When their first report came out a few years ago, I thought their fit to an orbital cycle looked like wishful thinking. I gather from the Kipling quote on their web site that a number of other people thought so too, and they have some hard feelings about it.
Well, I'm sorry about that. They got the money to expand the experiment's size (sodium iodide crystals under the Gran Sasso mountain), and this time their signal modulation looks much more believable. The preprint is here at Arxiv. It looks like they understand the backgrounds pretty well, and the statistics are now much better. Single interaction (single flash) events which dump 2-6 KeV of energy into the detector happen more often the more the Earth's motion around the Sun matches up with the direction of the Sun's movement through the galaxy. More energetic events don't show this modulation with time of year.
They've dealt with a lot of the common background possibilities (even with seasonal changes in water in the rock changing the amount of neutron moderation in the granite). Their results seem to be consistent with a "gas" of weakly interacting massive particles ("WIMP"s) that is more or less at rest and which the Sun/Earth system is speeding through. Every now and then a WIMP scatters off a nucleus, whose recoil energy shows up as a flash of light in the NAI scintillator. The WIMPs must be slow enough that the difference in the Earth's orbit with and against the Sun's speed (± 3 E4 m/sec) makes a noticeable difference in the interaction rate at this scattering energy.
Interesting. A cloud no bigger than a man's hand?
No comments:
Post a Comment