Cosmological constraints on light flavons

The Froggatt-Nielsen mechanism is a well-motivated framework for generating the fermion mass hierarchy. This mechanism introduces flavons, complex scalars which are singlet under the Standard Model gauge symmetry and charged under a new global family symmetry. We make use of a leptophilic flavon to produce the charged lepton Yukawa matrix. The real part of the flavon mixes with the Higgs boson and introduces lepton flavour violating interactions which are bounded by experiment. The imaginary part of the flavon, $\eta$, is a long-lived light particle, whose abundance is restricted by cosmological observations. For $m_\eta < 2m_e$ where the decay of $\eta$ to charged leptons is kinematically forbidden, we identify allowed regions of $m_\eta$ with respect to the vacuum expectation value of the flavon field where all experimental and cosmological constraints are satisfied.