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Katherine Copic | USLHC | USA

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almost superluminal physics chatter

Everyone is talking about a new result that will be released soon, claiming the observation of neutrinos travelling faster than the speed of light. A seminar is scheduled for Friday at 4 pm at CERN to discuss the results. Because I work on the ATLAS experiment at CERN, and CERN is in a lot of the headlines, I’ve gotten a bunch of emails/IM/Google+ questions in the last few hours, which mostly boil down to: “Is this thing real?!?” I don’t know if it’s real, but here is what I do know.

The best story I’ve seen so far is Can Neutrinos Move Faster Than Light?, where the experiment is described this way:

The data come from a 1300-metric-ton particle detector named Oscillation Project with Emulsion-tRacking Apparatus (OPERA). Lurking in Italy’s subterranean Gran Sasso National Laboratory, OPERA detects neutrinos that are fired through the earth from the European particle physics laboratory, CERN, near Geneva, Switzerland. As the particles hardly interact at all with other matter, they stream right through the ground, with only a very few striking the material in the detector and making a noticeable shower of particles.

Over 3 years, OPERA researchers timed the roughly 16,000 neutrinos that started at CERN and registered a hit in the detector. They found that, on average, the neutrinos made the 730-kilometer, 2.43-millisecond trip roughly 60 nanoseconds faster than expected if they were traveling at light speed. “It’s a straightforward time-of-flight measurement,” says Antonio Ereditato, a physicist at the University of Bern and spokesperson for the 160-member OPERA collaboration. “We measure the distance and we measure the time, and we take the ratio to get the velocity, just as you learned to do in high school.” Ereditato says the uncertainty in the measurement is 10 nanoseconds

Thing 1) The first thing I want to see is a paper explaining what the scientists who performed this measurement did. So far, no one I know has seen a paper, only news articles are available, and they don’t have enough detail to evaluate what was done. Ideally, this paper would be submitted to a peer-reviewed journal and accepted by a journal before everyone gets too excited.

Thing 2) The most important part of the paper we want to see will explain how the uncertainty on the measurement was obtained. Whether or not the result is exciting will depend on how well they can measure the speed of the neutrinos. In the same article linked above, this explanation is given by an expert in the field working at a different lab:

Jung, who is spokesperson for a similar experiment in Japan called T2K, says the tricky part is accurately measuring the time between when the neutrinos are born by slamming a burst of protons into a solid target and when they actually reach the detector. That timing relies on the global positioning system, and the GPS measurements can have uncertainties of tens of nanoseconds. “I would be very interested in how they got a 10-nanosecond uncertainty, because from the systematics of GPS and the electronics, I think that’s a very hard number to get.”

Thing 3) After reading the paper, the next thing we’ll think about is how other experiments can measure the same thing. In another good article from today, with a link to an actual paper, MINOS results from Fermilab are discussed. MINOS also measured their neutrinos going faster than expected a few years ago, but they didn’t claim that they had enough resolution to make a definitive statement. To compare with the 10 nanosecond number above, the MINOS paper quotes an uncertainty of 64 nanoseconds. If the OPERA experiment measured the same thing – the neutrinos going 60 nanoseconds faster than the speed of light – with a 64 nanosecond uncertainty instead, then the result would be consistent with the expectation and no news at all. We can also compare to the data from neutrinos coming from supernovas, which Matt Strassler discusses in his blog post.

I’ll certainly be tuned in early tomorrow morning (for me, on West Coast time) to see what the OPERA folks have to say, but so far the physicists I know haven’t been too kind. On various social networking sites, these (unnamed) friends have been making comments like “Time to work on neutrinos! That… or laugh when some systematic effect is discovered in a few months.” or “The OPERA Spokesman feels the need to personally announce their result to Reuters before the CERN press release and the paper out. Weird!” One friend sums up his feelings succinctly this way: “superluminal neutrinos my @%$.”

Update! The OPERA paper is posted here: http://arxiv.org/abs/1109.4897 — time to read it and see what it says.

  • Steven Herrin

    I’m skeptical. Neutrinos from SN1987A were detected almost exactly the same time as light from the supernove reached earth. 60 ns over 730 km corresponds to 4 years over 168000 light years.

  • Isaac

    My expert friend AK notes, “Another difference is that the supernova neutrino that Kamiokande detector detected was electron neutrino whereas OPERA detected muon neutrino. Electron neutrino is supposed to be a mix of roughly 1/3 nu1, 1/3 nu2 and 1/3 nu3 according to solar neutrino result, whereas muon neutrino is 1/2 nu2 and 1/2 nu3. If some of the neutrinos from nu2 or nu3 mass eigenstates component arrived the Kamiokande detector 4years earlier, the detector was not operational.” Not a defense of the OPERA measurement but a comment that thinks can get complex fast. Even when we’re talking distance = rate * time!

  • Georges

    It is interesting that the measurement of the mass squared of the electron as well as muon neutrino masses have long been measured to be negative, even if the error is large (see: http://www-pdg.lbl.gov/2011/listings/rpp2011-list-neutrino-prop.pdf). This means that p^2 > E^2, and hence beta=p/E > 1.

  • Brian

    Thanks Kathy. It’s articles like this that help your farmer friend understand high energy physics just a little more. Yay for sound science! Keep it up. I’ll hug a cow for you.

  • S

    Since star mechanics are heavily dependent on relativity, would the supernova results really form a convincing counterargument – since this experiment if confirmed, might mean that relativity is off?

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  • NeverGotEntryVisa

    Did those 16’000 neutrinos all have entry visa to Italy or they have been arrested by the border police of Italy?