Abstract
We consider an enlarged color sector which solves the strong CP problem via new massless fermions. The spontaneous breaking of a unified color group into QCD and another confining group provides a source of naturally large axion mass m_a due to small size instantons. This extra source of axion mass respects automatically the alignment of the vacuum, ensuring a low-energy CP-conserving vacuum. The mechanism does not appeal to a Z_2 “mirror” copy of the SM, nor does it require any fine-tuning of the axion-related couplings at the unification scale. There is no very light axion and uncharacteristically the lighter spectrum contains instead sterile fermions. The axion scale f_a can be naturally brought down to a few TeV, with an exotic spectrum of colored pseudoscalars lighter than this scale, observable at colliders exclusively via strong interactions. The {m_a, f_a} parameter space which allows a solution of the strong CP problem is thus enlarged well beyond that of invisible axion models.
Highlights
Phenomenological analyses based on chiral perturbation theory and supported by lattice computations indicate that all Standard Model (SM) quarks have non-zero masses
We will develop in what follows a new solution to the strong CP problem via massless fermions, in which the issue of the different θ parameters that arise in the presence of two or more confining groups is solved via color unification
The θ parameter of the auxiliary SU (3 ) gauge group can be made unphysical by the addition of a massless fermion field χ that transforms as a fundamental of SU (3 ) and is an EW and SU (6) singlet
Summary
Phenomenological analyses based on chiral perturbation theory and supported by lattice computations indicate that all Standard Model (SM) quarks have non-zero masses. Taking into account the SM quark sector and the SM η , three flavor-singlet pseudoscalars result in the low-energy spectrum for only two instanton sources of masses: the η , a very heavy axion with mass ∼ ̃ and a second axion almost massless and obeying Eq (2) Because of this last axion, the axicolor construction can be seen as an ultraviolet dynamical completion of the invisible axion paradigm. We will develop in what follows a new solution to the strong CP problem via massless fermions, in which the issue of the different θ parameters that arise in the presence of two or more confining groups is solved via color unification. The necessary separation of QCD and a larger confining scale will naturally follow. We will develop in detail two realistic ultraviolet (UV) completions
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
