Search for correlations of the arrival directions of high-energy cosmic neutrinos and highest-energy charged cosmic rays

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The origin of charged cosmic rays remains a mystery, even more than 100 years after their discovery. Main reason is their deflection in galactic and extra-galactic magnetic fields. Only at highest energies is the expected deflection sufficiently small to attempt directional astronomy. However, even with the accumulated statistics of more than a decade operation of the world-wide largest observatories Pierre Auger and Telescope Array, no sources could be identified to date. A promising approach is to correlate the directions of high-energy cosmic rays with observations of high-energy neutral particles, neutrinos and photons. These are not deflected and are expected to be produced also by or near the sources that accelerate cosmic rays. While high-energy photons above several tens of TeV are absorbed already within the local universe, high-energy neutrinos are promising messengers because they can propagate almost unaffected over cosmological distances. Recently the IceCube Neutrino Observatory has discovered a flux of cosmic neutrinos extending to above PeV energies. The sources of this flux have not been identified yet, but the flux appears largely isotropic indicating an extra-galactic origin. Furthermore, the flux normalization is consistent with the Waxman-Bahcall expectation that was derived from the observed flux of ultra-high-energy cosmic rays. In order to evaluate a possible connection between these fluxes and to identify the astrophysical sources that are accelerating cosmic rays, scientists from the Antares, IceCube, Pierre Auger and Telescope Array Observatories are analyzing their data for directional correlation. As a result, weak indications were found initially, which were, however, not confirmed by the most recent data. In this project we propose a follow-up analysis, which strongly improves in terms of statistics, directional information of the neutrino data and the analysis methods. The statistics of all data sets is increased by more than a factor two and particularly the statistics of track-like events with good pointing is improved by about a factor four. The analysis which is using these track-like events will be based on a new, magnetic-deflection-independent, likelihood method, by explicitly fitting for the most probable location of common sources. A positive observation would be a scientific breakthrough, as it would be direct evidence that at least a fraction of the sources of the astrophysical neutrino flux are relatively close to our galaxy. The good pointing of neutrinos would allow to directly pin-point the sources of cosmic rays and to connect these two observations in a multi-messenger context. Even a negative result compatible with a no-correlation hypothesis can be used to constrain the relation of cosmic rays and neutrinos as well as source and propagation models.