Astroparticle physics originated in 1912 with the discovery of cosmic rays, when particles of extraterrestrial origin were first observed traversing the Earth’s atmosphere. This discovery opened up the possibility of studying the Universe and astrophysical objects not only through optical telescopes, but also using instruments developed for particle physics, thereby providing complementary information to traditional astrophysics. Today, the composition of cosmic rays is well understood, being primarily made of nuclei (mostly protons), yet their origin remains an open question. Identifying their sources is particularly challenging: as charged particles, cosmic rays are deflected by electromagnetic fields, making it difficult to trace back their trajectories. For this reason, the search for their sources relies on other electrically neutral messengers produced in the same acceleration and propagation processes, namely gamma rays and neutrinos. Gamma rays, i.e. high-energy photons, are observed with techniques similar to those used for cosmic rays, and several very-high-energy sources are now known. However, distinguishing the origin of gamma-ray emission is not straightforward, as it can arise from both leptonic and hadronic processes; moreover, gamma rays may be absorbed during propagation. Neutrinos, on the other hand, interact only weakly with matter and can therefore carry information about their production mechanisms almost unaltered. In addition, unlike gamma rays, neutrinos are produced exclusively in hadronic processes, making them a unique probe of cosmic-ray acceleration. However, their weak interaction also makes them extremely difficult to detect, requiring large detection volumes such as water or ice. Neutrino telescopes are based on the detection of Cherenkov radiation emitted by charged particles produced in neutrino interactions. In 2013, the IceCube experiment, located at the South Pole with an instrumented volume of about 1 km³, reported the first observation of a flux of astrophysical neutrinos; in 2017, it also identified a source candidate, the blazar TXS 0506+056. These discoveries highlighted the need for cubic-kilometer-scale detectors and motivated the construction of new neutrino telescopes, including KM3NeT, currently under construction in the Mediterranean Sea. KM3NeT consists of two detectors: ORCA, dedicated to atmospheric neutrino studies, and ARCA, designed for high-energy neutrino astronomy. In its final configuration, ARCA will comprise 230 detection lines equipped with photomultiplier tubes to measure Cherenkov light, instrumenting a volume of about 1 km³. At present, about 20% of the detector is deployed and taking data. In 2023, with 21 active lines, ARCA detected the highest-energy neutrino ever observed (KM3-230212A), highlighting the experiment’s scientific potential. This PhD work, carried out within the KM3NeT Collaboration, focuses on the search for neutrino emission from point-like sources. The results presented are based on data collected by ARCA between May 2021 and September 2023, spanning detector configurations from 6 lines (ARCA6) up to 21 lines (ARCA21). The event selection procedure developed for the ARCA19 and ARCA21 datasets is also discussed, as neutrino telescopes are affected by a significant background of atmospheric neutrinos and muons. Finally, a likelihood-based method used for the data analysis is described. In addition to the KM3NeT/ARCA Point Source results, three complementary studies are presented: a follow-up analysis of the KM3-230212A event, a joint point source search analysis combining ARCA and ANTARES (the predecessor of KM3NeT), and a study of Galactic sources in which gamma-ray observations are used to model the expected neutrino signal.
Follow-up of the neutrino emission from known gamma-ray celestial sources
PARISI, VITTORIO
2026-07-09
Abstract
Astroparticle physics originated in 1912 with the discovery of cosmic rays, when particles of extraterrestrial origin were first observed traversing the Earth’s atmosphere. This discovery opened up the possibility of studying the Universe and astrophysical objects not only through optical telescopes, but also using instruments developed for particle physics, thereby providing complementary information to traditional astrophysics. Today, the composition of cosmic rays is well understood, being primarily made of nuclei (mostly protons), yet their origin remains an open question. Identifying their sources is particularly challenging: as charged particles, cosmic rays are deflected by electromagnetic fields, making it difficult to trace back their trajectories. For this reason, the search for their sources relies on other electrically neutral messengers produced in the same acceleration and propagation processes, namely gamma rays and neutrinos. Gamma rays, i.e. high-energy photons, are observed with techniques similar to those used for cosmic rays, and several very-high-energy sources are now known. However, distinguishing the origin of gamma-ray emission is not straightforward, as it can arise from both leptonic and hadronic processes; moreover, gamma rays may be absorbed during propagation. Neutrinos, on the other hand, interact only weakly with matter and can therefore carry information about their production mechanisms almost unaltered. In addition, unlike gamma rays, neutrinos are produced exclusively in hadronic processes, making them a unique probe of cosmic-ray acceleration. However, their weak interaction also makes them extremely difficult to detect, requiring large detection volumes such as water or ice. Neutrino telescopes are based on the detection of Cherenkov radiation emitted by charged particles produced in neutrino interactions. In 2013, the IceCube experiment, located at the South Pole with an instrumented volume of about 1 km³, reported the first observation of a flux of astrophysical neutrinos; in 2017, it also identified a source candidate, the blazar TXS 0506+056. These discoveries highlighted the need for cubic-kilometer-scale detectors and motivated the construction of new neutrino telescopes, including KM3NeT, currently under construction in the Mediterranean Sea. KM3NeT consists of two detectors: ORCA, dedicated to atmospheric neutrino studies, and ARCA, designed for high-energy neutrino astronomy. In its final configuration, ARCA will comprise 230 detection lines equipped with photomultiplier tubes to measure Cherenkov light, instrumenting a volume of about 1 km³. At present, about 20% of the detector is deployed and taking data. In 2023, with 21 active lines, ARCA detected the highest-energy neutrino ever observed (KM3-230212A), highlighting the experiment’s scientific potential. This PhD work, carried out within the KM3NeT Collaboration, focuses on the search for neutrino emission from point-like sources. The results presented are based on data collected by ARCA between May 2021 and September 2023, spanning detector configurations from 6 lines (ARCA6) up to 21 lines (ARCA21). The event selection procedure developed for the ARCA19 and ARCA21 datasets is also discussed, as neutrino telescopes are affected by a significant background of atmospheric neutrinos and muons. Finally, a likelihood-based method used for the data analysis is described. In addition to the KM3NeT/ARCA Point Source results, three complementary studies are presented: a follow-up analysis of the KM3-230212A event, a joint point source search analysis combining ARCA and ANTARES (the predecessor of KM3NeT), and a study of Galactic sources in which gamma-ray observations are used to model the expected neutrino signal.
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