The astroparticle physics program addresses questions of fundamental physics in astrophysical systems. Current research topics include Solar Neutrinos, Sterile Neutrino Searches, Neutrino Oscillations, direct Dark Matter searches, and Ultra-High Energy Cosmic Rays.
Solar Neutrinos: Milano has a major role in the Borexino Solar Neutrino Experiment at the Gran Sasso Laboratory. The detector consists of 1300 tons of ultra-pure liquid scintillator, taking data continuously since 2007. Borexino made the first direct-counting measurement of low-energy neutrinos from the Sun (7Be and pep neutrinos) and in 2014 the first observation of the fundamental and very elusive pp-neutrinos, a direct proof of the sun stability over a 10^4 year period that achieved publication on the journal Nature. These measurements have contributed to establish fundamental features of neutrino oscillations and at the same time they allow to understand the complex physics of the Sun and of stellar evolution. The experiment has also measured geo-neutrinos for the first time, an important step toward the understanding of the Earth composition.
Sterile Neutrino Searches: Several experimental hints accumulated in the last decade pointing to the possible existence of a sterile neutrino at the few eV mass scale. The Borexino detector is perfectly suited to host a short baseline neutrino oscillation experiment that can shed light on this intriguing hypothesis by observing the associated oscillation signature. The experiment SOX is financed by two prestigious ERC grants and will be carried out in 2016 by using a 144Ce-144Pr anti-neutrino source placed close to the detector. The SOX result is eagerly awaited in the neutrino physics community, as it can finally solve one of the most puzzling questions of the last twenty years.
Neutrino Oscillations: The JUNO detector will exploit the liquid scintillation technology to address one of the main remaining unknowns of the neutrino oscillation paradigm, i.e. the neutrino mass hierarchy, essential to progress in our knowledge of the PMNS neutrino mixing matrix. JUNO will target the mass hierarchy by measuring the spectral features of the intense anti-neutrino flux from two powerful nuclear reactor plants in the South of China. Its physics program will include also the precise determination, at the sub-percent level, of the “solar” oscillation parameters θ12 and Δm21, as well as of Δm31. Moreover, it will address the typical astroparticle physics program of a liquid scintillator detector, i.e. the study of solar neutrinos, geo-neutrinos and supernova neutrinos, and related exotic searches.
θUltra High Energy Cosmic rays: The Pierre Auger Observatory (PAO) is an international cosmic ray observatory designed to detect ultra high energy cosmic rays. These are sub-atomic particles (protons or other nuclei) with energies beyond 10^20 eV. Such high energy particles have an estimated arrival rate of 1 per square kilometer per century. Therefore, in order to record a sufficiently large number of these events, the Auger Observatory has created a detection area the size of 3000 square km. The PAO is the largest ultra-high energy cosmic ray detector in the world and it is located on the vast plain of Pampa Amarilla in Argentina. The PAO is the first experiment that combines both ground and fluorescence detectors at the same site, thus allowing cross-calibration and reduction of systematic effects that may be peculiar to each technique.
Direct Dark Matter Searches: There is an overwhelming evidence that some 30% of the universe is in the form of cold dark matter, but the nature of this matter remains a mystery. According to the most attractive theory, it is made of Weakly Interacting Massive Particles (WIMPs) that froze out of the early universe. DarkSide at Gran Sasso Laboratory is an innovative experiment for the direct detection of WIMPs in the galactic halo. The design concept involves two-phase, liquid argon Time-Projection Chambers (LAr-TPC) in which the energy released in WIMP-induced nuclear recoils can produce both scintillation and ionization. DarkSide is operating a 50kg detector and is designing the next ton-scale phase of the project, featuring major technological advances like SiPM photo-sensors and the directionality sensitivity of WIMP scattering.WIMP-nucleus interaction rate is also expected to undergo a yearly modulation due to the variation of the relative speed of the Earth in the DM galactic halo during the revolution around the Sun. The DAMA/LIBRA high significance observation of this modulation points to a WIMP mass of ~10GeV. This result is however controversial due to lack of observation by several other experiments, which investigated the same parameter space with different technologies.
A new project is therefore starting that aims to detect the yearly modulation using the same technology, NaI scintillating crystals, with the novel advantage of an active liquid scintillator background rejection. Two twin detectors will be exposed in the opposite hemispheres, at LNGS and in Australia, decoupling the signal from any season-related modulation.