The Phantom of OPERA

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Those working in science are accustomed to receiving emails starting with “dear sir/madam, please look at the attached file where I’m proving einstein theory wrong”. This time it’s a tad more serious because the message comes from a genuine scientific collaboration… As everyone knows by now, the OPERA collaboration announced that muon neutrinos produced at CERN arrive to a detector 700 kilometers away in Gran Sasso about 60 nanoseconds earlier than expected if they traveled at the speed of light (incidentally, trains traveling the same route arrive always late)….. The paper is available on arXiv, and the video from the CERN seminar is here.

OPERA is an experiment who has had some bad luck in the past. Its original goal was to study neutrino oscillations by detecting the appearance of tau neutrinos in a beam of muon neutrinos. However due to construction delays their results arrive too late to have any impact on measuring the neutrino masses and mixing; other experiments have in the meantime achieved a much better sensitivity to to these parameters. Moreover, the “atmospheric” neutrino mass difference, which enters the probability of a muon neutrino oscillating into a tau one, turned out to be at the lower end of the window allowed when OPERA was being planned. As a consequence, a fairly small number of oscillation events is predicted to occur on the way to Italy, leading to the expectation of about 1-2 tau events to be recorded during experiment’s lifetime (they were lucky to already get 1). However they will not walk off the stage quietly. What was meant to be a little side analysis returned the result that neutrinos travel faster than light, confounding the physics community and wreaking havoc in the mainstream media.

I’m not very original in thinking that the result is almost certainly wrong. The main experimental reason, already discussed on blogs, is the observation of neutrinos from the supernova SN1987A. Back in 1987, three different experiments detected a burst of neutrinos, all arriving within 15 seconds and 2-3 hours before the visible light (which agrees with models of supernova explosion). On the other hand, if neutrinos traveled as fast as OPERA claims, they should have arrived years earlier. Note that the argument that OPERA is dealing with muon neutrinos while supernovae produce electron ones is not valid: electron neutrinos have enough time to oscillate to other flavors on the way from the Large Magellanic Clouds. One way to reconcile OPERA with SN1987A would be to invoke a strong energy dependence of the neutrino speed (it should be steeper than Energy^2), since the detected supernova neutrinos are in the 5-40 MeV range, while the energy of the CERN-to-Gran-Sasso beam is 20 GeV on average. However OPERA does not observe any significant energy dependence of the neutrino speed, so that is an unlikely explanation either.
From the point of view of theory the chances that the OPERA result being true are no better as there is no sensible model of tachyonic neutrinos. At the same time, we’ve been observing neutrinos in numerous experiments and in various different settings, for example in beta decay, from terrestrial nuclear reactors, from the Sun, in colliders as missing energy, etc. Each time they seem to behave like ordinary fermions obeying all rules of the local Lorentz invariant quantum field theory.

We should weigh this evidence against the analysis of OPERA which does not appear rock solid. Recall that OPERA was conceived to observe tau neutrino appearance, not to measure the neutrino speed, and indeed there are certain aspects of the experimental set-up that call for caution. The most worrying is the fact that OPERA has no way to know the precise production time of a neutrino it detects, as it could be produced anytime during a 10 microsecond long proton pulse that creates the neutrinos at CERN. To go around this problem they need a statistical approach. Namely, they measure the time delay of the neutrino arrival in Gran Sasso with respect to the start of the proton pulse at CERN. Then they fit the time distribution to the templates based on the measured shape of the proton pulse, assuming various hypotheses about the neutrino travel time. In this manner they find the best fit is for the travel time of 988 ns, instead of the expected 1048 ns. However, one could easily imagine that the systematic errors of this procedure have been underestimated, for example, the shape of the rise and the fall-off of the proton pulse have been inaccurately measured. OPERA does a very good job arguing that the distance from CERN to Gran Sasso can be determined to 20cm precision, or that synchronizing the clocks in these two labs is possible to 1 nanosecond precision, but the systematic uncertainties on the shape of the proton pulse are not carefully addressed (and, during the seminar at CERN, the questions concerning this issue were the ones that confounded the speaker the most).

So what’s next? Fortunately OPERA appears to be open for discussion and scrutiny, thus the issue of systematic uncertainties should be resolved in the near future. Simultaneously, the MINOS collaboration should be able to repeat the measurement with similar if no better precision, and I’m sure they’re already sharpening their screwdrivers. In the longer timescale, OPERA could try to optimize the experimental setting for the velocity measurement. For example, they might install a near detector on the CERN site (where there should be no effect if the current observation is due to neutrinos traveling faster than light, or there should be a similar effect if there is an unaccounted for systematic error in the production time). Or they could use shorter proton pulses, so that the neutrino production time can be determined without statistical gymnastics (it appears feasible – the LHC currently works with 5 ns bunches). I bet, my private level of confidence being 6 sigma, that the future checks will demonstrate that neutrinos are not superluminal… in the end the character from the original book turned out to be 100% human. But, of course, the ultimate verdict belongs not to our preconceptions but to experiment.

Written by physicsgg

September 24, 2011 at 11:16 am

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