neutrinos

Denouement: How OPERA’s Mystery Was Solved

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On Friday I learned, and reported to you, that the OPERA experiment’s investigations into its early-arriving-neutrino anomaly (widely reported as `faster-than-light neutrinos’), performed with help from the nearby LVD experiment, have basically confirmed that a combination of (1) an optical fiber within the main timing system that was incorrectly screwed in, and (2) a timing … Read more

OPERA’s Timing Issue Confirmed? Yes!

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[QUICK UPDATE April 2: I’ve now finished an article giving more details of how OPERA, with LVD’s help, solved the mystery.]

[UPDATE March 31 2 a.m.: following study of the slides from a mini-workshop recording the results of investigations by OPERA and LVD, I now have the information to remove all the guesswork from my original post; you’ll see outdated information crossed out and newer and more precise information written in orange.  I’ve also added figures from the talks.]

March 30 5:30 p.m. Two main scientists at OPERA, one leading the OPERA team as a whole and the other leading the neutrino speed measurement, resigned their leadership positions today.  The suggestion from the press is that this is due to personal and scientific conflicts within the OPERA experiment, rather than due directly to the errors made in the neutrino speed experiment; but of course the way the measurement was publicized by OPERA caused serious internal conflicts at the time and are surely part of the issue.    [Oh, and meanwhile, back over at the CERN lab, some good news: collisions at the Large Hadron Collider with 8 TeV of energy per collision were achieved this afternoon.]

The mystery surrounding OPERA, the Gran Sasso experiment which (apparently through a technical problem) measured that neutrinos sent from the CERN lab to the Gran Sasso lab in Italy arrived earlier than expected by 60 nanoseconds, seems to be on the verge of being is resolved.  Statements made by an OPERA scientist in the Italian language press, pointed out to me by commenters (Titus and A.K.), seem to imply that OPERA has more or less confirmed that the problematic fiber optic cable (along with the clock problem, to a lesser extent) was responsible for a 60 nanosecond (billionth-of-a-second) shift in the timing, creating the false result.  We do not yet have official information from OPERA about this, but talks given at a mini-workshop a couple of days ago make clear that this is the case.

The way this was done if I/we understand the Italian correctly is something like is the following  with all details still very uncertain.

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This Time, ICARUS Really DOES Refute OPERA

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Well, ICARUS flies even higher, and so far shows no sign of losing its wings.

Remember OPERA, the experiment that claimed neutrinos sent from the CERN lab in Switzerland to the Gran Sasso lab in Italy arrive earlier than they were expected to? And that a couple of weeks ago had to admit they’d found a couple of problems that were large enough to scrap their result for the moment, and that require additional investigation?

And remember ICARUS, OPERA’s neighbor in the same Gran Sasso lab in Italy, which measured the energies of neutrinos from the CERN neutrino beam, and showed they were not altered in flight? And thus proved that if the neutrinos really were traveling faster than light, they did not exhibit anything like the variant of Cerenkov radiation that was suggested by and calculated by Cohen and Glashow?

Now, ICARUS’s result from the fall didn’t directly refute the OPERA experiment (despite some claims, even by them) but it certainly added to the aura of extreme implausibility that surrounded the whole story.

Well, this time ICARUS refutes OPERA. Essentially, they did the same measurement as OPERA-2, as I called the short-pulse variant of OPERA’s original experiment.  They took data at the same time as OPERA-2, in the same neutrino beam, in the same laboratory.  It took them a while to do all the distance and timing calibrations that OPERA had done many months ago, but they’re finished now. And whereas OPERA-2 gets the same result as OPERA-1— an early arrival of 60 nanoseconds (billionths of a second) — ICARUS finds a result consistent with an on-time arrival. Same measurement, different answer. At least one experiment made a mistake; and one result is vastly more plausible than the other, so I think the consensus is pretty clear in the matter.

ICARUS's 7 neutrinos (dark blue histogram), measured in October and November, arrived as expected to within 10 nanoseconds (billionths of a second). OPERA's result (but not its neutrinos) is shown at right, at approximately 58 nanoseconds early arrival.

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Why the Curtain Has Not Fallen on OPERA

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For those of you who read the news reports about OPERA, and its potentially (not) superluminal neutrinos, on Thursday or on Friday morning, and stopped following after that, I have news for you: almost everything that appeared in the press up to that point was wrong in some important details.  Thanks to my readers and their comments and detective work, we’ve collectively managed to figure out much more clearly what’s actually going on.  I put up a relevant post Thursday morning and another Thursday afternoon, but I especially recommend Friday morning’s post (and comments) and Friday afternoon’s post (and comments).  I really emphasize the value of the comments; I have some very well-informed and insightful readers who contributed a great deal.  You can read this summary post first, and then go back to the older posts and read through the earlier viewpoints and the detailed commentary.  [The science press has caught up, though; here’s an accurate article from 2/27 in Nature.]

Most press reports on Wednesday, Thursday and Friday boiled down to this statement: “The OPERA folks found a loose wire, and when they fixed it their timing shifted by 60 nanoseconds [billionths of a second], bringing neutrino speeds right back to where they were supposed to be.”  That’s certainly what the original Science Insider article implied, from which many articles took their cue.  This is illustrated in the Figure below (labeled (b) to be consistent with a figure from an earlier post. )  The original OPERA result — that neutrinos arrived 60 nanoseconds before they were expected to — is shown as (a).

But this statement is completely wrong.

(a) OPERA originally claimed neutrinos arrived early by 60 nanoseconds (ns), with an uncertainty of about 10 nanoseconds, shown by the black vertical bar. (b) Incorrect press reports widely suggested that OPERA had found a mistake (a "loose wire") that caused a 60 nanosecond shift and brought the measurement back into agreement with expectations. (d/e) But in fact the two problems identified so far by OPERA, a sensitivity to an optical fiber's exact orientation and a miscalibrated timing oscillator, are both large compared to the original measurement, are both imprecisely known, and point in opposite directions. This makes the situation entirely unclear for the moment.

In fact the OPERA press release made clear that there were two problems (a problematic fiber-optic cable and a miscalibrated oscillator), causing shifts in opposite directions, and mentioned that a re-run of the experiment would be necessary.  Still, most press articles seemed to give this lip service, and assume the correct reading of the situation was that the fiber was the main source of the problem, and that a re-run of the experiment was just pro forma.  They mostly stuck with the simplistic idea that the OPERA people found a mistake and now everything agrees nicely with Einstein.  A few, such as the New York Times, did a somewhat better job.  But they still missed key points.

So what is the real story?  

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Everybody’s a Critic

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I wanted to make a few assorted comments about the OPERA experiment’s painful climb-down, and about yesterday’s widespread response to it, which bothered me a lot.  You may want to read my initial post from yesterday, and also my attempt to sharpen the main question OPERA left unanswered in my second post.  [ALSO: look ahead to the next post, in which many of the confusions that were still present at the time of this post were resolved.]

Over the past day I’ve learned enough to be pretty convinced (but not certain) that the situation that we are in is case (e) [or a version of case (d)] as described in yesterday’s post: that probably the previous OPERA experimental data is tainted and we can draw no conclusions from it at all.  It’s not that they found a problem that shifts their data so that it is consistent with Einstein’s relativity [case (b) from yesterday], and they can say that the neutrinos travel as expected.  (Press reports that said so are just wrong.) It’s that they found a problem that means their data from last year can’t be interpreted at all… at least, not at the moment, and maybe not ever.  If true, this would indeed mean that there is no longer any data from OPERA that can be used to measure neutrino speeds to good accuracy, and we’re back where we were before OPERA ran in the first place: with no reason to think there’s anything amiss with Einstein’s relativity equations.  As for OPERA, the only way forward is to rerun the experiment (apparently in March-April-May.)

[The NY Times article that appeared today (which attributes OPERA, a non-CERN experiment, to CERN; what has journalism come to these days?) has some additional details, but if you read it carefully, those details don’t change anything written in this post.  See the first comment at the very end of this post.]

Ok, some comments.

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Synopsis of the OPERA Situation

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I thought I’d better try to clarify what the logical issues are regarding OPERA before things get too confusing.  [Read the previous post first.] What’s going on is sketched in the Figure below. a) In September (and confirmed in November) OPERA claimed to have found that, compared to expectations, neutrinos from CERN seemed to be arriving … Read more

OPERA in Question

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[UPDATE: Some extensive comments added below.]

[UPDATE: Journalists and Bloggers: PLEASE NOTE: OPERA is not a CERN experiment.  The CERN laboratory does not deserve the bad press it is getting (though they certainly unwisely put themselves in a position to receive it.)]

Many of you are probably already aware of various rumors running around By now you are all aware of yesterday’s initial report that the OPERA experiment — famous for announcing that neutrinos traveling from CERN to the Gran Sasso Laboratory in Italy arrived 60 nanoseconds (billionths of a second) earlier than expected, thus suggesting that neutrinos can travel slightly faster than the speed of light — had found a problem with a cable connector that exactly explains the 60 nanosecond timing shift.

But the immediate source of this rumor was a science journalist, and the article was based on an anonymous source who is not described as being in the OPERA experiment. And the details quoted in the article didn’t add up, in my view. Given the number of wrong reports and rumors that I have read over the past months about this experiment, my reaction was to wait.

I didn’t have to wait long. Presumably Perhaps to avoid misinformation from hitting the headlines, it appears that OPERA has released a statement  that indicates that the article from earlier today yesterday is not true. [Update: To be clearer, I probably should have written, “true in its details.”]  But this statement itself contains big news.

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OPERA’s Next Act

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Can’t take a breath this week without stumbling over another particle physics result of note. (It isn’t usually like this, folks — this year has been very odd.) OPERA (the experiment that claims their neutrinos travel faster than light does, which if true would require some kind of modification of Einstein’s relativity principles) is back, and they’ve done a very important cross-check many of us were hoping they would do, which is very good news indeed. Since they say that it confirms their previous result, the plot now thickens considerably; the experiment’s technique is now harder to question, and the long list of possible sources of problems with the experiment is considerably shorter. Obviously this news deserves a long post, explaining exactly what they did and why it is such an improvement. I’ll produce one for you before the weekend is over, possibly as soon as tomorrow. Watch this space!

In the meantime, here’s a guide to past posts, which cover most of what you need to know:

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