By Devin Powell
A new study puts the brakes on faster-than-light neutrinos.
In September, a group at Italy’s OPERA experiment reportedly clocked neutrinos traveling the 730 kilometers from CERN in Switzerland to Italy’s underground Gran Sasso National Laboratory about 60 nanoseconds faster than light would have covered that distance in a vacuum (SN: 10/22/11, p. 18). But if this were true, most of the neutrinos would have shed energy en route, a new analysis by Boston University physicists suggests.
OPERA should have detected this radiation, say the physicists, if its claims are to be believed. It didn’t.
“I would be ecstatic to see some kind of new physics coming from this experiment,” says Andrew Cohen, a theoretical physicist who, with Nobel Prize–winner Sheldon Glashow, reports the new finding in an upcoming Physical Review Letters. “It’s just hard to accommodate that, given this [lack of] radiation.”
To follow up on this idea, a second neutrino experiment at Gran Sasso called ICARUS searched for signs of this radiation and found none, another group reports online October 17 at arXiv.org.
A similar type of energy loss has long been studied in water and other materials in which light travels slower than it does in empty space. Particles that travel faster than light in these substances give off flashes of energy known as Cerenkov radiation. Cerenkov detectors exploit this effect to spot such particles, including those created by cosmic rays.
But Cohen and Glashow’s paper is the first to extend this idea to neutrinos allegedly outpacing light in a vacuum. The standard model of particle physics dictates that they too must give up energy, spitting out pairs of electrons and positrons, says Cohen.
Exotic theories do exist that allow faster-than-light neutrinos to hold on to their energy. But the lack of this signature adds support to the prevailing opinion that the OPERA team has been misled by some unaccounted-for uncertainty in their measurements.
“We’re pretty much convinced that the experiment is wrong,” says Glashow. “But I don’t think anyone has identified the error, if there is an error, as of yet.”
Gilles Henri, a theoretical astrophysicist at the Institute of Planetary Science and Astrophysics in Grenoble, France, wonders whether fluctuations in the neutrino beam may be to blame. In a paper posted online October 2 at arXiv.org, he suggests that some neutrinos in the crowd that traveled to Italy may have started their journey earlier than thought, throwing off the average speed calculated for the bunch.
The two atomic clocks used by the OPERA team to time their neutrinos have also been scrutinized. Carlo Contaldi, a theoretical physicist at Imperial College London, suggests that Einstein’s general theory of relativity could have caused the clocks to tick at different rates — thanks to gravity pulling harder on the clock located at the start of the neutrino’s journey in Switzerland than on its partner, deep underground in Italy. To check his idea, Contaldi is waiting on the OPERA team to explain the details of their experiment more fully.
He’s not the only one.
“Until further details come out as to how they did the various bits of their experiment,” says Contaldi, “it’s not clear how to proceed.”
