Tag Archives: neutrinos

This Time, ICARUS Really DOES Refute OPERA

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

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?   Continue reading

Everybody’s a Critic

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. Continue reading

Synopsis of the OPERA Situation

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 early, by about 60 nanoseconds, with a six sigma significance.  That is sketched in the figure as a bar, centered at 60 nanoseconds, with a width of 10 nanoseconds in each direction (so if you made the bar six times larger its bottom would reach zero nanoseconds.)    Such a result is convincing — convincing that either they made a mistake or they’ve discovered a new effect in nature.  The new effect in this case — neutrinos going faster than the universal speed limit — is very implausible for many reasons that I’ve covered in the past, but such an experimental result cannot simply be ignored.

(a) What OPERA said originally. (b) What yesterday's newspaper article claimed OPERA was now saying. (c),(d),(e) Three possible interpretations of OPERA's statement. In each case, a bar whose center lies at the most likely result of the measurement, and whose width indicates the degree of uncertainty, is shown. If we take OPERA's statement at face value, we still don't know if it is inconsistent with zero but less certain than before, consistent with zero, or simply unknown (in which case the previous measurement must be discarded.)

b) On February 22nd, a news report said that OPERA had found a mistake — a problem with the connection in an optical fiber — that explained the 60 nanosecond shift.  That would mean that the result was now — with confidence — consistent with Einstein’s relativity.

But the OPERA statement contradicts b).  Unfortunately it doesn’t clarify whether we have the situation c), d) or e):

c) The uncertainty on the result is larger than it was before, although the result is still not particularly consistent with what you would expect from Einstein’s relativity.  However, the reduced confidence in the result would mean that instead of it being an impressive result it would become a much less impressive one, much easier to ignore.

d) The uncertainty on the result is so much larger than it was before that the result is actually consistent with Einstein’s relativity, though so weak that it can’t distinguish at all between zero and the original non-zero value that OPERA claimed.

e) There is no sensible estimate of the uncertainty on the result. The result simply cannot be used and will have to be discarded.

From the point of view of the plausibility of the original OPERA result, (b), (c), (d) and (e) are all bad.  From the point of view of the experiment’s credibility, though, case (b) would look the best: the experimenters could at least say that they figured out what they did wrong and knew what it was.  Cases (c), (d) and (e) are increasingly bad; they indicate less and less knowledge of how their own experiment worked.  [But see the first comment below, which criticizes the way I expressed this, and emphasizes that a lack of knowledge is not always the fault of the experimenters.]  However, the situation might change over time; right now they might be in case (d) [because they’re still working on understanding what happened] but soon it might switch to case (b), or back to case (a), once they have it straight.  So we do have to let things play out a little bit, and not jump to conclusions.

But I think it is clear we are not currently in case (b).  Because if we were, why would one would emphasize the re-running of the experiment?  There would be no need.

OPERA in Question

[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. Continue reading

OPERA’s Next Act

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: Continue reading

ICARUS weighs in; but we knew this

Another experiment has weighed in on superluminal neutrinos — indirectly.  The ICARUS experiment has measured the energy spectrum of neutrinos traveling from CERN to Gran Sasso.  They confirm that the Cohen-Glashow effect, which I described in some detail here,  is not occurring in the beam of neutrinos that OPERA is also using.  The distribution of neutrino energies in the beam is undistorted.   If standard Cerenkov processes take place, then these neutrinos are not traveling faster than light.

At least one blogger has said this refutes the OPERA experiment.  Hmmm… for my part, I don’t think it really changes the situation much at all.  It only checks something that we were already pretty sure of. As I explained in my post on the subject, Cohen-Glashow emission would have had a big effect on the OPERA neutrino beam, one that OPERA itself would have detected.   That was Cohen and Glashow’s main point.  OPERA didn’t see any hint of such an effect.  All ICARUS has done, as far as I can tell,  is confirm that indeed there’ s no such effect, to significantly higher precision.  If you want to say that ICARUS has refuted OPERA, you really have to say that OPERA refuted itself first… that the experiment was self-contradictory.

What’s the loophole?  It’s small, but it’s still there.  We already knew, both from Cohen-Glashow emission and from the calculation of Giudice et al. that OPERA almost certainly cannot be right unless relativity is altered in such a way as to shut off Cerenkov-like processes at high energy.  That loophole may well be too narrow for any decent modification of relativity to get through.  But I don’t think ICARUS’s result changes the location or nature of the loophole.

UPDATE: In a question below it was asked whether sterile neutrinos [new types of neutrino-like particles that aren’t affected by the weak nuclear force and therefore would not have large Cohen-Glashow emission] could also pose an acceptable loophole, by oscillating with ordinary neutrinos.  I can’t make a clean argument against it without going a calculation, so for now I have to say that, though I’m dubious, I am not sure that loophole is closed yet.

UPDATE: I hadn’t had time when I first put this post up to explain why ICARUS and OPERA are measuring the same neutrino beam.  Both experiments are in the Gran Sasso lab, quite near each other.  By the time the neutrinos arrive at Gran Sasso, the beam of neutrinos is quite spread out — a couple of kilometers wide, I believe — so the Gran Sasso experiments are sitting in the same neutrino beam.  We can expect other Gran Sasso experiments to weigh in with their own observations over time.

UPDATE: Here’s a picture comparing my sketch (from my  post ten days ago on Cohen-Glashow emission) of what CG emission predicts for the OPERA beam (an effect that OPERA itself would have seen)  to the measurement from ICARUS.  Note the peak and the tail, marked by the black arrow and violet arrow on both plots; if Cohen-Glashow emission were operating, the tail would be gone and the peak would have shifted to the left.

Left: Sketch of how CG emission would have strongly distorted the beam at OPERA, in a way that OPERA itself would have detected. The red curve is expectation, the green curve is what CG emission would have done to the beam. In this way, Cohen-Glashow argued, OPERA is self-contradictory. Right: The ICARUS experiment, sitting in the same beam, confirms that data (blue points) and expectation (red curve) for the distribution of neutrino energies agree, with a peak at 20 GeV (black arrow) and a tail extending far higher (violet arrow). This observation rules out any significant amount of Cohen-Glashow emission, constraining any theoretical attempt to explain OPERA's result.

Comments welcome.

Another Speed Bump for Superluminal Neutrinos

Here’s another major strike against the OPERA experiment’s claim of superluminal neutrinos, in addition to the Cohen-Glashow argument I described last week. It comes from a very natural place: the weak nuclear force. The theory (i.e. the equations) that we use, with great success, to predict the behavior of the weak nuclear force inextricably links some of the properties of neutrinos and charged leptons (electrons, muons and taus.) Because of this linkage, you simply can’t make neutrinos travel faster than light without making electrons do it too — by a smaller amount, to be sure, but still bigger than a part in a billion. And it turns out the effect is large enough that it should already have been detected by existing experiments, putting OPERA’s result further in doubt. Continue reading