Cosmological Limits on the Neutrino Mass and Lifetime. (arXiv:1909.05275v1 [hep-ph])

<a href="http://arxiv.org/find/hep-ph/1/au:+Chacko_Z/0/1/0/all/0/1">Zackaria Chacko</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Dev_A/0/1/0/all/0/1">Abhish Dev</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Du_P/0/1/0/all/0/1">Peizhi Du</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Poulin_V/0/1/0/all/0/1">Vivian Poulin</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Tsai_Y/0/1/0/all/0/1">Yuhsin Tsai</a>

At present, the strongest upper limit on $sum m_{nu}$, the sum of neutrino

masses, is from cosmological measurements. However, this bound assumes that the

neutrinos are stable on cosmological timescales, and is not valid if the

neutrino lifetime is less than the age of the universe. In this paper, we

explore the cosmological signals of theories in which the neutrinos decay into

invisible dark radiation on timescales of order the age of the universe, and

determine the bound on the sum of neutrino masses in this scenario. We focus on

the case in which the neutrinos decay after becoming non-relativistic. We

derive the Boltzmann equations that govern the cosmological evolution of

density perturbations in the case of unstable neutrinos, and solve them

numerically to determine the effects on the matter power spectrum and lensing

of the cosmic microwave background. We find that the results admit a simple

analytic understanding. We then use these results to perform a Monte Carlo

analysis based on the current data to determine the limit on the sum of

neutrino masses as a function of the neutrino lifetime. We show that in the

case of decaying neutrinos, values of $sum m_{nu}$ as large as 0.9 eV are

still allowed by the data. Our results have important implications for

laboratory experiments that have been designed to detect neutrino masses, such

as KATRIN and KamLAND-ZEN.

At present, the strongest upper limit on $sum m_{nu}$, the sum of neutrino

masses, is from cosmological measurements. However, this bound assumes that the

neutrinos are stable on cosmological timescales, and is not valid if the

neutrino lifetime is less than the age of the universe. In this paper, we

explore the cosmological signals of theories in which the neutrinos decay into

invisible dark radiation on timescales of order the age of the universe, and

determine the bound on the sum of neutrino masses in this scenario. We focus on

the case in which the neutrinos decay after becoming non-relativistic. We

derive the Boltzmann equations that govern the cosmological evolution of

density perturbations in the case of unstable neutrinos, and solve them

numerically to determine the effects on the matter power spectrum and lensing

of the cosmic microwave background. We find that the results admit a simple

analytic understanding. We then use these results to perform a Monte Carlo

analysis based on the current data to determine the limit on the sum of

neutrino masses as a function of the neutrino lifetime. We show that in the

case of decaying neutrinos, values of $sum m_{nu}$ as large as 0.9 eV are

still allowed by the data. Our results have important implications for

laboratory experiments that have been designed to detect neutrino masses, such

as KATRIN and KamLAND-ZEN.

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