Dark Matter really is dark!

Hi, it's been awhile, but I've been busy. Hm. Understatement of the year I think. 14 hour days start to get to you.. In fact, I've noticed that all this fancy learnin' has actually begun to push other information out of my head. When I was a kid I was able to remember license plates of all my friends and their parents. I remembered telephone numbers from when I was a kid... Now all that is gone (though not really missed). Now I forget conversations and events, I forget when they happened, something that happened two days ago I claim happened weeks ago... I'm actually becoming an absent minded graduate student. Anyway, hopefully it's not all for naught. I've come to the conclusion that graduate school, so far anyway, would be very satisfying if it weren't for the classes. On that note, I'll segue to a discussion of my research....

Dark matter. I'm working on an experiment called LUX, which I think stands for some amalgamation of liquid, underground, and xenon. Of course, the underground isn't liquid, the xenon is.... Anyway, this is an experiment to detect dark matter directly. Dark matter has been detected indirectly, which is why it was postulated to begin with. Dark matter was theorized to account for the rotation of galaxies among other things. Gravity (Newton's, as well as Einstein's General Relativity) tell us how galaxies should rotate given a certain amount of mass at their centers. However, galaxies rotate differently than these theories predict for the amount of mass that we can see. The key here is 'see'. We look at a galaxy with visible light telescopes, as well as X-Ray telescopes (among others) and can see how much regular matter is there, because it glows (because it has some non-zero temperature). When it glows, it is releasing radiation, some of which we can see (stars), and some of which is in the non-visible part of the spectrum, which we 'see' with X-Ray telescopes. Anyway, the point is, we can tell how much regular matter is in a galaxy, but it behaves as if there is more matter there. This cannot necessarily be described with just a black hole, which can be very massive, and as light doesn't not escape it, very dark. However, we can tell the size of the black hole at the center of galaxies by the Hawking radiation emanating from it's event horizon (this is radiation emitted from matter that is accelerated towards the black hole). Also, the galaxy appears to rotate not as if there is some extra matter can't see at the middle, but as if it is distributed in a sphere extending in a 'halo' around the galaxy! Since we can't see what matter could be doing this, we postulate that it is dark. This means that it doesn't give off radiation like the normal matter that we're made of, as well as the stars and all the other glowing matter that I mentioned earlier (known as baryonic matter). This implies that it doesn't interact with regular matter via the electromagnetic force, which causes the radiation that we see. So we think that it must be weakly interacting, yet still very massive. This gives rise to the particle known as a WIMP (Weakly Interacting Massive Particle), which we believe is dark matter!

So, LUX is an experiment to detect WIMPs. Remember, they're weakly interacting, not non-interacting. So we try to detect how they interact with regular matter by bumping into the nucleus, rather than the electrons around the nucleus, which is predominantly how baryonic matter interacts. The basic setup for the experiment is a bucket of liquied xenon (very large nucleus, good for WIMPS to bump into) that is very cold, and quiet. When a WIMP streams through the detector (they're streaming through all of us right now, without interacting, or barely interacting with us) it may bump into the xenon nucleus, which gets heated up slightly from the interaction, and emits radiation (light) that we detect. That's it, pretty simple huh? Yes and no. There is a lot of other stuff streaming through us right now, besides WIMPs, which is more strongly interacting. So if we were to turn our detector on right now, on the surface, without any shielding, it would light up like a Christmas tree. This is because all the other particles (cosmogenic muons from the sun, radiation from normal matter that decays in and around us, etc.) interact in the detector too, causing a lot of background noise (signals we don't care about, that are not WIMPS). So we put massive water shields around the detector and go deep underground where most of the radiation from the sun cannot reach. The water shield absorbs most of the radiation from the cavern rock and all the other stuff in the room. Then we use very clever veto techniques to rule out interactions as being WIMPs. For example, a neutron that decayed from the cavern wall might enter the detector, but it when it hits the xenon nucleus it will impart a lot more energy than a WIMP, so we call it a high energy veto, which means anything depositing energy over a certain threshold, is not a WIMP, so we ignore it. Also, WIMPs are so weakly interacting, they are sure to interact only once in the detector, if at all. Other particles like neutrons may interact more than once. So we use a multiple scatter veto to rule them out. Many other techniques like this are used to sift out signals that we know aren't WIMPs, to look for ones that might be.

So far, experiments like this one haven't turned up any verifiable detection of dark matter WIMPs, but LUX will be the most sensitive dark matter detector in the world when it is turned on. Even if it doesn't see anything, we'll be able to rule out a lot of theories for dark matter, which is just as exciting, because it opens up new physics. The great thing about experimental physics is that whether your see what your were looking for or not, you have revealed some great truth about nature that will keep theorists busy for years.

Anyway, that should be enough to keep you busy for awhile, please post any questions, as I'm sure you will have some, unless this is completely uninteresting to you, in which case, thanks for reading anyway.