Detectability of μ, μ₁, μ₂ and μ₃. The upper panel shows the limits for ϵ_X = f_X/z_X, while the lower panel uses the standard yield variable, E_visY_X (cp., Kawasaki et al. 2005). For a given particle lifetime, we compute the required value of ϵ_X for which a 1σ-detection of the corresponding variable is possible with PIXIE. The violet shaded area is excluded by measurements of the primordial ³He/D abundance ratio (1σ-level, adapted from fig. 42 of Kawasaki et al. 2005).