LEANOR takes place of multi-messenger astronomy (MMA), a new, multidisciplinary approach, which combines electromagnetic observations, neutrinos, and gravitational waves in view of understanding the most energetic phenomena in our Universe. While astrophysical sources have been studied using light for centuries, the possibility to observe them through different messengers has been made possible in the past 10 years with the first observation of high-energy astrophysical (HE) neutrinos in IceCube and the first detection of gravitational waves by LIGO and Virgo.
Neutrinos play a special role in MMA: being neutral and weakly interacting particles, they can travel over cosmological distances, without being deflected by magnetic fields nor absorbed by matter and radiation. The unique scientific value of measuring cosmic neutrinos has motivated the advent of neutrino telescopes in the last decade. These telescopes challenge the smallness of the interaction rates through the instrumentation of large volumes to detect the Cherenkov light emitted by secondary particles produced in neutrino interactions with the water or the ice. The largest operating neutrino telescope, IceCube, is a 3D matrix of sensors deployed over a volume of 1 km^3 deep in the ice of the South Pole. The next-generation detector of comparable size, KM3NeT, has started construction in the Mediterranean Sea.
Large neutrino telescopes have been working towards the identification of the astrophysical sources producing high-energy astrophysical neutrinos. While recent multi-messenger observations suggest that blazars may be the first identified sources of high-energy neutrinos, other popular candidates are gamma-ray bursts (GRBs), the brightest electromagnetic events observed in the Universe. The observation of a short GRB in coincidence with the gravitational wave event GW170817 confirmed the hypothesis that such events are produced by binary neutron star mergers. The joint observation of a neutrino signal would be another breakthrough. So far, none of the neutrino searches revealed a significant signal.
HE neutrino telescopes are optimized for the TeV-PeV range. At lower energies, neutrino experiments have focused on the characterization of the atmospheric neutrino flux. The GeV energy domain remained poorly explored in terms of astrophysical observations, but as astrophysical neutrino fluxes typically exhibit a power-law decrease with energy, one can expect that neutrino telescopes sensitive to such low energies (LE) could allow us to probe larger neutrino fluxes and possibly identify new astrophysical neutrino sources!
ORCA is the low-energy branch of KM3NeT. It is optimized for the detection of neutrinos in the 5 – 100 GeV range, mainly focusing on fundamental neutrino physics. The ORCA digital optical modules (DOMs) rely on the innovative KM3NeT design featuring 31 small photosensors in one glass sphere. The first ORCA detection unit (DU), a flexible line about 350m high and supporting 18 DOMs equally spaced by 9m, has been installed in September 2017 offshore Toulon, France. In June 2022, 10 DUs have been deployed on the ORCA site.
LEANOR proposed to enlarge the science scope of ORCA by turning it into a GeV neutrino telescope and seized this experimental opportunity to develop an innovative, promising and cost-effective way to observe astrophysical GeV neutrinos, with two main objectives:
-develop the potential of ORCA to detect LE astrophysical neutrinos: LEANOR could not only demonstrate the feasibility of astrophysical neutrinos searches with ORCA in the GeV range, but also contributed to the improvement of astrophysical neutrino detection at lower (MeV) and higher (GeV-TeV) energies!
-promote low-energy neutrinos as new promising messengers to study the Extreme Universe: LEANOR led to the first constraints on the GeV neutrino flux using IceCube for gravitational wave candidates detected by LIGO and Virgo. LEANOR also led to the development of multi-detector techniques that significantly enhance the potential of low-energy astrophysical neutrino detection.
