Faster-Than-Light Result to Be Scrutinized

Part of the OPERA detector.

OPERA Collaborative

The OPERA collaboration, which made headlines around the world last month when it announced that it had apparently observed neutrinos traveling faster than the speed of light, has decided that it will carry out a new set of very precise measurements in order to check its controversial result. The decision means the group will delay submitting its result to a peer-reviewed journal by up to a month. The move also comes in the wake of heated disagreements between collaboration members regarding the solidity of its superluminal claim—and whether it was announced prematurely.

OPERA (Oscillation Project with Emulsion-Tracking Apparatus) uses a detector located under the Gran Sasso mountain in central Italy to study the properties of neutrinos that travel some 730 kilometers through the Earth's crust after being produced at the CERN laboratory in Geneva. In a paper uploaded to the arXiv preprint server on 22 September, the collaboration reported that neutrinos observed between 2009 and 2011 appeared to have traveled faster than light, arriving about 60 nanoseconds earlier than precise calculations predicted they should.

Some members of the 160-strong collaboration, however, believe that further checks are needed to be absolutely sure that the result was not due to an error. And there has been intense discussion within the collaboration about whether those extra checks should be carried out before submitting the result to peer review. It now appears that those urging caution have prevailed, with a new set of measurements to be carried out ahead of any submission.

The new measurements will involve a change in the CERN neutrino beam. CERN makes the particles by colliding proton pulses with a graphite target, with each pulse being about 10,500 nanoseconds long. CERN has now split these pulses up so that each one consists of bunches lasting 1 to 2 nanoseconds; bunches are separated by gaps of 500 nanoseconds. The change means that it will be possible to tie each neutrino event recorded inside OPERA to a specific proton bunch, thus enabling a very precise measurement of the time it takes neutrinos to travel between the two labs. Previously, the collaboration had to carry out a statistical analysis to establish the time-of-flight, which involved comparing the temporal distribution of protons generated at CERN with that of the neutrinos detected at Gran Sasso. Critics had argued that this analysis might not be reliable.

Sources suggest that the collaboration will carry out the measurements over a period of 10 days, probably starting next week, and that in that time it should intercept around 12 neutrinos. It's possible that will generate enough data to disprove the announced result, or else to confirm an important part of the analysis behind the result.

The new checks follow scores of papers posted to arXiv by other physicists, either speculating on the theoretical implications of the superluminal claim or arguing that the claim is flawed in some way. One such paper, written by Andrew Cohen and Sheldon Glashow of Boston University, argues that the energy distribution of neutrinos observed by OPERA is not what would be expected if neutrinos within the beam were traveling faster than light, since these neutrinos would lose significant amounts of energy through the creation of pairs of electrons and positrons. Indeed, the ICARUS collaboration, which operates a second detector in Gran Sasso to intercept the CERN neutrinos, says that the energy distribution it has recorded is exactly that expected from non-faster than light neutrinos.