For the last few months, physicists have been attempting to explain the apparent discovery of neutrinos traveling faster than the speed of light. No one has as yet refuted this finding, but some other particles may refute these neutrinos' existence.
In September, physicists at the OPERA experiment announced that they had observed neutrinos traveling through the Earth from CERN to an underground detector in Gran Sasso seemingly faster than the speed of light, arriving 60 nanoseconds earlier than the laws of physics would allow. Right now, physicists around the world are attempting to replicate these findings and to sort through all possible objections and potential sources of error. It's early days yet, but the OPERA anomaly has stood up fairly well to these attempts thus far.
But now Washington University St. Louis physicist Ramanath Cowsik and his team have come up with what is quite possibly an impossible problem for these faster-than-light neutrinos to overcome. Instead of focusing on the neutrinos themselves, Cowsik looked at the other subatomic particles in the experiment that were smashed together to create the neutrinos.
Here's how the OPERA experiment worked: Protons were shot towards a stationary object, which produced a pulse of particles known as pions. These are low-mass subatomic particles that are composed of a pair of quarks. (For more on pions, check out our particle physics field guide.) These pions were magnetically forced through a long tunnel, and there they decayed into neutrinos and muons, which are like a more massive cousin of electrons.
When the particles reached the end of the tunnel, the muons smashed into the wall and came to a stop, but the extremely light neutrinos slipped right through and made their way to Gran Sasso, a journey they seemingly completed 60 nanoseconds too quickly.
The problem, from a theoretical perspective, is how to fit the pions into this story. To reach superluminal speeds, the neutrinos must have possessed extreme amounts of energy. The law of conservation of energy and momentum demands that that energy came from somewhere — specifically, the pions. But Cowsik calculated that if pions were to have enough energy to create faster-than-light neutrinos, then their lifetimes would also increase.
That expanded pion lifespan would dramatically change the particles' decay patterns, and that's where the problem comes in. As the lifetime of the pion increases, the neutrinos would carry a smaller and smaller fraction of the total energy of the particle — meaning they wouldn't have enough energy to reach faster-than-light speeds. And that problem only gets worse the more energy you add to the pions.
This is a major problem for the existence of superluminal neutrinos, though Cowsik doesn't overstate his findings, simply observing "that in the present framework of physics, superluminal neutrinos would be difficult to produce." It would appear that pions are exactly the wrong decay source for such neutrinos, since there just doesn't seem to be a way to transfer enough energy from pions to neutrinos in our current understanding of physics.
Cowsik also explains that the IceCube neutrino detector seems to offer observational support for his team's theoretical work:
"IceCube has seen neutrinos with energies 10,000 times higher than those the OPERA experiment is creating. Thus, the energies of their parent pions should be correspondingly high. Simple calculations, based on the conservation of energy and momentum, dictate that the lifetimes of those pions should be too long for them ever to decay into superluminal neutrinos. But the observation of high-energy neutrinos by IceCube indicates that these high-energy pions do decay according to the standard ideas of physics, generating neutrinos whose speed approaches that of light but never exceeds it."
So, what does this mean for the OPERA anomaly? Since the results themselves seem to require previously unknown physics to explain them, it's at least conceivable that some other exotic phenomenon could explain this pion decay problem. But right now, this appears to be some of the most damning evidence yet against the existence of the faster-than-light neutrinos.