Looking for a diffuse flux of astrophysical muon neutrinos with IceCube

A few days ago, the IceCube Collaboration presented strong evidence for an extraterrestrial neutrino flux from an analysis that looked at neutrino-induced events inside the IceCube detector. However, scientists will not be completely sure about its origin until they have an observation of an astrophysical neutrino flux in all possible detection channels. And, more importantly, by looking at the astrophysical neutrino signal in every channel we can learn more details about the origin of cosmic neutrinos and of ultra-high-energy cosmic rays (UHECR).

In another analysis published today, the IceCube Collaboration reports on a search for a diffuse astrophysical neutrino signal, looking at high-energy upward-going muon tracks, with data taken between May 2009 and May 2010, when the detector was running in its 59-string configuration. The search found a high-energy neutrino excess of 1.8σ compared to the background scenario of a pure conventional atmospheric model, a measurement consistent with the astrophysical neutrino flux described in Science. The results of this research have been submitted to Physical Review D.

The best-fit astrophysical muon neutrino flux is found to be $$ E^{2}\cdot \Phi (E) = 0.25\cdot 10^{-8} GeV cm^{-2}s^{-1}sr^{-1}$$ , with a zero prompt component. The upper limit on this flux, at the 90% confidence level, is a factor of 1.5 above the Waxman and Bahcall upper bound. However, with this analysis, IceCube is the first neutrino telescope to reach a sensitivity for astrophysical neutrinos below the Waxman and Bahcall upper bound. Thus, upcoming results on muon neutrino searches with the IceCube detector will be able to probe the astrophysical diffuse neutrino flux with a significance close to or above the one reached in the results just published in Science.

The current analysis also sets further constraints on the prompt atmospheric neutrino flux, lowering previous results by one order of magnitude. A precise estimate of the prompt component, which cannot be currently measured with particle accelerators, becomes crucial for measurements of the extremely high-energy neutrino flux, with energies around 100 PeV or higher.

Limit on a muon neutrino astrophysical flux from this analysis in comparison to theoretical flux predictions and limits from other experiments. The black lines show the expected atmospheric neutrino flux with and without a prompt component. The red dashed line marks the Waxman-Bahcall upper bound. Green dashed lines represent various model predictions for astrophysical neutrino fluxes. Horizontal lines show limits and sensitivities from different experiments. The pink solid line is the 90% CL upper limit of this analysis, the orange solid line shows its sensitivity. Image: IceCube Collaboration.

Why look for a diffuse flux if no cosmic high-energy neutrino sources have been identified so far? The reason is quite simple: “if there are enough sources of high-energy neutrinos out there, the integrated flux of all of them can be detected before the flux of any of the individual sources reaches the detection threshold,” explains Anne Schukraft, an IceCube postdoctoral researcher at RWTH Aachen and one of the corresponding authors of this paper.

Waxman and Bahcall postulated a model independent limit on the flux of very high-energy neutrinos produced in cosmic ray sources, which allowed setting a limit on the direct neutrino flux by only taking into account the observed UHECR flux. This flux became a benchmark against which to measure the astrophysical neutrino component of high-energy neutrino fluxes, and reaching this sensitivity will allow IceCube to test a variety of theoretical models for cosmic neutrinos in the near future.

This analysis also sets a limit on a prompt muon neutrino contribution, which is an improvement over previous results but is still a factor of 4 to 10 above most of the current model predictions. The prompt component is produced by the decay of heavy mesons after the interaction of cosmic rays with the Earth’s atmosphere. The conventional atmospheric neutrino component is due to the decay of charged pions and kaons.

Month by month, IceCube has also been improving its sensitivity to test these prompt models. The current analysis disfavors the intrinsic charm model of Bugaev et al at a confidence level of more than 90%. But other models will be either rule out or preferred once new data sets are analyzed.

Info “Search for a diffuse flux of astrophysical muon neutrinos with the IceCube 59-string configuration,” IceCube Collaboration: M.G. Aartsen et al. Physical Review D89 (2014) 6, arXiv.org:1311.7048