Possible mixing between elementary and bound state fields in the $t\bar{t}$ production excess at the LHC

TL;DR

The study addresses a near-threshold $t\\bar{t}$ production excess observed at the LHC by proposing mixing between a toponium bound state $\\eta_t$ and an elementary field $\\Psi$, analyzed within a BHL compositeness framework under the Multicritical Point Principle (MPP). It derives one-loop RGEs and an effective potential, applying MPP at a high scale $\\mu_c$ (\\lesssim 10^{12.3} GeV) and BHL boundary conditions at a cutoff $\\Lambda$ (370–400 GeV) to constrain the Yukawa couplings and the mixing angle; the framework is explored in two realizations: a minimal mixing scenario and an embedding into 2HDM Types II/Y. The results yield mixing bounds $|\\theta|\\le 13^\circ$ in the minimal case and $|\\theta|\\le 1^\circ$ in the 2HDM scenarios, with a toponium mass near 345 GeV and near-degenerate heavier states, under the assumed scales. The analysis suggests the minimal scenario is more natural and testable, while the 2HDM embedding tends toward decoupling; future work should include higher-order corrections and broader collider and flavor constraints to sharpen tests of the proposed link between the $t\\bar{t}$ excess and new scalar/bound-state mixing.

Abstract

Recent report by CMS Collaboration on the excess of top and anti-top pair production is studied, under the hypothesis of the coexistence of a toponium $(η_t)$ and an additional elementary field $(Ψ)$. We examine the scenario where toponium and an additional field are mixed, and consider the plausible scenarios in that case. Two scenarios are examined: one is the minimal model with $Ψ$ close to the inert Higgs doublet, and the other is embedded into the two Higgs doublet models (2HDM), where $Ψ$ is one of the two Higgs scalars after transforming the basis. The value of the each coupling constant is restricted by the Multicritical Point Principle (MPP). Consistency with the data gives constraints on a mixing angle $θ (-45^\circ\leθ\le45^\circ)$, with which the mass eigenstate $Ψ^\prime$ contributing to the excess is defined by $Ψ^\prime=Ψ\cos θ+ η_t\sin θ$. The obtained results are $|θ| \le 13^{\circ}$ for the minimum scenario, and $|θ| \le 1^{\circ}$ for the second scenario of 2HDM(Type II and Y). We also briefly discuss the comparison with Type I and X.

Figures (2)

• Figure 1: The $x$-axis and $y$-axis show $\log_{10}(\frac{\mu}{\mathrm{GeV}})$ and the value of each line. The blue line and orange line are $\frac{V_{\mathrm{eff}}}{\mu^4}$ and $\frac{1}{\mu^3}\frac{dV_{\mathrm{eff}}}{d\mu}$ in the case of $M_t=172.69\ \mathrm{GeV},\ \alpha_s(M_Z)=0.1189, \Lambda = 400\ \mathrm{GeV},\ y_\Psi(M_t) = 0.3,$ and $\kappa_1(M_t) =0.29$. The red line shows zero on the $y$-axis. It can be seen that the MPP condition is satisfied around $\mu_c=10^{12.3}$ GeV.
• Figure 2: The production and decay process involving the mixed mass eigenstates is $gg\rightarrow R \rightarrow t\bar{t}$. $R= \eta_t^\prime,\ \Psi^\prime$