Neutrinos are the most famously shy of particles, zipping through just about everything—your body, Earth, detectors specifically designed to catch them—with nary a peep. But compared with their heretofore hypothetical cousin the sterile neutrino, ordinary neutrinos are veritable firecrackers. Sterile neutrinos don’t even interact with ordinary matter via the weak force, the ephemeral hook that connects neutrinos to the everyday world. Recently, however, new experiments have revealed tantalizing evidence that sterile neutrinos are not only real but common. Some of them could even be the stuff of the mysterious dark matter astronomers have puzzled over for decades.
Physicists aren’t quite ready to make such dramatic pronouncements, but the results "will be extremely important—if they turn out to be correct,” says Alexander Kusenko of the University of California, Los Angeles.
How did scientists go about looking for particles that are virtually undetectable? Kusenko and Michael Loewenstein of the NASA Goddard Space Flight Center reasoned that if sterile neutrinos really are dark matter, they would occasionally decay into ordinary matter, producing a lighter neutrino and an x-ray photon, and it would make sense to search for these x-rays wherever dark matter is found. Using the Chandra x-ray telescope, they observed a nearby dwarf galaxy thought to be rich in dark matter and found an intriguing bump of x-rays at just the right wavelength.
Another piece of evidence comes from supernovae. If sterile neutrinos really do exist, supernovae would shoot them out in a tight stream along magnetic field lines, and the recoil from this blast would kick the pulsars out through the cosmos. It turns out astronomers observe precisely that: pulsars whizzing through the universe at speeds of thousands of kilometers a second.
Astronomers don’t have to rely on the skies for evidence of sterile neutrinos, though. Scientists at Fermi National Accelerator Laboratory recently verified a 16-year-old experiment that sought the first evidence of these particles. The Fermilab scientists fired ordinary neutrinos through Earth at a detector half a kilometer away. They found that in flight, many of these neutrinos changed their identities in just the way they should if sterile neutrinos do in fact exist.
The next step is to confirm the results. Loewenstein and Kusenko recently repeated their experiment on another space-based x-ray telescope, the XMM-Newton, and Fermilab scientists are also setting up another run. The shyest elementary particles may not be able to evade their seekers for long.