Flavor structures of charged fermions and massive neutrinos (1909.09610v4)

Abstract: Most of the free parameters in the Standard Model (SM) -- a quantum field theory which has successfully elucidated the behaviors of strong, weak and electromagnetic interactions of all the known fundamental particles, come from the lepton and quark flavors. The discovery of neutrino oscillations has proved that the SM is incomplete, at least in its lepton sector; and thus the door of opportunity is opened to exploring new physics beyond the SM and solving a number of flavor puzzles. In this review article we give an overview of important progress made in understanding the mass spectra, flavor mixing patterns, CP-violating effects and underlying flavor structures of charged leptons, neutrinos and quarks in the past twenty years. After introducing the standard pictures of fermion mass generation, flavor mixing and CP violation in the SM extended with the presence of massive Dirac or Majorana neutrinos, we briefly summarize current experimental knowledge about the flavor parameters of quarks and leptons. Various ways of describing flavor mixing and CP violation are discussed, the renormalization-group evolution of flavor parameters is illuminated, and the matter effects on neutrino oscillations are interpreted. Taking account of possible extra neutrino species, we propose a standard parametrization of the $6\times 6$ flavor mixing matrix and comment on the phenomenological aspects of heavy, keV-scale and light sterile neutrinos. We pay particular attention to those novel and essentially model-independent ideas or approaches regarding how to determine the Yukawa textures of Dirac fermions and the effective mass matrix of Majorana neutrinos, including simple discrete and continuous flavor symmetries. An outlook to the future development in unravelling the mysteries of flavor structures is also given.

An Analysis of "Flavor Structures of Charged Fermions and Massive Neutrinos"

The paper "Flavor structures of charged fermions and massive neutrinos" by Zhi-zhong Xing offers an extensive review and analysis of the complex flavor phenomena within the context of elementary particles. The text methodically highlights advancements in the understanding of mass spectra, flavor mixing patterns, CP-violating effects, and the underlying flavor structures associated with quarks and leptons.

The Standard Model (SM) of particle physics successfully explains the strong, weak, and electromagnetic interactions among fundamental particles but leaves open questions regarding the flavor structures, particularly due to neutrino oscillations which confirm its incompleteness in the lepton sector. Neutrino oscillations signal the non-zero mass of neutrinos, hence motivating the exploration of physics beyond the SM.

Overview of Fermion Mass Generation and Flavor Mixing

The article thoroughly explores both Dirac and Majorana mass terms. The Dirac mass framework necessitates right-handed neutrinos, introducing Yukawa interactions similar for all fermions leading to lepton number conservation. In contrast, the Majorana approach involves possible neutrino self-coupling violating lepton number, which subtly links neutrino masses to physics beyond the SM through mechanisms like the seesaw, unifying small neutrino masses with heavier counterparts. The paper explains the seesaw mechanism, where integrating out heavy states results in effective light neutrino masses proportional to the inverse of heavy masses, offering a natural explanation for the tiny size of neutrino masses. This naturally leads to the seesaw mechanisms being favored over pure Dirac formulations.

On flavor mixing, the paper examines the quantitative experimental data that anchor our understanding of quark and lepton flavor mixing. In the quark sector, the Cabibbo-Kobayashi-Maskawa (CKM) matrix provides a framework for quark flavor mixing, further expanded via the precise calculation of its hierarchical structure. For the lepton sector, the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix encompasses neutrino mixing, with current data supporting a near tri-bimaximal mixing form, albeit with small deviations.

CP Violation and Its Cosmological Implications

The paper discusses the significance of CP violation, an essential ingredient for explaining the matter-antimatter asymmetry in the universe via baryogenesis mechanisms like leptogenesis. In leptogenesis, heavy Majorana neutrinos within the seesaw models decay, generating a lepton asymmetry that is transformed into a baryon asymmetry by sphaleron processes. The Jarlskog invariant precisely measures the extent of CP violation inherent in the flavor sector, and current experimental data hint at significant CP violation in the lepton sector, unlike the quark sector where it's minuscule.

Challenges and Future Prospects

One of the strong points of the paper is its consideration of the open issues in flavor physics, such as the hierarchy problem in fermion masses and the flavor puzzle concerning the differences between the quark and lepton mixing angles and CP phases. The paper posits that understanding these anomalies might require novel theoretical frameworks, possibly involving new symmetries either discrete or continuous, and could even indicate new particles such as sterile neutrinos or other extensions beyond the SM like grand unified theories or supersymmetry.

Furthermore, precision measurements in future experiments like JUNO, DUNE, and Hyper-Kamiokande could shed light on these issues. They are expected to determine the mass ordering of neutrinos, the octant of the mixing angle $\theta_{23}$, and the violation of CP symmetry with unprecedented precision, potentially guiding theorists toward viable beyond-SM theories.

Conclusion

This paper offers insightful perspectives and mathematical rigor in addressing fundamental flavor physics issues, reflecting on both established and novel methodologies to explore these questions. It unequivocally establishes a roadmap for future experimental and theoretical inquiries, emphasizing the interplay between observable parameters and the profound implications built upon hypothetical but testable new physics frameworks. The research deepens our understanding of how nature's underlying flavor puzzle could unveil the limitations of current physics paradigms and encourage explorations into new territory bearing definitive answers.