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.

  • $\begingroup$ What do you mean, "why"? They are defined to be like that, and it is hoped (or rather, the theory is built that way because we know) that its predictions correctly reproduce our observations. $\endgroup$ – ACuriousMind Mar 8 '15 at 1:26

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.

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