If you go down to the bottom of Soudan Mine in Minnesota, you’ll see an usual site for a mine: a group of giant physics experiments! Why? At the bottom of mineshafts, you have up to hundreds of feet of Earth protecting you from all the particles and radiation coming from the Sun, from space, and from terrestrial sources. This way, when you’re looking for a very rare physical event, you can be confident that what you’re seeing isn’t contaminated by other, mundane processes.
Perhaps the most important experiment going on down there is the Cryogenic Dark Matter Search (CDMS), which searches for Dark Matter particles by hoping that they bounce off of their detectors (above). When a dark matter particle bounces off, you can detect the recoil of other particles.
But there’s a problem here: other things bounce off your detector, too, despite your best attempts at shielding it. You cannot stop neutrinos, you cannot stop radioactive particles inside the Earth from decaying, and you cannot stop some cosmic rays (like muons) from getting through. These all hit your detector also, and create what we call “background” events. You can calculate what your expected background is; anything you see above that is likely your signal.
So what you do is reduce your background (which is bad) as much as you can. You try to catch all the muons and block all the radioactivity, and you find a way to try and tell light, fast-moving neutrinos apart from heavy, slow-moving WIMPs. At CDMS, they got their background down so far that, over two years of looking for these dark matter particles, they expected about one background event.
They saw two. What’s the appropriate response to that, Matt Foley?
That’s right, when you expect to see one and you see two, it doesn’t tell you anything conclusive. On Monday, I went through a hypothetical situation about CDMS, and now here’s the revised version — as best as I can reconstruct it — based on CDMS’ released results.
Looking for dark matter is like playing a one-in-a-million lottery. There are two interesting ways to play it for you: you can either buy a whole bunch of different tickets for one drawing, which gives you the best chance of winning exactly once, or you can buy tickets one-at-a-time for a bunch of successive lotteries.
CDMS, based on how long they waited (two years) and what their detector size was (30 hockey-puck-sized detectors), had the equivalent of about 900,000 tickets. If you could buy 900,000 tickets and you bought them all at once, you’d have a 10% chance of not winning and a 90% chance of winning once. CDMS can’t do that; each particle at each moment is an entirely different lottery drawing. When you work out the math for that, you find that:
- 40.66% of the time, after two years, you observe 0 events.
- 36.59% of the time, after two years, you observe 1 event.
- 16.47% of the time, after two years, you observe 2 events.
- 4.95% of the time, after two years, you observe 3 events.
- 1.11% of the time, after two years, you observe 4 events.
- 0.22% of the time, after two years, you observe 5+ events.
That’s what you expect to see from your background alone. The fact that CDMS saw two events? That doesn’t even count as a “hint” for dark matter; you would’ve needed to have seen four event to scientifically count as a hint, and you would’ve needed about 8 events to have some very good evidence.
The scientists who released these results are good, and they said as much in their release (emphasis is mine):
We estimate that there is about a one in four chance to have seen two
backgrounds events, so we can make no claim to have discovered WIMPs.
That’s right. What we saw? It’s probably background, and there’s no evidence at all that what we saw is anything more than that.
This doesn’t mean dark matter doesn’t exist, and it doesn’t mean that dark matter isn’t made of WIMPs either. But it means that we haven’t detected it yet, either, and stating that these two events mean anything other than this is just unscientific wishful thinking.