Why are neutrinos emitted by core-collapse supernovae? How can these neutrinos be detected? How can they be distinguished from solar neutrinos?

Neutrinos are elusive subatomic particles that are fundamental to our understanding of the cosmos, particularly during monumental events like core-collapse supernovae. When a massive star exhausts its nuclear fuel, its core collapses under the force of gravity. This rapid collapse triggers a catastrophic explosion, known as a supernova, with the creation of either a neutron star or a black hole at the center.

During this dramatic finale of a star's life, a flurry of neutrinos is emitted. These neutrinos are generated from an array of nuclear reactions and particle decays occurring in the extreme conditions of the dying star. Astonishingly, they are responsible for carrying away almost 99% of the supernova's energy. Unlike other forms of matter, neutrinos rarely interact with other particles, allowing them to escape the dense core almost immediately and provide a glimpse into the processes occurring within.

Spotting these ghostly particles requires cutting-edge technology. Neutrinos are famously known for being 'weakly interacting'; this means they can pass through most matter without leaving a trace. However, on the rare occasion that they do interact with a nucleon, they can be detected.

How do we catch something so elusive?

Scientists utilize detectors with large volumes of water or other substances such as heavy water or liquid argon. When a neutrino interacts with a particle within this medium, it causes a reaction that emits a flash of light, known as Cherenkov radiation. These light flashes are then captured by sensitive equipment lining the walls of the detector.

Enormous detectors, like the Super-Kamiokande in Japan or IceCube at the South Pole, are stationed underground to shield them from cosmic rays and other background noises, ensuring that the rare neutrino interactions are not drowned out by other signals.

Understanding the difference between supernova neutrinos and solar neutrinos is crucial for astrophysics. Solar neutrinos originate from the nuclear fusion reactions that power the sun. Unlike the sporadic nature of supernovae, the sun is a stable source of neutrinos radiating a steady stream of these particles with lower energies, usually in the Mega-electron-volt (MeV) range.

In contrast, neutrinos from a supernova come with much higher energies, typically tens of MeV, a direct consequence of the more violent and energetic processes. Moreover, the neutrinos are released in a massive burst, preceding the explosion's light, which can take hours or even days to emerge. This offers a unique opportunity: the detection of supernova neutrinos signals that a supernova is happening or is about to be visible, serving as an early warning system for astronomers. By scrutinizing the energy signatures and the flux patterns, scientists can distinguish between the steady, low-energy solar neutrinos and the high-energy, burst-like emission from a supernova.