Search for high-energy cosmic neutrinos


The origin and nature of cosmic rays at very high energy (UHECRs) is a long-standing puzzle that is still not completely resolved after a century of theoretical and experimental efforts. Neutrinos can travel untouched from their source, thus indicating their locations. Neutrinos of cosmic origin are expected to be produced via the decay of charged pions and kaons, generated in hadronic interactions of cosmic rays with gas or radiation in their acceleration sites or during their propagation. The decay of neutral pions also produces gamma-rays intimately connecting the three messengers: cosmic rays, neutrinos and gamma-rays. High-energy neutrinos (HEN), in contrast to gamma-rays, are solely produced through scenarios implying the acceleration of cosmic-rays (except for exotic models). For this reason, their observation would constitute an unambiguous signature of a source of UHECRs.
            Neutrino telescopes instrument large volumes of water or ice with photomultipliers (PMT) to detect the Cherenkov radiation inferred by charged leptons induced neutrino that interact with the target transparent medium, inside or near the instrumented volume. Depending on their size and of the density of the PMT configuration, such detectors can be used for different physics purposes: 
- gigaton-scale detectors focus on the detection of cosmic HEN (TeV--PeV) originating from astrophysical sources. 
- megaton-scale, denser detectors can address the fundamental properties of neutrinos, and in particular their mass ordering needed to constrain the models that seek to explain the origin of mass in the leptonic sector and the differences within the mass spectrum of all elementary particles. They focus on lower energy (~GeV) neutrinos produced in the atmosphere. 
            Within this double perspective, the international KM3NeT [1] project is deploying the next-generation neutrino telescope in the Mediterranean Sea, with one site dedicated to TeV-PeV neutrino astronomy in Italy (ARCA), and one site dedicated to the measurement of the neutrino mass hierarchy in France (ORCA). KM3NeT builds on the expertise gained in the operation of the smaller detector ANTARES [2], deployed at a depth of ~2500m, off Toulon, still taking data. A major technological upgrade of KM3NeT consists in the use of novel multi-PMT optical modules (OM) whose first prototypes are now successfully tested undersea. The construction of the detector on both sites has already started and is expected to extend during the period of this project. It is therefore expected that the PhD students enrolled in the project will have access to these data and should be able to provide first results with real data, in addition to sensitivity estimates related to the full expected detector.
Among the various candidates proposed as cosmic ray accelerators, Active Galactic Nuclei (AGN) are of particular interest. They are the observational effect of accretion of matter onto supermassive black holes in the center of Galaxies, and are the most luminous, persistent objects in the sky. In a class of AGNs, the accretion of matter is associated with an outflow in the form of a pair of relativistic jets of plasma. When one of the jets points in the direction of the Earth, the source appears extremely bright and is dubbed blazar. Blazars indeed dominate the extragalactic gamma-ray sky in the GeV and TeV energy bands. Electromagnetic radiation from AGN jets can be produced via a variety of non-thermal radiative processes: if hadrons are accelerated in jets, physics of hadronic interactions (proton-photon and proton-proton collisions) imply that the photons produced in blazars must be accompanied by a flow of neutrinos. 
The first association between a HEN and a cosmic source has actually been reported by IceCube [3] in September 2017, involving the so-called blazar TXS 0506+056. A following study looking at IceCube’s archival data in the direction of the blazar has also shown the presence of a 3.5σ candidate neutrino flare between September 2014 and March 2015. However, in this case the gamma-ray flux of the source was one order of magnitude lower and showed no sign of time variability. If the neutrino signal is genuine, this implies that the relation between the observed gamma-ray flux and the HEN flux is not simple [4], which underline the importance of making contact with phenomenologist experts as foreseen in this project 
Other recent findings using IceCube data and involving radio blazars are of particular interest [5,6]. Radio galaxies are a subclass of active galaxies that could be considered as possible sources of high-energy neutrinos. The neutrinos could be produced by the hadronic interaction of accelerated cosmic rays within the jets or in the giant lobes at the end of the jets. A sample of 65 soft gamma-ray selected radio galaxies was recently probed by the ANTARES Collaboration reporting a small excess with a pre-trial p-value p = 4.8 · 10-3, equivalent to a 2.8 σ excess, reduced to 1.6 σ post-trial [7].
In the light of the above-mentioned context, the scope of the PhD project is to study the astrophysical potential of KM3NeT in the search for neutrinos from blazars (including radio sources) and perform first analyses with the available data from KM3NeT/ARCA (potentially combining them with the full ANTARES dataset). Being part of the ANTARES and KM3NeT Collaborations offers the possibility to use a reliable modeling of the detector response, which is necessary for a robust analysis. 
The final analysis that will be performed should lead to a publication, reporting our findings possibly ranging from constraints on the models (upper limits on the relevant parameters) to a signal identification…
[3] IceCube collaboration, Science 361, 147 (2018).
[4] M. Cerruti e al., MNRAS: Letters, Volume 483, Issue 1, February 2019, Pages L12–L16,
[5] A. V. Plavin, Y. Y. Kovalev, Yu. A. Kovalev, and S. V. Troitsky, Astrophys. J. 894, 101 (2020), arXiv:2001.00930
[6] A. V. Plavin, Y. Y. Kovalev, Yu. A. Kovalev, and S. V. Troitsky, Astrophys. J. 908, 157 (2021), arXiv:2009.08914
[7] Antares, A. Albert et al., To appear in ApJ,


Julien Aublin et Antoine Kouchner






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