2022/03/17 by Joe Zhiyu Chen, Isabel M. Oldengott, Chen, Joe Zhiyu +5 · 6 citations
Physics and Astronomy · #Neutrino Physics Research #Astrophysics and Cosmic Phenomena #Particle physics theoretical and experimental studies
paper · pdf · doi:10.48550/arxiv.2203.09075
We consider invisible neutrino decay νH → νl + ϕ in the ultra-relativistic limit and compute the neutrino anisotropy loss rate relevant for the cosmic microwave background (CMB) anisotropies. Improving on our previous work which assumed massless νl and ϕ, we reinstate in this work the daughter neutrino mass mνl in a manner consistent with the experimentally determined neutrino mass splittings. We find that a nonzero mνl introduces a new phase space factor in the loss rate Γ\rm T proportional to (Δmν2/mνH2)2 in the limit of a small squared mass gap between the parent and daughter neutrinos, i.e., Γ\rm T ∼ (Δmν2/mνH2)2 (mνH/Eν)5 (1/τ0), where τ0 is the νH rest-frame lifetime. Using a general form of this result, we update the limit on τ0 using the Planck 2018 CMB data. We find that for a parent neutrino of mass mνH \lesssim 0.1 \rm eV, the new phase space factor weakens the constraint on its lifetime by up to a factor of 50 if Δmν2 corresponds to the atmospheric mass gap and up to 105 if the solar mass gap, in comparison with naive estimates that assume mνl=0. The revised constraints are (i) τ0 \gtrsim (6 → 10) × 105~\rm s and τ0 \gtrsim (400 → 500)~\rm s if only one neutrino decays to a daughter neutrino separated by, respectively, the atmospheric and the solar mass gap, and (ii) τ0 \gtrsim (2 → 3) × 107~\rm s in the case of two decay channels with one near-common atmospheric mass gap. In contrast to previous, naive limits which scale as mνH5, these mass spectrum-consistent τ0 constraints are remarkably independent of the parent mass and open up a swath of parameter space within the projected reach of IceCube and other neutrino telescopes in the next two decades.