First observation of PeV-energy neutrinos with IceCube (1304.5356v2)

Abstract: We report on the observation of two neutrino-induced events which have an estimated deposited energy in the IceCube detector of 1.04 $\pm$ 0.16 and 1.14 $\pm$ 0.17 PeV, respectively, the highest neutrino energies observed so far. These events are consistent with fully contained particle showers induced by neutral-current $\nu_{e,\mu,\tau}$ ($\bar\nu_{e,\mu,\tau}$) or charged-current $\nu_{e}$ ($\bar\nu_{e}$) interactions within the IceCube detector. The events were discovered in a search for ultra-high energy neutrinos using data corresponding to 615.9 days effective livetime. The expected number of atmospheric background is $0.082 \pm 0.004 \text{(stat)}^{+0.041}_{-0.057} \text{(syst)}$. The probability to observe two or more candidate events under the atmospheric background-only hypothesis is $2.9\times10^{-3}$ ($2.8\sigma$) taking into account the uncertainty on the expected number of background events. These two events could be a first indication of an astrophysical neutrino flux, the moderate significance, however, does not permit a definitive conclusion at this time.

• The paper presents the first detection of peta-electronvolt neutrinos with measured energies around 1.04 and 1.14 PeV.
• It utilizes 615.9 days of IceCube livetime to identify cascade-like events from high-energy neutrino interactions.
• Statistical analysis reveals a low atmospheric background probability, indicating an emerging astrophysical neutrino flux.

Insightful Overview of "First observation of PeV-energy neutrinos with IceCube"

The academic paper, titled "First observation of PeV-energy neutrinos with IceCube," presents the first detections of neutrinos with energies in the peta-electronvolt (PeV) range, made by the IceCube Neutrino Observatory. These events, which possessed deposited energies of $1.04 \pm 0.16$ and $1.14 \pm 0.17$ PeV, respectively, are amongst the highest energies for neutrinos ever observed, and they add a significant contribution to the paper of high-energy cosmic phenomena.

Summary of Findings

IceCube recorded two events consistent with particle showers initiated by either neutral-current or charged-current interactions from neutrinos within the IceCube detector, marking the first-ever PeV-energy neutrinos detected. These neutrinos were found through a comprehensive analysis of data that spanned 615.9 days of effective livetime. The paper suggests a low probability of these events being attributed solely to atmospheric background, considering it to be approximately $2.9 \times 10^{-3}$ (equivalent to $2.8\sigma$ significance). This indicates these detections might be an initial indication of an astrophysical neutrino flux.

Methodological Approach

The IceCube observatory, an installation featuring 5,160 optical sensors embedded in Antarctic ice, operates by detecting Cherenkov photons emitted from secondary particles generated by high-energy neutrino interactions. Operating since 2005 and attaining full capacity by 2010, IceCube's configuration and its dense sub-array DeepCore, enable the detection of these neutrino-induced particle showers. In particular, the neutrino events are distinguished as either track-like (from muon neutrinos) or cascade-like (from neutrinos interacting via neutral-current interactions), with this paper's observed events falling into the latter category.

Results and Statistical Considerations

The effective collection of data over two years resulted in the detection of two significant cascade events. The paper provides a thorough statistical analysis, which supports the claim that these events are not easily explained by background atmospheric neutrino flux, although uncertainties in the atmospheric model (such as those from decay of charmed mesons) require further investigation. The simulations performed indicate a substantially larger expected signal for any proposed astrophysical flux, contingent on continued data collection and analysis.

Implications and Future Directions

These findings hold substantial implications for astroparticle physics, suggesting the potential for IceCube to detect and paper extremely high-energy cosmic neutrinos, which would otherwise be challenging due to a lack of deflection in cosmic magnetic fields. Such neutrinos are critical for probing the dynamic processes at work in sources like gamma-ray bursts or active galactic nuclei. Identification and measurement of high-energy neutrinos offer unprecedented insight into these astrophysical bodies and the extreme environments from which they originate.

Going forward, the IceCube Observatory will continue to refine its sensitivity and methodology, expecting further detections of high-energy neutrinos that might signify an astrophysical origin. Additionally, enhancements in neutrino interaction models, particularly concerning sources within our galaxy, could further elucidate the nature and distribution of such high-energy particle sources.

In summary, while the moderate significance of the findings suggests cautious interpretation, the observation of PeV energy neutrinos presents a significant milestone in neutrino astronomy and opens the door for more in-depth studies of astrophysical phenomena at the highest energies.