Search for a diffuse astrophysical neutrino emission from the Galactic Ridge with the first KM3NeT/ARCA data and strategies for real-time follow-up searches of rapid transient phenomena

Multi-messenger astronomy has allowed a completely and revolutionary new approach to the observation of the Universe. Within this picture, neutrino astrophysics has started in recent years to consolidate the theoretical framework at its basis through the first detection of neutrino sources. However, the complexity of astrophysical environments requires more observational data to fully characterise their internal mechanisms. For this reason, second-generation neutrino telescopes are currently under construction worldwide at different locations. Specifically, KM3NeT inheriting the experience of the ANTARES telescope is a detector under construction and deployed in the abyssal site of the Mediterranean Sea. Thanks to its design, it is already taking data and will be capable, in the next few years, of complementing the observations conducted up to now and of shedding light on important unanswered questions. In June 2023 the IceCube Collaboration reported the first observation of astrophysical neutrinos that originated in the centre of our Galaxy. Within the work of this thesis, the first data collected by the KM3NeT experiment have been analysed, mainly focusing on the search for neutrino emission from this location in the sky. In fact, thanks to the position and to the excellent angular resolution of the KM3NeT detectors, the Galactic plane is visible for most of the time, representing one of the main physics objectives to address for the experiment. Furthermore, in the observation of fast transient phenomena, neutrino signals could be invaluable precursors of electromagnetic light ejected by sources. For this reason, the last chapter of the thesis is focused on the first correlation studies ever performed with KM3NeT data with respect to external alerts sent by other observatories.