Decays of the Littlest Higgs Z_H and the Onset of Strong Dynamics

TL;DR

This paper investigates the decay properties of the Z_H gauge boson in the Littlest Higgs model, revealing signs of strong dynamics and potential breakdown of perturbation theory as the model approaches certain parameter regimes.

Contribution

It provides analytical formulas for Z_H decay widths, catalogs three-body decay modes, and identifies the onset of strong coupling effects in the Littlest Higgs model.

Findings

The partial width of Z_H to Z_L H H exceeds Z_H to Z_L H for certain mixing angles.

Decay widths increase significantly as the model approaches strong coupling.

Analytical expressions for all two-body Z_H decays are derived, including mass effects.

Abstract

The Little Higgs mechanism, as realized in various models, requires a set of new massive gauge bosons, some of which mix with gauge bosons of the Standard Model. For a range of mixing angles the coupling of gauge bosons to scalars can become strong, ultimately resulting in a breakdown of perturbative calculation. This phenomenon is studied in the Littlest Higgs model, where the approach to strong dynamics is characterized by increasing tree-level decay widths of the neutral Z_H boson to lighter gauge bosons plus multiple scalars. These increasing widths suggest a distinctive qualitative collider signature for the approach to the strong coupling regime of large Higgs and other scalar multiplicities. In this work we catalog the kinematically allowed three-body decays of the Z_H, and calculate the partial width of the process Z_H to Z_L H H. This partial width is found to be larger than the comparable two-body decay Z_H to Z_L H for values of the SU(2) mixing angle cosine(theta) less than 0.13, indicating divergence of the littlest Higgs sigma field expansion at values of cosine(theta) larger than a simple parametric calculation would suggest. Additionally, we present analytical expressions for all two-body decays of the Littlest Higgs Z_H gauge boson, including the effects of all final-state masses.