I am fascinated by neutrinos!  These puzzling elementary particles vastly outnumber all other particles in our universe, except photons. Neutrinos interact very weakly. Three distinct families or “flavors” of neutrinos exist and they oscillate (convert) into each other by flavor mixing.

Spanning an incredibly large energy range, neutrinos are copiously produced in a variety of astrophysical environments, ranging from  stars like the Sun to the most extreme astrophysical transients like core-collapse supernovae and neutron star mergers. Many cosmic accelerators, such as gamma-ray bursts or active galactic nuclei, should also produce neutrinos. Owing to their feeble interactions, neutrinos  have the extraordinary ability to escape, almost unimpeded, from cosmic sources and carry  information on sites not otherwise accessible. If you are curious to know more about neutrinos and their sources, check our review paper (Vitagliano, Tamborra, Raffelt, 2020).

Grand Unified Neutrino Spectrum at Earth, integrated over directions and summed over all flavors. Solid lines are for neutrinos, dashed or dotted lines are for antineutrinos, superimposed dashed and solid lines are for sources of both neutrinos and antineutrinos (Vitagliano, Tamborra, Raffelt, 2020).

My research mainly revolves around neutrinos in astrophysics and cosmology, as well as multi-messenger astrophysics. Below, I provide a glimpse on my work.

Core-collapse supernovae

Neutrinos are crucial to the dynamics of the most spectacular events in our galaxy:
Core-collapse supernovae (SN). According to the delayed neutrino-driven explosion mechanism, neutrinos provide new energy to the stalled shock wave to trigger the SN explosion. The most recent SN simulations in 3D suggest that neutrinos and gravitational waves carry experimentally detectable imprints of the SN physics. Neutrinos also drive the LESA hydrodynamical instability. LESA consists of an astonishingly large asymmetric emission of electron neutrinos with respect to electron antineutrinos with possible major implications for  flavor conversions and nucleosynthesis. Moreover, neutrinos can be used as gyroscopes to probe the SN rotation and may be the only detectable probes of black-hole forming stellar collapses.

Electron neutrino lepton number flux as a function of viewing direction for an 11 Msun SN model. The LESA asymmetry in the neutrino number flux emission can reach up to the 20% with respect to its average  (Tamborra et al., 2014).

Supernovae are neutrino-dense environments where neutrino-neutrino interactions are not negligible. Neutrino self-interactions are responsible for coupling the flavor evolution of neutrinos of different energies in a non-linear fashion. Recent developments concern the possibility that fast pairwise neutrino conversion occurs, possibly leading to flavor equilibration (see Tamborra & Shalgar, 2020 for a review on the topic). If this conjecture is confirmed, the SN explosion mechanism as well as the synthesis of the heavy elements will be dramatically affected.

A SN explodes somewhere in the Universe every second. The detection of the cumulative flux of neutrinos from all SNe, the so-called Diffuse Supernova Neutrino Background (DSNB), is now within reach. The DSNB will allow to constrain properties of the SN population, such as the fraction of black-hole forming collapses and the local supernova rate. If you are curious to learn more about the role of neutrinos in SNe, check our review paper on the topic (Mirizzi, Tamborra et al., 2016).

Compact Binary Mergers

The massive neutron star (NS) or black hole (BH) accretion disk resulting from NS–NS or NS–BH mergers is dense in neutrinos, similarly to SNe. In compact binary merger remnants, the flux density of electron antineutrinos is larger than the one of electron neutrinos, differently from core-collapse supernovae. Flavor instabilities could occur everywhere above the neutrino emitting surfaces in mergers because of the merger geometry and the natural excess of electron antineutrinos. As a consequence, an enhancement of the lanthanide mass fraction by more than a factor of a thousand is expected. Our findings hint towards a potentially relevant role of neutrino flavor oscillation for the prediction of the kilonova lightcurves.

Mass fraction as a function of mass number for the neutrino-driven ejecta of a compact binary merger remnant. Because of flavor conversion, the element production shifts towards elements with heavier mass number (Wu, Tamborra et al., 2017).

Cosmic Accelerators

The IceCube Neutrino Observatory, a giant neutrino telescope deployed deep in the Antartic ice, routinely detects  cosmic neutrinos with PeV energy, the highest energy ever observed. We do not know yet the origin of these neutrinos, although it is clear that they are generated from cosmic accelerators out of our Galaxy.

By employing the most recent electromagnetic data, collected at different wavelengths, we investigate the particle production and acceleration in a range of cosmic accelerators, such as star-forming galaxies, galaxy clusters, supernovae,  gamma-ray bursts, and fast blue optical transients. In particular, despite the fact that we only have hundreds of neutrino events, we are able to constrain the physics of cosmic accelerators by employing multi-messenger constraints.

Diffuse neutrino intensity from star-forming galaxies as a function of the energy (pink band). The IceCube estimated  flux is marked in blue. Star-forming galaxies could explain the high-energy neutrino events observed from IceCube for E< 0.5 PeV (Tamborra, Ando & Murase, 2014).

Physics Beyond the Standard Model in Astrophysical Sources

Neutrinos of astrophysical origin provide a fascinating window into Physics Beyond the Standard Model. For example, if neutrinos convert in keV-mass sterile neutrinos in the SN core, a non-negligible lepton asymmetry generates with possibly major implications on the SN physics. At the same time, a self-consistent treatment of the production of keV-sterile neutrinos in the SN environment leads to very different bounds on the mass-mixing of these particles. Similar conclusions hold for light sterile neutrinos and other scenarios involving New Physics.

The high-energy neutrinos observed by the IceCube Observatory can also carry observable imprints of New Physics occurring within the source or during propagation to Earth. Hence, astrophysical neutrino data already allow to place stringent independent bounds on New Physics.

Ratio between the energy emitted in sterile neutrinos without feedback (left panel) and with feedback (right panel) on the SN dynamics and the benchmark gravitational energy. The dark yellow region is disfavored. The dynamical feedback due to the production of sterile states on the SN dynamics considerably relaxes the excluded region of the parameter space of sterile neutrinos (Suliga, Tamborra, Wu, 2019).

Neutrino Cosmology

The Early Universe is dense in neutrinos. Neutrinos are usually assumed to be fully thermalized at the BBN epoch. However, this assumption may be modified (with a non-negligible impact on the observables) because of non-standard physics affecting the neutrino sector and flavor conversion.  A sophisticated modeling of the neutrino propagation in the Early Universe is still missing given our superficial understanding of neutrino conversions in dense media, we are working to asses the impact that neutrino physics may have on the cosmological observables.

2D marginalized 68%, 95% and 99% credible regions for the neutrino mass and thermally excited number of degrees of freedom. Pre-Planck data are compatible with one or two extra sterile neutrino families with sub-eV mass (Hamann et al., 2010).