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Fermilab | Batavia, IL | USA

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COUPP: How to make dark matter triggered bubbles

Hugh Lippincott installs a COUPP bubble chamber.

In keeping with the introductory series of posts you’ve seen in this blog from Fermilab experiments, I guess I’ll introduce myself as well. My name is Hugh Lippincott. I’m a postdoc at Fermilab, where I work on an experiment looking for dark matter called COUPP (the experiment is COUPP, not the dark matter. The dark matter is WIMPs, or weakly interacting massive particles).

If you’ve been reading these posts, you’ll know by now that all experiments and many physics concepts have some kind of acronym or cute name, and ours is no different. The acronym stands for Chicagoland Observatory for Underground Particle Physics, but no one thinks of it like that, it’s just COUPP. The real debate is whether the two Ps are silent or not, and we’ve been known to have long debates inside the collaboration about this. I tend to think of it as silent, the way the P sounds when you say coup,  as in overthrowing the state. But if you prefer to think of sounding more like the P in coupe,  a small car, be my guest. Maybe we can have an Internet poll or something in the future and solve that problem once and for all.

This is not actually my first attempt at blogging (although I really do detest that verb and will try to avoid using it henceforth). I wrote several posts at physicsformom.blogspot.com where I attempted to explain a somewhat significant chunk of dark matter physics in a way that could hold my mother’s attention. In this, as in so many things, I came up a bit short, and I haven’t posted anything there for months, but I actually think the three introductory posts on what dark matter is hold up OK. So, instead of going back over all that here, I’ll risk losing half of my audience by being a lazy scientist and ask you to review those posts if you want more information.

Double click on the above icons to see the bubble chamber in action.

I’ll talk more about COUPP and anything else going on as I continue writing. For now, I’ll just say the COUPP collaboration is building a series of bubble chambers , which essentially means it is  literally watching a jar of fluid waiting for bubbles to appear. For example, the accompanying movie shows a neutron (produced by a neutron source placed near the detector) that has scattered four times in our chamber we’ve recently installed in a deep underground site called SNOLAB in Ontario, Canada). This particular event is pretty recent, from a chamber called COUPP-4 since it has 4 kg of fluid.

As I mentioned here ( I’m referencing myself again), bubble chambers were used in the heyday of particle physics when it seemed like new particles were being discovered and understood every two weeks.  We’re now using the same technology, just in a new way.  A bubble chamber is a jar filled with a superheated liquid, or liquid that is hotter than its boiling point. The liquid wishes it were boiling but can’t because there is nowhere to make a bubble. I’m not sure if that entirely makes sense, so I’ll try again. When you boil a pot of water, you see bubbles form first on the metal of your pot. That’s partly because a bubble needs a place to be born, called a nucleation site. In general, this can be an impurity or a rough surface like the metal of the pot or anywhere where a little pocket of gas can form and then grow. Without these impurities or surfaces, the liquid can’t boil, and instead becomes superheated – a very unstable state where any input at all (such as an interacting dark matter particle) that can nucleate a bubble will cause rapid boiling.

Some of you may be familiar with this phenomenon if you’ve ever tried to boil clean water in a ceramic mug in the microwave. In fact, there was a Mythbusters episode about it and a host of other videos on YouTube. What they show is that superheated water will boil (or explode) very suddenly as soon as anything that can create a bubble touches the water.

In our bubble chambers, the bubble is created by particles interacting in the chamber. For example, in the movie above, a neutron scattered and deposited heat in four places, creating four bubbles. We superheated the fluid, making sure that there was nothing else in there to nucleate bubbles, and then waited until some radioactive particle zipped through. When we saw a bubble, we knew something had interacted in the fluid.  And that’s how a bubble chamber works.

— Hugh Lippincott

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