[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by OPERA and LVD, I now have the information to remove all the guesswork from my original post; you’ll see outdated information crossed out and newer and more precise information written in orange.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. Inside the Gran Sasso Laboratory, which is deep underground, the LVD experiment and OPERA experiment are not too far apart , though I haven’t been able to determine the distance; probably a few hundred meters just 160 meters (a meter is about three feet). If a muon (which, depending on how energetic it is, can travel tens or hundreds of meters through solid rock) passes through OPERA, there is some probability that it will later pass through LVD as well, a half a millionth of a second or so later. If the same particle passes through both detectors, it can be used to check whether the clocks at the two experiments are synchronized.
Where might such a muon come from?
Well, this is one point of uncertainty. Cosmic rays (high energy particles from space colliding with an atom in the upper atmosphere) create showers of pions, and from there a shower of muons and neutrinos (and their anti-particles). Either (1) a muon (or anti-muon) from the shower can penetrate an exceptionally long distance through the rock and into the Gran Sasso Lab, or (2) a neutrino (or anti-neutrino) from the shower can pass effortlessly through the rock until it hits an electron or an atomic nucleus and creates a muon (or anti-muon) which then makes it to LVD and OPERA. I am still unsure whether (1) or (2) is more common; whichever is more common is what they will use. They use process (1), which is common because there is one direction from which there are an exceptionally large number of muons making it through to the underground lab, due to an unusual thinning in the rock to one side of the lab — and by chance, this direction is one which makes it possible for such muons to sometimes pass first through OPERA and then through LVD. Over the past five years they have about 300 such muons.
These muons will be traveling at (just a tiny bit below) the speed of light, and will traverse the distance between the two detectors in a time that can be calculated (since the distance between the two detectors can be precisely measured inside the lab). Now, if you measure the muon’s arrival time in OPERA relative to the clock that OPERA uses, and you measure the same muon arriving in LVD relative to the clock that LVD uses, this gives you the relative timing between the two detectors.
At least that’s my assumption about what they must be doing; I might not be quite right about this. Confirmed!! Apparently, the time difference between when muons arrive at LVD and then arrive at OPERA, or vice versa, as measured by their respective clocks, shifted by about 73 +- 9 nanoseconds some time in around May to August 2008, relative to what it was before 2008 that time. And then it shifted back, at around the same time that the suspect optical fiber was screwed back in the way it should have been. This strongly suggests that the badly adjusted optical fiber was responsible for the whole majority of the 60 nanosecond shift, and that this shift was stable over all or almost all of the entire period of both versions of the experiment, which I called OPERA-1 and OPERA-2. The question of stability of the fiber’s orientation over that whole period was one of my main worries about whether the real problem at OPERA had yet been found; the new information from the LVD/OPERA timing comparison would suggest strongly that this worry is unfounded.
A big question that the OPERA leadership that resigned today has to answer: why didn’t they do this cross-check before they made their result public? Did no one think of it til recently? And if not, why not? Was it harder than it sounds? Or did they just miss an obvious opportunity?
Anyway, it would appear that the mystery is now solved: that the fiber was the main problem, and that the second problem they identified (with their main clock) was
irrelevant less important but not irrelevant; the clock’s frequency is slightly off, so if a neutrino arrives early in a grouping of neutrino bunches its time will be accurately recorded, but if it arrives later in a grouping of neutrino bunches, its time will be recorded a bit later than it should be. This effect counteracts and reduces the 73 nanosecond shift from the fiber down to about the observed 60 nanoseconds. We’d like to hear more details about the technique used in comparing LVD and OPERA;
Also, OPERA reports multiple cross-checks were made internally within OPERA, and all of them agree with the conclusions obtained via the method I’ve described. (These are quite interesting and I’ll try to cover them at a later time.)
Puzzle apparently solved, through excellent detective work by OPERA, both in collaboration with LVD and on its own.
…and we’ll see in a few months, after LVD, OPERA, ICARUS and BOREXINO all measure the neutrino timing independently, whether they all get the same answer. But meanwhile, as often happens during the final aria of a tragic OPERA, a few heads are rolling.