Archive for March 16, 2012
(Somehow there is a good pun lurking there, but at the tail end of a bout with the flu, I’m not finding it…)
Anyway, this plot says it all:
I’ve taken this image from a new paper posted by the ICARUS Collaboration (arXiv:1203.3433).
The ICARUS experiment sees the same neutrino beam that OPERA sees, and it is located in the same Gran Sasso underground laboratory. Their experimental technique is based on an ultra-pure liquid Argon chamber that produces exquisite images of neutrino interactions, such as this one:
The neutrino enters from the right and interacts after traveling a few cm into the liquid Argon volume. An energetic muon is produced that flies forward to the left and exits the volume; other charged particles coming from the collision with an Argon nucleus move slower and are visible as stubby tracks coming from a common vertex. There is no doubt that this is a good neutrino interaction, and its position in the detector is very clear.
The timing capabilities of the ICARUS detector sound very impressive. The photomultiplier tubes can collect the earliest signals from the interaction and establish the time with precision in the ns range, including the inevitable systematic uncertainties coming from channel-to-channel variations, etc. Given the precision of the timing and the position measurement, the time of the neutrino arrival at, say, the wall 11 m in front of the detector can be established to better than 10 ns.
Recall that the OPERA anomaly corresponds to 60 ns.
The challenge is in making an absolute TOF measurement. The issues and challenge are essentially the same as for OPERA, and indeed ICARUS based the absolute time reference on the system set up by OPERA. They conducted a distance measurement again based on GPS that is accurate at the cm level. Although there is no independent, non-GPS cross check of the absolute time stamp, one presumes that the accuracy is as stated: much better than 10 ns. And in any case, if there were a systematic problem with the absolute time standard, it should be the same for ICARUS and OPERA (well, at least in some cases…).
CERN provided a special neutrino beam last fall in which the neutrino bunches were 3 ns wide rather than 10 us. Concerns about the treatment of pulse shapes, variations in horn currents and efficiencies across a 10 us pulse, possible technical failures of the equipment that record the pulse shapes all are swept away. The much narrower pulse leads to a much cleaner statistical analysis; the 3 ns spread is essentially negligible for the purposes of testing a 60 ns offset. This beam is ideal and indeed OPERA already reported some results earlier on.
ICARUS recorded 7 events consistent with the known neutrino flux. Three of these have interaction points inside the Argon (the event picture above is one of these three), and the other four have interaction points inside the rock upstream of the detector; muons produced in the interaction passed through the Argon allowing an accurate time measurement. The three contained events have δTOF values of 0, -5 and +3 ns. As seen in the plot above, the set of seven events is nicely centered on zero with absolutely no indication of a 60 ns offset. The ICARUS Collaboration quantify their result as δTOF = 0.3 ± 4.0 ± 9.0 ns.
This measurement, if correct, completely refutes the OPERA result. I expect the community will scrutinize the ICARUS measurement, which is reported in far less detail than the OPERA one. Given the doubts associated with cable connections in the OPERA detector, however, it is nearly impossible to believe that neutrinos indeed travel faster than the speed of light…