Cosmic Messengers: IceCube's Hunt for Extremely High-Energy Neutrinos

* **Introduction:** This episode discusses the search for extremely-high-energy neutrinos (EHEν) using 12.6 years of data from the IceCube Neutrino Observatory. EHEνs are unique messengers from the distant universe, traveling without being deflected by magnetic fields or attenuated by interactions with background photons.

* **IceCube Detector:** The IceCube detector, located at the South Pole, consists of 5160 Digital Optical Modules (DOMs) distributed on 86 strings, instrumenting a cubic kilometer of ice. The detector observes Cherenkov light produced by charged particles from neutrino interactions. A surface array called IceTop measures cosmic-ray air showers.

* **EHEν Detection:** EHEν events in IceCube are observed as tracks (from muon or tau neutrinos) or cascades (from all-flavor neutral-current interactions and electron neutrinos). The search focuses on downgoing or horizontal neutrinos because higher energy neutrinos are absorbed by the Earth.

* **Backgrounds:** The main background is from downgoing atmospheric muon bundles. Other backgrounds include atmospheric neutrinos, which are divided into conventional and prompt components, and astrophysical neutrinos.

* **Analysis:** The analysis uses quality cuts of high-energy events and an IceTop veto to improve the signal-to-noise ratio. The event direction is reconstructed, and energy loss profiles are used to distinguish between single muons and muon bundles.

* **Results:** The non-observation of cosmogenic neutrinos places constraints on the cosmological evolution of ultra-high-energy cosmic ray (UHECR) sources. The study constrains the proton fraction of UHECRs above approximately 30 EeV to be less than 70% at a 90% confidence level, assuming that the source evolution is comparable to or stronger than the star formation rate. This result disfavors the "proton-only" hypothesis for UHECRs.

* **Significance:** This research complements direct air-shower measurements by being insensitive to uncertainties associated with hadronic interaction models. The study also provides the most stringent limit on cosmogenic neutrino fluxes to date.

* **Methodology:** The analysis fits data using a binned Poisson likelihood in the space of reconstructed direction and energy. The study uses the CRPropa package to model cosmogenic fluxes and includes energy losses from photo-pion production and pair production on the cosmic microwave background (CMB) and extragalactic background light (EBL).

* **Event Selection:** The event selection involves several steps including: charge and hit cuts, track quality cuts, muon bundle cuts, and IceTop veto.

* **Differential Limit**: The differential upper limit on the neutrino flux above 5 x 10^6 GeV is presented in the study and compared to various cosmogenic neutrino models.

* **Systematics**: Systematic uncertainties are taken into account through pseudo-experiments. Parameters considered include: the optical efficiency of the DOMs, the neutrino cross section, average neutrino inelasticity, and atmospheric muon and neutrino fluxes.

* **Reference:** The research is detailed in the article "A search for extremely-high-energy neutrinos and first constraints on the ultra-high-energy cosmic-ray proton fraction with IceCube".

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: NSF, IceCube