Some recent particle experiments – dark matter and neutrinos

Recently two new particle detectors have released their first set of results – the Large Underground Xenon (LUX) experiment and Ice Cube. Both machines are trying to track down the extremely rare interactions of particles which only interact weakly with other matter. These are generally the kind of interactions you cannot study with something like the LHC, because even though many, many, (many) particles are produced every second at the LHC, the detector is too small to catch those that interact weakly. The solution is to build giant underground tanks of some material (in this case, Xenon or Ice), and look for the extremely rare collision of a weakly interacting particle with the detector material.

LUX is an experiment designed to look for dark matter. A class of dark matter candidates are WIMPS – Weakly Interacting Massive Particles. They are weakly interacting because they do not interact electromagnetically (they are “dark”), and they are massive because the claim is that they are responsible for a bunch of phenomena which can be explained by adding mass to astrophysical objects. I won’t rant too much about this, but the logic goes something like this:

  1. Model a system with Newtonian gravity (galactic rotation curves, gravitational lensing*, etc).
  2. Try to verify your model, and find that it doesn’t quite work.
  3. Without trying the full theory of general relativity, give up on a gravitational solution to a gravitational phenomena.
  4. Propose a new form of matter which lies outside the standard model, which must be dark and massive.

* Yes, I guess you should argue this is not “Newtonian” since you are letting photons interact gravitationally even though they don’t have mass. This should probably say “Linearized GR”…

So LUX fills this big underground tank in South Dakota with Xenon and looks for WIMPS. If they exist, they should (very rarely!) perturb the Xenon atoms around, which will then produce a flash of light (“scintillate”) and signal a detection. Of course, all kinds of other things are flying into this vat of Xenon, so the clever folks there focus the search in the very center of the detector, which should have the best shielding from the outside because there is so much Xenon in the way.

Anyway, the recently announced that the first dataset is “consistent with the background-only hypothesis” at the 90% confidence level (http://arxiv.org/abs/1310.8214). In other words, they have not detected anything other then what the standard model predicts. This is in contrast to some other recent results from similar experiments. For those of us who love the standard model and think we just need to work on gravity a little harder, this is one for the win column. Even if I’m the only one counting it as such…

The Ice Cube detector is based on the same principle, but is looking for neutrinos. These little guys are the last piece of the standard model which contains some uncertainty (now that we have found Higgs, anyway…). We know they exist, we just need to fill in some details like their exact masses and exact character. They also interact only weakly, but in order to get the largest possible sample, you want the largest possible amount of matter in your detector. This material should also be relatively transluscent, since you are looking for flashes of light. So, they drilled a bunch of holes in the Antarctic Ice, lowered some cameras in, and looked for flashes of light from neutrinos interacting with frozen water! I think this is such a cool idea – their detector is effectively a cubic kilometer in size! The Xenon detector is a cylinder 6 m tall with a radius of under 4 m. So that’s around a million times the size of Xenon. Despite being so damn huge, they found a paltry 28 neutrino candidates (http://www.sciencemag.org/content/342/6161/1242856). Which is actually more then twice what was predicted!

At 28 neutrinos, I guess one can’t get overly excited about the numbers, but there is something quite striking about their energies – from 30 TeV to 1200 TeV. Compare this with the LHC, which is currently operating at 8 GeV – these neutrinos are over 1000 times the energy present in LHC collisions. These kind of experiments allow us to probe energy scales which are generally impossible to reach on Earth. Basically, Universe is much better at accelerating than we could ever be.

In my opinion, these types of experiments are the future – they allow us to directly answer questions about unexplained phenomena, and they do it pretty cheap (relatively). Although people are already talking about the next big machine after the LHC, without direct detection of dark matter such a prospect seems highly unlikely, and would likely leave the neutrino problem completely untouched. It seems that Astroparticle physics is a bit of a growth area for the physical sciences, and has the potential to open up entirely new “eyes” on the universe. Very Exciting!

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