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Barenboim, G., Turner, J., & Zhou, Y. L. (2021). Light neutrino masses from gravitational condensation: the Schwinger-Dyson approach. Eur. Phys. J. C, 81(6), 511–12pp.
Abstract: In this work we demonstrate that non-zero neutrino masses can be generated from gravitational interactions. We solve the Schwinger-Dyson equations to find a non-trivial vacuum thereby determining the neutrino condensate scale and the number of new particle degrees of freedom required for gravitationally induced dynamical chiral symmetry breaking. We show for minimal beyond the Standard Model particle content, the scale of the condensation occurs close to the Planck scale.
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de Medeiros Varzielas, I., Neder, T., & Zhou, Y. L. (2018). Effective alignments as building blocks of flavor models. Phys. Rev. D, 97(11), 115033–21pp.
Abstract: Flavor models typically rely on flavons-scalars that break the family symmetry by acquiring vacuum expectation values in specific directions. We develop the idea of effective alignments, i.e., cases where the contractions of multiple flavons give rise to directions that are hard or impossible to obtain directly by breaking the family symmetry. Focusing on the example where the symmetry is S-4, we list the effective alignments that can be obtained from flavons vacuum expectation values that arise naturally from S-4. Using those effective alignments as building blocks, it is possible to construct flavor models, for example by using the effective alignments in constrained sequential dominance models. We illustrate how to obtain several of the mixing schemes in the literature, and explicitly construct renormalizable models for three viable cases, two of which lead to trimaximal mixing scenarios.
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Fu, B. W., King, S. F., Marsili, L., Pascoli, S., Turner, J., & Zhou, Y. L. (2025). Non-Abelian domain walls and gravitational waves. J. High Energy Phys., 04(4), 142–29pp.
Abstract: We investigate the properties of domain walls arising from non-Abelian discrete symmetries, which we refer to as non-Abelian domain walls. We focus on S4, one of the most commonly used groups in lepton flavour mixing models. The spontaneous breaking of S4 leads to distinct vacua preserving a residual Z2 or Z3 symmetry. Five types of domain walls are found, labelled as SI, SII, TI, TII, and TIII, respectively, the former two separating Z2 vacua and the latter three separating Z3 vacua. We highlight that SI, TI and TIII may be unstable for some regions of the parameter space and decay to stable domain walls. Stable domain walls can collapse and release gravitational radiation for a suitable size of explicit symmetry breaking. A symmetry-breaking scale of order 100 TeV may explain the recent discovery of nanohertz gravitational waves by PTA experiments. For the first time, we investigate the properties of these domain walls, which we obtain numerically with semi-analytical formulas applied to compute the tension and thickness across a wide range of parameter space. We estimate the resulting gravitational wave spectrum and find that, thanks to their rich vacuum structure, non-Abelian domain walls manifest in a very interesting and complex phenomenology.
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King, S. F., Leontaris, G. K., Marsili, L., & Zhou, Y. L. (2024). Leptogenesis in realistic flipped SU(5). J. High Energy Phys., 12(12), 211–18pp.
Abstract: We study thermal leptogenesis in realistic supersymmetric flipped SU(5) x U(1) unification. As up-type quarks and neutrinos are arranged in the same multiplets, they exhibit strong correlations, and it is commonly believed that the masses of right-handed (RH) neutrinos are too hierarchical to fit the low-energy neutrino data. This pattern generally predicts a lightest RH neutrino too light to yield successful leptogenesis, with any lepton-antilepton asymmetry generated from heavier neutrinos being washed out unless special flavour structures are assumed. We propose a different scenario in which the lightest two RH neutrinos N1 and N2 have nearby masses of order 109 GeV, with thermal leptogenesis arising non-resonantly from both N1 and N2. We show that this pattern is consistent with all data on fermion masses and mixing and predicts the lightest physical left-handed neutrino mass to be smaller than about 10-7 eV. The Dirac phase, which does not take the maximal CP-violating value, plays an important role in leptogenesis.
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