Why are right hand neutrinos unaffected by all forces except gravity

Question

I'm curious as to something I read on Berkeley's website. Does anyone happen to know why, according to this model,right hand neutrinos are unaffected by all forces except gravity?

(Model taken from http://ctp.berkeley.edu/neutrino/neutrino5.html)

An alternative way to make right-handed neutrinos extremely weakly interacting was proposed in 1998 by Nima Arkani-Hamed at the Stanford Linear Accelerator Center, Savas Dimopoulous of Stanford University, Gia Dvali of the International Centre for Theoretical Physics in Trieste and John March-Russell of CERN. They exploited an idea from superstring theory in which the three dimensions of space with which we are familiar are embedded in 10- or 11-dimensional spacetime. Like us, all the particles of the Standard Model — electrons, quarks, left-handed neutrinos, the Higgs boson and so on — are stuck on a three-dimensional "sheet" called a three-brane.

One special property of right-handed neutrinos is that they do not feel the electromagnetic force, or the strong and weak forces. Arkani-Hamed and collaborators argued that righthanded neutrinos are not trapped on the three-brane in the same way that we are, rather they can move in the extra dimensions. This mechanism explains why we have never observed a right-handed neutrino and why their interactions with other particles in the Standard Model are extremely weak. The upshot of this approach is that neutrino masses can be very small.

Answer 1

The short answer to your question is that, out of the known non-gravitational forces, neutrinos only respond to the weak force, but the weak force only acts on left-handed particles, so right-handed neutrinos would not respond to any of those forces.

As for the paper by Arkani-Hamed et al, the idea is that the non-gravitational forces only act within the "3-brane" which is our observable universe, and the right-handed neutrinos are traveling in the extra dimensions around the brane. That concept is due to string theory, where the non-gravitational forces are carried by open strings whose ends are attached to the brane, whereas gravity is due to closed strings (forming a loop, like a circle) which are separate from the brane.

I guess I'll add that this paper is not exactly a work of string theory, it's just string-inspired, and the details of its proposals (like the proposals of many "large extra dimensions" papers) look problematic from a string theory perspective, e.g. the reference to "messenger fields" in the bulk. Those would be non-gravitational forces and so they just shouldn't be there. Or at least I don't know a legitimate string construction for them. But maybe the option where the right-handed neutrinos are "massless bulk fermions" could work, if we were talking about components of bulk gravitinos, which ought to be there by supersymmetry.