Measurements of neutrino speed

Existing measurements provided upper limits for deviations from light speed of approximately 10−9, or a few parts per billion.

It was assumed for a long time in the framework of the standard model of particle physics that neutrinos are massless.

However, velocity differences predicted by relativity at such high energies cannot be determined with the current precision of time measurement.

The reason why such measurements are still conducted is connected with the theoretical possibility that significantly larger deviations from light speed might arise under certain circumstances.

For instance, it was postulated that neutrinos might be some sort of superluminal particles called tachyons,[2] even though others criticized this proposal.

[3] While hypothetical tachyons are thought to be compatible with Lorentz invariance, superluminal neutrinos have also been studied in Lorentz invariance violating frameworks as motivated by speculative variants of quantum gravity, such as the Standard-Model Extension according to which Lorentz-violating neutrino oscillations can arise.

[4] Besides time-of-flight measurements, those models also allow for indirect determinations of neutrino speed and other modern searches for Lorentz violation.

Fermilab conducted in the 1970s a series of terrestrial measurements, in which the speed of muons was compared with that of neutrinos and antineutrinos of energies between 30 and 200 GeV.

The remaining hadrons were stopped by a secondary dump, so that only neutrinos and some energetic muons can penetrate the earth- and steel shield of 500 meter length, in order to reach the particle detector.

Since the protons are transferred in bunches of one nanosecond duration at an interval of 18.73 ns, the speed of muons and neutrinos could be determined.

The difference of approximately three hours was explained by the circumstance, that the almost noninteracting neutrinos could pass the supernova unhindered while light required a longer time.

In order to generate neutrinos (the so-called NuMI beam) they used the Fermilab Main Injector, by which 120-GeV-protons were directed to a graphite target in 5 to 6 batches per spill.

In the OPERA experiment, 17-GeV neutrinos have been used, split in proton extractions of 10.5 μs length generated at CERN, which hit a target at a distance of 743 km.

The neutrinos traveled further to the Laboratori Nazionali del Gran Sasso (LNGS) 730 km away, where the OPERA detector is located.

[12][13][14] To exclude possible statistical errors, CERN produced bunched proton beams between October and November 2011.

The measurement of twenty neutrino events again gave an early arrival of about 62 ns, in agreement with the previous result.

[15][16] In February and March 2012, it was shown that there were two mistakes in the experimental equipment: An erroneous cable connection at a computer card, making the neutrinos appearing faster than expected.

[19] Finally, in July 2012 the OPERA collaboration published a new analysis of their data from 2009 to 2011, which included the instrumental effects stated above, and obtained bounds for arrival time differences (compared to the speed of light): and bounds for speed differences: Also the corresponding new analysis for the bunched beam of October and November 2011 agreed with this result: Although at the extremes of error these results still allow for superluminal neutrino velocities, they are predominantly consistent with the speed of light, and the

[21] For the 2011 data, they evaluated 36 neutrino events and obtained an upper limit for time of flight differences: For the May 2012 measurements, they improved their equipment by installing a new analogue small–jitter triggering system and a geodetic GPS receiver coupled to a Rb clock.

62 neutrino events could be used for the final analysis, giving a more precise upper limit for time of flight differences[21] corresponding to The LVD collaboration first analyzed the beam rerun of Oct.–Nov.

[25] An additional, independent timing system and four different methods of analysis were used for the evaluation of the neutrino events.