Neutrino astrophysics in the multi-messenger era

Since 2013, the detection of a diffuse flux of high-energy neutrinos (0.1-10 PeV), in excess with respect to the atmospheric one, has opened a new window to the Universe, revealing the existence of extremely energetic astrophysical neutrino sources. Within the context of standard acceleration scenarios, this measurement implies that the processes responsible for the production of these particles proceed all the way up to multi-PeV energies.

In addition, in 2017 the first extra-galactic source of high-energy neutrinos was announced, namely TXS 0506+056. This was identified into a blazar, known since 1983 through its radio emission, and recently observed in flaring activity of multi-GeV gamma rays by Magic and Fermi. The observation of a multi-TeV neutrino emission from this source has assessed the fundamental role of neutrinos in the context of multi-messenger studies, since the presence of neutrinos as part of the flux originated by the astrophysical source can probe the physics of accelerated hadrons, and possibly test physics beyond the Standard Model.

Since the neutrino fluxes from cosmic sources are expected to be faint, of the order of few particles per km³ per year, and given the small cross-section of neutrino interactions, a Neutrino Telescope requires a large instrumented volume. This can be achieved by using the Cherenkov light revelation technique in transparent media, sucha as sea-water or ice.

The IceCube experiment at the South Pole, in operation for more than a decade, has reached the km³ dimension, paving the way with its discoveries to the era of neutrino astrophysics. The future generation of Neutrino Telescopes will consist of three-dimensional arrays of photomultipliers, deployed deep into the water of the Mediterranean Sea (at about few kilometres depth): it will observe the Cherenkov light induced in water by the passage of an ultra-relativistic charged particle, namely the secondary product of neutrino interaction with the surrounding water.

A Rome group, let's refer to it as “astrophysical neutrino group,” has been promoting for more than two decades the construction of a multi-km³ Neutrino Telescope in the Mediterranean Sea: at the beginning by participating in the NESTOR Collaboration, then with the activities performed within the INFN-NEMO program for the measurements of the environmental properties of deep-sea Mediterranean sites. Recently the astrophysical neutrino group in Sapienza University of Rome is involved in the operation of the current major neutrino telescope in the Northern Hemisphere, ANTARES, as well as in the construction of the next-generation cubic-kilometre detector, KM3NeT.

The KM3NeT project will consist of two detectors, built with the same technology, dedicated to two different aspects of neutrino physics:

• the Astronomy Research with Cosmics in the Abiss (ARCA) will be located close to the Sicily coast at 3500m depth and will be devoted to high-energy neutrino astrophysics. The relative distance of the optical modules will be on average ~40m in the vertical and ~100m in the horizontal direction, respectively, in order to have optimal detection for neutrinos with E_ν > 1 TeV.
• The Oscillation Research with Cosmics in the Abiss (ORCA) will be much more compact (the optical modules distance will be on average ~9m and ~20 in the vertical and horizontal direction, respectively) and, being capable of reconstructing neutrinos with E_ν > few GeV will allow the study of intrinsic neutrino properties, such as the mass hierarchy and the atmospheric neutrino oscillation.

In particular, at present the group has expertise in several research areas, ranging from the analysis of data collected by the aforementioned experiments, to the temporal and charge calibration of the detectors, as well as to the design and validation tests of the electronics used in the apparatus. In the context of multi-messenger data analysis, the group has been mainly working in correlation studies among high-energy neutrinos and high-energy gamma rays from Gamma-Ray Bursts (GRBs), the most energetic explosions of the Universe. In addition, in the same context of multi-messenger studies, the group has been active in correlations among high-energy neutrinos and ultra-high-energy cosmic rays (UHECRs). Concerning astronomical studies, the group has performed searches for dark matter from neutrino annihilation in the Galactic Center.

The group consists of 4 senior scientists, namely 1 full professor, 2 technologists and 1 University researcher, plus 2 post docs, 1 PhD student and 1 Master student:

• Prof. Antonio Capone, full professor. He has been the pioneer, in Italy, of “high-energy neutrino astroparticle physics” within the NESTOR project (1996-1998) and later within the INFN-NEMO project (1997-2005). Since the year 2000, he is member of the ANTARES Collaboration. He has been one of the promoters of the KM3NeT Collaboration since 2005. He has in particular carried out a series of measurements (1998-2002) for the characterization of the environmental properties of deep-sea water sites in the Mediterranean Sea in order to select the best location for the future Neutrino Telescope. He is acting as supervisor on the analysis activity of the Rome group; at present he is member of the Publication Committee of the ANTARES and KM3NeT Collaborations.
• Dr. Fabrizio Ameli is a Staff Senior Technological Researcher at INFN-Rome. He participated to the activity of the astrophysical neutrino group since the beginning of its scientific activities with his expertise in analog and digital electronics. After the realization of front-end and transmission electronics for NESTOR and NEMO experiments, he is currently contributing to the KM3NeT submarine infrastructure with an electronic board devoted to acoustical and optical calibration of the detector.
• Dr. Carlo Nicolau is a Staff Technological Researcher at INFN-Rome. After contributing to the realization of the front-end electronics for the NEMO experiment, he is currently contributing to the design, realization and validation of the power distribution network for the KM3NeT infrastructure.
• Dr. Irene Di Palma is a University researcher. She is a member of ANTARES and KM3NeT Collaborations since more than decade and also part of LIGO-Virgo Collaboration since 2009. Her major interest is astrophysical data analysis, with a specific focus on multi-messenger astroparticle physics which will allow us to study cosmic processes and violent relativistic events such as the collisions of black holes or neutron stars, supernovae, and gamma-ray bursts, and objects unreachable through conventional methods.
• Dr. Paolo Fermani, post-doc. He is a member of ANTARES and KM3NeT since 2010, and his main research expertise concerns into dark matter data analysis and detector calibrations. He already performed an indirect search of DM from the Galactic Centre using ANTARES data, he is planning to analyse the KM3NeT-ORCA data to search again for an indirect signature of DM.
• Dr. Silvia Celli, post-doc. She joined the ANTARES and KM3NeT Collaborations in 2015, being responsible for the ANTARES data analysis of neutrinos from GRBs. As a member of the CTA Consortium, her research activities have also been dedicated to cosmic-ray and gamma-ray physics in the context of particle acceleration, particle propagation and particle escape from supernova remnants.
• Angela Zegarelli, PhD student. She is a member of ANTARES and KM3NeT Collaborations since 2018, when she started to work on her master thesis. She is actually involved in the search of neutrinos from GRBs, collaborating in the data analysis.
• Michela Fasano, Master student in Particle and Astroparticle Physics; her thesis project consists in classifying GRBs, focusing on the Supernovae-GRB connection, in order to investigate the “choked GRB” theory and to explore neutrino flux predictions.