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Darker Matter

August 4, 2016

Svengoolie is a weekly television show that's presented nationwide in the US on one of the minor cable television channels. While having other humorous content to hold audience attention, its intended purpose is to broadcast the old monster-themed films from Universal Studios. My wife and I have watched quite a few of these, including the many movies based on The Invisible Man by H. G. Wells.

Statue of the Invisible ManStatue of The Invisible Man.

My inspiration for this image was a lifetime of viewing New Yorker cartoons!

(Base image, Statue in Duke's Wood Commemorative Plantation, by James Hill, via Wikimedia Commons and modified using GIMP.)

Films of this genre can be enjoyed only with a willing suspension of disbelief, and this is quite difficult for a scientist. While I usually grant my nihil obstat to quite a few plot points in the older science fiction films, ignoring such things as traveling to Mars in a day using conventional propulsion, the invisible man concept of using an injected drug to render invisibility is quite far-fetched.

However, current research into invisibility through use of metamaterials seems to indicate that a physical device for invisibility could be forthcoming; something like an invisibility suit. As Arthur C. Clarke said in his Third Law: Any sufficiently advanced technology is indistinguishable from magic.

Science has given visibility to many things that were once invisible, such as atoms. As I pointed out in an earlier article (Imaging Atoms, September 26, 2012), when we claim to "see" atoms, this is not actually true. While optical microscopes act as magnifiers for the light that reaches our eyes and we see microscope images using our own eyes, atomic force microscopes are not magnifiers of anything that can be seen. What appears on a computer display is just a representation of atoms without a direct linkage to the human sense of sight.

One thing in the universe that's remained invisible for nearly a century after the discovery of its far-reaching affect is dark matter, previously called "missing mass" by astronomers. Many years ago, the American Physical Society had a limerick contest, and I submitted quite a few entries. I was a finalist for a limerick about Schrödinger's cat, but one of my other submissions was about missing matter.
The accountant was ranting and hissing!
Such an audit was not of our wishing!
But such was our state,
An astronomer's fate,
Since some of our mass was found missing!
Dark matter was first inferred through observations of the gravitational binding of galaxies in the Coma Cluster by astronomer, Fritz Zwicky (1898-1974). It was Zwicky who gave dark matter its name. Current estimates are that about 85% of the mass of the universe is dark matter; so, it's really important, and it's strange how it's never been found.

Modern technology has now given us some unique tools in our search for dark matter, one of which is the Large Underground Xenon experiment, also called LUX. Particle physicists are accustomed to find their matter as subatomic particles, so LUX was designed to search for weakly interacting massive particles (WIMPS). Such supposed particles are candidate dark matter, since they are both "massive," and "weakly interacting." I wrote about LUX in a previous article (LUX Dark Matter Search, November 20, 2013).

Figure caption
We've come a long away, both in civility and technology. LUX soap products have been produced since 1899. The politically incorrect advertisement on the left was published c. 1915. The right image is the Large Underground Xenon experiment, also called LUX. (Left image, from the Wellcome Trust, image GC EPH169-L0069079, and the right image, are both via Wikimedia Commons.)

LUX is a tank of 368 kilograms of liquidxenon surrounded by detectors and shielded by a 72,000 gallon water envelope and nearly a mile of dirt. Xenon, an inert gas with atomic number 54, has many stable isotopes with nuclei that contain 70 or more neutrons that give a high interaction cross-section. Xenon is somewhat rare, existing at just 0.087 parts-per-million by volume in Earth's atmosphere.

LUX is located in a laboratory 4,850 ft underground, the Sanford Underground Research Facility, Lead, South Dakota. This is the same underground site where Ray Davis did his experiment on solar neutrinos. I wrote about the Davis experiment in a previous article (Bacterial Iron Isotopes, July 22, 2013).

The LUX team presented their final report at the Identification of Dark Matter 2016 Conference (IDM2016, July 18-22, 2016).[1-2] They summarized their data from October, 2014, through May, 2016, that showed not a single WIMP. This null result was despite the fact that the sensitivity of LUX was four times that of its original design.[1] Said Rick Gaitskell, a professor of physics at Brown University and co-spokesperson for the LUX experiment,
"LUX has delivered the world's best search sensitivity since its first run in 2013... With this final result from the 2014 to 2016 search, the scientists of the LUX Collaboration have pushed the sensitivity of the instrument to a final performance level that is four times better than the original project goals. It would have been marvelous if the improved sensitivity had also delivered a clear dark matter signal. However, what we have observed is consistent with background alone"[1]

LUX DetectorAs pretty as a work of art.

The LUX photodetectors are capable of detecting a single photon.

(Image by Matt Kapust, Sanford Underground Research Facility, via Brown University.)

The sensitivity was measured by simulating WIMPs with a beam of neutrons, and also by using radioactive gases to find how well the detectors could distinguish between a WIMP signal and the ambient radioactivity that's always present in Earth's crust.[1] Simon Fiorucci, a physicist at Lawrence Berkeley National Laboratory and science coordination manager for the experiment, emphasized that "The result is unambiguous data we can be proud of and a timely result in this very competitive field - even if it is not the positive detection we were all hoping for."[1]

The WIMP search will continue after LUX with the LUX-ZEPLIN (LZ) experiment, also sited at the Sanford Underground Research Facility. LUX-ZEPLIN will contain ten tons of liquid xenon, about thirty times the xenon of LUX. LUX-ZEPLIN will reside inside the same radiation shield of 72,000-gallons of water used by LUX.[1] LUX was funded by the U.S. Department of Energy Office of Science, and the National Science Foundation. The collaboration included twenty research universities and national laboratories in the United States, the United Kingdom, and Portugal.[1]

References:

  1. Kevin Stacey, "World's most sensitive dark matter detector completes search," Brown University Press Release, July 21, 2016.
  2. Katyanna Quach, "Nope, we can't find dark matter either, says LUX team," The Register (UK), July 21, 2016.
  3. Large Underground Xenon dark matter experiment web site.

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