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Michael DuVernois | Wisconsin IceCube Particle Astrophysics Center | USA

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What can we learn from “faster-than-light” neutrinos?

I’ve let the news aspect of this story die back a little before writing about it. It now appears that the OPERA results were due to a mistake in the end. Rumors have it that it was a bad connector on a fiber optic link between a GPS and a computer that gave a 60ns time shift. New data taking with the tightened connector will be required to verify that this was, in fact, the cause of the problem. See also the Nature page.

Of course this was not a grand surprise, the vast majority of physicists felt that a mundane explanation would be found in the experiment rather than a rewrite of much of fundamental physics. But what I want to explore here is instead, what would you do? And, how does this illuminate the differences between theory and experiment?

The first question was fairly explicitly asked to me by colleagues at a meeting just days after the announcement of the preliminary OPERA “faster-than-light” neutrinos. I stumbled over an answer that I can abstract as “if you make a measurement, you can think about it, and even not believe it, but eventually you publish it” presumably with enough caveats that you aren’t misleading the readers into a different level of confidence in the results that you yourself hold. But I’m not sure if this answer (though probably close to what any official answer would be) is truly correct.

Extraordinary claims do demand extraordinary proof. And at first look the OPERA folks seemed to be extraordinarily careful in their review of their own work. Since the velocity measurement  in the experiment fundamentally comes just from the distance and the time of flight of the particles, a lot of effort went into the metrology and surveying for the distance measurement and a careful evaluation of the clocks involved. But a loose connection seems to have been missed before public announcements and the wild theorist party (see below) that emerged from the smoke at the initial CERN lecture cum press conference.

Connectors are the bane of an experiment. From the horrid Lemo 00 connectors still found all too often in particle and nuclear physics, to the stiff cadmium-plated circular military connectors beloved of the aerospace concerns, down to the simple is-it-really-connected-securely screw terminals on the back of an old power supply, this is where so much debugging time and effort goes. So it seems plausible that the error could be there. But when should it have been found?

I’d want to tear the experiment down and build it back up, re-cable, re-connect, tear everything apart before I’d be willing to claim a major discovery. At the time, the word was that the OPERA folks had put lots of time and effort into trying to find the problem, the mistake, but couldn’t locate it, so the news was released and the world started talking (and writing papers for arxiv) about faster-than-light neutrinos. I think we still don’t have a good enough picture as to the level of due diligence at the experiment. Did folks rebuild all of the timing system multiple times? Did the full signal chain get carefully looked at?

We tend to not be too critical of other physicists, and without knowing what happened within the OPERA collaboration, it’s easy for me to ask these questions without a real response. What experimentalists, in my opinion, need to take away from this is a real understanding of responsibility for being self-critical especially, but not exclusively, if there is a lot at stake. We well know the “solid four sigma” results which fade away in a few months, and yet it happens again and again. We know what will play in the popular press, and we’re careless about how we explain ourselves. (But enough about quantum teleportation illustrated with Star Trek visuals.)

I suspect that more than a few folks within the experiment, as well as outside, got terribly excited by the slim possibility of a major discovery. Within the group, this hopefully did not affect the critical thinking and tear-down of the experiment. Outside, in the larger community, certainly every neutrino experiment discussed very seriously what could be done to make such a measurement, and the theorists started producing papers. Why the results were wrong. Why the results were right, and agreed with their favored ideas. What it means for the rest of physics.

At times like this the cultural divide between theory and experiment never seems larger! A flood of papers since even a slight touch on a big discovery is worth something it seems. And now what? After the experimental error seems to be, well, an experimental error that wasn’t caught for a very long time, what do we think of all of theory papers? Presumably they just fade away, a light bright (?) spot of activity in late 2011 that someone will write a book about in five years, “The Faster-than-Light Neutrino Craze of ’11?” Some people got a little bit of publicity for misunderstanding GPS or for boldly extrapolating the neutrino velocity to higher energies. Is there regret over the waste of time? Or just a little exclamation, “ahhh…those experimentalists not checking their cables.”

More on this as the story develops, and as the water-cooler arguments continue.



  • http://backreaction.blogspot.com/ Bee

    I thought you might be interested to know that some members of the OPERA collaboration were not in favor of publishing the results prematurely. This was published in a German newspaper, so it probably didn’t spread very widely. You can find a partial translation and link to the original article here.

  • Michael DuVernois

    Thanks for the pointer! It must have been a difficult process for the authors, deciding whether or not to sign on to a potentially exciting, but likely wrong, result. Those must have been interesting meetings…

  • LarryJayCee

    It is, of course, much easier to identify the error (assuming that the connector is the source of the error) in hindsight. There is one issue that puzzled me though. The first measurements were made with long pulses of protons at CERN. later they used much shorter pulses to prove that it wasn’t an error in fitting to the pulse shape. Why didn’t they look at the time measurement and how it was transferred from the GPS receiver to the laboratory at the same time as they set up the experiment at CERN to use much shorter pulses? Could it have been “tunnel vision” – because they were particle physicists, they were looking principally at particle physics causes.

  • Michael DuVernois

    There was an effort looking at the time transfer. I had understood that standard clocks had been transferred and checked. That’s a standard astronomical practice. I once sat next to a technician who was taking a cesium beam clock up to one of the Hawaii observatories to transfer the calibration. Specifically on this problem, it still is hard to understand exactly how a loose connection could cause the problem. Presumably the reflected signal was used rather than the direct, but it’s hard to think of a setup in which only the (late) reflected pulse was setting up the clock and the direct wasn’t used.

  • Michael DuVernois

    The ICARUS T600 experiment reports non-super-luminal neutrinos from CERN. http://arxiv.org/abs/1203.3433