# Why is the Majorana mass of a neutrino so big compared to the

[Moderator's note: Quoted-printable stuff NOT corrected. Sometimes I
correct small errors, but a) there are several here and b) it is not
obvious what the meaning should be. Please submit only 100% 7-bit ASCII
can't. -P.H.]

I read the abstract of Prof. Vafa's recent paper on string
phenomenology. (arXiv:0806.0102)
But, I don't really understand the following part:

"This effect can simultaneously generate a viably small =A5=EC term as well
as an acceptable Dirac neutrino mass on the order of 0.5=A1=BF 10^(-2=A1=BE0=
..5)
eV. In another scenario, we find a modified seesaw mechanism which
predicts
that the light neutrinos have masses in the expected range while the
Majorana mass term for the heavy neutrinos is =A1=AD 3=A1=BF10^(12=A1=BE1.5)=
GeV."

So, it seems that the Majorana mass of neutrino is much much bigger
than the Dirac mass of neutrino.
Why is it so? As far as I know, the mass of neutrino is very small.
But, how can this light neutrinos have so big Majorana mass? Or,
rather, as stated in the excerpt, is there something called "the heavy
neutrinos" different from the light neutrinos which I am familiar
with?

Or, rather, even though it's unlikely, is Prof. Vafa suggesting that
"another scenario" of his is incorrect, since it predicts a big
Majorana mass?

I am confused,

Youngsub.

yy...@fas.harvard.edu wrote:
> Or, rather, even though it's unlikely, is Prof. Vafa suggesting that
> "another scenario" of his is incorrect, since it predicts a big
> Majorana mass?

It has nothing per se to do with string theory, but is a somewhat
standard part of folklore in neutrino physics. The *left* neutrino and
*right* anti-neutrino have small masses. The *right* neutrino and
*left* anti-neutrino, on the other hand, are usually stipulated to
have large masses. This is part of the See Saw mechanism and is used
to explain why the left neutrino (and right anti-neutrino) have such
small masses.

On Jun 7, 8:49 pm, yy...@fas.harvard.edu wrote:
> So, it seems that the Majorana mass of neutrino is much much bigger
> than the Dirac mass of neutrino.
> Why is it so? > Youngsub.

Stimulated by Rock B's explanation, I've just learned from the
references in Wikipedia that explain this whole matter
straightforwardly:

http://en.wikipedia.org/wiki/Neutrino

http://en.wikipedia.org/wiki/Sterile_neutrino

http://en.wikipedia.org/wiki/Seesaw_mechanism

Regards - P