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Faster than light neutrinos?

The astonishing report that neutrinos appeared to be moving faster than light recently emerged, although tentatively.  The OPERA experiment, that produced this result, generates a beam of neutrinos by colliding high energy protons with a carbon rod.  The neutrinos are then sent through rock before interacting with the OPERA detector in central Italy. The neutrino detectors comprise over 1000 tons of lead, interspersed by photographic media and detectors for identifying the interaction of neutrinos with the lead. 

OPERA is designed to detect neutrino oscillations using artificial sources.  Time-of-flight (TOF) measurements made with the OPERA apparatus indicates that the μ-neutrinos arrive about 60 nsec before a light beam would have arrived.  This corresponds to a speed of about 1.000025 c, or about 7.5 km/sec faster than light.  This is a six-sigma result — near statistical certainty. 

The OPERA equipment is amazingly effective at detecting these "ghost" particles, but was not specifically designed to take neutrino time of flight (TOF) measurements.  The reported TOF measurement is statistically built from the number of neutrino interactions as a function of time from the beginning of the proton burst. There are well-known uncertainties associated with this procedure, but most of these would increase the measured TOF.

To the credit of the OPERA collaborators, they spent years accumulating data and analyzing their equipment for consistent errors before announcing the anomaly.  In addition, they did not announce discovery of faster than light particles, but only that their experiment indicated a TOF that was shorter than expected, and that they were unable to explain the finding. 

The most obvious factor with potential to produce this result is that the measured distance between source and detector is incorrect, so that the path is 18 meters (60 nsec) shorter than expected.  However, it appears that distance and timing errors should result in no more than 3-5 nanoseconds of error.

Another theory for what is happening comes from string theorists. Tachyons are a proposed class of particles which are Lorentz invariant, but must always travel faster than the speed of light.  One result of this is negative mass; in view of which it is interesting to note that attempts to measure the neutrino mass have tended to indicate a negative mass, although in nearly all cases the error bars encompass very small positive masses as well.  String theorists have suggested that extension of extremely high energy neutrinos into a nearby membrane could move faster than light through an analogy of the theoretically predicted Scharnhorst effect.  A theory known as double special relativity is consistent with velocities greater than light. 

An interesting point made by most commentators is that there did not appear to be a significant time difference between the initial neutrino flux from Supernova 1987a and the appearance of light, suggesting that the neutrino velocity was very close to light velocity.  However, the neutrinos in the OPERA experiment had nearly 1000 times more energy than the neutrinos from the supernova. This being the case, energy-dependent terms in the dynamics of particle motion could have come into play at the larger energies. 

Faster than light neutrinos?  Maybe.  Stay tuned to this fascinating story — given the extremely tiny interactions of neutrinos with matter, I suspect it will take a decade or two to clear up what is really happening.

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