What are singular and double seesaw?

[Riotto]

I have just started reading about neutrino physics and recently came across two terms called double seesaw and singular seesaw. Although I’m familiar with other seesaw mechanisms (such as type-I and II) for explaining smallness of neutrino mass. I’m completely at dark about double and singular seesaw. If someone can help me understand those it would be great.

 

[jedishrfu]

I can't really help here but I did find this paper describing some of the math behind the principles which might help you gain some understanding of the differences.

https://journals.aps.org/prd/pdf/10.1103/PhysRevD.70.053012

and this older paper on the singular variety:

http://people.hws.edu/tjallen/PDF/tjallen/neutrino.pdf