[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 plain-text posts. Even if YOU can read something fancier, many readers 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.