Observation and Characterization of a Cosmic Muon Neutrino Flux from the Northern Hemisphere using six years of IceCube data (1607.08006v3)

Abstract: The IceCube Collaboration has previously discovered a high-energy astrophysical neutrino flux using neutrino events with interaction vertices contained within the instrumented volume of the IceCube detector. We present a complementary measurement using charged current muon neutrino events where the interaction vertex can be outside this volume. As a consequence of the large muon range the effective area is significantly larger but the field of view is restricted to the Northern Hemisphere. IceCube data from 2009 through 2015 have been analyzed using a likelihood approach based on the reconstructed muon energy and zenith angle. At the highest neutrino energies between 191 TeV and 8.3 PeV a significant astrophysical contribution is observed, excluding a purely atmospheric origin of these events at $5.6\,\sigma$ significance. The data are well described by an isotropic, unbroken power law flux with a normalization at 100 TeV neutrino energy of $\left(0.90^{+0.30}_{-0.27}\right)\times10^{-18}\,\mathrm{GeV^{-1}\,cm^{-2}\,s^{-1}\,sr^{-1}}$ and a hard spectral index of $\gamma=2.13\pm0.13$. The observed spectrum is harder in comparison to previous IceCube analyses with lower energy thresholds which may indicate a break in the astrophysical neutrino spectrum of unknown origin. The highest energy event observed has a reconstructed muon energy of $(4.5\pm1.2)\,\mathrm{PeV}$ which implies a probability of less than 0.005% for this event to be of atmospheric origin. Analyzing the arrival directions of all events with reconstructed muon energies above 200 TeV no correlation with known $\gamma$-ray sources was found. Using the high statistics of atmospheric neutrinos we report the currently best constraints on a prompt atmospheric muon neutrino flux originating from charmed meson decays which is below $1.06$ in units of the flux normalization of the model in Enberg et al. (2008).

• The paper establishes a significant astrophysical muon neutrino flux between 194 TeV and 7.8 PeV, excluding a purely atmospheric origin at 5.6σ.
• It utilizes charged current muon neutrino events with vertices outside the instrumented volume to expand the effective detection area.
• The study constrains prompt atmospheric neutrino fluxes and reveals a harder-than-expected power-law spectrum, prompting further multi-messenger research.

Analysis of Cosmic Muon Neutrino Flux with IceCube

The paper supported by the IceCube Collaboration focuses on a detailed examination of the cosmic muon neutrino flux observed in the Northern Hemisphere. Utilizing data accumulated over six years between 2009 and 2015, the research leverages the capabilities of the IceCube Neutrino Observatory. This observatory, situated at the South Pole, is notable for its extensive cubic-kilometer size, which allows for the measurement of high-energy astrophysical neutrinos.

Methodology and Findings

A pivotal aspect of the research is the utilization of charged current muon neutrino events, particularly those where the interaction vertex occurs outside the instrumented volume of the detector. This approach has increased the effective area, albeit with a limitation in the field of view to the Northern Hemisphere. Through a likelihood analysis based on reconstructed muon energies and zenith angles, the paper conclusively identifies a significant astrophysical contribution at energies between 194 TeV and 7.8 PeV. An exclusion of purely atmospheric origin at a significance level of 5.6σ strengthens the astrophysical signal hypothesis.

The data supports an isotropic power-law flux with a normalization at 100 TeV of approximately 0.90 x 10^-18 GeV^-1 cm^-2 s^-1 sr^-1, with a hard spectral index γ = 2.13 ± 0.13. This observation proposes a potentially harder spectrum than previous analyses with lower energy thresholds, suggesting an unknown origin break in the astrophysical neutrino spectrum. Furthermore, the event with the highest energy observed has a reconstructed muon energy of about 4.5±1.2 PeV, strongly indicating an astrophysical source given the statistical improbability (< 0.005%) of an atmospheric origin.

Implications for Astroparticle Physics

The detection and characterization of high-energy cosmic neutrinos have essential implications for astrophysical source investigation and understanding cosmic-ray accelerators. The identified isotropic muon neutrino flux is consistent with an all-sky diffuse high-energy astrophysical neutrino flux, yet presents potential tension with the Southern Hemisphere results, where a softer spectral index has been observed. Such differences underline the complexity of universal neutrino flux characteristics across various regions and energy thresholds. This prompts further consideration of regional contributions, such as a possible subdominant galactic component.

Additionally, the constraints placed on prompt atmospheric neutrino fluxes originating from charmed meson decays establish a significant advancement in background modeling for high-energy neutrino research. This paper reports the most stringent constraints to date, pushing the upper limits below predictions based on the reference model used in the analysis.

Future Directions in Neutrino Research

In light of these findings, ongoing and future research could focus on resolving tensions between hemispheric observations by employing more sophisticated detection and analysis techniques. Strategies could also explore potential source identification correlating with other cosmic messengers, such as gamma rays, to provide complementary constraints on neutrino origin models. The continued refinement of theoretical models to accurately predict atmospheric and astrophysical components remains a cornerstone for progress, as does expanding the effective global collaboration to substantiate findings across different observational platforms.

The IceCube detector's capabilities have been vital in enhancing our comprehension of high-energy neutrinos, and its findings continuously shape the dynamic narrative of astroparticle physics. This paper, through its rigorous methodological adherence and significant discoveries, reinforces the potential for new discoveries in the pursuit of cosmic exploration.