Singular and double seesaw are theoretical frameworks used in particle physics to explain the origin of neutrino masses. They involve the introduction of heavy particles (singlet and doublet fermions) to the Standard Model of particle physics.
The main difference between singular and double seesaw is the number of heavy particles introduced. In singular seesaw, only one heavy singlet fermion is added, while in double seesaw, both a heavy singlet and a heavy doublet fermion are added.
The introduction of heavy particles in singular and double seesaw leads to the violation of lepton number conservation, which allows for the generation of small neutrino masses through a mechanism called "seesaw mechanism". This mechanism explains why neutrino masses are much smaller than other particle masses.
Currently, there is no direct experimental evidence for singular and double seesaw. However, the predictions of these frameworks are consistent with the observed properties of neutrinos, such as their small masses and oscillation patterns.
The existence of singular and double seesaw would support the idea of grand unification, where all fundamental forces are unified at high energies. It could also shed light on the nature of dark matter and the matter-antimatter asymmetry in the universe.