A New Type of Lepton Seesaw Model in a Modular <i>A</i>4 Symmetry

Abstract We propose a new type of lepton seesaw model introducing a modular A4 flavor symmetry in which isospin doublet vector fermions play an important role in constructing seesaw mechanisms for both the charged-lepton mass matrix and the neutrino one. The charged-lepton mass matrix is induced via the Dirac seesaw with a two-by-two block mass matrix. On the other hand, the neutrino mass matrix is generated via a seesaw with four-by-four block mass matrix where the right-handed neutral fermions are also added. Remarkably, the neutral fermion mass matrix provides us with a new type of seesaw formula for the active neutrino mass. Through our chi-squared analysis, we discuss some tendencies of observables, focusing on two fixed points $\tau =i,\ \omega$.