The flavor-blind principle: A symmetrical approach to the Gatto-Sartori-Tonin relation

TL;DR

This paper investigates the Gatto-Sartori-Tonin relation in fermion mixing, introducing the flavor-blind principle which explains the symmetry-breaking pattern of Yukawa couplings across multiple fermion families.

Contribution

It introduces the flavor-blind principle, providing a symmetry-based explanation for the Gatto-Sartori-Tonin relation and the structure of Yukawa matrices in fermion families.

Findings

Necessary conditions for the Gatto-Sartori-Tonin relation without approximation.

Decomposition of Yukawa matrices into flavor-blind and symmetry-breaking parts.

Sequential symmetry breaking pattern for fermion families.

Abstract

We perform a systematic study of the generic Gatto-Sartori-Tonin relation, $\tan^2\theta_{ij}= m_i/m_j$. This study of fermion mixing phenomena leads us to the necessary conditions that are needed in order to obtain it without any approximation. We begin by considering two Dirac fermion families. By means of the hierarchy in the masses, it is found that a sufficient and necessary condition is to have a normal matrix with $m_{11}=0$. This matrix can be decomposed into two different linearly independent contributions. The origin for such two independent contributions can be naturally explained by what we shall call the flavor-blind principle. This principle states that Yukawa couplings shall be either flavor-blind or decomposed into several sets obeying distinct permutation symmetries. In general, it is shown that the symmetry properties of the introduced set of Yukawa matrices follow for $n$ fermion families the unique sequential breaking $S_{nL}\otimes S_{nR} \rightarrow S_{(n-1)L}\otimes S_{(n-1)R} \rightarrow \cdots \rightarrow S_{2L}\otimes S_{2R} \rightarrow S_{2A}$. The particular case of three fermion families explains why the four mass ratios parametrization that we recently proposed can be used even in the case of no hierarchical masses.