Neutrinos occupy the third row of the table of fermions. Because they are electrically neutral – by definition having no electric charge – there are only three flavors.
Neutrinos are at once ubiquitous and nearly impossibly to detect. The electric neutrality means none of the electromagnetic forces act upon them. They are nearly massless – for a long time they were thought to be entirely massless – and are the tiniest members of the fermions.
But they are ubiquitous. Some 65 billion of them sleet through every square centimeter of matter on earth every second. Most are created by the fusion process in the Sun, but they are also created by novae and supernovae.
And by scientists. Firing a proton beam at a denser mass creates neutrinos. But they are devilishly difficult to detect. In fact, they have not and are not likely to ever be directly observed.
Oak Ridge National Lab is home to the Spallation Neutron Source. The SNS accelerates a beam of protons and then smashes them into a tank of mercury. The atomic particle collision creates subatomic debris that includes lots of neutrons, which are used for a variety of scientific purposes, and the primary purpose of the SNS. But theory says the subatomic debris also includes some neutrinos that are otherwise lost in the spray of subatomic flotsam and jetsam that streams out of the collisions.
43 years ago, particle physcists suggested that a neutrino give some of its momentum to the nucleus of a heavier atom by exchanging a Z boson with one of the nucleus’ quarks, after which the nucleus would that energy by emitting a photon. And photons, of course, represent light. Until now, the reaction had not been detected.
But the physicists at SNS thought their fairly low-speed neutrinos might be detectable. They found an area near the accelerator that screened most of the neutrons and other particles; the neutrinos, of course, almost all blow through stuff that stops other particles. The physicists built a detector with cesium iodide, two relatively heavy elements. Because those elements are similar in mass, the photons they emit after a neutrino interaction should be similar in energy, ensuring that a single detector can pick up any interactions.
Then they watched for a little more than a year.
Then they looked over the year-plus of data, checking for more photon flashes around bursts of protons into the mercury. Solar neutrinos set a fixed background rate; increased activity when the SNS fired would indicate detection of manmade neutrinos.
And they found them. About nine excess events a month, right after a pulse of protons hit the mercury tank in the SNS.1 That’s about consistent with predictions from the Standard Model. And it’s nearly seven standard deviations higher than what you’d expect if you weren’t seeing neutrino interactions.
Why does this matter? Because if neutrinos are shown to “bump” into atoms at a predictable rate, and semi-portable neutrino detectors can be built, then science may be able to “see” with neutrinos, just as we “see” with x-rays.” Except that neutrinos would allow “seeing” through things that are utterly opaque to the electromagnetic spectrum. We might “see” the interior of the Earth, or the interior of the Sun.
Plus, another confirmation of a prediction made by the Standard Model.
[WC expects Randall Munroe will shortly be publishing a cartoon explaining all this far more clearly and accurately than any explanation WC could possible muster. Watch for it.]
- Note to particle physicists: yes, WC understands that the results also modeled the predicted levels of each of the kinds of neutrinos, which is even cooler. But WC’s readers really don’t want to know about the three kinds of neutrinos and how they can oscillate between those kinds. Sorry. ↩