Saturday, July 23, 2016


LUX looked for WIMPs using liquid Xenon. Their presentation is here.

Summary: they didn't find any, and put even stronger limits on their presence than ever before. (No, I have no idea what that 8B point on the figure means.) In the figure below, they say anything below the dark line at the bottom is still possible, but above it--not likely.

I don't mean that kind of WIMP. We've strong evidence for dark matter--stuff that doesn't interact strongly with normal matter, has to be uncharged, etc. One easy model proposes that dark matter is made of massive particles that don't interact strongly, but still do interact. WeaklyInteractingMassiveParticles. Since they interact weakly, maybe they interact via the Weak Interaction? If so, every now and then one of these WIMPs streaming through space should kick a nucleus, just as every now and then a neutrino does.

Several experiments have looked for them. LUX is the best so far in this regime--their curve is lower than everybody else's, including their own earlier (less data) work.

They lucked out, I suppose. The "Brazil" bands of yellow and green (named because of a resemblence to the flag of--guess) represent where they thought their sensitivity would be--they did even better, presumably thanks to some statistical fluctuation.

In one sense, there's still plenty of room for WIMPs to exist: you can always add another decade to the bottom of the log plot and make it look like there's more space. And, though it isn't so obvious, there are some assumptions shared by all the experiments shown involving the expected density of dark matter at this radius of the galaxy. If those estimates are badly wrong the raw numbers on the plot change. (The curves stay the same relative to each other, though.)

Long ago they ruled out interpreting the DAMA (or DAMA/Libra) results as WIMPs.

DAMA assumed that our solar system's orbit in the galaxy intersects a stream of dark matter flowing at some speed and direction. So long as it isn't at right angles to the Earth's rotation around the Sun, the Earth (and detectors flying aboard it) should intersect this stream with different speeds at different points in the Earth's orbit. If dark matter interacts at all with nuclei, never mind how, there should be an annual modulation in the interaction rate according as we hit the dark matter faster or slower. They see an annual modulation. They think it is dark matter. Most people don't, anymore.

So, where is the dark matter? If it doesn't interact at all (besides gravitationally) with normal matter, how in the world did the big bang manage to evolve two different types of matter? MACHOs, anyone?

I tend to favor a model in which the dark matter interacts only with neutrinos. Unfortunately, there's almost no way to test this. A successor to IceCube might be able to do it eventually, but first you have to have a good handle on extra-galactic neutrino sources, good enough to predict what rates ought to be and look for shortfalls--and that won't be good enough for years yet. Or you can hope for a conveniently placed supernova or three.

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