My research activity is focussed on astroparticle physics, with particular emphasis on neutrinos, axions and other low-mass weakly interacting particles and their role in astrophysics and cosmology.
Summaries of my research works are presented here.
SUPERNOVAE AND NEUTRINOS
Example of feedback effects on SN neutrino flavor evolution in a simplified, spherically symmetric bulb model. The energy spectra at the sources (dashed curves) and at a radius r = 350 km downstream (solid curves) are compared for electron neutrino νe (black) and non-electron neutrino νx (red), as well as for the corresponding antineutrinos. Notice the collective neutrino flavor swap around E~10 MeV. Figure taken from A. Mirizzi et al., “Supernova Neutrinos: Production, Oscillations and Detection,” Riv. Nuovo Cim. 39, no. 1-2, 1 (2016)
Neutrinos are the most elusive of all elementary particles. The three different kinds (flavors) of neutrinos can convert into each other by their nonzero masses and flavor mixing – a macroscopic quantum effect named neutrino flavor oscillations. The possibility of neutrino flavor oscillations, theoretically suggested more than half a century ago, is now an experimentally established fact. After many years of research with atmospheric, solar, accelerator and reactor neutrinos, we have achieved a mature understanding the oscillation phenomenon, crowned in 2015 with the Nobel Prize in Physics awarded to T. Kajita and A. B. McDonald “… for the discovery of neutrino oscillations which show that neutrinos have mass”. For a simple introduction to the neutrino oscillation saga give a look to the book “Neutrino: the mutant particle”
Now that the three-flavor oscillation framework is well determined (apart from the still unknown mass ordering and possible CP violation), neutrino physics enters a new phase, where these particles can be used to probe distant astrophysical and cosmic sources. In this context, the recent detection of an astrophysical neutrino flux at highest energies (E~PeV) by the Icecube experiment represents a landmark achievement. Further observations of this flux will provide valuable information on both the neutrino properties and on the nature of the cosmic accelerators. At the other end of the spectrum, the next frontier at low-energy (E~MeV) is the detection of a high-statistics neutrino signal from the next galactic core-collapse supernova (SN), as well as the observation of the integrated (diffuse, DSN) flux from all past SN. Indeed, SNe represent the most powerful source of neutrinos in the Universe: during a supernova explosion, 99% of the emitted energy (~1053 erg) is released by neutrinos and antineutrinos of all flavors.
The first (and so far only) SN neutrino burst observed in 1987 also led to a Nobel Prize in 2002 (awarded to M. Koshiba) and continues to generate a wide scientific interest and to inspire further research. Even if only two dozens of neutrinos were detected from SN 1987A, this signal provides us with a broad-brush picture of the explosion phenomenon and the associated neutrino emission, confirming the salient features of their physical understanding. This observation also allowed us to put strong constraints on exotic neutrino properties (e.g., decays, charges) that would have grossly altered the supernova neutrino emission. Of course, the signal of SN 1987A has also been analyzed in the light of neutrino oscillations, with increasing attention to the peculiar high-density features of the SN interior. In particular the impact of high-density background matter on neutrino flavor transformations has been studied since the very beginning through the celebrated Mikheyev-Smirnov-Wolfenstein (MSW) effect- a flavor-birefringent term of the neutrino dispersion relation in a medium, arising from an intriguing conspiracy between the very small neutrino mass differences and their very weak interactions.
More recently, it has been realized that, in the SN core, for a few seconds the neutrino gas can be so dense to provide itself a birefringent background to neutrino flavor transformations, which thus become a highly nonlinear process, giving rise to novel feedback phenomena on both neutrino oscillations and on the SN dynamics. In this context, one of my specialties is the theory and phenomenology of supernova neutrinos flavor conversions. In this context, I gained a significant experience in the study of non-linear equations that describe neutrino flavor evolution associated with the neutrino-neutrino interactions. These result dominant in the deepest supernova regions, where high neutrino densities are present. The characterization of these effects is crucial to predict the neutrino signal from a galactic supernova and may have an important impact on the explosion mechanism. My works on this topic are standard references with a significant impact on the development of the field. In particular, I recently wrote a highly cited review on these topics A. Mirizzi, I. Tamborra, H. T. Janka, N. Saviano, K. Scholberg, R. Bollig, L. Hudepohl and S. Chakraborty, “Supernova Neutrinos: Production, Oscillations and Detection,” Rivisita del Nuovo Cimento 39, no. 1-2, 1 (2016)
STERILE NEUTRINOS IN THE EARLY UNIVERSE
Exclusion plot for the active-sterile neutrino mixing parameter space from first data of the Planck satellite experiment. Figure taken from A. Mirizzi et al., Physics Letters B 726, 8 (2013)
Despite the success of the neutrino oscillation paradigm, there are a number of experimental results that appear anomalous in the context of the standard 3 neutrino framework, and can be explained by a sterile neutrino with mass around 1 eV. At the same time, there are a number of results which are in conflict with this interpretation. The situation at the moment appear contradictory. However, if these new neutrino states exist they would a deep impact on cosmological observations. Notably a thermal population of light sterile neutrinos would be one possible cause of a non-standard effective number of neutrino families Neff. This quantity would have a big impact on both the Cosmic Microwave Background (CMB) anisotropy map, and the Big Bang Nucleosynthesis (BBN) nuclear species yields.
In this context, I worked on active-sterile neutrino flavor conversions in the early universe, with particular interest towards the effect of low-mass sterile neutrinos on cosmological observables. In this context in A. Mirizzi, G. Mangano, N. Saviano, E. Borriello, Carlo Giunti, Physics Letter B 726, 8 (2013) we placed strong bound on active-sterile neutrino mixing using the first data of the Planck satellite experiment on Neff.
AXIONS IN ASTROPHYSICS AND COSMOLOGY
Exclusion plot for ultralight axion-like-particles in the plane mass ma vs axion-photon coupling g_ag. Figure taken from M. Meyer, M. Giannotti, A. Mirizzi, J. Conrad and M.A. Sanchez-Conde, Physical Review Letters 118, no.1, 011103 (2017)
The Standard Model (SM) of Particle Physics is an extremely successful theory, explaining all interactions between the known elementary particles as observed in the laboratory to a very high accuracy. However, there is an enormous evidence about the existence of physics beyond the SM. Indeed, the discovery of neutrino oscillations implies that neutrinos should have a sub-eV mass that is not accounted for in the SM. In this sense neutrinos represent the first example of weakly interacting slim particles (WISPs) pointing towards the need of new physics beyond the SM. Moreover, from astrophysics and cosmology we know that most of the matter in the universe appears to be composed of non-relativistic dark matter (DM) particles with at most feeble interactions with SM particles.
The discovery of the Higgs boson proves that fundamental scalar bosons exist in nature, and thus the search for further light scalar or pseudoscalar particles is timely and well-motivated. The prime example of a very light pseudo-Goldstone boson is the axion, introduced to solve the strong CP problem in QCD. Nevertheless, beside the “QCD axion”, there could be other (pseudo)scalar particles with properties very similar to the axion. These emerge in many theoretically appealing ultraviolet completions of the SM and are generically called axion-like particles or ALPs. In these theoretical frameworks the presence of light hidden sector U(1) gauge bosons, named hidden or dark photons, is also expected. All these WISPs offer a convincing physics case in connection to the puzzle of DM, and provide a variety of opportunities for experimental and observational searches.
In this context, I am actively working on the physics of these low-mass weakly interacting particles (axions, dark photons). In particular, I am interested in the cosmological and astrophysical role of these particles. In this context, I am carrying on an extensive investigation on astrophysical axion bounds e.g.,
A. Ayala, I. Domínguez, M. Giannotti, A. Mirizzi and O. Straniero, “Revisiting the bound on axion-photon coupling from Globular Clusters,” Physical Review Letters 113, no. 19, 191302 (2014)
A. Payez, C. Evoli, T. Fischer, M. Giannotti, A. Mirizzi and A. Ringwald, “Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles,” Journal of Cosmology and Astroparticle Physics 1502, no. 02, 006 (2015)
P. Carenza, T. Fischer, M. Giannotti, G. Guo, G. Martinez-Pinedo and A. Mirizzi, “Improved axion emissivity from a supernova via nucleon-nucleon bremsstrahlung”, Journal of Cosmology and Astroparticle Physics 10, no.10, 016 (2019)
These related publications represent the state-of-the-art in the field and are mentioned in reviews on axions in Particle Data Group