Neutrinoless Double Beta Decay: 2015 Review

Figure 2

Massive fields in their rest frames. The arrows show the possible directions of the spin. (Left) the 4 states of Dirac massive field. The signs indicate the charge that distinguishes particles and antiparticles, for example, the electric charge of an electron. (Right) the 2 states of Majorana massive field. The symbol “zero” indicates the absence of any charge: particles and antiparticles coincide.