Solar neutrinos have an interesting feature: they can change into another kind of neutrino on their way to Earth. This is "neutrino oscillation." Why does this happen?
It has to do with the wavelike nature of neutrinos. Waves oscillate back and forth. Add two waves together and you get a new composite wave.
For instance, when two very similar musical notes are played together, there's an interference effect that causes the sound to wobble between loud and soft, producing "beats." Similarly, oscillating neutrinos are comprised of three different waves that combine in different ways as they travel through space. The "beats" are caused by small physical differences in mass that lead to those telltale interference effects.
Scientists can also observe neutrino oscillations in particle accelerators. Physicists have been studying this phenomenon for the last 15 years or so, but while several experiments clearly showed evidence for neutrino oscillations as they travel long distances through space, they still wondered: Could, say, an electron neutrino emerge from a beam of muon neutrinos through the oscillation process?
Last year, researchers at the OPERA experiment at Gran Sasso National Laboratory made the first direct observation of a tau neutrino emerging from a muon beam - another very rare event in high-energy physics. But T2K actually started with a controlled beam of muon neutrinos and detected the electron neutrinos produced through oscillation, offering compelling new evidence in support of the phenomenon.
To create just the right conditions, T2K scientists sent a beam of muon neutrinos through a complicated system of detectors close to the targets, before the particles make a 295-kilometer journey across Japan to the Super-Kamiokande neutrino detector, which is capable of distinguishing between muon and electron neutrinos with great precision. Then they analyze that data to see if any muon neutrinos oscillated into electron neutrinos during the journey.
The physicists detected 88 candidate events for the oscillation of muon neutrinos into electron neutrinos, based on data collected between January 2010 and March 11, 2011, when a magnitude 9 earthquake struck Japan. Of those, they identified six events that provide evidence of this rare type of interaction.
Origins of Mass
So if neutrinos have mass (and it now seems that they do, although they are far lighter than the other subatomic particles), where does it come from? The answer might lie in the fact that we live, literally, in a material world because of a slight asymmetry in the amount of matter and antimatter in the earliest phases of the cosmos.
A long time ago, when our universe was still in its earliest birthing throes, matter and antimatter were colliding and annihilating each other out of existence constantly. This process slowed down as our universe gradually cooled, but there should have been equal parts matter and antimatter - and there weren't. Instead, there were slightly more matter particles than antimatter.