For ten points, name this neutral lepton with little or no mass.
It's the other particle produced in the fusion of two protons to form a deuteron and a positron.  The SAGE experiment uses gallium, Super-Kamiokande uses water, and the Homestake mine uses cleaning fluid to detect their weak interaction with matter.  For ten points, name this neutral lepton with little or no mass.
It's the other particle produced in the fusion of two protons to form a deuteron and a positron.  
electron neutrino