Condition for neutrino oscillation instability in Stars

Neutrinos, Stars, and Collective Oscillations

Neutrinos are tiny elementary particles. These particles, which come in three flavors, have the unusual ability to periodically change their flavor from one to another via neutrino oscillations. Discovery of neutrino oscillation in the 1990s revealed that neutrinos have mass and behave in a manner predicted by quantum mechanics.

Supernova are massive stars that explode at the end of their life-cycle. They produce large numbers of neutrinos via nuclear reactions. The three flavors are created with distinct spectra, i.e., with unequal abundances with energy and direction of emission. Most of these neutrinos leak out of the star, though some deposit their energy back in the star and help in the creation of heavy nuclear elements. In the dense stellar environment, neutrino oscillation can become unstable -- its amplitude growing exponentially with time. These novel instabilities, predicted by Jim Pantaleone, Alan Kostelecky, and Stuart Samuel in the 1990s and by Ray Sawyer in the 2000s, have to do with collective oscillations where all neutrinos oscillate in synchrony. Such collective behavior has often been compared with how a dense flock of birds, a school of fish, or a crowd at the train station is forced to move in the same direction.

Condition for Instability

A critical question remained  -- What are the conditions under which exponential growth of flavor conversions can take place? In a study recently published in the Physical Review Letters, Basudeb Dasgupta at the Tata Institute of Fundamental Research in Mumbai has shown that exponential growth can occur only if the spectra for two of the neutrino flavors cross each other at some energy or emission angle. This result, based on a method invented by Taiki Morinaga at Waseda University, guarantees that observation of neutrino oscillation instabilities will reveal new information from deep within the star.

Observations: Present and Future

The observation of neutrinos from supernovae, and their impact on stellar heating and nucleosynthesis, will therefore provide information on neutrinos deep inside them.

In 1987, about 20 neutrinos were seen from a supernova just outside our galaxy. This is the only time astronomers have seen neutrinos from any dense star. Unfortunately, these few neutrinos were not enough to reveal the collective oscillation instabilities.

In the future, it is hoped that existing neutrino telescopes, such as Super-K in Japan and IceCube in Antarctica, will record more than ten thousand neutrinos from a supernova in our galaxy. Such an event, expected to occur only a few times in a century, would prove to be a bonanza for the study of neutrinos and supernovae. A key and unique outcome of a galactic supernova observation would be insight into these possible neutrino oscillation instabilities, which would also constitute the first direct evidence for interactions of neutrinos with each other.

Multi-messenger detections of neutron star mergers, such as the detection of the GW170817 event by the gravitational wave interferometer LIGO and subsequent observations by optical and infrared telescopes, gave detailed information about the creation of heavier elements in these stellar collisions. Based on this observation, several groups of astronomers have established that rare elements like gold and silver, and physiologically important elements such as oxygen, calcium, and iodine, are dominantly produced in these stellar collisions. Neutrino oscillations in the neutron star merger environment affect the efficiency of synthesis of these elements. Future observations of such binary neutron star mergers, may reveal evidence for collective neutrino oscillation instabilities from the observed pattern of elemental abundances.

Neutrinos are famous for springing new surprises and resolving inconsistencies in the theories of particle physics. Dr. Dasgupta says "experimental discovery of collective oscillations will open a new window to peek into the depths of faraway stars, adding yet another feather in the cap of neutrinos -- the great problem solver among elementary particles."

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