More neutrino news

November 22, 2011

I am catching up on recent developments in the superluminal neutrino story. You will recall that in September the OPERA collaboration claimed that they had measured neutrinos travelling from CERN, near Geneva, to Gran Sasso, in Italy, at a speed in excess of the speed of light. This is forbidden by all of modern physics, and so attracted a lot of attention.

I wrote a few weeks ago about an interesting little paper that pointed out a relativistic time synchronization effect that might have produced the anomalous measurement. I thought it sounded like just the sort of thing that was needed. A little to my surprise, that paper has not attracted much attention in the scientific literature, so maybe someone knows something I don’t.

This week the OPERA collaboration announced that they had repeated the experiment, addressing some criticisms about their method, but had arrived at the same anomalous result.

In the meantime, Andrew Cohen and Sheldon Glashow published a very nice (as you would expect from these physicists) paper in which they showed that superluminal neutrinos, if they exist, should radiate electron-positron pairs at a rate sufficient to deplete the high-energy end of the neutrino spectrum. Picking up on that idea, the ICARUS experiment, also located at Gran Sasso, measured the energy spectrum of the neutrinos coming from CERN and showed that the neutrinos were not radiating their energy away. In other words, this is more evidence that the supposedly superluminal neutrinos are not actually superluminal.

This still leaves open the question of exactly what is wrong with the original experiment, but it does help me to sleep a little more peacefully at night.

(Thanks to Maclin for pointing me to the ICARUS result.)

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2 Responses to “More neutrino news”

  1. Vince Says:

    Although the Cohen and Glashow paper does provide very strong evidence that the OPERA neutrinos did not travel faster than light, it’s not definitive proof. If those neutrinos really were superluminal, then special relativity isn’t the whole story. But special relativity is necessary for the results of the Cohen and Glashow paper to apply. I confess, I did not read the paper, but I’ve read *about* the paper, so I don’t know how universal their results are.

    One thing I read somewhere was that some sort of clock used in the experiment runs at 20 MHz, which means there’s some granularity in their timing events of 50 ns, which might introduce something fishy.

  2. cburrell Says:

    As far as I could see in my quick perusal, they relied principally on kinematic considerations, though they did throw in some time dilation factors at one step. (For the non-initiated, this means that they used special relativity.)

    It poses an interesting puzzle: what reasoning can you use to assess something that strikes at the root of your reasoning? Time dilation, I would think, is pretty well attested experimentally — though not necessarily for neutrinos specifically. There’s the rub.

    That clock granularity does seem like an obvious potential problem!


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