Light scalar as the messenger of electroweak and flavor symmetry breaking

We propose a new framework for understanding the hierarchies of fermion masses and mixings. The masses and mixings of all standard model (SM) charged fermions other than top arise from higher dimensional operators involving a messenger scalar S and flavon scalars F_i. The flavons spontaneously break SM flavor symmetries at around the TeV scale. The SM singlet scalar S couples directly to the Higgs H and spontaneously breaks another U(1) at the electroweak scale. At the TeV scale, SM quarks and charged leptons have renormalizable couplings to S, but not to H or F_i. These couplings involve new heavy vectorlike fermions. Integrating out these fermions produces a pattern of higher dimensional operators that reproduce the observed hierarchies of the SM masses and mixings in terms of powers of the “little hierarchy”: the ratio of the electroweak scale to the flavor-breaking scale. The framework has important phenomenological implications. Flavor-changing neutral currents are within experimental limits but D⁰-[overline D]⁰ mixing and B_s-->µ⁺µ^- could be close to current sensitivities. The neutral scalar s of the messenger field mixes with the light Higgs of the SM, which can have strong effects on Higgs decay branching fractions. The s mass eigenstate may be lighter than the Higgs, and could be detected at the Tevatron or the LHC.

©2009 The American Physical Society