2011 OPERA faster-than-light neutrino anomaly

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.

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.

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.

[5] The OPERA collaboration stated in their initial press release that further scrutiny and independent tests were necessary to definitely confirm or refute the results.

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 September 23, 2011, the researchers announced that neutrinos had been observed traveling at faster-than-light speed.

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).

[3] 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".

The difference between the measured and expected arrival time of neutrinos (compared to the speed of light) was approximately 6.5 ± 15 ns.

[4] In March 2012, the co-located ICARUS experiment refuted the OPERA results by measuring neutrino velocity to be that of light.

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.

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.

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.

To link the surface GPS location to the coordinates of the underground detector, traffic had to be partially stopped on the access road to the lab.

1 shows, the time measuring system included the neutrino source at CERN, the detector at LNGS (Gran Sasso), and a satellite element common to both.

[31] In addition, highly stable cesium clocks were installed both at LNGS and CERN to cross-check GPS timing and to increase its precision.

After OPERA found the superluminal result, the time calibration was rechecked both by a CERN engineer and the German Institute of Metrology (PTB).

In the main November analysis, all the existing data were reanalyzed to allow adjustments for other factors, such as the Sagnac effect in which the Earth's rotation affects the distance traveled by the neutrinos.

This comparison indicated neutrinos had arrived at the detector 57.8 nanoseconds faster than if they had been traveling at the speed of light in vacuum.

An alternative analysis in which each detected neutrino was checked against the waveform of its associated proton spill (instead of against the global probability density function) led to a compatible result of approximately 54.5 nanoseconds.

This meant a detected neutrino could be tracked uniquely to its generating 3 nanoseconds pulse, and hence its start and end travel times could be directly noted.

After the initial report of apparent superluminal velocities of neutrinos, most physicists in the field were quietly skeptical of the results, but prepared to adopt a wait-and-see approach.

Nobel laureates Steven Weinberg,[36] George Smoot III, and Carlo Rubbia,[37] and other physicists not affiliated with the experiment, including Michio Kaku,[38] expressed skepticism about the accuracy of the experiment on the basis that the results challenged a long-held theory consistent with the results of many other tests of special relativity.

[40] Previous experiments of neutrino speed played a role in the reception of the OPERA result by the physics community.

John Ellis, theoretical physicist at CERN, believed it difficult to reconcile the OPERA results with the SN 1987A observations.

Physicists affiliated with the experiment had refrained from interpreting the result, stating in their paper: Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly.

[10] Theoretical physicists Gian Giudice, Sergey Sibiryakov, and Alessandro Strumia showed that superluminal neutrinos would imply some anomalies in the velocities of electrons and muons, as a result of quantum-mechanical effects.

[47][48][13][16] A vote of no confidence among the more than thirty group team leaders failed, but spokesperson Ereditato and physics coordinator Autiero resigned their leadership positions anyway on March 30, 2012.

"[51] In the wake of the discovery, the minister of education Mariastella Gelmini issued a press release praising the experiment and her government for financing the construction of a tunnel between the two laboratories.

Fig. 1 Faster than light neutrinos. What OPERA saw. Leftmost is the proton beam from the CERN SPS accelerator. It passes the beam current transformer (BCT), hits the target, creating first, pions and then, somewhere in the decay tunnel, neutrinos. The red lines are the CERN Neutrinos to Gran Sasso (CNGS) beam to the LNGS lab where the OPERA detector is. The proton beam is timed at the BCT. The left waveform is the measured distribution of protons, and the right that of the detected OPERA neutrinos. The shift is the neutrino travel time. Distance traveled is roughly 731 km. At the top are the GPS satellites providing a common clock to both sites, making time comparison possible. Only the PolaRx GPS receiver is above-ground, and fiber cables bring the time underground.
Fig. 1 What OPERA saw. Leftmost is the proton beam from the CERN SPS accelerator. It passes the beam current transformer (BCT), hits the target, creating first, pions and then, somewhere in the decay tunnel, neutrinos . The red lines are the CERN Neutrinos to Gran Sasso (CNGS) beam to the LNGS lab where the OPERA detector is. The proton beam is timed at the BCT. The left waveform is the measured distribution of protons, and the right that of the detected OPERA neutrinos. The shift is the neutrino travel time. Distance traveled is roughly 731 km. At the top are the GPS satellites providing a common clock to both sites, making time comparison possible. Only the PolaRx GPS receiver is above-ground, and fiber cables bring the time underground.
Fig. 2 Analysis of the internal replication. Distribution of the early-arrival values for each detected neutrino with bunched-beam rerun. The mean value is indicated by the red line and the blue band.
Fig. 2 Analysis of the internal replication in November. Distribution of the early-arrival values for each detected neutrino with bunched-beam rerun. The mean value is indicated by the red line and the blue band.