Technicolor with a Massless Scalar Doublet

TL;DR

This work proposes a minimal technicolor model in which a massless scalar doublet couples the ordinary and technicolor sectors, enabling fermion mass generation once technicolor condensation induces a vacuum expectation value for the scalar. The authors construct the low-energy effective chiral Lagrangian, incorporate Coleman–Weinberg radiative corrections, and analyze flavor-changing neutral currents and oblique electroweak parameters to map the viable parameter space in terms of the quartic coupling $\boldsymbol{\lambda}$ and the Yukawa combination $h=(h_++h_-)/2$. They find regions where FCNCs are under control and the $S,T$ parameters remain within experimental bounds, with the model requiring only two additional parameters beyond the Standard Model and no explicit mass scale in the fundamental Lagrangian. The study argues for the phenomenological viability of this simplest possible extension and discusses implications for collider phenomenology and the interpretation of a dimensionful-parameter-free theory.

Abstract

We consider a minimal technicolor model in which the ordinary and technicolor sectors are coupled by a {\it massless} scalar doublet. When technicolor interactions become strong, the resulting technicolor condensate not only breaks the electroweak symmetry, but also causes the scalar to develop a vacuum expectation value. With the appropriate choice of the scalar's Yukawa couplings, fermion masses are generated, giving us the conventional pattern of flavor symmetry breaking. Although no explicit scalar mass term appears in the full lagrangian of the model, the pseudoscalar states that remain in the low-energy effective theory gain sufficient mass through technicolor interactions to evade detection. We show that this model does not generate unacceptably large flavor changing neutral currents, and is consistent with the experimental constraints on oblique electroweak radiative corrections. We determine the experimentally allowed region of the model's parameter space, and discuss the significance of a phenomenologically viable model that has no arbitrary dimensionful parameters. In terms of parameter counting, our model is the simplest possible extension of the standard model.

Figures (10)

• Figure 1: Non-standard box diagrams for one and two pion exchange.
• Figure 2: The $m_\sigma=48$ GeV line. Solid line is $m_t=110$ GeV, $\delta=0$. Dashed line is $m_t=150$ GeV, $\delta=1$. The dotted line is $h f'=4\pi f$.
• Figure 3: Parameter space excluded by $K^0$-$\overline{K^0}$ mixing; the region above the K-line and to the right of the 48 GeV contour is allowed.
• Figure 4: Parameter space excluded by $B^0$-$\overline{B^0}$ mixing; the region above the B-line and to the right of the 48 GeV contour is allowed.
• Figure 5: Parameter space showing the $m_\pi=m_Z/2$, $m_\pi=m_W$ and $h f'=0.2f$ lines.