Lab researcher contributes to new understanding of supernova neutrinos

New calculations address some of the complexity surrounding neutrinos

Cygnus loop supernova explosion Credit: NASA/courtesy of nasaimages.org

July 8, 2010—Supernova explosions caused by the collapse of stellar cores play an essential role in the evolution of the Universe, from controlling the temperature of the gas and the rate of star formation in the galactic disk to synthesizing and dispersing heavy elements. Therefore, researchers need a better understanding of how these events occur. One way is to analyze the time evolution of the emitted neutrinos that are copiously generated during the first 10 seconds after a star's collapse. By observing the signatures of the expanding shock and the postshock region in the neutrino signal, scientists could learn about the development of the explosion during the crucial first 10 seconds. This information could be inaccessible in other ways.

However, neutrinos are chameleons of the particle world—they change identities from one flavor (electron, muon, tau) to another as they pass from the collapsing core. This makes understanding a neutrino signal exceedingly complex. In contrast to the situation for solar neutrinos, in supernova explosions the density of the streaming neutrinos near the core region is so large that their interactions with each other, called "self-refraction," become important. Neutrinos in a core-collapse supernova undergo coherent þavor transformations in their own background. These interactions lock the oscillating neutrinos of different energies into a collective mode. Instead of dealing with a single-particle evolution problem, scientists are presented with a problem of the system evolving as a whole. In a paper in Physical Review Letters, Alexander Friedland of the Lab's Nuclear and Particle Physics, Astrophysics and Cosmology group reports new calculations that address some of this complexity.

Friedland calculated the self-refraction effect several seconds into the explosion, as the collapsed core began to cool. He found that a full three-flavor computation reveals new effects. In particular, unusual patterns in the neutrino energy spectrum occur as one flavor changes into another. In the antineutrino spectrum a new "mixed" spectrum appears. It will be vital to understand such effects, because the resulting predictions for the signal expected at the planned detector in the Deep Underground Science and Engineering Laboratory (DUSEL) are qualitatively different from those of earlier two-flavor studies. Physical Review Letters also featured the work as an "Editor's suggestion." The Laboratory Directed Research and Development (LDRD) project, "Probing Physics Beyond the Standard Model with Supernovae" supported the research.