Flavor Changing Neutrinos
August 23, 2013

The Curious Case Of Flavor-Shifting Neutrinos

John P. Millis, PhD for redOrbit.com - Your Universe Online

Protons, neutrons, and electrons – it is these tiny particles that make up most of the normal matter (that is, non-dark matter) in the Universe. Yet it is another, nearly massless particle that may turn out to be the most revealing about the nature of the cosmos.

The neutrino plays a crucial role in nuclear processes such as fusion. But because of its small mass and neutral charge, it is very difficult to measure and study and as a result has been a source of frustration to physicists and astronomers.

One of the major revelations came when physicists attempted to confirm that the process that powered the Sun was, in fact, fusion in the stellar core. The data should have revealed that the rate of neutrinos coming from the Sun was equivalent to the fusion rate that would be required to sustain our central star. Startlingly, however, what researchers found was that the measured flux was only about a third of what they expected.

Prior to this study, neutrinos were thought to be massless particles, somewhat like photons of light, but this new result indicated that these particles had to have at least some minimum amount of mass. And not only that, but neutrinos could actually oscillate between different types – known as 'flavors' – as they mixed together in space.

The theory fit nicely, explaining why the observed flux was only a third of the expected amount, yet there was still a problem. To know for sure, researchers would have to measure these neutrinos in their other flavors, looking for the slight differences in the neutrino masses by a process called mass splitting.

“Mass splitting represents the frequency of neutrino oscillation,” explains Kam-Biu Luk of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. “Mixing angles, another measure of oscillation, represent the amplitude. Both are crucial for understanding the nature of neutrinos.”

To investigate, researchers developed the Daya Bay Neutrino Experiment, a purpose-built detector tasked with exploring these neutrino oscillations. “Understanding the subtle details of neutrino oscillations and other properties of these shape-shifting particles may help resolve some of the deepest mysteries of our universe,” said Jim Siegrist, Associate Director of Science for High Energy Physics at the U.S. Department of Energy (DOE).

The three flavors of neutrino are electron, muon, and tau. Each of these flavors can be represented as a mixture of three different masses. Measuring how the neutrinos oscillate between the different flavors allows researchers to calculate the probability of a neutrino being in a particular state of mass (its mixing angle) as well as what the differences are between the masses themselves (mass splitting).

In the Daya Bay experiment electron neutrinos are created at incredibly high rates – a billion trillion every second – and they travel down two kilometers of pipe to underground detectors. Along the way some of them seem to vanish, when in fact they are simply oscillating into a different flavor, rendering them invisible to the detectors. By accurately measuring the fraction that oscillates, the research team can determine the mixing angle. And by varying the energy over which the neutrinos are created, the mass splitting can also be determined.

Daya Bay has now released its latest results, refining earlier measurements from 2012. Additional data has led to greater precision and now confirms that the electron neutrino has all three mass states and behaves similarly to the muon neutrino, as studied by MINOS.

“These new precision measurements are a great indication that our efforts will pay off with a deeper understanding of the structure of matter and the evolution of the universe – including why we have a universe made of matter at all,” says Steve Kettell, a Senior Scientist at BNL and U.S. Daya Bay Chief Scientist.