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Neutrinos and the origin of the cosmic rays

Periodic Reporting for period 3 - NEUCOS (Neutrinos and the origin of the cosmic rays)

Reporting period: 2018-09-01 to 2020-02-29

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 focuses 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 is the efficient description of cosmic ray accelerators producing heavier nuclei and, possibly, neutrinos. Another example is 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.
See image: Nuclear Cascade - Photonuclear cross section measurements (red/yellow) compared to required information for cosmic ray astrophysics (blue).
From: “Nuclear Physics Meets the Sources of the Ultra-High Energy Cosmic Rays” by D. Boncioli, A. Fedynitch, W. Winter, Scientific Reports 7 (2017) 4882
"A multi-disciplinary working group has been formed to address one of the key challenges of the NEUCOS project, which is the description of astrophysical accelerators loaded with nuclei (heavier than protons) and its consequences for multi-messenger astroparticle physics. It has been found that the measurements of the relevant photo-nuclear cross sections are rare, and the corresponding uncertainty has been quantified (Boncioli, Fedynitch, Winter, Scientific Reports 7, 2017, 4882). The nuclear cascade developing in the sources and its consequences for the neutrino production have been computed for different astrophysical object classes, such as Gamma-Ray Bursts (GRBs; Biehl, Boncioli, Fedynitch, Winter, Astronomy&Astrophysics 611, 2018, A101), Active Galactic Nuclei (AGN; Rodrigues, Fedynitch, Gao, Boncioli, Winter, Astrophysical Journal 854, 2018, 54) and jets from Tidal Disruption Events (TDEs; Biehl, Boncioli, Lunardini, Winter, Scientific Reports 8 (2018) 10828). For example, it has been demonstrated that, for Gamma-Ray Bursts, the expected neutrino flux hardly depends on the injection composition, which means that the neutrino constraints to the ultra-high energy cosmic ray origin of neutrinos apply to sources loaded with nuclei as well.
A possible common origin for cosmic rays and neutrinos has been found to be jetted TDEs or low-luminosity GRBs, which describe the observed neutrinos and cosmic rays at the highest energies from the same emission region – which implies that the radiation densities must be high, and that the nuclear cascade develops (Biehl, Boncioli, Lunardini, Winter, Scientific Reports 8 (2018) 10828; Boncioli, Biehl, Winter, Astrophysical Journal 872 (2019) 110). In order to describe the neutrino observations including spectrum and spatial distribution in a self-consistent way, a multi-component model has been proposed (Palladino, Winter, Astronomy&Astrophysics 615 (2018) A168) which includes several populations contributing to the neutrino flux – including one from TDEs (or a similar contribution) at the highest energies which can be connected with the origin of ultra-high energy cosmic rays.
Apart from the question where the diffuse neutrino and cosmic ray fluxes come from, two multi-messenger events have been drawing a lot of attention recently: a short gamma-ray burst associated with a binary neutron star merger gravitational wave event, and neutrinos associated with an AGN named TXS 0506+056. Within the project, it has been demonstrated that no neutrinos were expected from the gravitational wave event, in consistency with observations (Biehl, Heinze, Winter, Monthly Notices of the Royal Astronomical Society 476, 2018, 1191). For the AGN flare, the signature of the cosmic ray loading in the electromagnetic spectrum has been identified, and self-consistent flare models for the neutrino emission have been developed (Gao, Fedynitch, Winter, Pohl, Nature Astronomy 3 (2019) 88; Rodrigues, Gao, Fedynitch, Palladino, Winter, Astrophysical Journal Letters 874 (2019) L29).
Regarding the optimization of future neutrino telescopes, the role of the flavor composition of astrophysical neutrinos and the Glashow resonance have been studied for searches for physics beyond the Standard Model in particle physics, and for astrophysical neutrino production diagnostics (Biehl, Fedynitch, Palladino, Weiler, Winter, Journal of Cosmology and Astroparticle Physics 11, 2017, 009). 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 has been quantified (Winter, Nuclear Physics B908, 2016, 250, special issue ""Neutrino Oscillations: Celebrating the Nobel Prize in Physics 2015""). At the highest neutrino energies, future radio detection experiments are being discussed and optimized; therefore neutrino flux predictions (cosmogenic neutrinos) for interactions of UHECRs with cosmic background photons"
The technology developed for studying nuclei in astrophysical sources is currently being applied to leading source class candidates for the cosmic rays observed at the highest energies, and detected astrophysical neutrinos associated with multi-messenger signals.
Among the most significant results of the project so far have been leading interpretations for the common origin of the diffuse fluxes of UHECRs and neutrinos, some of the most accepted multi-messenger interpretations of the neutrinos from the AGN blazar TXS 0506+056, which is the first astrophysical neutrino source discovered during the progress of this project, and critical conclusions for the optimization of future instruments, such as regarding the expected cosmogenic neutrino fluxes.
Eventually, plausible scenarios for the origin of cosmic rays, tested with neutrinos, will be highlighted or constrained; several concluding articles are work in preparation.
Nuclear Cascade Image (D. Boncioli, A. Fedynitch, W. Winter; Scientific Reports 7 (2017) 4882)