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Discrete Flavor Symmetries and Models of Neutrino Mixing (1002.0211v2)

Published 1 Feb 2010 in hep-ph

Abstract: We review the application of non abelian discrete groups to the theory of neutrino masses and mixing, which is strongly suggested by the agreement of the Tri-Bimaximal mixing pattern with experiment. After summarizing the motivation and the formalism, we discuss specific models, based on A4, S4 and other finite groups, and their phenomenological implications, including lepton flavor violating processes, leptogenesis and the extension to quarks. In alternative to Tri-Bimaximal mixing the application of discrete flavor symmetries to quark-lepton complementarity and Bimaximal Mixing is also considered.

Citations (855)

Summary

  • The paper demonstrates that non-abelian discrete groups, notably A4, naturally yield the tribimaximal mixing pattern observed in neutrino experiments.
  • It provides a detailed numerical analysis of neutrino mass spectra and mixing angles under various hierarchical scenarios.
  • The work discusses implications for quark mixing and grand unification, highlighting challenges in extending these discrete symmetries beyond the lepton sector.

Discrete Flavor Symmetries and Models of Neutrino Mixing

The paper by Guido Altarelli and Ferruccio Feruglio provides a comprehensive examination of the role of discrete flavor symmetries in shaping our understanding of neutrino masses and mixing. In particular, the work explores the theoretical basis and phenomenological implications of models that utilize non-abelian discrete groups, such as A4A_4 and S4S_4, aiming to explain the observed patterns in neutrino oscillations.

Theoretical Framework

The paper begins by discussing the empirical foundations leading to the acceptance of neutrinos as massive particles, as evidenced by neutrino oscillation experiments. The tribimaximal (TB) mixing pattern, which remarkably matches experimental data, emerges as a focal point of their investigation. TB mixing suggests that the neutrino mixing matrix (UPMNSU_{\text{PMNS}}) can be represented through simple angles, indicative of underlying geometric symmetries.

Discrete Symmetries in Neutrino Models

A central argument in the paper is that discrete flavor symmetries, particularly non-abelian groups, may naturally lead to the TB mixing observed in neutrino experiments. Among these groups, A4A_4, the group of even permutations of four objects, is highlighted as especially suitable for this purpose. The group A4A_4 has the property that it allows for specific vacuum alignments that lead to TB mixing. This is due to its structure that supports invariant subgroup symmetries which, when spontaneously broken, can manifest in the observed neutrino mixing angles.

The authors also discuss other groups like S4S_4, presenting them as alternatives that offer similar outcomes but with different symmetry properties and predictions for corrections to the TB pattern.

Implications for Quarks and Grand Unification

Regarding the extension to quarks, the paper acknowledges the challenges in directly applying the same discrete symmetries to the quark sector. While the structure of groups like A4A_4 can account for certain aspects of lepton mixing, the spontaneous symmetry breaking patterns relevant for quark mixing are not as straightforward. Nonetheless, the possibility of embedding these symmetries in a grand unified theory (GUT), which could encompass both quarks and leptons within a coherent framework, is discussed as an intriguing avenue, albeit with significant obstacles.

Numerical and Phenomenological Outcomes

The paper does not shy away from the numerical intricacies of such models, offering detailed analysis of neutrino mass spectra under different hierarchical scenarios and their corresponding predictions. The inclusion of results from various models underscores the theoretical flexibility and predictive power of using discrete symmetries.

Future Direction and Challenges

The authors highlight the necessity of both theoretical and experimental advancements to further refine these models. The potential discovery of deviations from TB mixing, particularly in the θ13\theta_{13} angle, could provide critical insight into the validity of these symmetries. Moreover, the paper suggests that interdisciplinary progress involving cosmology (via leptogenesis) and collider physics (through tests of new physics) could significantly bolster or challenge the framework presented.

Conclusion

Overall, the paper by Altarelli and Feruglio systematically elucidates how discrete symmetries, particularly non-abelian groups like A4A_4 and S4S_4, offer a robust theoretical scaffold for understanding the peculiarities of neutrino oscillations, as embodied in the TB pattern. While challenges remain, particularly in integrating quarks and extending these ideas to GUTs, this research marks a significant step in the continuous effort to unveil the fundamental symmetries governing particle masses and mixings. Future experimental data will be pivotal in testing these theoretical predictions and refining the models further.