What can solve the strong CP problem? by David E. Kaplan, Tom Melia, Surjeet Rajendran

“…Abstract Three possible strategies have been advocated to solve the strong CP problem. The first is the axion, a dynamical mechanism that relaxes any initial value of the CP violating angle θ ¯ $$ \overline{\theta} $$ to zero. …”
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Noninvertible Peccei-Quinn Symmetry and the Massless Quark Solution to the Strong CP Problem by Clay Córdova, Sungwoo Hong, Seth Koren

“…Such symmetries exist solely because the standard model has the same number of generations as colors, N_{g}=N_{c}, which leads to a massless down-type quark solution to the strong CP problem in an ultraviolet SU(9) theory of quark color-flavor unification. …”
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Three-zero texture of quark-mass matrices as a solution to the strong CP problem by Qiuyue Liang, Risshin Okabe, Tsutomu T. Yanagida

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Spontaneous CP violation in supersymmetric QCD by Shota Nakagawa, Yuichiro Nakai, Yaoduo Wang

“…Abstract We investigate a composite model of spontaneous CP violation based on a new supersymmetric QCD as a solution to the strong CP problem. The scalar components of the meson chiral superfields obtain complex vacuum expectation values to break CP symmetry spontaneously. …”
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Extension of SUSY SU(5) GUTs with Nelson-Barr models by Junji Hisano, Moe Kuroda

“…The NB models are a candidate for the solution of the strong CP problem. We show that the parameters in the CKM matrix are reproduced while the mass ratios of the down-type quarks and leptons in the second and third generations are explained in the minimal extension.…”
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More about quark Yukawa textures from selection rules without group actions by Tatsuo Kobayashi, Yume Nishioka, Hajime Otsuka, Morimitsu Tanimoto

“…They lead to interesting phenomenology such as a solution to the strong CP problem without axion.…”
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Cosmology of the companion-axion model: Dark matter, gravitational waves, and primordial black holes by Zhe Chen, Archil Kobakhidze, Ciaran O'Hare, Zachary S. C. Picker, Giovanni Pierobon

“…The companion-axion model introduces a second QCD axion to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. …”
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Five dimensional $$SU(5) \times U(1)_\textrm{PQ}$$ S U ( 5 ) × U ( 1 ) PQ grand unification by Norimi Yokozaki, Junhao Zhu

“…Abstract We propose a grand unified model in five dimensions that addresses the strong CP problem. In this framework, the boundary conditions that break the unified gauge group into the Standard Model gauge group simultaneously explain the mass splitting of the new matter content required for gauge coupling unification and the emergence of the QCD axion as the $$U(1)_\textrm{PQ}$$ U ( 1 ) PQ gauge field. …”
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Seesaw limit of the Nelson-Barr mechanism by A.L. Cherchiglia, A.G. Dias, J. Leite, C.C. Nishi

“…We investigate how the solution to the strong CP problem and the explanation for the observed fermion mass hierarchies can be intrinsically related. …”
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CTAO sensitivity to axion-like particles by Schiavone Francesco, Di Venere Leonardo, Giordano Francesco

“…Axion-like particles (ALPs) are a common feature in several extensions of the StandardModel, arising, for example, as a solution to the strong CP problem in quantum chromodynamics, or as a prediction of string theories. …”
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Fine-tuning problems in type IIA string theory by Yang Liu

“…We show that the strong CP problem can be effectively “solved” in type IIA orientifold constructions, particularly in the type IIA T 6/(ℤ 2 × ℤ 2) model. …”
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A holographic perspective on the axion quality problem by Peter Cox, Tony Gherghetta, Minh D. Nguyen

“…Abstract The axion provides a compelling solution to the strong CP problem as well as a candidate for the dark matter of the universe. …”
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CPon Dark Matter by Ferruccio Feruglio, Robert Ziegler

“…Abstract We study a class of supersymmetric models where the strong CP problem is solved through spontaneous CP violation, carried out by a complex scalar field that determines the Yukawa couplings of the theory. …”
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A Correlation between the Higgs Mass and Dark Matter by Mark P. Hertzberg

“…We assume that Peccei-Quinn (PQ) dynamics enters to solve the strong CP problem and, for a PQ-scale in this energy range, may also remove the unstable region. …”
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Radiative mass mechanism: addressing the flavour hierarchy and strong CP puzzle by Gurucharan Mohanta

“…Abstract We propose a class of models based on the parity invariant Left-Right Symmetric Model (LRSM), which incorporates the mechanism of radiative generation of fermion masses while simultaneously possessing the solution to the Strong CP problem. A flavour non-universal gauged abelian symmetry is imposed on top of LRSM, which helps in inducing the masses of second and first-generation fermions at one-loop and two-loop, respectively, and thereby reproduces the hierarchical spectrum of the masses. …”
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High quality QCD axion via electric-magnetic duality by Shota Nakagawa, Yuichiro Nakai, Junxuan Xu, Yufei Zhang

“…Since the model naturally accommodates a mechanism to suppress the axion isocurvature fluctuation, it provides a complete solution to the strong CP problem as well as the identity of DM.…”
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Spontaneous CP violation, sterile neutrino dark matter and leptogenesis by Yanjin Jiang, Norimi Yokozaki

“…We constructed a model for spontaneous CP symmetry breaking in five-dimensional space-time that has a potential to solve the strong CP problem. To explain the nature of dark matter and the baryon asymmetry of the universe, three right-handed neutrinos and U(1)B−L gauge interaction are introduced in the bulk, in addition to the field contents of the Bento-Branco-Parada model. …”
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A light QCD axion with hilltop misalignment by Raymond T. Co, Tony Gherghetta, Zhen Liu, Kun-Feng Lyu

“…Consequently, this “hilltop” misalignment mechanism opens new regions of axion parameter space, with the correct relic abundance while still solving the strong CP problem, that could be explored in future experiments.…”
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