Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector (1311.5238v2)

Abstract: We report on results of an all-sky search for high-energy neutrino events interacting within the IceCube neutrino detector conducted between May 2010 and May 2012. The search follows up on the previous detection of two PeV neutrino events, with improved sensitivity and extended energy coverage down to approximately 30 TeV. Twenty-six additional events were observed, substantially more than expected from atmospheric backgrounds. Combined, both searches reject a purely atmospheric origin for the twenty-eight events at the $4\sigma$ level. These twenty-eight events, which include the highest energy neutrinos ever observed, have flavors, directions, and energies inconsistent with those expected from the atmospheric muon and neutrino backgrounds. These properties are, however, consistent with generic predictions for an additional component of extraterrestrial origin.

• The paper demonstrates 28 high-energy neutrino events that significantly exceed atmospheric background levels, achieving a 4σ confidence level for extraterrestrial origin.
• It employs a robust veto mechanism and detailed energy and directional analyses using Antarctic ice Cherenkov detection to accurately classify neutrino events.
• The results support an E^-2 power-law spectrum, implying contributions from cosmic accelerators such as blazars and gamma-ray bursts in high-energy neutrino production.

High-Energy Extraterrestrial Neutrinos Observed by IceCube

The paper "Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector" presents a comprehensive analysis of neutrino events detected by the IceCube observatory. This research, conducted from May 2010 to May 2012, focuses on identifying high-energy neutrinos that potentially have an extraterrestrial origin. The findings indicate that 28 neutrino events with energies ranging from 30 TeV to 1.2 PeV were observed, marking a significant deviation from purely atmospheric expectations.

Key Findings

A total of 26 additional events were observed in IceCube, a continuation of previously detected PeV neutrino events. These events significantly exceed the expected number from atmospheric muon and neutrino backgrounds, which was estimated to be approximately 10.6 events. The combined analysis of these events allows the rejection of a purely atmospheric origin at a $4\sigma$ level confidence. The distribution in neutrino flavors, directions, and energies suggests a component that aligns with theoretical predictions of an extraterrestrial source.

Methodology and Results

IceCube utilizes a large-volume array of photomultiplier tubes located deep in Antarctic ice to detect Cherenkov radiation from secondary particles generated by neutrino interactions. By focusing on events that are well-contained within the detector volume and excluding those potentially originating from external muons, the analysis provides a relatively uniform sensitivity across different neutrino flavors and directions.

The analysis implemented a veto mechanism to minimize background events primarily caused by cosmic-ray muons interacting outside the detector. These efforts resulted in a high rejection rate of atmospheric muons while maintaining sensitivity to neutrino interactions within IceCube.

The spectrum and zenith angle distribution of the 28 detected events were analyzed and contrasted with expectations from known atmospheric backgrounds. The observed data presented a hard energy spectrum inconsistent with the softer spectra expected from atmospheric neutrinos, particularly those resulting from decays of charmed mesons, which have not been observed at substantial levels. Moreover, the spatial distribution in the southern sky of IceCube's neutrino events is inconsistent with the background expectations that suggest a northern bias due to accompanying muon tracks.

Discussion and Implications

The findings support the presence of a new component in the astrophysical neutrino flux, possibly indicating neutrinos being generated in extragalactic sources or other yet unidentified astrophysical phenomena. The best-fit model to the observed data corresponds to an $E^{-2}$ power-law spectrum that is consistent with predictions for neutrino production in cosmic accelerators. This suggests the events could be attributed to high-energy astrophysical processes such as those occurring in blazars, gamma-ray bursts, or other cosmic accelerators.

A test for spatial clustering did not yield conclusive evidence for specific point sources or temporal clusters, leaving open questions about the locations or types of specific sources of these high-energy neutrinos. This result implies that the detected neutrinos likely originate from a diffuse extraterrestrial source rather than a small number of discrete sources.

Conclusions and Future Directions

The disparity between observed high-energy neutrino events and atmospheric expectations is consistent with a component of astrophysical origin. These results pave the way for further investigations into the potential sources of cosmic rays and the processes that produce high-energy neutrinos.

The significance of these results highlights the evolving role of neutrino astronomy in identifying and understanding cosmic ray sources and the broader universe's high-energy processes. As IceCube continues to gather data, ongoing analyses may refine the clarity on the isotropy of the signal and potentially identify specific sources or classes of astrophysical objects responsible for these high-energy neutrinos. The continued development of neutrino detectors and international collaboration promises to illuminate the high-energy universe further.