Color unified dynamical axion

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 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.

Dynamical axions and gravitational waves

“…Models in Ref. [25] and [30] have a confining exotic color (gauge) group with an SU (3) L × SU (3) R (global) chiral symmetry. The SU (3) flavor symmetry is explicitly broken by QCD.…”

“…It is seen that in this case, the ratio is not bounded from above. For N F = 4, we will study a larger range, m η /m ϕ = [1,10], noting that a large mass ratio is a natural expectation of models such as [30]. We repeat the calculation described in the previous subsection, for 50 parameter points as before.…”

“…The behavior of the η can be studied in the framework of new exotic confining color groups, which are prevalent in model building to address the strong CP problem [20][21][22][23][24][25][26][27][28][29][30][31]. Heavy or visible axion models predominately utilize exotic color groups to alter the typical (m a , f a ) relationship [22][23][24][25][26][28][29][30][31][32][33]. There are generic features of chiral phase transitions in dynamical axion models that set them apart from general confining hidden sector models.…”

Dynamical axion misalignment with small instantons

“…2 Similar models in color unified theory have been developed in Ref. [34], while others have been studied in the context of lepton flavor universality violation in B-meson decays [35]. 3 For the potential in Eq.…”

UV sensitivity of the axion mass from instantons in partially broken gauge groups

“…This possibility opens up new regions of the parameter space on the m 2 a , f a plane. Interesting models implementing the mechanism of [12] were proposed in [14][15][16][17][18], applications to models of inflation were studied in [19]. Other approaches to modify the axion mass within QCD were proposed in [20][21][22][23] while in [24][25][26][27][28][29][30] the axion mass is raised by coupling it to a new confining gauge group.…”