Invisible neutrino decay in precision cosmology

Gabriela Barenboim; Joe Zhiyu Chen; Steen Hannestad; Isabel M. Oldengott; Thomas Tram; Yvonne Y. Y. Wong

doi:10.1088/1475-7516/2021/03/087

paper

Invisible neutrino decay in precision cosmology

arXiv:2011.01502 · doi:10.1088/1475-7516/2021/03/087

Abstract

We revisit the topic of invisible neutrino decay in the precision cosmological context, via a first-principles approach to understanding the cosmic microwave background and large-scale structure phenomenology of such a non-standard physics scenario. Assuming an effective Lagrangian in which a heavier standard-model neutrino $ν_{H}$ couples to a lighter one $ν_{l}$ and a massless scalar particle $ϕ$ via a Yukawa interaction, we derive from first principles the complete set of Boltzmann equations, at both the spatially homogeneous and the first-order inhomogeneous levels, for the phase space densities of $ν_{H}$ , $ν_{l}$ , and $ϕ$ in the presence of the relevant decay and inverse decay processes. With this set of equations in hand, we perform a critical survey of recent works on cosmological invisible neutrino decay in both limits of decay while $ν_{H}$ is ultra-relativistic and non-relativistic. Our two main findings are: (i) in the non-relativistic limit, the effective equations of motion used to describe perturbations in the neutrino--scalar system in the existing literature formally violate momentum conservation and gauge invariance, and (ii) in the ultra-relativistic limit, exponential damping of the anisotropic stress does not occur at the commonly-used rate $Γ_{T} = (1/ τ_{0}) (m_{ν H} / E_{ν H})^{3}$ , but at a rate $\sim (1/ τ_{0}) (m_{ν H} / E_{ν H})^{5}$ . Both results are model-independent. The impact of the former finding on the cosmology of invisible neutrino decay is likely small. The latter, however, implies a significant revision of the cosmological limit on the neutrino lifetime $τ_{0}$ from $τ_{0}^{old} ≳ 1.2 \times 1 0^{9} s (m_{ν H} /50 meV)^{3}$ to $τ_{0} ≳ (4 \times 1 0^{5} \to 4 \times 1 0^{6}) s (m_{ν H} /50 meV)^{5}$ .

arXiv comment53 pages, 10 figures, v2 matches the accepted JCAP version

Invisible neutrino decay in precision cosmology

Abstract

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