The work proposed by the PI lies at the intersection of astrophysics, particle physics and many-body theory. It is in three main areas, neutrino physics in a dense medium, electron-positron pair plasmas, and processes in plasmas in strong magnetic fields. The PI proposes to work on the effects of neutrino-neutrino interactions on neutrino propagation in dense astrophysical media such as core collapse supernovae. The PI has found a new type of instability in neutrino-neutrino coherent forward flavor-changing interactions which allows to trade flavor at a rate faster than ordinary oscillations between neutrinos with different angular distributions and energy spectra. This could help equalize the energy spectra of the three flavors that emanate from the neutrino sphere of a supernova. The PI proposes to parameterize the angle and energy distributions of the different neutrino and antineutrino flavors to allow this instability to be studied in large scale simulations. The PI will study electron-positron pair (and photon) relativistic plasma created by neutrinos near a black hole in the last minutes of a black hole- neutron star merger. The PI also proposes to study processes in plasmas in strong magnetic fields using methods of many body quantum field theory. The broader impacts of this research are that Neutrino-neutrino interactions are expected to play a very important role in core-collapse supernovae. Since such supernovae are one of the proposed sites of r-process nucleosynthesis,. The results of this research could potentially impact our understanding of the origin of elements.
", For most of the period of this grant the "media" that were being investigated were astrophysical; situated in the interiors of evolving stars or on the periphery of a supernova explosion. The "particle physics" that is involved has little to do with the barrage of press coverage of new results from the Large Hadron Collider, or the new particle zoo that may or may not emerge from these experiments. Instead it deals with reactions involving the standard and well-understood particles like electrons, protons, neutrons, neutrinos and photons. The two threads need to be brought together because the surrounding astrophysical media influence particle reactions greatly, sometimes increasing reaction rates by orders of magnitude, sometimes inhibiting them, sometimes introducing multi-particle reactions that cannot be performed in the laboratory. Thus understanding laboratory experiments often does not tell one how things work in the interior of a star. And a lot has to be done theoretically at the purely microscopic level in order to obtain the building blocks for modeling the whole object. This modeling, especially when it is of transient behavior in highly evolved stars, involves large numbers of physics inputs of different kinds, and massive numerical calculations. Astronomical data then typically do not tell one about the correctness of any one microphysical pieces of input, because there are too many and they are too diverse. Still, to have credible modeling, it is necessary to address these individual issues as well as can be done. Turning to the specifics of the project, and the problem areas in which the most progress was made, we choose two: a) The evolution of neutrino "flavors" ( the term distinguishing the three kinds of neutrinos observed up until now), in the huge pulse of energy expelled from a supernova in the form of neutrinos. The recent work under the grant has resulted in a prediction of complete mixing among the flavors of and energies of the neutrinos, where conventional pictures had quite different energy distribution for the different flavors. This will matter in the modeling of the dynamics of the explosion; sooner or later we will know by how much. Also it should increase the rate of synthesis of heavy nuclei in the supernova envelope. b) The effects of the surrounding plasma (ionized gas) on certain nuclear fusion rates in the interiors of highly evolved stars. This is an old, old topic, but we have made a major advance in the formal underpinning, and used it in a critique of a particular model for a "super-burst" in a neutron star that is accreting matter.
