Scientific Method —

Detector finally catches neutrinos in act of flavor changing

An experiment that shoots a beam of neutrinos from CERN to a detector in …

Over the weekend, a detector in the Gran Sasso facility in Italy detected a single tau neutrino in a beam that should have been purely muon neutrinos. That may not sound like a stunning turn of events, but it's actually yet another demonstration that the Standard Model of physics, which explains the behavior of elementary particles, doesn't quite explain everything. It's also the product of decades of somewhat confusing physics along with an experimental setup that involves shooting a beam of particles through hundreds of kilometers of the Earth's crust, only to have most of them go undetected. Welcome to the very strange world of the neutrino.

The American Association for the Advancement of Science meeting had an entire session on neutrinos, and Fermilab's Kurt Riesselmann introduced it by asking everyone to hold out their hand for three seconds. During that time, a trillion neutrinos would pass through the space occupied by your hand. Despite these massive quantities, Riesselmann said, a typical human would have a grand total of three neutrinos actually collide with an atom in their bodies over the course of an entire lifetime.

Neutrinos and normal matter simply don't interact well, which has given the former a long and somewhat bewildering history. At AAAS, André de Gouvêa described how Pauli first proposed their existence in 1930 in order to balance the books on nuclear decay reactions. Unfortunately, he called them neutrons, only to have someone else use the same term for the relatively massive particle we now know as a neutron. Fermi eventually sorted this out by using the term neutrino.

It took 25 years and the development of nuclear reactors before we had a sufficiently intense source of neutrinos to actually confirm they exist. About a decade later, they were causing trouble again. The nuclear reactions that were thought to power the sun should produce prodigious amounts of neutrinos—in fact, it's possible to take a picture of the sun from deep in a mine using neutrinos. Unfortunately, the detectors in use consistently detected fewer neutrinos than the theorists said they should have.

This left three unappealing possibilities: theorists had the sun all wrong, particle physicists weren't building the detectors properly, or the neutrinos were doing something strange that violated the Standard Model of physics. Eventually, the theorists and physicists trusted each others' work well enough that they started taking this last possibility seriously.

There are three different types of neutrinos: electron, muon, and tau. The detectors that had been used were specialized—they only picked up a specific type of neutrino. One possible explanation for the apparent neutrino deficit is that a given neutrino isn't a fixed type, but instead oscillates among the three. When it hits the detector, there's a chance that it's not the expected type of neutrino; it has "changed flavor" in physics parlance.

Flavor changes would explain the apparent deficit. They'd also mean that the neutrino has mass, which would also violate the Standard Model, so physicists took a fair bit of convincing. Still, the deficit showed up in enough different experiments that it's now generally accepted that neutrinos have mass. The results from Gran Sasso are the first indication that we'll be able to experimentally confirm this.

The CERN/OPERA collaboration is one of at least three that are performing similar searches for direct evidence of neutrino oscillations—Fermi is running an experiment called MINOS, and there's another one running at Japan's Super-Kamiokande neutrino detector. All of them start with the same basic principle: a single physical process involving a particle accelerator that produces a pure beam of a single type of neutrino. At the detector end, any neutrinos of a different type are evidence of a flavor change somewhere between the production point and the detector.

The challenge with this work is that flavor changes become more probable with time, and these neutrinos move fast. That means you have to set up the detector a long distance from the source in order to have a decent chance of detecting an oscillation. Here's where the fact that neutrinos don't tend to interact with normal matter becomes an advantage: you can simply shoot them through the Earth's crust from the source to a detector hundreds of kilometers away. So, both Fermi/Minos and CERN/OPERA have set up the detectors about 730km from the neutrino source.

Fermi's are sent to Minnesota, but there are plans afoot to open a new detector even further away in South Dakota. CERN's travel from the Swiss/French border well into Italy. If you're located appropriately, the beam travels under your feet without any hint of its presence.

The event that was announced was the first detection of what appears to be a tau neutrino in a beam that should be nothing but muon neutrinos. Obviously, physicists aren't going to be satisfied with a single event, so the experiment is going to continue in an effort to build some statistical power into the detection. The events are expected to be so rare, however, that, over the entire lifetime of the project, it is likely to detect only 15 tau neutrinos.

CERN has a video with another take on the results, complete with some mildly cheesy music and a spokesperson sent out into an empty field that happens to be located directly above the neutrino source.

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