Out of Tune

November 06, 2008

Welcome to the second installment of Neutrino Week at Twisted Physics! In the last post, we learned about Ray Davis and his seminal experiment to detect neutrinos from the sun. The problem was, he only saw about a third of the expected "events." What happened to the missing two-thirds? This is why science is such an exciting field: even when an experiment works, it tends to raise more questions than it answers. (That's a sign you're doing it right, one might say.) And in this case, it turned out that the "missing" neutrinos were there all along -- literally hiding in plain sight.

I should point out here that we're talking specifically about solar neutrinos. There are three different kinds of neutrinos, or "flavors." Electron (solar) neutrinos are produced during the nuclear fusion of the sun. Muon neutrinos and tau neutrinos only come about from nuclear decay processes that require higher energies. Tau neutrinos, for example, are only produced during supernova explosions. And each flavor of neutrino has its own unique "signature."

Naturally, all the experiments to detect solar neutrinos were specifically designed to pick out the signatures of, well, solar neutrinos, and to ignore the other two flavors. Then came the Sudbury Neutrino Observatory in Ontario, Canada, an experiment designed to look for the other two flavors as well that announced its results in 2002 -- solving the solar neutrino problem in the process. Like Davis' experiment, SNO was built deep underground to reduce background noise from all the cosmic rays constantly bombarding our atmosphere.

SNO used heavy water as a detector fluid, in which two hydrogen atoms in each water molecule are replaced by an isotope called deuterium. Whenever a neutrino collided with a deuterium atom -- about 10 times a day -- the atom ripped apart.

There are two ways that atom can split after such a collision. If it's a solar neutrino you'll get two protons and an electron, along with a telltale flash of light (Cherenkov radiation), and by measuring the flashes it's possible to tell the number of solar neutrinos. For the other two kinds of neutrinos, salt was added to the heavy water (sodium and chlorine atoms). If a muon or tau neutrino collides with a deuterium atom, it splits into a proton and neutron. The salt will then absorb the neutron and emit high-energy gamma rays -- which are very easy to detect.

When the SNO researchers collected and analyzed all their data, and added all three neutrino flavors together, they got the expected total number of solar neutrinos predicted by the theoretical model. That was a weird and unexpected twist, because by definition, the sun only produces solar neutrinos. Clearly, something had happened to the neutrinos as they traveled from the sun's core to the Earth: they gradually became "detuned" and turned into one of the other two flavors. So the "missing" neutrinos weren't missing at all; they'd just changed their tune.

This is called neutrino oscillation. Think of specific piano strings tuned to specific musical notes, such as G, E or C (a C Major chord). Scientists had assumed that if a neutrino started life as a "C", it would always be a C. But just like the strings of a piano can gradually shift their tone over time, so neutrinos can shift their flavor. A G can become an E, or a C. This also means that scientists had been wrong when they assumed neutrinos have no mass. They have a very, very tiny bit of mass; that's why they're able to change flavors.

I once saw astrophysicist Janet Conrad demonstrate the phenomenon with a pair of tuning forks, each tuned to the same frequency, except one had a tiny bit of extra mass attached to one of its tines. She struck one, then the other, and that tiny difference in their masses produced a "wah-wah-wah" kind of warble. This, she explained, is due to the wavelike nature inherent in all subatomic particles (they are both particle and wave). Those waves oscillate back and forth, sometimes adding together into a new composite wave. When two very similar notes are played together, there's an interference effect that causes the  sound to wobble between loud and soft, producing some semblance of a "beat." That's pretty much what happens to neutrinos as they travel through space. The "beats" are caused by those tiny differences in mass, giving rise to interference effects. And over time, the neutrinos change flavors.

That's the basic idea, anyway, although scientists are only beginning to investigate the complex phenomenon of neutrino oscillations with new experimental facilities, like Fermilab's MiniBooNE (short for Mini Booster Neutrino Experiment), which announced its results in 2007, and is now running a new version of the experiment. Like SNO and the Homestake Mine facility before it, MiniBooNE is deep underground. But there are other projects underway that will search for neutrinos in even more extreme environments: in the depths of the ocean, and at the frigid South Pole. More on those to come!

Photo: Artist's conception of Sudbury Neutrino Observatory. Source: SNO.

Comments

[Brian]

Seriously, how do you detect the destruction of a single atom in such a large tank of water if it's only happening ten times a day. I could understand if it was occurring several times per second. Sheer probability would dictate that eventually your sensor would be pointed in the right place.

But how do you monitor an entire giant tank of water and be able to pick out when a single atom discharges energy?

Warps my mind. Great post.