Nuclear physics and nuclear astrophysics stand at the intersection point of two of the most exciting recent developments in science: (1) the experimentally-driven revolution in neutrino physics; and (2) the startling recent advances in the capabilities of observational astronomy, especially in cosmology. The chief research objective of the supported work is to exploit this intersection, leveraging new developments in astronomy and astrophysics to probe fundamental neutrino and nuclear physics issues not easily explored in the lab, and vice-versa, using new insights in nuclear and neutrino physics to better understand the cosmos. For example, one key objective of the proposed work is to calculate how neutrinos change from one kind to another in the dense environment of a supernova and in the very early universe, and what this process might mean for the synthesis of the elements. Another objective is to use what we know about how the light nuclei are formed in the first seconds of the universe, together with results of observations of the cosmic microwave background radiation, to probe unmeasured neutrino properties, e.g., the neutrino rest mass.
That this work will have the potential to fuel developments in two seemingly disjoint fields, with nuclear physics as the essential fulcrum, is one obvious broader impact. Another is that this project should serve as an excellent training ground for graduate students. Likewise, the Quarks-to-Cosmos Summer School and the UCSD Center for Astrophysics and Space Sciences outreach efforts will convey the excitement of nuclear astrophysics to young people, from new postdocs and graduate students to undergraduates to secondary school students at a variety of institutions.
The primary goal of this project was to investigate one of the most exciting and rapidly developing subjects in modern physics: the elementary particles called neutrinos and the way these particles are produced by, and interact with matter, particularly atomic nuclei, in the cosmos. Along the way this research has given us insights into how stars explode and cook elements, and it has forced us to examine key new aspects of humanity's basic theory of how nature works - quantum mechanics. This research is theoretical, i.e., it involved calculations, some of which are large and had to be performed on supercomputers. Neutrinos experience only the aptly named "weak interaction" (the nuclear interaction that converts protons into neutrons and vice versa and which is some twenty orders of magnitude weaker than the electromagnetic interaction that governs light!) and gravitation. As a consequence neutrinos can escape from very dense and hot environments that other kinds of particles are trapped in. In fact, neutrinos can more than make up for their feeble interactions with huge numbers. And indeed, they are produced copiously in the very early universe and in the collapse and explosions of massive stars. They may carry a dominant or significant fraction of the energy in both of these venues. Both of these cosmic environments are important because they are prime candidate sites for the origin of the light elements (e.g., helium, deuterium, lithium) in the case of the very early universe, and the heavier elements (e.g., iron, gold, and uranium) in the case of massive stars. The objective of this project was to use what we have measured about neutrinos to try to better understand how the cosmos got to be the way it is and, concomitantly, to use what we have observed in astronomy to probe the properties of neutrinos and other particles in ways we could never do in a laboratory on earth. All of this constitutes the rationale for the intellectual merit of this research. The results of this research are in some cases startling and unexpected. We have found that neutrinos in core collapse supernova explosions can undergo collective neutrino oscillations, wherein all the neutrinos are all transforming their "flavors" (i.e., whether they are electron type, muon type, or tau type) in a kind of synchronized dance. This pattern, if detected on earth from a relatively nearby supernova, could give us insights into the way neutrino rest masses are arranged and split. This information is a key target of earth-based accelerator experiments. If these experiments measure this arrangement of masses, the so-called neutrino mass hierarchy, then our calculations would allow the detection of a supernova neutrino burst to give insights into how those explosions come about and possibly whether the very heaviest elements can be made there. The "flavor" of a neutrino determines how it interacts with matter. This is why it is important to know how neutrino flavors change as they move through the supernova or early universe medium. This research project investigated this process in regimes where direction-changing scattering of neutrinos was important and where it was not. We discovered what we called the "Neutrino Halo", wherein a tiny population of direction-scattered neutrinos can have inordinate influence on flavor transformation. We derived for the first time the equations which govern this process in general conditions. We discovered an unexpected and unknown physical property of neutrinos - the medium they move through produces a coupling which can interconvert neutrinos and antineutrinos. This may have implications for our understanding of where the heaviest elements are made. Perhaps ultimately even more important is that this problem appears to be a new kind of quantum mechanics problem and our work suggests new insights into how quantum coherence can be lost in particle interactions in matter. We also investigated how the very hot, highly excited, and very heavy nuclei expected to exist in collapsing/exploding stars interact with and produce neutrinos. This is extreme nuclear physics, currently impossible to duplicate in a terrestrial laboratory - but collapsing stars and the neutrinos and elements they produce provide us a "laboratory" to study this physics and the work in this project has helped us exploit this "laboratory". Likewise, we used what has been measured in the cosmic background radiation leftover from the early uinverse to probe and constrain speculative types of neutrinos with properties that cannot be probed in the lab. All of this work was intimately wrapped up with the training and development of young nuclear physicists and nuclear/particle astrophysicists. A number of UCSD graduate students did PhD thesis work under funding for this project. Undergraduate students were also involved. The PI reached graduate students around the USA and the world with lectures and talks based on this research. This research and student training constitutes the broader impact of this project.
