[Paper Review] New limits on neutrino decay from the Glashow resonance of high-energy cosmic neutrinos

Mauricio Bustamante|arXiv (Cornell University)|Apr 15, 2020

Astrophysics and Cosmic Phenomena1 references4 citations

TL;DR

This paper uses the first observed Glashow resonance candidate in IceCube (6.3 PeV anti-electron neutrino event) to set new lower limits on the lifetimes of cosmic neutrino mass eigenstates ν₁ and ν₂, leveraging their decay into ν₃. The method exploits the fact that if ν₁ and ν₂ decay significantly, the surviving flux would lack sufficient electron-flavor content to trigger the Glashow resonance; thus, detecting even one such event constrains their lifetimes, with the ν₂ limit being the current best and ν₁'s approaching it.

ABSTRACT

Discovering neutrino decay would be strong evidence of new physics. Presently, there are only lax lower limits on the lifetime $ au$ of neutrinos, of $ au/m > 10^{-3}$ s eV$^{-1}$ or worse, where $m$ is the neutrino mass. Fortunately, TeV-PeV cosmic neutrinos offer superior sensitivity to decay due to their cosmological-scale baselines. We employ a promising method, recently proposed, that uses the Glashow resonance $\bar{ u}_e + e o W$, triggered by $\bar{ u}_e$ of 6.3 PeV, to test decay with only a handful of detected events. Based on the recent detection of the first Glashow resonance candidate in IceCube, we place new lower limits on the lifetimes of $ u_1$ and $ u_2$ in the inverted mass ordering. For $ u_2$, our limit is the current best. For $ u_1$, our limit is close to the current best and will surpass it soon.

Motivation & Objective

• To constrain the lifetimes of high-energy cosmic neutrinos ν₁ and ν₂, which are candidates for physics beyond the Standard Model.
• To exploit the Glashow resonance (ν̄ₑ + e⁻ → W⁻) at 6.3 PeV as a probe of neutrino decay, since decay into ν₃ would suppress the resonance signal.
• To use the first observed Glashow resonance candidate in IceCube (4.6 years of data) to derive conservative, model-independent lower bounds on ν₁ and ν₂ lifetimes.
• To project future sensitivity with IceCube and IceCube-Gen2, assuming detection of 2–4 Glashow resonance events.

Proposed method

• The method relies on the Glashow resonance, a process with a cross-section ~200× larger than standard νN scattering at 6.3 PeV, making it a sensitive probe for high-energy cosmic neutrinos.
• If ν₁ and ν₂ decay into the stable ν₃, their electron-flavor content is suppressed, so the flux would not produce Glashow resonance events; thus, observing such an event implies that not all ν₁ and ν₂ have decayed.
• A Bayesian analysis is performed using the observed single Glashow resonance candidate in 4.6 years of IceCube data, with priors on mixing parameters from the NuFit 4.1 global fit and wide priors on neutrino lifetimes.
• The analysis accounts for detector effects, particle physics uncertainties (mixing angles, CP phase), and astrophysical flux parameters, ensuring conservative limits.
• Projected limits are derived assuming 2–4 observed events in IceCube and 2 events in future IceCube-Gen2, with mixing parameters fixed at best-fit values.
• Upper limits on effective couplings (scalar + pseudoscalar) to a new light boson are derived from lifetime limits, assuming hierarchical mass models.

Experimental results

Research questions

• RQ1 Can the first observed Glashow resonance candidate in IceCube be used to constrain the lifetimes of cosmic ν₁ and ν₂?
• RQ2 How do the resulting limits compare to existing constraints from solar neutrino experiments?
• RQ3 What is the expected improvement in sensitivity with future detections of additional Glashow resonance events?
• RQ4 How do uncertainties in neutrino mixing parameters and astrophysical fluxes affect the derived lifetime bounds?
• RQ5 What are the implications for models of neutrino decay involving new light bosons?

Key findings

• The observed Glashow resonance candidate in 4.6 years of IceCube data sets the current best lower limit on the lifetime of ν₂, with τ₂/m₂ > 10^6.81 s/eV at 90% C.L.
• For ν₁, the limit is τ₁/m₁ > 10^8.03 s/eV at 90% C.L., which is comparable to the best existing limit from solar neutrino experiments.
• With the detection of a second Glashow resonance event, the ν₁ lifetime limit will improve by more than an order of magnitude, surpassing current constraints.
• Projected limits with 2 Glashow events in IceCube (9.2 years) will improve the ν₁ limit to τ₁/m₁ > 10^6.81 s/eV and ν₂ to τ₂/m₂ > 10^6.81 s/eV, with further gains expected from IceCube-Gen2.
• The 90% C.L. upper limit on the combined coupling of ν₁ to a new light boson is 4.77×10⁻⁶ (eV/m₁), improving to 3.17×10⁻⁷ (eV/m₁) with two events.
• The upper limit on ν₂'s coupling is 7.24×10⁻⁶ (eV/m₂), improving to 4.41×10⁻⁶ (eV/m₂) with two events, outperforming solar neutrino constraints in the ν₂ case.