The neutrino problem in string models

Abstract

At the standard model of electroweak interactions neutrinos are massless and mixing among the neutrinos in different families is not predicted. Also the standard model does not include gravity. I discuss the first appearance of sterile neutrinos in D-brane models from string theories. In particular, I focus on a 5-stack intersecting D6-brane string model from IIA orientifolds. The sterile neutrinos couple to the right handed neutrino and the left handed neutrino of a single family. The existence of sterile neutrinos is independent of string RR tadpole cancellation conditions satisfaction.