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Register a new userCosmological Constraints on Invisible Neutrino Decays Revisited
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Invisible neutrino decay modes are difficult to target at laboratory experiments, and current bounds on such decays from solar neutrino and neutrino oscillation experiments are somewhat weak. It has been known for some time that Cosmology can serve as a powerful probe of invisible neutrino decays. In this work, we show that in order for Big Bang Nucleosynthesis to be successful, the invisible neutrino decay lifetime is bounded to be $tau_ u > 10^{-3},text{s}$ at 95% CL. We revisit Cosmic Microwave Background constraints on invisible neutrino decays, and by using Planck2018 observations we find the following bound on the neutrino lifetime: $tau_ u > (1.3-0.3)times 10^{9},text{s} , left({m_ u}/{ 0.05,text{eV} }right)^3$ at $95%$ CL. We show that this bound is robust to modifications of the cosmological model, in particular that it is independent of the presence of dark radiation. We find that lifetimes relevant for Supernova observations ($tau_ u sim 10^{5},text{s}, left({m_ u}/{ 0.05,text{eV} }right)^3$) are disfavoured at more than $5,sigma$ with respect to $Lambda$CDM given the latest Planck CMB observations. Finally, we show that when including high-$ell$ Planck polarization data, neutrino lifetimes $tau_ u = (2-16)times 10^{9},text{s} , left({m_ u}/{ 0.05,text{eV} }right)^3$ are mildly preferred -- with a 1-2 $sigma$ significance -- over neutrinos being stable.
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Within the standard three-neutrino framework, the absolute neutrino masses and their ordering (either normal, NO, or inverted, IO) are currently unknown. However, the combination of current data coming from oscillation experiments, neutrinoless double beta decay searches, and cosmological surveys, can provide interesting constraints for such unknowns in the sub-eV mass range, down to O(0.1) eV in some cases. We discuss current limits on absolute neutrino mass observables by performing a global data analysis, that includes the latest results from oscillation experiments, neutrinoless double beta decay bounds from the KamLAND-Zen experiment, and constraints from representative combinations of Planck measurements and other cosmological data sets. In general, NO appears to be somewhat favored with respect to IO at the level of ~2 sigma, mainly by neutrino oscillation data (especially atmospheric), corroborated by cosmological data in some cases. Detailed constraints are obtained via the chi^2 method, by expanding the parameter space either around separate minima in NO and IO, or around the absolute minimum in any ordering. Implications for upcoming oscillation and non-oscillation neutrino experiments, including beta-decay searches, are also discussed.
We revisit our previous work [Phys. Rev. D 95, 096014 (2017)] where neutrino oscillation and nonoscillation data were analyzed in the standard framework with three neutrino families, in order to constrain their absolute masses and to probe their ordering (either normal, NO, or inverted, IO). We include updated oscillation results to discuss best fits and allowed ranges for the two squared mass differences $delta m^2$ and $Delta m^2$, the three mixing angles $theta_{12}$, $theta_{23}$ and $theta_{13}$, as well as constraints on the CP-violating phase $delta$, plus significant indications in favor of NO vs IO at the level of $Deltachi^2=10.0$. We then consider nonoscillation data from beta decay, from neutrinoless double beta decay (if neutrinos are Majorana), and from various cosmological input variants (in the data or the model) leading to results dubbed as default, aggressive, and conservative. In the default option, we obtain from nonoscillation data an extra contribution $Deltachi^2 = 2.2$ in favor of NO, and an upper bound on the sum of neutrino masses $Sigma < 0.15$ eV at $2sigma$; both results - dominated by cosmology - can be strengthened or weakened by using more aggressive or conservative options, respectively. Taking into account such variations, we find that the combination of all (oscillation and nonoscillation) neutrino data favors NO at the level of $3.2-3.7sigma$, and that $Sigma$ is constrained at the $2sigma$ level within $Sigma < 0.12-0.69$ eV. The upper edge of this allowed range corresponds to an effective $beta$-decay neutrino mass $m_beta = Sigma/3 = 0.23$ eV, at the sensitivity frontier of the KATRIN experiment.
We propose a method for testing the Dirac neutrino hypothesis by combining data from terrestrial neutrino experiments, such as tritium beta decay, with data from cosmological observations, such as the cosmic microwave background and large scale structure surveys. If the neutrinos are Dirac particles, and if the active neutrinos sterile partners were once thermalized in the early universe, then this new cosmological relic would simultaneously contribute to the effective number of relativistic species, $N_text{eff}$, and also lead to a mismatch between the cosmologically-measured effective neutrino mass sum $Sigma m_ u$ and the terrestrially-measured active neutrino mass sum $Sigma_i m_i$. We point out that specifically correlated deviations in $N_text{eff} gtrsim 3$ and $Sigma m_ u gtrsim Sigma_i m_i$ above their standard predictions could be the harbinger revealing the Dirac nature of neutrinos. We provide several benchmark examples, including Dirac leptogenesis, that predict a thermal relic population of the sterile partners, and we discuss the relevant observational prospects with current and near-future experiments. This work provides a novel approach to probe an important possibility of the origin of neutrino mass.
Given the elusive nature of neutrinos, their self-interaction is particularly difficult to probe. Nevertheless, upper limits on the strength of such an interaction can be set by using data from terrestrial experiments. In this work we focus on additional contributions to the invisible decay width of $Z$ boson as well as the leptonic $tau$ decay width in the presence of a neutrino coupling to a relatively light scalar. For invisible $Z$ decays we derive a complete set of constraints by considering both three-body bremsstrahlung as well as the loop correction to two-body decays. While the latter is usually regarded to give rather weak limits we find that through the interference with the Standard Model diagram it actually yields a competitive constraint. As far as leptonic decays of $tau$ are concerned, we derive a first limit on neutrino self-interactions that is valid across the whole mass range of a light scalar mediator. Our bounds on the neutrino self-interaction are leading for $m_phi gtrsim 300$ MeV and interactions that prefer $ u_tau$. Bounds on such $ u$-philic scalar are particularly relevant in light of the recently proposed alleviation of the Hubble tension in the presence of such couplings.
Unparticles ($U$) interact weakly with particles. The direct signature of unparticles will be in the form of missing energy. We study constraints on unparticle interactions using totally invisible decay modes of $Z$, vector quarkonia $V$ and neutrinos. The constraints on the unparticle interaction scale $Lambda_U$ are very sensitive to the dimension $d_U$ of the unparticles. From invisible $Z$ and $V$ decays, we find that with $d_U$ close to 1 for vector $U$, the unparticle scale $Lambda_U$ can be more than $10^4$ TeV, and for $d_U$ around 2, the scale can be lower than one TeV. From invisible neutrino decays, we find that if $d_U$ is close to 3/2, the scale can be more than the Planck mass, but with $d_U$ around 2 the scale can be as low as a few hundred GeV. We also study the possibility of using $V (Z)to gamma + U$ to constrain unparticle interactions, and find that present data give weak constraints.
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