Neutrino Astrophysics (D. Semikoz, C. Volpe)
Neutrinos tell us stories from sources far away in space and time. After the discovery of neutrino oscillations, an impressive progress has been made in our knowledge of neutrino properties in the past decade. Still, they remain intriguing elementary particles that still have key unknown properties to reveal. These include their absolute mass and mass ordering, leptonic CP violation, the neutrino Majorana or Dirac nature and the existence of a fourth sterile neutrino. Their measurement will bring fundamental bricks for physics beyond the Standard Model of particles and interactions. Neutrinos are also intimately connected with major astrophysics questions, of the energy production in stars, of the end of life of massive stars and of the origin of the elements that make up stars and our Universe. In particular, uncovering how neutrinos change flavor in such environments is essential for these issues. The cold background of cosmological neutrinos is yet to be discovered, while it has left an imprint on the nucleosynthesis of light elements (big-bang nucleosynthesis) and on large scale structures. The APC theory group works at the forefront of this research, investigating fundamental aspects, conceiving new avenues, or in close connection with experiments.
Neutrino flavor evolution in astrophysical environments (C. Volpe)
Important progress is ongoing in our understanding of how neutrinos change flavor in astrophysical environments, in particular core-collapse supernovae. In fact, neutrinos change flavor in unexpected ways in massive stars, compared to the case of our Sun. It is now established that the solar neutrino deficit is due to the Mikheev-Smirnov-Wolfenstein effect - a resonant adiabatic flavor conversion phenomenon due to the neutrino interaction with matter. In massive stars, novel neutrino conversion phenomena are being uncovered due to the presence of the neutrino interaction with neutrinos and to dynamical aspects related to the star explosion - shock waves and turbulence. While many features are now understood important open questions need to be addresses in the future. Theoretically, various methods can be employed for the description of neutrino propagation in astrophysical and cosmological environments, going across domains, such as systems of spins moving in effective magnetic fields, algebraic methods, or many-body approaches known in the study of atomic nuclei, of clusters or of condensed matter. Phenomenologically these studies are essential, to put on a solid ground, predictions of neutrino supernova signals associated with an (extra)galactic explosion, or of the diffuse supernova neutrino background yet to be discovered, or to assess the impact on the supernova dynamics and stellar nucleosynthesis.
UHE neutrinos (D. Semikoz)
Ultra-High Energy (UHE) cosmic rays produce secondary charge pions on background fields in the sources or in intergalactic space. Neutrinos produced during propagation are called "cosmogenic neutrinos". Flux of those neutrinos depends on the unknown distribution of sources and on the initial proton spectrum produced at those sources. Experiments like ANITA, AUGER and ICECUBE will be able to study this flux. For direct neutrino flux from the sources even backgrounds are unknown or at least model dependent. This make predictions of neutrino flux from sources even more difficult. We are developing theoretical models of UHE neutrino sources.
Cosmological neutrinos at the epoch of big-bang nucleosynthesis (D. Semikoz, C. Volpe)
Big Bang Nucleosynthesis (BBN) is one of key stones of cosmology. When the Universe cools down to sub-MeV temperatures, the plasma is not hot enough anymore to destroy light nuclei, produced from protons and remaining neutrons. The abundances of light elements is then governed by the neutron-to-proton ratio, which in turn depends on reactions with electron neutrinos and anti-neutrinos as well as neutron decay, and on the total energy density of Universe at that time. The observed abundances of light elements strongly restrict any new physics connected with MeV scales. This offers a poweful tool to constrain the parameter spaces of exotic models or novel particles such as sterile neutrinos.
Low energy weak interaction and neutrino physics (C. Volpe)
Low energy weak interaction and neutrino physics has brought milestone in the build up of the Standard Model. Nowadays it is a powerful tool to search for new physics beyond it. The group has been strongly contributing to this domain by predicting neutrino-nucleus cross sections crucial for the interpretation of oscillation experiments, exploring the connection with the lepton-flavor violating neutrinoless doble-beta decay, proposing experiments nearby existing facilities such as spallation source ones (ESS), investigating other low energy weak processes. The group is also renown for the proposal of a novel neutrino facility in the 100 MeV energy range based on a new concept : the low energy beta-beam. This facility is of great interest for nuclear physics, neutrino and supernova physics.