An underground detector reports zero detections of weakly interacting massive particles (WIMPs), the top candidate for mysterious dark matter.

Davis Cavern
The Davis cavern, deep within what used to be the Homestake Mine, before the placement of the LUX experiment.
LUX / Sanford Underground Research Facility

Founded in 1876, the town of Lead in South Dakota hummed along as a mining community for more than a century. Homestake Mine employed thousands in the largest, deepest, and most productive gold mine in the Western Hemisphere.

Now scientists are using it to mine for gold of a darker kind.

More than a mile underground, where miners once accessed precious ore, sits a 3-foot-tall, dodecagonal cylinder of liquid xenon. The 122 photomultiplier tubes at the container’s top and bottom await the glitter of light that would signal an elusive dark matter shooting through the cylinder and interacting with one of the xenon atoms. But after more than a year of data collecting, the Large Underground Xenon (LUX) experiment announced last week at the Identification of Dark Matter 2016 conference that they’re still coming up empty-handed.

A Physicist’s Gold Mine

Weakly interacting massive particles (WIMPs) are the top candidates for dark matter, the invisible stuff that makes up about 84% of the universe’s matter. By definition, dark matter doesn’t interact with light, nor does it interact via the strong force that holds nuclei together. And while we know it interacts with gravity, that interaction leaves only indirect evidence of its existence, such as its effect on galaxy rotation.

Bottom view of LUX
This bottom view shows the photomultiplier tube holders in the LUX experiment. Find more images on LUX's Flickr account.
LUX / Sanford Underground Research Facility

But WIMP theory says dark matter particles should also interact via the weak force, a fundamental force that governs nature on a subatomic level — including the fusion within the Sun. So a WIMP particle should very rarely smash into a heavy nucleus, generating a flash of light. The chance for a direct hit is very, very low, but 350 kilograms (770 pounds) of liquid xenon in the LUX experiment should have good odds.

After just three months of operation, in 2013 the LUX experiment had already reported a null result. At the time, the experiment had probed with a sensitivity 20 times that of previous experiments (check out the graph here to see how three months of LUX ruled out numerous WIMP scenarios).

A new 332-day run began in September 2014, and the preliminary analysis announced last week probes four times deeper than the results before. Yet despite a longer run time, increased sensitivity, and better statistical analysis, the LUX team still hasn’t found any WIMPs.

Simply put: either WIMPs don’t exist at all, or the WIMPs that do exist really, really don’t like interacting with normal matter.

It’s also worth noting that LUX isn’t just looking for WIMPs. The WIMP scenario is the primary one it’s testing, and the one that last week’s announcement focused on. But more results are forthcoming about LUX results on dark matter alternatives, such as axions and axion-like particles.

Not All That’s Gold Glitters

The non-finding may not win any Nobel Prizes, but in a way it’s great news for physicists. Numerous experiments (such as CDMS II, CoGeNT, and CRESST) had found glimmers of WIMP detections, but none had found results statistically significant enough to be claimed as a real detection. The LUX results have been helpful in ruling out those hints of low-mass WIMPs.

LUX: cross-section vs. mass plot
For the technically minded, this is the result that was presented at the Identification of Dark Matter conference in Sheffield, UK. The plot shows the possibilities for dark matter in terms of its cross-section — the bigger the value, the more easily it interacts with normal matter — and its mass. (The mass is given in gigaelectron volts per speed of light squared, which translates to teeny tiny units of 1.9 x 10-27 kg.) LUX's most recent results rule out any dark matter particles with mass and cross-section that place them above the solid black line. The upshot is that LUX, the most sensitive dark matter experiment to date, is narrowing the playing field, especially for low-mass WIMP scenarios.


“It turns out there is no experiment we can think of so far that can eliminate the WIMP hypothesis entirely,” says Dan McKinsey (University of California, Berkeley). “But if we don't detect WIMPs with the experiments planned in the next 15 years or so . . . physicists will likely conclude that dark matter isn't made of WIMPs.”

That’s why — despite not finding any WIMPs this time around — the LUX team continues to work on the next-gen experiment: LUX-ZEPLIN. Its 7 tons of liquid xenon should begin awaiting flashes from dark matter interactions by 2020.

Three years of data from LUX-ZEPLIN will probe WIMP scenarios down to fundamental limits from the cosmic ray background. In other words, if LUX-ZEPLIN doesn’t detect WIMPs, they don’t exist — or they’re beyond our detection capabilities altogether.


Image of Mike-Cavedon


July 27, 2016 at 1:54 pm

Dark matter fills 'empty' space. Dark matter strongly interacts with matter. Dark matter is displaced by matter.

What physicists mistake for the concentration of dark matter is the state of displacement of the dark matter.

'[0903.3802] The Milky Way's dark matter halo appears to be lopsided'

"the emerging picture of the dark matter halo of the Milky Way is dominantly lopsided in nature."

The Milky Way's halo is not a clump of dark matter traveling along with the Milky Way. The Milky Way's halo is lopsided due to the matter in the Milky Way moving through and displacing the dark matter, analogous to a submarine moving through and displacing the water.

What ripples when black holes collide is what waves in a double slit experiment, the strongly interacting dark matter which fills 'empty' space.

Einstein's gravitational wave is de Broglie's wave of wave-particle duality, both are waves in the strongly interacting dark matter.

Dark matter displaced by matter relates general relativity and quantum mechanics.

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Howard Ritter

July 29, 2016 at 6:12 pm

I don't think you'll find much published support for what you claim. "What ripples when black holes collide" is NOT "what waves in a double-slit experiment". The former is well understood to be the geometry of spacetime, as explained by General Relativity; the latter is a manifestation of the wave-particle duality of quantum theory. Even though the latter is one of the most contentious issues in quantum theory, what is not in doubt is that it is clearly not a matter of oscillation in spatial geometry. The two simply have no area even of overlap except for the word "wave".

There is no basis for equating gravitational waves with de Broglie's pilot wave, which incidentally is intended to do away with wave-particle duality, not to explain it. Nor is there any indication that dark matter interacts strongly with anything, including itself; read the Wikipedia article on the "Bullet Cluster" and observe the passage of the dark-matter halos of two galaxy clusters right through each other without interacting:

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August 10, 2016 at 4:41 pm

"Robert B. Laughlin, Nobel Laureate in Physics, endowed chair in physics, Stanford University, had this to say about ether in contemporary theoretical physics:
It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed [..] The word 'ether' has extremely negative connotations in theoretical physics because of its past association with opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum. . . . Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that any such matter must have relativistic symmetry. [..] It turns out that such matter exists. About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with 'stuff' that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo."

Matter, quantum solids and fluids, a piece of window glass and 'stuff' have mass and so does the dark matter.

It's the 'stuff' that ripples when galaxy clusters collide and it is what waves in a double slit experiment.

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Howard Ritter

July 29, 2016 at 5:29 pm

Yet another result that fails to refute my suggestion that dark matter is "nonstandard" matter: Particles that are distinct from the particles of "standard" matter that are part of the Standard Model of physics.

Why should we assume that the Big Bang created only "standard" matter? I suggest that it did not, that it produced additional particles that are neither predicted nor able to be accommodated by the Standard Model. Being nonstandard matter, these particles do not feel the strong, weak, or electromagnetic forces, which are mediated by particles that are part of the Standard Model (elementary bosons – photons of the electromagnetic force, gluons of the strong nuclear force, and the W and Z bosons of the weak force as well as the Higgs boson of the Higgs field).

I further suggest that gravity is in fact the gravity of General Relativity, which is why it has not been unified with quantum theory as being a force mediated by a fifth elementary boson, the graviton, which has also eluded detection. I suggest that, given the above, nonstandard matter simply does not interact with standard matter through the three boson-mediated forces (which is why it has never been directly detected), but, having mass and existing in our spacetime continuum, DOES interact with standard matter through the force that is actually the geometry of space: Gravity (which is why dark matter has been detected at all – through its gravitational effects). In addition, I propose that particles of dark or nonstandard matter do not interact even with each other (a la neutrinos) or interact weakly, as is seen in the "Bullet Cluster", where two colliding galaxy clusters' dark matter complements passed right through each other, as indicated by gravitational-lensing modeling that shows the dark matter still moving along with (in the gravitational field of) the visible matter, in contrast to the respective complements of intergalactic gas (standard matter), which collided violently, with the generation of X-ray emission that shows the gas collections largely being halted at the interface of the colliding clusters by their collision.

I predict that as dark matter goes on and on not being identified, the default conclusion will come to be just as above – just as Pauli predicted the totally out-of-left-field neutrino to fulfill the need for a heretofore unknown and unpredicted particle to account for an otherwise unexplained effect. By definition, it may be rather more difficult to prove this than Pauli's prediction!

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