In the last two decades milestones have been made in our knowledge of neutrinos as well as in our understanding of neutrino flavor evolution in astrophysical and cosmological environments. Crucial open questions remain on neutrino properties and on our understanding of how neutrinos change flavor in dense astrophysical environments, i.e. core-collapse supernovae (CC SNe) and binary neutron star mergers (BNS).
M.C. Volpe has published articles, focussed on key open questions at the forefront of neutrino physics and astrophysics.
With A. Chatelain, she has been working on neutrino flavor evolution in BNS remnants and performed the first investigation of non-standard interactions and show that a complex pattern of flavor mechanisms arise, even for very small values of non-standard couplings, which can influence r-process nucleosynthesis in such sites. Moreover the role of wrong helicity contributions due to the neutrino mass have also been studied based on detailed BNS simulations. Based on a perturbative argument valid both for BNS and CC SNe, these contributions are shown not to impact flavor evolution due non linear feedback. These investigations are important in connection with kilonova observations.
Effects of gravitational fields nearby compact objects on neutrino propagation and flavor evolution are still little explored. With A. Chatelain, M.C. Volpe has perfomed the first study of decoherence effects due to gravitational fields on neutrino wavepackets. Extending the density matrix formalism to curved spacetime, she has shown that gravity can impact significantly the decoherence proper time.
Fast conversion modes have triggered an intense activity since they can occur on short time scales and potentially influence CC SNe explosion dynamics. It was speculated in the literature that such modes could introduce flavor equilibration, which would simplify the complexity of predicting supernova neutrino signals of a future (extra)galactic supernova. With S. Abbar, she has investigated on a schematic two beam model that has clearly shown that fast modes do not produce flavor equilibration, contrarily to what is believed. Moreover, analysis of neutrino angular distributions has shown the occurrence of fast modes for the first time in two- and three-dimensional supernova simulations and the conditions for their occurrence have been identified.
With A. Gallo Rosso and A. Vissani, M.C. Volpe has studied the ability to reconstruct supernova neutrino spectra in Cherenkov detectors, if a galactic supernova explodes. A ten-dimensional likelihood analysis of the neutrino spectra has shown that we will be able to reconstruct the gravitational binding energy of the newly formed neutron star with 11 % precision in Super-Kamiokande and 3 % precision in Hyper-Kamiokande, by combining neutrino electron scattering and inverse beta-decay. This measurement and knowledge of neutron star equation of state allows to determine the neutron star compactness and mass-radius relation. With S. Abbar as well, she has been investigating the possibility to pin down information on extended theories of gravity (f(R) theories) through a precise determination of the neutron star radius.
As for cosmological neutrinos at the epoch of Big Bang Nucleosynthesis, with J. Froustey and C. Pitrou, M.C. Volpe rederived the neutrino quantum kinetic equations, based on the BBGKY hierarchy that she had already been using to extend mean-field equations for astrophysical applications. Numerical calculations including for the first time the full collision term, as well as QED corrections, have given a precise determination of the effective number of degrees of freedom N_{eff} = 3.0440.
M.C.Volpe (DR CNRS), S.Abbar (postdoc 2017-2019), A. Chatelain (PhD 2016-2018), J. Froustey (IAP, PhD 2019-), A. Gallo Rosso (PhD 2016-2019)