In 2011, the Oscillation Project with Emulsion-tRacking Apparatus (OPERA) experiment mistakenly observed neutrinos appearing to travel faster than light. Even before the source of the error was discovered, the result was considered anomalous because speeds higher than that of light in vacuum are generally thought to violate special relativity, a cornerstone of the modern understanding of physics for over a century.
On June 8, 2012, after further research and analysis, CERN research director Sergio Bertolucci declared that the speed of neutrinos is consistent with that of light. The press release, made from the 25th International Conference on Neutrino Physics and Astrophysics in Kyoto, states that the original OPERA results were wrong, due to equipment failures.
On July 12, 2012, OPERA updated their paper by including the new sources of errors in their calculations. They found agreement of neutrino speed with the speed of light.
Detection The experiment created a form of neutrinos, muon neutrinos, at CERN's older SPS accelerator, on the Franco–Swiss border, and detected them at the LNGS lab in Gran Sasso, Italy. OPERA researchers used common-view GPS, derived from standard GPS, to measure the times and place coordinates at which the neutrinos were created and detected. As computed, the neutrinos' average time of flight turned out to be less than what light would need to travel the same distance in vacuum. In a two-week span up to , the OPERA team repeated the measurement with a different way of generating neutrinos, which helped measure travel time of each detected neutrino separately. This eliminated some possible errors related to matching detected neutrinos to their creation time. The OPERA collaboration stated in their initial press release that further scrutiny and independent tests were necessary to definitely confirm or refute the results.
First results In a analysis of their data, scientists of the OPERA collaboration reported evidence that neutrinos they produced at CERN in Geneva and recorded at the OPERA detector at Gran Sasso, Italy, had traveled faster than light. The neutrinos were calculated to have arrived approximately 60.7 nanoseconds (60.7 billionths of a second) sooner than light would have if traversing the same distance in vacuum. After six months of cross checking, on , the researchers announced that neutrinos had been observed traveling at faster-than-light speed. Similar results were obtained using higher-energy (28 GeV) neutrinos, which were observed to check if neutrinos' velocity depended on their energy. The particles were measured arriving at the detector faster than light by approximately one part per 40,000, with a 0.2-in-a-million chance of the result being a false positive, assuming the error were entirely due to random effects (significance of six sigma). This measure included estimates for both errors in measuring and errors from the statistical procedure used. It was, however, a measure of precision, not accuracy, which could be influenced by elements such as incorrect computations or wrong readouts of instruments. For particle physics experiments involving collision data, the standard for a discovery announcement is a five-sigma error limit, looser than the observed six-sigma limit.
The preprint of the research stated "[the observed] deviation of the neutrino velocity from c (the speed of light in vacuum) would be a striking result pointing to new physics in the neutrino sector" and referred to the "early arrival time of CNGS muon neutrinos" as an "anomaly". OPERA spokesperson Antonio Ereditato explained that the OPERA team had "not found any instrumental effect that could explain the result of the measurement". James Gillies, a spokesperson for CERN, said on September 22 that the scientists were "inviting the broader physics community to look at what they [had] done and really scrutinize it in great detail, and ideally for someone elsewhere in the world to repeat the measurements".
Internal replication
In November, OPERA published refined results where they noted their chances of being wrong as even less, thus tightening their error bounds. Neutrinos arrived approximately 57.8 ns earlier than if they had traveled at light-speed, giving a relative speed difference of approximately one part per 42,000 against that of light. The new significance level became 6.2 sigma. The collaboration submitted its results for peer-reviewed publication to the Journal of High Energy Physics.
In the same paper, the OPERA collaboration also published the results of a repeat experiment running from to . They detected twenty neutrinos consistently indicating an early neutrino arrival of approximately 62.1 ns, in agreement with the result of the main analysis.
Measurement errors In February 2012, the OPERA collaboration announced two possible sources of error that could have significantly influenced the results. A link from a GPS receiver to the OPERA master clock was loose, which increased the delay through the fiber. The glitch's effect was to decrease the reported flight time of the neutrinos by 73 ns, making them seem faster than light. A clock on an electronic board ticked faster than its expected 10 MHz frequency, lengthening the reported flight-time of neutrinos, thereby somewhat reducing the seeming faster-than-light effect. OPERA stated the component had been operating outside its specifications.
In March 2012 an LNGS seminar was held, confirming the fiber cable was not fully screwed in during data gathering. LVD researchers compared the timing data for cosmic high-energy muons hitting both the OPERA and the nearby LVD detector between 2007 and 2008, 2008–2011, and 2011–2012. The shift obtained for the 2008–2011 period agreed with the OPERA anomaly. The researchers also found photographs showing the cable had been loose by October 13, 2011.
Correcting for the two newly found sources of error, results for neutrino speed appear to be consistent with the speed of light.
End results On July 12, 2012, the OPERA collaboration published the end results of their measurements between 2009 and 2011. The difference between the measured and expected arrival time of neutrinos (compared to the speed of light) was approximately . This is consistent with no difference at all, thus the speed of neutrinos is consistent with the speed of light within the margin of error. Also, the re-analysis of the 2011 bunched beam rerun gave a similar result.
Independent replication
In March 2012, the co-located ICARUS experiment refuted the OPERA results by measuring neutrino velocity to be that of light. ICARUS measured speed for seven neutrinos in the same short-pulse beam OPERA had checked in November 2011, and found them, on average, traveling at the speed of light. The results were from a trial run of neutrino-velocity measurements slated for May.
In May 2012, a new bunched beam rerun was initiated by CERN. Then in June 2012, it was announced by CERN that the four Gran Sasso experiments OPERA, ICARUS, LVD, and BOREXINO measured neutrino speeds consistent with the speed of light, indicating that the initial OPERA result was due to equipment errors.
In addition, Fermilab stated that the detectors for the MINOS project were being upgraded. Fermilab scientists closely analyzed and placed bounds on the errors in their timing system. On June 8, 2012, MINOS announced that according to preliminary results, the neutrino speed is consistent with the speed of light.
The measurement The OPERA experiment was designed to capture how neutrinos switch between different identities, but Autiero realized the equipment could be used to precisely measure neutrino speed too. An earlier result from the MINOS experiment at Fermilab demonstrated that the measurement was technically feasible. The principle of the OPERA neutrino velocity experiment was to compare travel time of neutrinos against travel time of light. The neutrinos in the experiment emerged at CERN and flew to the OPERA detector. The researchers divided this distance by the speed of light in vacuum to predict what the neutrino travel time should be. They compared this expected value to the measured travel time.
Overview The OPERA team used an already existing beam of neutrinos traveling continuously from CERN to LNGS, the CERN Neutrinos to Gran Sasso beam, for the measurement. Measuring speed meant measuring the distance traveled by the neutrinos from their source to where they were detected, and the time taken by them to travel this length. The source at CERN was more than away from the detector at LNGS (Gran Sasso). The experiment was tricky because there was no way to time an individual neutrino, necessitating more complex steps. As shown in Fig. 1, CERN generates neutrinos by slamming protons, in pulses of length 10.5 microseconds (10.5 millionths of a second), into a graphite target to produce intermediate particles, which decay into neutrinos. OPERA researchers measured the protons as they passed a section called the beam current transducer (BCT) and took the transducer's position as the neutrinos' starting point. The protons did not actually create neutrinos for another kilometer, but because both protons and the intermediate particles moved almost at light speed, the error from the assumption was acceptably low.
The clocks at CERN and LNGS had to be in sync, and for this the researchers used high-quality GPS receivers, backed up with atomic clocks, at both places. This system timestamped both the proton pulse and the detected neutrinos to a claimed accuracy of 2.3 nanoseconds. But the timestamp could not be read like a clock. At CERN, the GPS signal came only to a receiver at a central control room, and had to be routed with cables and electronics to the computer in the neutrino-beam control room which recorded the proton pulse measurement (Fig. 3). The delay of this equipment was 10,085 nanoseconds and this value had to be added to the time stamp. The data from the transducer arrived at the computer with a 580 nanoseconds delay, and this value had to be subtracted from the time stamp. To get all the corrections right, physicists had to measure exact lengths of the cables and the latencies of the electronic devices. On the detector side, neutrinos were detected by the charge they induced, not by the light they generated, and this involved cables and electronics as part of the timing chain. Fig. 4 shows the corrections applied on the OPERA detector side.
Since neutrinos could not be accurately tracked to the specific protons producing them, an averaging method had to be used. The researchers added up the measured proton pulses to get an average distribution in time of the individual protons in a pulse. The time at which neutrinos were detected at Gran Sasso was plotted to produce another distribution. The two distributions were expected to have similar shapes, but be separated by 2.4 milliseconds, the time it takes to travel the distance at light speed. The experimenters used an algorithm, maximum likelihood, to search for the time shift that best made the two distributions to coincide. The shift so calculated, the statistically measured neutrino arrival time, was approximately 60 nanoseconds shorter than the 2.4 milliseconds neutrinos would have taken if they traveled just at light speed. In a later experiment, the proton pulse width was shortened to 3 nanoseconds, and this helped the scientists to narrow the generation time of each detected neutrino to that range.
Measuring distance