# Research

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.

Other than in the Early Universe, 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 these cosmic sources and carry  information on sites not otherwise accessible.

My research revolves around active neutrinos and their sterile cousins in  astrophysics and cosmology. Below, I provide you with 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). The element formation in SNe is also dramatically affected by neutrinos.

#### Neutrinos and Stellar Dynamics

According to the delayed neutrino-driven explosion mechanism, neutrinos are thought to provide new energy to the stalled shock wave to trigger the SN explosion. Looking at the the neutrino emission properties from the first world-wide 3D SN simulations, we found that the neutrino flux strongly depends on the emission direction, contrarily to the commonly adopted spherical symmetry assumption. Moreover, neutrinos carry experimentally detectable imprints of the pre-explosion SN dynamics, such as the bouncing of the shock wave that happens just before the explosion, the so-called SASI sloshing motions (Tamborra et al., 2013, Tamborra et al., 2014).

We also discovered an astonishing completely new effect: The LESA instability. LESA is a large asymmetric emission of electron neutrinos with respect to electron antineutrinos (Tamborra et al., 2014). LESA could have major implications for  flavor oscillations, nucleosynthesis and neutron-star kicks.

#### Neutrino flavor conversions in supernovae

Supernovae are neutrino-dense environments where neutrino-neutrino interactions are not negligible; self-interactions are responsible for coupling the flavor evolution of neutrinos of different energies,  modifying the flavor ratio expected on Earth, and potentially affecting the stellar dynamics and nucleosynthesis. The most known effect of such interactions is the  “spectral split,” a complete flavor change in certain energy domains. Although a lot of work has been done, we are still far from fully grasping the role of neutrino-neutrino interactions in astrophysical neutrino-dense environments (see, e.g, Mirizzi, Tamborra et al., 2016). Recent developments in this context concern the possibility that fast pairwise neutrino conversions occur, possibly leading to flavor equilibration (Izaguirre et al., 2017, Tamborra et al., 2017).

#### Neutrinos and nucleosynthesis

Flavor oscillations, especially in sterile neutrinos (neutrinos that oscillate in active states, not interacting with other particles),  could create a  favorable environment for the
formation of neutron-rich nuclei heavier than iron (r-process).  We simulated  the feedback of oscillations on the r-process, also including sterile neutrinos and integrated oscillations in nucleosynthesis networks (Tamborra et al., 2012, Pllumbi et al. 2015). It seems that the alpha-effect contrasts the neutron-rich environment  formation, but still further investigations are needed on this front.

#### Diffuse supernova neutrino background

A SN explodes somewhere in the Universe every second, although a galactic burst is a rare event. The cumulative flux of neutrinos from all SNe, the so-called Diffuse Supernova Neutrino Background (DSNB), has not been detected yet, but experimental limits are very close to its theoretical predictions. Neutrino-neutrino interactions seem to have negligible effects on the DSNB (Lunardini & Tamborra 2012). The detection of the DSNB in next-generation neutrino detectors will allow to constrain general properties of the supernova population, such as the fraction of black-hole forming supernovae and the local supernova rate (Moller et al., 2018).

If you are curious to learn more about the role of neutrinos in core-collapse supernovae, 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 core-collapse supernovae. In compact mergers the flux density of electron antineutrinos is larger than the one of electron neutrinos, differently from core-collapse supernovae. We have investigated the role of the neutrino angular distributions in the flavor conversions above the remnant disk (Wu & Tamborra, 2017). Our findings suggest that flavor instabilities could occur everywhere above the neutrino emitting surface. As a consequence, a significant enhancement of nuclei with mass numbers A>130 is expected as well as a change of the lanthanide mass fraction by more than a factor of a thousand. Our findings hint towards a potentially relevant role of neutrino flavor oscillations for the prediction of the kilonova lightcurves (Wu, Tamborra et al., 2017).

### Cosmic Accelerators

The high-energy neutrino astronomy era just begun! IceCube, a giant neutrino telescope deployed deep in the Antartic ice, recently detected  cosmic neutrinos with PeV energy, the highest energy ever observed. We do not know yet the exact origin of these neutrinos, although it seems plausible that they are generated from cosmic accelerators out of our galaxy.

I investigated the diffuse origin of such events by employing the most recent photon data, collected at different wavelengths. We pointed  out as starburst galaxies could be sources of the IceCube events (Tamborra, Ando & Murase 2014); tomographic constraints support such hypothesis (Ando, Tamborra & Zandanel 2015).  I also worked on the diffuse neutrino emission  from galaxy clusters (Zandanel, Tamborra, Gabici & Ando 2015) and  gamma-ray bursts (Tamborra& Ando 2015, Tamborra & Ando 2016). In particular, low-luminosity and choked gamma-ray bursts could provide a substantial contribution to the diffuse background of high-energy neutrinos, but cannot explain the low energy tail of the IceCube energy spectrum (Denton & Tamborra, 2018).

We only have a bunch of neutrino events, nevertheless we are already able to constrain the physics of cosmic accelerators by employing these data, e.g. the IceCube neutrinos tell us that up to the 1% of existing core-collapse supernovae can harbor jets and most of these jets are expected to be choked (Denton & Tamborra, 2017).

### Neutrino Cosmology

Using pre-Plank cosmological data, we found hints for extra-radiation in the universe, possibly explained through sterile neutrinos (Hamann et al. 2010). However, constraints on the number of neutrino species are usually derived by assuming their thermalization at the BBN epoch. By numerically solving  the neutrino quantum kinetic equations in the early universe, we pointed out under which conditions full thermalization should occur (Hannestad, Tamborra & Tram 2010).