one loop decays to $γγ$ through charged vector and Higgs bosons, as well as top-, bottom- and electron-like exotic particles that arise naturally from the condition of anomaly cancellations of the $SU(3)_{C}\otimes SU(3)_{L}\otimes U(1)_{X}$ models.

Motivation & Objective

• To assess the viability of $SU(3)_C \otimes SU(3)_L \otimes U(1)_X$ models in explaining the 750 GeV diphoton resonance observed by ATLAS and CMS.
• To determine whether the diphoton decay of a 750 GeV scalar boson can be mediated through loop diagrams involving exotic vector bosons, charged Higgs bosons, and exotic fermions.
• To evaluate the impact of interference effects between fermionic, vector bosonic, and Higgs contributions on the diphoton production cross section.
• To constrain model parameters such as exotic quark masses and Yukawa couplings based on the observed 6 fb cross section and 45 GeV width of the resonance.

Proposed method

• The study employs the $SU(3)_C \otimes SU(3)_L \otimes U(1)_X$ gauge group with spontaneous symmetry breaking via scalar triplets $\chi$, $\rho$, and $\eta$, leading to massive exotic states.
• Fermion content is determined by the $\beta$ parameter in the electric charge generator $Q = T_3 + \beta T_8 + X I$, with $\beta = \pm \sqrt{3}$ and $\pm 1/\sqrt{3}$ considered.
• The diphoton decay amplitude is computed at one-loop level using form factors $F_0$, $F_{1/2}$, and $F_1$, incorporating contributions from exotic fermions $\psi$, charged vector bosons $K^\pm$, and charged Higgs bosons $h^{\pm}$ or $h^{\pm\pm}$.
• Interference effects between different particle contributions in the loop are systematically evaluated using the full amplitude expression, including sign differences between fermionic and bosonic contributions.
• Constraints are applied using the observed diphoton cross section of 6 fb and width of 45 GeV, with limits from LHC on exotic quark masses.
• Numerical contours of the production cross section $\sigma(pp \to \xi_\chi \to \gamma\gamma)$ are generated for different $\beta$ values and model variants (A and B), varying masses and couplings.

Experimental results

Research questions

• RQ1 Can $SU(3)_C \otimes SU(3)_L \otimes U(1)_X$ models explain the 750 GeV diphoton resonance through one-loop decays into two photons?
• RQ2 How do interference effects between exotic fermions, charged vector bosons, and charged Higgs bosons modify the diphoton production cross section in these models?
• RQ3 What are the allowed parameter ranges for exotic quark masses and Yukawa couplings that reproduce the observed 6 fb cross section and 45 GeV width?
• RQ4 Why does the model with $\beta = \sqrt{3}$ exhibit the smallest Yukawa coupling values among the considered $\beta$ choices?
• RQ5 Under which conditions does the inclusion of vector boson contributions exclude the model despite acceptable fermionic and Higgs contributions?

Key findings

• For $\beta = 1/\sqrt{3}$, the model remains viable with exotic quark masses $m_T > 900$ GeV and Yukawa couplings $h/4\pi < 0.5$, particularly when only fermionic contributions are considered.
• For $\beta = -1/\sqrt{3}$, the model allows for allowed regions with $m_T > 900$ GeV when including both fermionic and charged Higgs contributions, though vector boson contributions strongly suppress the cross section.
• The model with $\beta = \sqrt{3}$ is excluded for $h/4\pi < 0.5$ and $m_T > 1.8$ TeV due to dominant destructive interference from vector bosons.
• The model with $\beta = -\sqrt{3}$ shows similar behavior, with strong suppression from vector boson contributions, but an allowed region emerges when all contributions are included.
• Interference effects are most significant for vector bosons ($K^\pm$), which produce the largest suppression, while charged Higgs contributions have the smallest effect.
• The green (fermionic-only) and blue (full loop) lines in the figures show that the full amplitude can restore allowed regions even when individual contributions are excluded, highlighting the importance of interference.