The claim by a team of researchers in Italy that neutrinos can travel faster than the speed of light will require extra checks before being submitted to a peer-reviewed journal. That is the position of a number of researchers in the OPERA collaboration, which announced on 23 September that it had observed superluminal neutrinos travelling from the CERN particle-physics lab near Geneva to the Gran Sasso underground lab in central Italy.

The announcement made headlines around the world, since it appears to contradict Einstein's special theory of relativity. However, not everyone within OPERA was happy to release the results publicly, with several of the 30 group leaders within the 160-strong collaboration being opposed to the release of a paper on the arXiv preprint server and the accompanying seminars and press release without further tests of possible systematic errors being carried out. Now, a larger fraction of the group leaders is concerned about the paper being submitted to a research journal. One member of OPERA, who does not wish to be named, says there is a "lot of tension" within the collaboration and that up to half of the members are opposed to an immediate submission.

Precision measurements

Neutrinos are produced by accelerating protons at CERN's Super Proton Synchrotron accelerator and colliding proton bunches 10 µs in length into a graphite target, generating mesons that in turn decay into neutrinos. The 1300-tonne OPERA detector, which has been running since June 2008, measures the properties of muon neutrinos as they travel 730 km through the Earth's crust from CERN to Gran Sasso.

The experiment was originally designed to study the oscillation of muon neutrinos into tau neutrinos, but following tentative results in 2007 from the MINOS experiment in the US that showed neutrinos appearing to travel faster than light, researchers realized accurate velocity measurements could also be carried out with OPERA. Researchers installed atomic clocks at both ends of the neutrino beam to establish exactly when the neutrinos are created and detected, and used GPS-based measurements to precisely measure the length of the baseline – the velocity being derived by dividing the baseline by the time of flight.

Collecting more than 16,000 events between 2009 and 2011, the OPERA collaboration calculated that muon neutrinos arrive on average 60.7 ns earlier than they would have done had they travelled at the speed of light, which corresponds to a fractional increase over light speed of 25 parts in a million. Having accounted for a host of possible systematic errors, including uncertainties relating to the precise moment of creation and detection of the neutrinos plus errors introduced by cabling and clock synchronization, the researchers arrived at a total systematic error of 7.4 ns, comparable with the statistical error of 6.9 ns.

The OPERA collaboration calculated a confidence level of "6σ", or a one in a billion chance the result was a statistical fluke, and this persuaded most of the collaboration that the result was solid enough to publish. However, some members were worried that unknown sources of systematic error might potentially destroy the confidence level. They argued that before making an announcement, further checks should be carried out – a process that could take several months.

One such check regards the timing of the neutrinos' arrival at Gran Sasso, and involves carrying out an analysis of timing data collected by monitoring the charge, rather than the light, generated by particles passing through the detector. This analysis relies on a very precise and painstaking measurement of the length of the cabling used to collect the timing data, in order to isolate any systematic errors that may be present within the electronics or other parts of the timing system.

Another independent check involves the statistical analysis of the data collected by OPERA. The researchers are not able to track, and therefore time, individual neutrinos as they travel from Geneva to Gran Sasso, but instead they measure the temporal distribution of the protons within each bunch just before the protons hit the graphite target and then compare this with the distribution of the corresponding neutrinos as they are detected in OPERA – with the temporal offset between the two revealing the time of flight. Some members of the collaboration argue that this offsetting procedure needs to be carried out independently, in order to be sure that the temporal profile of the neutrinos leaving CERN can be inferred accurately from that of the protons that produced them.

Heated debate

Discussions about whether or not the collaboration was ready to publish took place in early September. As these discussions were quite animated, the decision was put to a vote, with collaboration spokesperson Antonio Ereditato from the University of Bern proposing that initially the research be published on arXiv while at the same time being presented in a series of scientific seminars, before later being submitted to a peer-reviewed journal. This strategy received a majority, but not a unanimous, vote. It was then left to individual researchers to sign the arXiv paper, with about 10 senior members out of a total of 170 people (including some non-official members) deciding not to do so.

There are so many things that people outside can't check. It is these things that we have to do before publishing Caren Hagner, Hamburg University

Ereditato says that the collaboration will continue to carry out checks but will do so in parallel with the journal submission. He maintains that no-one outside the collaboration, either at the seminars or via e-mail, has yet presented "smoking guns against what we have seen" and adds that "as experimentalists we have done everything we can". However, Caren Hagner, leader of the OPERA group at Hamburg University and one of the people whose name does not appear on the arXiv paper, believes that the collaboration should carry out the extra checks before submitting the paper for peer review. "Many of the collaboration are convinced that if a mistake is subsequently found it, won't be down to OPERA," she says. "But I am not really convinced. There are so many things that people outside can't check. It is these things that we have to do before publishing."

Laura Patrizii, who is leader of OPERA's Bologna group and who did sign the preprint, clarifies the motivation of the dissenters. "It is not that people think there is a mistake that is being hidden," she says. "But since something going faster than light would kill modern physics as we know it, some researchers would feel more at ease with these independent checks."

Looking to the outside

In addition to the checks that can be carried out within the collaboration, there are also some additional checks that CERN could perform, such as using detectors downstream of the graphite target to provide a better estimate of the profile of the departing neutrinos. The MINOS experiment is also currently improving its cabling and electronics, and collaboration co-spokesperson Jenny Thomas from University College London says that new data collected with the upgraded detector combined with a better analysis of existing data could allow MINOS to largely rule out the OPERA result within the next four to six months (but not to rule it in, given that this would require a higher level of accuracy).

Giovanni Amelino-Camelia, a theoretical physicist at the University of Rome "La Sapienza", believes that a confirmed OPERA result would lead to a "revolution" within physics. But he thinks that this confirmation is unlikely, pointing out that in the history of physics there have been many experimental "alarms" suggestive of a revolution but that only a small fraction of these, such as the Michelson–Morley experiment, have been confirmed.

With OPERA in the spotlight, collaboration members also disagree about their future research programme. Luca Stanco, leader of the OPERA group from the University of Padova and one of the people who did not sign the preprint, believes that the priority now should be further investigation of the superluminal-neutrino result, rather than neutrino oscillations. Ereditato, however, says that even though the collaboration will pursue superluminal research, "the main focus will continue to be oscillations".