Particle Physics and the Dark Side · 12:10pm Jul 30th, 2016
Last week saw some exciting news on the experimental search for dark matter. The LUX and PandaX experiments have released some new results. Spoiler alert – they haven’t found it.
Latest results in the hunt for dark matter
Scientists announce results from new detector
This gives me an excuse to write a short Twilight-and-Luna piece for Codex Equestria, to set the astronomical scene, and follow it up with a blog post on my adventures in astroparticle physics.
What is dark matter? The short answer is: we don’t know—it’s a mystery—one of the biggest unsolved mysteries in modern science. To astronomers, there is overwhelming evidence that the majority of the mass of the galaxy is in the form of invisible matter. They see the effects of its gravity, but they don’t know what it is.
This is where particle physics jumps forward with a hypothesis: it’s a new type of particle, which only interacts very weakly with ordinary matter. Zillions of them are out there holding the galaxy together, but we never see them as they just zip through the earth without hitting our detectors.
This isn’t just a wild guess—it fits in with theories of fundamental physics and cosmology—but there is not yet any direct evidence. So we need an experiment—or several experiments. LUX and PandaX are the current leading projects. The X in their names reflects their choice of xenon as a detector material.
The interaction rate isn’t zero—just really low—so if you have a big super-sensitive particle detector, you have a chance of seeing something. Except if you run it on the surface, any signal is swamped by the noise from cosmic rays and background radioactivity. You need to run it in a deep underground laboratory like the Sanford facility, in a former Gold Mine in South Dakota, or the China Jin-Ping underground Laboratory, in a hydroelectric tunnel complex in central China. Then you need to maintain ultra-clean conditions to keep out radioactive atoms like uranium, thorium, and carbon-14. Then you have to wait and hope a dark matter particle hits your detector.
Dark matter searches have been going on for over twenty years. When I got my degree (sixteen years ago) one my lecturers (a theoretical physicist), assured us that experimentalists would find these particles in five years. I did a PhD working on one such experiment. We didn’t find anything. A few false-positives aside, the history of dark matter searches has been a twenty-year string of null results.
But we are making progress. The early experiments had a sensitivity corresponding to a few hits a month in a kilogram detector; the latest results rule out theories with a predicted signal of just a few hit in a year in half a tonne of liquid xenon.
While nobody is making any predictions as to when we will see something, the next five to ten years should be interesting, as experiments can now probe some of the more promising theories. There is also a chance a dark matter particle will be produced and seen at a detector at the Large Hadron Collider.
But it’s just a hypothesis. We may not see anything. As we approach the limits of detector technology, it would be frustrating, but not unlikely, if the dark matter interaction rate turns out to be so low that we never see it, but neither can we rule it out. But we have to try.
Must be a pain when going through the fruit and veg section to take a geiger counter with you so you dont end up doing an Italian Job on your experiment.
I wonder if we will get to the point where research students in particle physics will have to spend the first two years living in secured accomodation, eating only filtered atmosphere food, to try and wash naural isotopes out of them for their third detection year?
Whatever the result of the experiments are, it should lead to more than just a single answer. I keep wondering why if LIGO needs such complex and accurate vibration compensation, that it isnt used as baseline for seismic activity. Between the various devices around the planet, we could learn more about the planets interior and maybe planetary motions from the vibrations than from teh signal they actually want.
Supercritical Xenon has a really weird relationshp between speed of sound, speed of light, which is like something from Birkhof? which I still think is Aether, or Planck Foam.
The answer is,
Yes?
The Answer Is,
Yes, Yes.
42.
A problem with underground particle detectors are all those darn annoying Tachyons all over the place!
I recall in a science magazine once a scientist came forward claiming that perhaps dark matter did not exist but that the discrepancies with gravity calculations could be explained by a "correction" in the equation used in the calculations. It was an interesting article from around 10 years ago but I do not recall much more than that.
4121991
That's one way to do it. But it might be difficult to get the impurity level down to the parts-per-billion level. And it might create problems recruiting students. Probably easier to ask them to go outside before starting data taking.
4121992
Fortunately Tachyons never hang around long enough to have much impact.
4122426
Sounds like MOND (MOdified Newtonian Dynamics). That theory has been around for years, but has dropped out of fashion over the last decade as evidence grew for dark matter (notably the Bullet Cluster). It worked quite well at explaining the orbital speed of stars around galaxies, but failed to explain the behaviour of galaxy clusters.
I like the chapter.
I have a movie-recommendation:
I heard that IceAge has a cameo of Professor Neal DeGrass Tyson. It does. I also say a poster advertising Vaxxed will soon arrive:
I cannot speak for all Americans, but you can have Mister Andrew Wakefield back. I shall sweeten the deal by throwing in Jenny McCarthy and Jim Carry too. They are all yours, you can keep them forever.
Frankly, prior to the Chandra X-ray Observatory and Hubble Space Telescope data release in 2015 regarding colliding galaxies, I was leaning towards 'dark matter' not actually being matter but an effect of something else (such as a modified gravitational theory), but now I'm leaning towards it being some form of weakly-interacting particles with mass. You'd know more about it than I would, though. I deal more in what happens to regular matter when you do weird stuff to it (to put it simply).
http://chandra.harvard.edu/photo/2015/dark/