Dirac or Majorana: The Factor of Two in Relic Capture Rates, Helicity Populations, and What Clustering Does to Each

Whether neutrinos are their own antiparticles changes the predicted relic capture rate by up to a factor of two. At the cosmic mean density a Majorana background gives 8.0 events per 100 grams per year against 4.0 for a Dirac background on the normalisation used here, a ratio of 2.00; if gravitational clustering flips half the helicity population, the ratio narrows to 1.33 because the Dirac rate rises to meet it. This paper explains the helicity bookkeeping behind the factor, models the clustering correction as a stated scenario parameter, and concludes that a capture-rate measurement alone cannot cleanly decide the Dirac-Majorana question unless the local density and helicity content are known independently, a cautionary negative result for an often-repeated claim. Computed, not measured.