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Neutrinos and the origin of the cosmic rays

Periodic Reporting for period 4 - NEUCOS (Neutrinos and the origin of the cosmic rays)

Reporting period: 2020-03-01 to 2020-08-31

The different observed messengers from the Milky Way and beyond are gamma-rays, cosmic rays, and neutrinos – in addition to the recently discovered gravitational waves. While theories for gamma-ray sources are often not conclusive regarding the presence of cosmic rays in the sources, cosmic rays interacting in the sources with matter or photons will lead to neutrino production. This means that cosmic rays and neutrinos are intimately connected. The observation of different messengers from the same source class can lead to powerful constraints of the theory of the sources, the mechanism of particle acceleration, and, eventually, to the identification of the origin of the cosmic rays.
Since the neutrinos point back directly to their sources, the recent discovery of cosmic neutrinos of presumably extragalactic origin opens new ways to search for the most powerful accelerators in the Universe. Neutrinos are also interesting from the particle physics perspective as their properties are not yet fully understood, and they may test new effects only showing up at extreme distances, energies, or densities.
The NEUCOS-project focused on the connection between neutrinos and cosmic rays with special attention to multi-disciplinary challenges requiring the expertise from both particle physics and astrophysics. One example was the efficient description of cosmic ray accelerators producing heavier nuclei and, possibly, neutrinos. Another example was the study of neutrino properties in ice or sea water, using the same infrastructure as the cosmic neutrino observations.
Identifying the origin of cosmic rays and neutrinos is one of the fundamental questions in particle astrophysics, which is of interest to a wider community and society.
A multi-disciplinary working group was formed to address one of the key challenges of the NEUCOS project, which was the description of astrophysical accelerators loaded with nuclei (heavier than protons) and its consequences for multi-messenger astroparticle physics. It was found that the measurements of the relevant photo-nuclear cross sections are rare, and the corresponding uncertainty has been quantified. The nuclear cascade developing in the sources and its consequences for the neutrino production were established for different astrophysical object classes which may be the primary candidates for the Ultra-High Energy Cosmic Rays (UHECRS), such as Gamma-Ray Bursts - GRBs, Active Galactic Nuclei – AGN and jets from Tidal Disruption Events – TDEs.
Jetted TDEs or low-luminosity were found to be a possible common origin for cosmic rays and, which describe the observed neutrinos and UHECRs from the same emission region – which implies that the radiation densities must be high, and that the nuclear cascade develops which may even describe the ankle feature in the cosmic ray spectrum. Regular GRBs were demonstrated to be able to describe the UHECR flux in multi-collision models; however, the energy requirements are challenging.
In order to describe the neutrino observations including spectrum and spatial distribution in a self-consistent way, a multi-component model was proposed, which includes several populations contributing to the neutrino flux – including one from a source population at the highest neutrino energies which can be connected with the origin of UHECRs. It, however, was also demonstrated during the project that a direct test of the UHECR origin with neutrinos by arrival directions is challenging. Neutrinos connected to the UHECRs may actually produce a flux at the highest energies which are in principle accessible by future radio detection experiments for AGNs; this information is relevant for the optimization of future instruments. On the other hand, it was demonstrated that the neutrinos from interactions with cosmic background light may be sub-dominant at these energies.
Apart from the question where the diffuse neutrino and cosmic ray fluxes come from, several multi-messenger discoveries were drawing a lot of attention during the project duration: a short gamma-ray burst associated with a binary neutron star merger gravitational wave event, neutrinos associated with an AGN named TXS 0506+056, and a neutrino produced by the TDE AT2019dsg. Within the project, it was demonstrated that no neutrinos were expected from the gravitational wave event, in consistency with observations. For the AGN and TDE neutrino detections, leading theoretical explanations have been developed within NEUCOS.
Regarding the optimization of future neutrino telescopes, the role of the flavor composition of astrophysical neutrinos and the Glashow resonance was studied for searches for physics beyond the Standard Model in particle physics, and for astrophysical neutrino production diagnostics. As one of the highlights, the potential of future atmospheric neutrino oscillation experiments to perform Earth tomography by neutrino coherent forward scattering with Earth matter was quantified. The NEUCOS project was extended by a WP 6: “Interdisciplinary application of simulation methodology to the new viral disease Covid-19”; a team member contributed to epidemiological modeling and an Italian outreach page, which had a major impact in the beginning of the pandemic (https://covstat.it/).
The results were disseminated in many workshop, summer school, conference and seminar contributions by the PI and the team members. The NEUCOS team also co-organized and co-sponsored the TeVPA 2018 conference as well as it co-organized the PAHEN 2019 workshop, both events took place in Berlin.
The main objectives of the NEUCOS project were achieved: several astrophysical neutrino emitters were identified, and it was demonstrated that there is apparently more than one astrophysical object class contributing to the diffuse neutrino flux. The origin of the dominant part of the diffuse UHECRs and neutrinos was studied for the leading candidate classes; for instance, LL-GRBs are an interesting object class to be studied in further observational campaigns and models. Recent evidence for UHECR arrival direction correlations with AGN and starburst galaxies have attracted the attention of NEUCOS towards the end of the project; corresponding studies will be continued.
Among the most significant results of the project were trendsetting interpretations for the common origin of the diffuse fluxes of UHECRs and neutrinos, leading multi-messenger interpretations of the neutrinos from the AGN blazar TXS 0506+056 and from the TDE AT2019dsg, which were the first astrophysical neutrino sources discovered during the progress of this project by multi-messenger follow-ups, and critical conclusions for the optimization of future instruments. The methodology developed during the project balances efficiency and precision in a way that it will path the way to the next discoveries.