STERILE neutrinos, hypothetical particles so aloof they may flit off into other dimensions at the drop of a hat, may finally be stepping into view. If their existence is confirmed, they might account for dark matter and could point the way to other exotic particles that do not fit into the standard model of physics.
The model is physicists’ best idea of how particles interact with three of the four fundamental forces – electromagnetism and the strong and weak nuclear forces. “So far we have no compelling measurement of a particle that doesn’t participate in the standard model interactions,” says of Vanderbilt University in Nashville, Tennessee.
Sterile neutrinos, by contrast, would be impervious to these forces and sensitive only to gravity, a force that lies outside the standard model’s jurisdiction. “Whoever finds the first [such] particle is opening a window to possibly a whole new set of particles,” says Weiler. These include the graviton, proposed to carry the gravitational force.
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Because sterile neutrinos only interact with gravity, the weakest of the four forces, they are not strongly coupled to matter in our universe. Some theories suggest this would allow sterile neutrinos to just zip away from the three spatial dimensions familiar to us. If that happens to gravitons, too, it could explain why gravity is so weak. “It could be that by finding a sterile neutrino, we’re also finding a probe of extra dimensions,” Weiler says.
So why do physicists think sterile neutrinos might exist? One reason is that they could act as a foil for ordinary neutrinos – subatomic particles that come in three types (see diagram). All three types spin to the left – a mystery since every other known type of subatomic particle can spin to the left or right. Sterile neutrinos would spin to the right, balancing out their lefty brethren. The fact that sterile neutrinos only interact via gravity also makes them a candidate to explain dark matter, the elusive additional gravitational glue that appears to hold galaxies and other cosmic structures together.
“Sterile neutrinos only interact via gravity, making them a candidate to explain dark matter”
But finding evidence of the particles has been difficult, to say the least. The past two decades have seen a roller-coaster of experimental measurements that alternately boost and bash the case for the sterile neutrino (see timeline). Many of the twists and turns have hinged on observations of one type of ordinary neutrino morphing into another at unexpected rates, suggesting they may have gone through an intermediate sterile phase.
Now the game of hide-and-seek may be drawing to a close. Three separate lines of evidence are starting to shore up the case for sterile neutrinos. “They’re all pointing in the same direction,” says of Princeton University. The new studies all involve observations of the sky or of neutrinos streaming in from space.
One line of evidence suggests that sterile neutrinos smoothed out clumps of matter in the early universe. These clumps began as quantum fluctuations in a soup of particles and radiation that filled space just after the big bang. Thanks to gravity, the clumps grew over time, eventually becoming galaxies.
The oldest light we can detect, the cosmic microwave background radiation, shows a snapshot of the universe’s structure when it was about 380,000 years old. Previous measurements of this light from NASA’s Wilkinson Microwave Anisotropy Probe hinted that these clumps were less clumpy than expected, assuming there were just three ordinary types of neutrino. Now the in Chile and the have borne this out at higher resolution, Spergel reported last month at a in Cambridge, UK. “One way to understand that is if we have more neutrino species,” Spergel says.
Neutrinos of any stripe hardly interact with matter, meaning they did not get sucked into the clumps but instead simply sped off into space. The upshot is that they smoothed over any differences between one part of the universe and another. “They’re contributing to the total energy of the universe, but they don’t stay in [gravitational] wells, so they don’t contribute to the growth of structure,” Spergel says.
The results are not statistically significant as yet, but ultrasharp observations from the European Space Agency’s Planck space telescope, which launched in 2009, should reveal this smoothing in 2013 if it is there.
The distances between galaxies might also expose the machinations of sterile neutrinos. In the universe’s infancy, photons pushing outwards from the clumps of subatomic particles created pressure waves that spread out like ripples from raindrops falling on a pond. Such ripples could only spread as long as the universe was so hot that electrons and protons remained separate, however. When the universe cooled enough for subatomic particles to combine into atoms, the pond effectively froze, leaving a ring with a radius of 500,000 light years around each clump.
Since the ripples were pressure waves where there was more matter, more galaxies formed on the rings and also in the original clumps. As the universe expanded, the rings did too, and so researchers believe galaxies should be clustered about 500 million light years apart today.
Yet new evidence from the Sloan Digital Sky Survey suggests they are not. Kushal Mehta of the University of Arizona in Tucson and colleagues say galaxies may instead cluster 480 million light years apart ().
Sterile neutrinos could be to blame for that, too: more neutrinos would make the universe expand at a faster rate, forcing the “pond” to freeze sooner. The ripples would have been smaller, explaining why galaxies appear to be clustered together more closely today.
Finally, the IceCube neutrino detector in Antarctica has recorded a dearth of muon neutrinos produced when cosmic rays slam into the atmosphere. That suggests they may have transformed into sterile neutrinos, since if they could only morph into their ordinary siblings, there would be more left over ().
Experimental uncertainties in all of the research above means physicists’ one-sided love affair with the sterile neutrino isn’t over quite yet. “We know the data will be better a year from now,” says Spergel. “Watch this space.”
Speedy neutrinos… so three-dimensional
Could sterile neutrinos explain the weirdest neutrino result to date?
In September last year, researchers with the OPERA experiment beneath the Gran Sasso mountain in Italy reported that neutrinos travelling there from CERN near Geneva, Switzerland, had outpaced the speed of light. That sent shock waves through the physics world, since it apparently violated Einstein’s special theory of relativity.
At first, Tom Weiler of Vanderbilt University in Nashville, Tennessee, thought sterile neutrinos might be able to explain the observation. Because they interact so weakly with matter, Weiler and colleagues thought they might have access to extra dimensions. They suggested that the OPERA neutrinos could have morphed into sterile neutrinos mid-flight, taken a shortcut through an extra dimension, and appeared at the detector 60 nanoseconds early.
But that option seemed less likely after the OPERA team reran their experiment with tighter bunches of particles over the next two months. The new data suggested that all the neutrinos were breaking the speed limit, not just some of them.
“That’s harder to accomplish,” Weiler says. “It’s harder to make all the neutrinos turn into steriles, go through extra dimensions and turn back into actives to be detected at OPERA.”
