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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| 2011 OPERA faster-than-light neutrino anomaly | 2/5 | https://en.wikipedia.org/wiki/2011_OPERA_faster-than-light_neutrino_anomaly | reference | science, encyclopedia | 2026-05-05T12:40:44.478096+00:00 | kb-cron |
=== 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 6.5 ± 15 ns. 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 730 kilometres (450 mi) 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.