# Dirac, Majorana & a missing factor of 2

1. Dec 17, 2007

### EL

A question concerning Feynman rules for Dirac vs Majorana neutrinos.

Take e.g. the scattering process:
electron + positron -> electron neutrino + electron antineutrino.

Following the electroweak Feynman rules we can calculate an expression for the unpolarized differential cross section.
The total cross section is then obtained by integrating over the angels, and, in the case of identical particles in the final state (as would be the case for Majorana neutrinos which are their own antiparticles), multiply by the symmetry factor 1/2.

So, to me it seems this procedure would give different results for Majorana and Dirac neutrinos. I'm definitely missing something here, since obviously this cannot be the case. (E.g. the number of neutrino species predicted from data from measurements of the Z width should hold no matter if the neutrinos are Majorana or Dirac.)

Where is the missing factor of 2?