A scale invariant extension of the Georgi Machacek model

Abstract: We propose a classically scale-invariant extension of the Georgi--Machacek model by augmenting its custodial (SU(2)_L \times SU(2)_R)-symmetric Higgs sector -- originally composed of a doublet and two triplets -- with a gauge-singlet scalar. Employing the Gildener--Weinberg formalism, we demonstrate that radiative symmetry breaking via the Coleman--Weinberg mechanism dynamically generates the electroweak scale along a flat direction. The scalar spectrum retains the quintet ((H_5)) and triplet ((H_3)) states of the original model while introducing three CP-even singlets: a pseudo-Goldstone boson (the \emph{scalon}), which acquires mass at one loop, and two additional massive scalars. One of these massive states corresponds to the observed (125\,\mathrm{GeV}) Higgs boson. We rigorously derive theoretical constraints from vacuum stability and perturbative unitarity, and we incorporate experimental bounds from electroweak precision tests (notably the (S) parameter) and Higgs signal strength measurements. Our parameter space analysis identifies viable regions where the scalon mass is below approximately (200\,\mathrm{GeV}) and the heavier scalar remains under (600\,\mathrm{GeV}). This framework addresses the hierarchy problem without fine-tuning, preserves custodial symmetry (with (\rho \approx 1) at tree level), and predicts testable deviations in Higgs couplings. Together with the extended scalar sector, these signatures provide promising avenues for direct investigation at collider experiments.