There was big news on the neutrino front last week, when the Japanese T2K (Tokai to Kamioka) experiment announced the first evidence (PDF) of a rare form of neutrino oscillation, whereby muon neutrinos turn into electron neutrinos. And this, in turn, gives physicist a potential clue to a critical mystery in cosmology: why there is something in the universe, rather than nothing.
It’s a small signal, just shy of of “3-sigma,” but statistically strong enough, given the rarity of the event, to be a genuine signal, not just background noise. The probability of this being real, and not simply due to chance, is around 99.3 percent.
“People sometimes think that scientific discoveries are like light switches that click from ‘off’ to ‘on,’ but in reality it goes from ‘maybe’ to ‘probably,’ to ‘almost certainly’ as you get more data,” Dave Wark of Imperial College London and a member of the UK contingent at T2K, said in the official press release. “Right now we are somewhere between ‘probably’ and ‘almost certainly.’”
Neutrinos are tiny subatomic particles that travel very near the speed of light. The two most defining features of neutrinos are that they have no charge and, until quite recently, physicists believed they had no mass. They’re extremely difficult to detect, because they very rarely interact with any type of matter, even though they’re the most abundant type of particle in the known universe. Only one out of every 1,000 billion solar neutrinos would collide with an atom on its journey through the Earth. Isaac Asimov dubbed them “ghost particles.”
The Standard Model of particle physics calls for three different kinds of neutrinos (electron, muon and tau, paired to leptons known as electron, muon and tau). The most common flavor is the solar (tau) neutrinos that come from the nuclear processes taking place at the core of the Sun. Trillions of neutrinos are produced by the sun every day.
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.
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.
We know this because we can see the remnants of that cosmic battle all around us: every bit of matter in our observable universe, from galaxies to dust mites and everything in between, exists because matter won that long-ago war of attrition. And physicists have no idea why that asymmetry should have existed in the first place.
Technically, it’s known as CP (charge-parity) violation, an effect first proposed by Russian physicist Andrei Sakharov, in which “when the charges and spins of particles are reversed, they should behave slightly differently.” Physicists think that neutrinos, with their teensy-tiny bits of mass, might have been the tipping point that tilted the scales to matter’s favor.
Like most subatomic particles, neutrinos have three “generations” in the Standard Model, arranged according to mass. Figuring out the hierarchy of neutrino masses — for example, are they light, heavy and very heavy, like other particles, or do they buck the trend and favor a pattern of light, heavy, and heavy? — is a very important first step to gaining insight into those subatomic processes that dominated in the few seconds after the Big Bang.
A Fermilab experiment currently under construction in Minnesota, called NOvA, is designed specifically to find that pattern. Its particle beam will travel farther than the one at T2K, making it easier to detect how they change from muon to electron neutrinos as they pass through the Earth. Whatever NOvA finds will be combined with data from the lab’s planned Long Baseline Neutrino Experiment, to be built in South Dakota, to determine if there is a fundamental difference between matter and antimatter neutrinos.
There are also short-baseline experiments under development, specifically Fermilab’s MINERvA and nuclear reactor experiments in China and France. Per the folks at Symmetry Breaking: “Short baseline experiments can’t compete in the hunt for why matter dominated antimatter, which requires tracking neutrinos across great distances, but they can provide the precision measurements that work like a rifle scope for particle hunters.”
The T2K experiment is currently offline because of the major earthquake that devastated Japan, cutting short the originally planned data run. But researchers expect to have the machine back online and taking more data by January 2012. With more data, the current 3-sigma signal should strengthen sufficiently to claim a solid discovery. All in all, these are exciting times for neutrino researchers.
Image credits: Super-Kamiokande