"My laboratory is a supernova," Yong Qian states matter-of-factly with a smile. Qian is a theoretical physicist studying neutrino oscillations, the change from one "flavor" to another as a neutrino moves through matter and space. This phenomenon occurs because a neutrino is created in one of three flavor states (or simply “flavors”), with three distinct mass states.
Using neutrinos produced by the sun, by interaction of cosmic rays with earth’s atmosphere, and by accelerators and nuclear reactors on earth, a number of experiments have shown that neutrinos oscillate and therefore have mass. Yet, some key parameters characterizing neutrino oscillations are unknown. Qian is particularly interested in the problem of neutrino mass hierarchy, or which flavor of neutrino dominates the heaviest, the intermediate, and the lightest mass state, respectively.
Qian thinks that nature may provide the best source of neutrino data in supernovae. The problem is that due to their weak interaction, a large number of neutrinos are required to extract their properties, and nearby supernovae providing sufficient neutrinos are rare. For example, in 1987 there was a supernova in the Large Magellanic Cloud, a close neighbor to our galaxy, but only a total of 20 or so neutrinos from this supernova were observed on earth. However, Super Kamiokande, a detector in operation now, can detect thousands of neutrinos from a supernova anywhere within our galaxy. Qian wants to find out whether supernovae can provide a novel approach to study neutrino oscillations before the next supernova occurs in our galaxy. Statistically, supernovae should occur once every century in our galaxy, but the Milky Way has not seen one for several hundred years. Qian figures that we are about due for another supernova.
He and his collaborators discovered that near the core of a supernova, neutrinos are so dense that interactions amongst themselves give rise to a new behavior called collective oscillations. In this phenomenon, neutrinos at certain energies form a unified block and experience the same flavor evolution. Further, the formation of such blocks depends on the neutrino mass hierarchy but is insensitive to the other unknown oscillation parameters. These results were obtained from both complex computer modeling and simple analytical studies by Qian and his collaborators, and were later confirmed by other research groups. Qian thinks that the collective oscillations of supernova neutrinos provide “a unique tool to figure out the neutrino mass hierarchy.”