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Register a new userRevisiting Neutrino Self-Interaction Constraints from $Z$ and $tau$ decays
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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.
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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.
Self-interaction in the active neutrinos is studied in the literature to alleviate the $H_0$ tension. Similar self-interaction can also explain the observed dips in the flux of the neutrinos coming from the distant astro-physical sources in IceCube detectors. In contrast to the flavour universal neutrino interaction considered for solving the $H_0$ tension, which is ruled out from particle physics experiments, we consider flavour specific neutrino interactions. We show that the values of self-interaction coupling constant and mediator mass required for explaining the IceCube dips are inconsistent with the strong neutrino self-interactions preferred by the combination of BAO, HST and Planck data. However, the required amount of self-interaction between tau neutrinos ($ u_tau$) in inverted hierarchy for explaining IceCube dips is consistent with the moderate self-interaction region of cosmological bounds at 1-$sigma$ level. For the case of other interactions and hierarchies, the IceCube preferred amount of self-interaction is consistent with moderate self-interaction region of cosmological bounds at 2-$sigma$ level only.
We revisit the current experimental bounds on fourth-generation Majorana neutrino masses, including the effects of right handed neutrinos. Current bounds from LEPII are significantly altered by a global analysis. We show that the current bounds on fourth generation neutrinos decaying to eW and mu W can be reduced to about 80 GeV (from the current bound of 90 GeV), while a neutrino decaying to tau W can be as light as 62.1 GeV. The weakened bound opens up a neutrino decay channel for intermediate mass Higgs, and interesting multi-particle final states for Higgs and fourth generation lepton decays.
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.
Hadronic tau decays offer the possibility of determining the strong coupling alpha_s at relatively low energy. Precisely for this reason, however, good control over the perturbative QCD corrections, the non-perturbative condensate contributions in the framework of the operator product expansion (OPE), as well as the corrections going beyond the OPE, the duality violations (DVs), is required. On the perturbative QCD side, the contour-improved versus fixed-order resummation of the series is still an issue, and will be discussed. Regarding the analysis, self-consistent fits to the data including all theory parameters have to be performed, and this is also explained in some detail. The fit quantities are moment integrals of the tau spectral function data in a certain energy window and care should be taken to have acceptable perturbative behaviour of those moments as well as control over higher-dimensional operator corrections in the OPE.
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