Saturday, October 2, 2010

Listening to bubbles to find dark matter

The departmental colloquium this last week was given by Peter Cooper who is part of the COUPP search for dark matter. Just like all the other dark matter searches the idea is to make your detector have as much material in it as possible and try to shield it as much as possible from any known interactions, cosmic rays, background radiation, etc. What you don't shield away you want to be able to discriminate anything we know about and then what you are left with must be dark matter.

In the case of the COUPP experiment they are using bubble chambers deep underground with several tons of scintilator oil on top. A bubble chamber works by keeping the fluid inside it at a temperature which is just above its boiling point. But because a bubble of gas takes more space than the liquid it takes a little bit of extra energy to make a bubble form. When a cosmic ray or other particle comes through the liquid it deposits energy and this extra little kick can make bubbles form in the liquid.

Usually in a bubble chamber you would be interested in the track that a particle leaves in the detector. But anything that leaves a track of bubbles interacts way too strongly to be dark matter. So the question is how do you discriminate between dark matter single bubble events and more boring single bubble events? The answer is you listen to the noise the bubbles make when they are made! alpha events in the chamber caused by trace amounts of radioactive materials in the device will show up as louder bubbles!

I can't help but think of a picture of a physicist carefully listening to the vibrations of tiny bubble floating deep in the dark and hoping to hear snatches of unknown harmonies of nature.

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