Matt Strassler [April 2, 2012]
This article is a transcription and cleaned-up version of a post, and its update, that appeared on March 30 and 31. If you want to read the original post, click here.
The article is based on slides from a March 28th mini-workshop recording the results of investigations by OPERA and LVD, along with a report on ICARUS’s results.
The mystery surrounding OPERA, the Gran Sasso experiment which (through a technical problem) measured that neutrinos sent from the CERN lab to the Gran Sasso lab in Italy arrived earlier than expected by about 60 nanoseconds, is now resolved. In this article I’ll explain to you the crucial technique which was used in the process. Here’s an article explaining the whole story in which this technique plays a key role, from when the problems were diagnosed to when the mystery was completely solved.
Inside the Gran Sasso Laboratory, which is deep underground, the LVD experiment and OPERA experiment are just 160 meters apart (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 (illustrated in Figure 1) 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? 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). A muon (or anti-muon) from the shower can sometimes penetrate an exceptionally long distance through the rock and into the Gran Sasso Lab. It turns out 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 are at very high energies and are traveling at (actually, just a tiny, 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 they
- measure the muon’s arrival time in OPERA relative to the clock that OPERA uses, and
- measure the same muon arriving in LVD relative to the clock that LVD uses,
- and compare the two arrival times as given by the respective clocks,
this gives them the relative timing of the two detectors’ clocks.
What the comparison of the cosmic-ray muons showed (see Figure 2, taken from the OPERA talk by M. Sioli at the March 28th workshop) was that the time difference between when muons arrive at OPERA and then at LVD, as measured by their respective clocks, shifted by about 73 +- 9 nanoseconds around May to August 2008, relative to what it was before 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 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 indicates that the cause of the shift was from how far the fiber had been screwed in, and this was stable.
A big question that the OPERA leadership (who resigned March 30th) 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 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 at both OPERA-1 and OPERA-2. For more details about how the cosmic-ray muons were used to show this with confidence, along with more general discussion about lessons to learn, read this article.