Pseudo-axions in Little Higgs models

TL;DR

This work analyzes pseudoscalar pseudo-axions (η) that arise generically in Little Higgs models due to broken U(1) subgroups. It characterizes η interactions—primarily Yukawa-coupled to fermions and anomaly-induced couplings to gauge bosons—and computes mass/decay patterns across three representative models: Littlest Higgs, the μ model, and the original Simple Group model. The authors explore rich phenomenology, showing that η can significantly affect heavy T-quark decays and can be directly produced at hadron colliders via gluon fusion, with potential diphoton signals particularly in the μ model, though visibility is model- and parameter-dependent. They extend the discussion to future colliders, including Linear and photon colliders, where ttη production and γγ→η resonant production offer complementary search channels, and conclude that η detection would provide a direct probe of the UV structure of Little Higgs theories.

Abstract

Little Higgs models have an enlarged global symmetry which makes the Higgs boson a pseudo-Goldstone boson. This symmetry typically contains spontaneously broken U(1) subgroups which provide light electroweak-singlet pseudoscalars. Unless such particles are absorbed as the longitudinal component of $Z'$ states, they appear as pseudoscalars in the physical spectrum at the electroweak scale. We outline their significant impact on Little Higgs phenomenology and analyze a few possible signatures at the LHC and other future colliders in detail. In particular, their presence significantly affects the physics of the new heavy quark states predicted in Little Higgs models, and inclusive production at LHC may yield impressive diphoton resonances.

Figures (9)

• Figure 1: Left: $M_T$ (solid) v. $\lambda_1$ in the Littlest Higgs model with $F=4\,{\rm TeV}$. Also plotted is $\lambda_2$ (dashed), which is constrained by the choice of $\lambda_1$ and the known value $m_t$. Right: Heavy $T$ quark branching ratios to $Wb$ (green), $tZ/th$ (blue), and $t\eta$ (red), for three choices of the $U(1)_\eta$ charge differences $\beta_{0,1,2}$: 1,1,1 (solid); 1,0,1 (dashed); 0,0,1 (dotted).
• Figure 2: Left: $m_\eta$ v. $\mu$ for four choices of the ratio $F_1/F_2$: 1 (solid), 1/2 (dashed), 1/4 (dot-dashed), and 1/6 (dotted). Middle: $m_H$ v. $\mu$ for fixed $F_2=2.0$ TeV and various choices of $F_1/F_2$: 1 (solid), 1/2 (dashed) and 1/4 (dot-dashed). Right: $m_H$ v. $\mu$ for fixed $F_1/F_2=1/4$ and various $F_1$ [TeV]: 0.5 (solid), 1.0 (dashed), 1.5 (dot-dashed), and 2.0 (dotted).
• Figure 3: $\eta$ branching ratios in the $\mu$ model for the Golden Point, as discussed in the text, as a function of $m_\eta$.
• Figure 4: Diphoton signal for $gg\to\eta\to\gamma\gamma$ at the LHC, shown as individual points which reflect the total signal cross section for each parameter space choice. The symbols for the Littlest Higgs points are shown in the plot legend along with the chosen $\beta_i$ sets. The symbols for the $\mu$ model points are shown with their corresponding choices for $F_1,F_2$. Two data points are shown for each value of $m_\eta$ in the Littlest Higgs model, representing the extremal allowed values of $\lambda_1$: $\lambda_t$ and $\sqrt{2}\lambda_t$. We have applied the known NLO K-factors to the signal cross sections. The continuum diphoton invariant mass distribution is shown as a differential cross section histogram with 4 GeV bins. It includes the direct photon, 1-fragmentation and 2-fragmentation contributions at NLO. All samples include kinematic cuts as described in the text, and an ID efficiency factor $\epsilon_\gamma=0.8$ for each photon.
• Figure 5: The $t\bar{t}\eta$ cross section at a 1 TeV linear collider, for three different values of $g_{t\bar{t}\eta}$.