The search for the sources of ultra-high-energy cosmic rays (UHECRs) is not a simple one. UHECRs, which are a mixture of protons and heavy nuclei, are the highest energy particles ever measured. They should produce “hotspots” of high-energy neutrinos if they interact with other particles near their point of origin. Six years ago, a first joint analysis of the IceCube, Pierre Auger, and Telescope Array collaborations looked for correlations between UHECRs and neutrinos. The results pointed to a correlation between extragalactic cosmic-ray sources and the highest energy neutrinos in IceCube, with energies between 20 TeV and a few PeVs. However, results were not conclusive and identified the need for a more advanced analysis.
And this is what an international team of more than 1,000 scientists is now presenting, with a new partnership of the IceCube, Pierre Auger, and Telescope Array collaborations that has grown to include the ANTARES collaboration. In contrast to the initial study, the new results, recently submitted to The Astrophysical Journal, looked at more data and used improved analysis techniques to find that the data are consistent with no significant correlation between UHECRs and neutrinos.
These results are still intriguing—and again point to the need for further improvements—since they could be explained by an assumption of no common sources of UHECRs and neutrinos, or they could just indicate that we still don’t know enough about them.
There are several hints that point to an extragalactic origin of most high-energy neutrinos in IceCube. First, they are distributed isotropically. Second, their intensity is compatible with what scientists expect from a diffuse flux originating from extragalactic populations of sources. In fact, the observed intensity is close to the Waxman-Bahcall flux, which sets an upper limit on the neutrino rate expected to escape from UHECR sources after the interaction of primary cosmic ray protons with photons or matter near the source.
Measurements of UHECR anisotropy by the Pierre Auger Collaboration indicate that UHECRs with energies above a few EeV are of extragalactic origin, because at lower energies cosmic rays would be trapped by interstellar magnetic fields in their host galaxies. However, at higher energies, UHECRs would escape the local magnetic field and travel to nearby galaxies, with arrival directions at detection that may differ slightly with respect to their initial directions as a result of magnetic field deflections. For UHECR sources beyond our local universe, intergalactic magnetic fields would scramble the direction of cosmic rays and delay their arrival at Earth, thus losing any correlation with neutrinos produced in the same source.
The searches presented in this multi-collaboration paper looked for correlations of nearby extragalactic sources of UHECRs with energies near 50 EeV and above. While neutrinos created in coincidence with UHECRs are expected to carry around 3-5% of the original proton energy, which means hundreds of PeV and above for the UHECRs selected in this study, the same sources would also emit lower energy cosmic rays with neutrinos that can be observed by the IceCube and ANTARES detectors. And, because neutrinos are direct tracers of hadronic interactions of cosmic rays that travel undeflected by magnetic fields, a combined analysis could unveil the location of UHECR sources.
To get the most out of the data, scientists have searched for correlations using full-sky neutrino and UHECRs data sets obtained by combining data from IceCube and ANTARES, and the Pierre Auger Observatory (Auger) and the Telescope Array (TA), respectively. Three analysis methods were used:
1) Researchers used muon neutrino tracks in IceCube and ANTARES, which have a good pointing resolution, to look for neutrino sources, i.e., an excess of neutrinos, that are coincident with the arrival directions of specific UHECRs.
2) Stacks of a few high-energy neutrinos with a high probability of astrophysical origin were used as markers of possible neutrino sources, which were then used to look for clusters of UHECRs around these neutrinos.
3) All pairs of UHECRs and these very high energy neutrinos within a certain angle were counted and compared to what we would expect from a random—not a common source—scenario.
The study involved three analyses that used different and complementary techniques to search for common sources of nearby UHECRs and neutrinos. The first two analyses used the most advanced techniques in IceCube searches for neutrino sources. The second and third analyses used the highest muon neutrino tracks and cascades that IceCube has measured. The third analysis also reduced the uncertainties since its techniques are not dependent on how we account for the magnetic deflection that UHECRs would go through on their way to Earth.
And all three yield the same main result: they did not find a correlation between UHECRs and neutrinos.
The follow-up question is uncomplicated to formulate: why did we not find any correlations? A straightforward explanation could be that nearby UHECR sources are not efficient neutrino emitters.
Unfortunately, current uncertainties are too large to make such a statement. However, researchers were able to set new exclusion limits on spatially correlated neutrino and UHECR fluxes.
So what´s next?
Well, improving the simulation of galactic magnetic fields is probably a must, since two of the methods strongly depend on the magnetic deflection and the charge of the UHECRs, which will also influence the deflection.
But, Lisa Schumacher, who led the first measurement, adds: “In my opinion, now that we know that there is not a strong coincident signal from UHECR and neutrino sources, we want to focus on better understanding what the common sources of UHECRs and neutrinos could be.” Schumacher is now a postdoctoral researcher at the Technical University of Munich and developed and performed the analysis used in the first measurement as a PhD student at RWTH Aachen University. She adds that knowing more about the sources will help them improve the search techniques and, thus, the chances of finding a significant correlation.
For example, the first evidence of a source of high-energy neutrinos, TXS 0506+056, identified neutrinos from a blazar located far from our local universe. This could point to more distant cosmic ray sources as the main producers of the highest energy neutrinos in IceCube, even though the primary cosmic rays would not reach Earth.
Other improvements might come from UHECR observatories and a better knowledge of the proton/nuclei composition of cosmic rays. “Though we didn’t find any significant correlation, it’s important that we move forward with multimessenger analyses. It’s likely that the understanding of the accelerators of the highest energy particles known will be obtained only by looking at different messengers. The game has just begun: with the upgraded AugerPrime detector. we will be able to have more information on the UHECR mass composition on an event-by event basis with large statistics, and we can foresee performing arrival directions studies like this one using only the events candidates to be light nuclei, which would be less deflected by magnetic fields,” explains Lorenzo Caccianiga, a researcher at INFN-Milan, who supervised the contribution of the Pierre Auger Collaboration to this work
Also, next-generation neutrino observatories, such as the planned 10-times-larger IceCube-Gen2 neutrino detector that will be more sensitive to the highest energy neutrinos, might well be the key to uncovering important mechanisms of hadronic acceleration in the most powerful cosmic engines.
“This study unravels the relevance of correlating various messengers measured by multiple experiments and at the same time points out that understanding the sources of the highest energy particles requires further investigation on the propagation of these messengers in the universe. Surely what impresses me is that these messengers are captured by incredibly challenging experiments!” says Teresa Montaruli, a professor at the University of Geneva, who supervised the second analysis of this work and has promoted this multimessenger partnership since it was first started.
+ Info: “Search for Spatial Correlations of Neutrinos with Ultra-High-Energy Cosmic Rays,” ANTARES, IceCube, Pierre Auger and Telescope Array Collaborations: M.G.Aartsen et al., Astrophysical Journal 934 (2022) 2, 164, iopscience.iop.org, arxiv.org/abs/2201.07313