Feynman Diagrams in 2 component notation When using two component notation people often prefer to refrain from using arrows in Feynman diagrams to denote charge flow as is done in four-component notation. Instead, if understand correctly, they use arrows to denote chirality. I'd like to know what is the prescription to draw out the diagrams. I have read here (pg. 39) that 

arrows indicate the spinor index structure, with fields of undotted indices flowing into any vertex and field of dotted indices flowing out of any vertices

(see the reference above for many examples).
However, trying this out on Majorana and Dirac mass terms, this doesn't seem to be correct. A Majorana mass term, $\psi ^\alpha \psi_\alpha +h.c.$, is thus composed only of undotted indices. With the reasoning above, it should have two arrows pointing into the vertex,

However, I'm pretty sure that this is a Dirac mass, and not a Majorana mass. What am I missing?
 A: When you say:
"However, I'm pretty sure that this is a Dirac mass, and not a Majorana mass."
that's where you are confused. (Why did you think you were sure of this?) A Majorana mass has that form, and so does a Dirac mass. They have the exact same Feynman rule arrow structure when you use 2-component notation. It is just that for a Majorana mass, the 2-component fields being connected are the same, and for a Dirac mass they are different (typically with opposite charge under some gauge or global symmetry).
The answers about the Majorana-Weyl condition are not relevant. In 4 dimensions, a Majorana fermion is simply a 2-component Weyl fermion with a mass term by itself. A Dirac fermion is a pair of 2-component Weyl fermions with a mass term connecting them.
A: Majorana-Weyl spinors exist only in some dimensions. Let the signature of spacetime be $(p,q)$ -- M-W spinors exist only when $p-q=0\mod 8$. Thus, they don't exist in $(1,3)$ Minkowski spacetime. You can impose only the Majorana conditions or the Weyl condition in this case. The notation that you mention can only be applied to Weyl spinors. I can't think immediately of a reference for the existence statement -- it might be discussed in Sohnius' Physics Reports article on supersymmetry. 
