Team of researchers in Italy claims to have directly detected dark matter

Short background: we know dark matter exists. We also know it must be made up of particles that are very difficult to detect (or else we’d see them, of course). A leading contender are WIMPs, or weakly interacting massive particles, specifically a weird particle called an axion. We still have not directly detected and identified an axion in any particle experiments, though LHC (and more on that later, of course!) may bag one soon.

DM particles form a cloud around and throughout the Milky Way. Although they don’t interact with normal matter terribly well (hence the weakly interacting part of their name) they do sometimes slam into normal matter. You can build a detector to look for that interaction — it would make little flashes of light — but again you need to look very carefully.

The Italian experiment looked very carefully. One of the things they looked for was a modulation in the signal, a change over time. They wanted to see the number of flashes of light from DM hitting normal matter go up and down by a few percent, with a maximum in June and a minimum number in December. Why?

Imagine you are in a car, driving through a cloud of bugs. If you hold your hand out the window, a bunch of bugs will hit it (ewwww). Now if you throw a ball out the window into the direction of the car’s motion, it will hit more bugs, because it’s moving into the cloud faster. If you throw a ball behind you, then the ball will be moving slower relative to the bugs, and fewer bugs will hit the ball.

A similar thing is happening with the Earth. The whole solar system is orbiting the center of the Milky Way at about 250 kilometers per second. But the Earth is also orbiting the Sun. When the Earth is at one part of its orbit, its velocity (30 kps) adds to that of the solar system, but six months later it’s headed the other way, and its velocity subtracts.

If the Earth is slamming into dark matter particles, then we should hit more when the Earth and solar system velocities are in the same direction, and hit fewer when the Earth is moving in the opposite direction of the solar system as a whole six months later. So not only should we see the number of hits go up and down every six months, but that oscillation must line up with the correct dates (June for the former, and December for the latter).

That is precisely what the Italian team found.

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Edit: I posted this before Phil updated his post with a link to Cosmic Variance that explains why the findings, while interesting, are nowhere near conclusive. We can say with a good bit of certainty that we’ve found something, but there is not necessarily any reason to believe it actually is dark matter. IF it is though, this would be one of the biggest discoveries in the history of physics.

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