Putting the Brakes on Superluminal Neutrinos

In our last update on the situation regarding OPERA’s superluminal neutrino measurement, I said that the true test of the result would be whether it stood up to independent verification.  Since the controversial result was announced last November, neutrino detector experiments have scrambled to help settle the neutrino time-of-flight with their own findings.  Now the first of those independent results has come in, and the outlook looks good for fans of special relativity: the ICARUS experiment, a time projection chamber located in the Gran Sasso tunnel, has made its own measurements of the same CERN beam used by OPERA, and has found no evidence of superluminal velocities.

Comparison of ICARUS and OPERA's measured deviations in neutrino time-of-flight from the light travel time. (Image from the ICARUS collaboration, arXiv:1203.3433v2)

ICARUS is another experiment located in the Gran Sasso tunnel, an Italian facility buried beneath a mountain in the Apennines.  Because it’s in the same place as OPERA, it can also make measurements of the beam fired from CERN.  Unlike OPERA, which measures neutrinos by watching for flashes in photoscintillation material, ICARUS is a time-projection chamber.  Time-projection chambers are a modern upgrade of bubble chambers, using a large cryogenic target volume (in this case, liquid argon) gridded with electronic detectors that pick up the signatures of collisions.  Using this method, ICARUS looked for neutrino interactions from the widely-spaced beam that CERN switched to in late 2011.  The long spacing between brief pulses – 3 ns bursts separated by 524 ns – makes it much easier for the experiments on the other end to be sure of the timing, because it’s clear which origin pulse each neutrino belongs to.

According to relativistic theory, neutrinos should be moving extremely quickly because they have such small masses.  Exactly how light neutrinos are is still an open question, but at ICARUS’ level of precision (and most everyone else’s), these particles should effectively appear to travel at the speed of light.  ICARUS reported 7 neutrino events in total that could be traced to the CERN beam, in line with the number they would expect given the flux sent from CERN; all 7 of them arrived at a time consistent with near-lightspeed velocities.

Other experiments are in the midst of collecting their own data to support or contest OPERA’s result, but the ICARUS results are the clearest evidence yet that the measurement made by OPERA was the result of – completely unintentional – error, and not a signal of new physics, and the significant controversy may now be considered to be over.  None of it should be considered wasted effort, however; the primary lesson we can all draw from this discussion is that the scientific process – and special relativity – are still in good, working order, and that’s always a reassuring thing to know.

About Elizabeth Lovegrove

I'm a graduate student at the University of California - Santa Cruz, working with Stan Woosley on simulations of supernovae. In the past I've tinkered with gamma-ray astronomy, galaxy evolution, exoplanet detection, and instrument design. I like supercomputers, aircraft, observing runs, loud techno, and videogames. I am on an unending quest to develop the nerdiest joke in the world. You can find me on Twitter at @GravityAndLight.

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