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Null results from Large Underground Xenon (LUX) dark matter detector

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October 30, 2013

A cross-sectional view of the LUX dark matter detector (Image: LUX)

A cross-sectional view of the LUX dark matter detector (Image: LUX)

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The Large Underground Xenon (LUX) experiment, buried nearly a mile beneath South Dakota, has completed 85 days of seeking dark matter particles. The results are consistent with a null result, and essentially rule out the 8.6 GeV dark matter candidate noted in the data of other experiments.

Back in the 1930s, astronomers began to realize that galaxies in galaxy clusters were moving far too rapidly to be gravitationally bound therein, and concluded that there must be a good deal of "missing mass" they could not see. Similarly, the rate at which stars orbit the center of their galaxy requires additional mass so that the galaxy does not disassemble over cosmic time spans.

As time went on, the evidence mounted that there was some form of matter in the Cosmos which we cannot directly see. Indeed, recent cosmological models based on a wide range of independent evidence suggest that there is about five times more so-called dark matter in the Universe than there is ordinary matter.

The evidence associated with the cosmic microwave background is particularly significant, in part because the data contained in the CMB has remained largely untouched since about 379,000 years following the Big Bang, when the Universe became transparent. Not only does the information encoded in angular fluctuations agree with the existence of massive amounts of dark matter, but in (essentially) the entire lifetime of the Universe, the CMB has accumulated no sign of interaction with the dark matter, leading to the idea that dark matter must not interact through electromagnetic or nuclear binding forces, but only through the weak force and gravity.

Despite having a good deal of inferential evidence for the existence of dark matter, efforts to detect a specific particle or influence responsible for this massive amount of matter have been fruitless to date. Many suggestions exist, such as a distribution of relatively small amounts of cold matter (MACHOS - Massive Compact Halo Objects), Weakly Interacting Massive Particles, or WIMPs, axions (suggested in order to plug a hole in the Standard Model of particle physics), and a host of others. Other suggestions, such as altering the theory of gravity to force agreement with observations without requiring any additional matter, so far fail to agree with enough of the relevant evidence for dark matter to be taken seriously.

At this point, cosmology and particle physics are in a quandary about dark matter, which explains why it is receiving so much attention by researchers. People tend to hop on any bit of statistically insignificant data that might point more or less toward the true resolution of this puzzle. For example, over the past two years two groups of observers have found some evidence that there exists a wholly unexpected candidate particle for dark matter with a mass of 8.6 GeV, or about nine times the mass of a proton. The mass is far below where WIMPs are expected to form, but science, if it is about anything, is about surprises.

LUX is presently the most sensitive recoil-based dark matter detector on the planet, and has just finished roughly three months of observations aimed at detecting the collision of particles of dark matter with xenon atoms. Such measurements are relatively independent of just what a dark matter particle is. If it interacts predominantly via the weak force, it could be detected by LUX.

The detector is located 1,480 m (4,850 ft) below ground at the Sanford Underground Research Facility in Lead, South Dakota. A 7.6-m (25-ft) diameter by 6.1-m (20-ft) tall cylindrical water tank containing ultrapurified water provides shielding to the detector from the radioactive elements present in the walls of the mile-deep canyon. The cosmic-ray background is already reduced by a factor of nearly a billion by the rock overhead.

The LUX detector holds 370 kg (816 lb) of liquid xenon, about 118 kg (260 lb) of which is active as the primary LUX scintillator (an atom that gives off light when struck by another particle). The remainder of the liquid Xenon acts to provide additional shielding for the active portion of the experiment, and to separate that portion from direct contact with the structural materials of the detector, which are ever so slightly radioactive.

Working principle of the LUX time-projection chamber (Image: LUX)

The active detector is a dual-phase time-projection chamber measuring 47 cm (18.5 in) in diameter and 48 cm (19 in) in height. When incident particles (including dark matter particles, if any) collide with xenon atoms in the chamber, the xenon atoms recoil and produce a prompt flash of light, which the lUX team call the S1 event.

Ionization of the xenon near the collision also occurs, if the collision is energetic enough. There is an electric field of 181 V/cm applied within the xenon liquid. The electrons freed by the ionization drift upwards under the influence of this field, and cause a second pulse of light (S2) when they pass into the gas above the liquid xenon. The time (S2-S1) it takes the electrons to drift to the surface tells how deep the S1 event was. The horizontal location of the S1 event is determined by analyzing the signals from the bottom array of 61 photomultiplier tubes.

The background counting rate of the detector in place underground was carefully studied. About 1,000 background events could be expected during the 85 live days of data collection, the vast majority of which would not show the proper signature to be confused with a recoil event.

Some 84 million events triggered the detector during the 85 live days of operation. Once multiple scattering events (recognized by multiple S1 and S2 events very closely spaced in time) were eliminated from consideration, 6.6 million remained. When events that did not have the signature of true recoil events occurring within the central core of the time-projection detector were also eliminated, only 160 events remained, a rate of about two per day.

The number of events are sufficient immediately to rule out the 8.6 GeV WIMP candidate possibly seen in the Berkeley CDMS II data. Given the collision cross-section of 2 x 10^-41 cm2 (derived from the data), LUX should have seen about 1,550 events attributable to dark matter during this data run. Instead the researchers saw none, which contradicts the CDMS II observations so dramatically that one of the experiments must be in error. As the LUX run comprised nearly 100 times more data, CDMS II is probably in error.

To extract possible dark matter collision events from an unknown residual from background radioactivity required complex statistical analysis of the 160 possible events. To make a long story short, the spectrum of possible dark matter events is consistent with no actual dark matter events being detected – in other words, a null result.

This question can be flipped on its head, to ask if a dark matter event could successfully hide from statistical analysis. When this was tried by the LUX team, it found that between 2.4 and 5.3 (depending on dark matter mass) dark matter events could hide in the data, but the error bars make these numbers indistinguishable from zero.

The bottom line is that this early LUX data run found no evidence for the existence of dark matter, and was able to tighten the bounds on the collision cross-section of dark matter and normal matter by a factor of at least three. The LUX team is now looking at longer data runs, but for the moment the best data we have does not disclose any form of weakly interacting dark matter. What the LUX experiment has accomplished is to narrow the field of acceptable candidate dark matter particles, a process that will hopefully continue until this puzzle is solved.

Source: LUX Experiment

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer.   All articles by Brian Dodson
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7 Comments

Contrary to what science still believes, at the time of the Big Bang there were no atoms but only waves carrying energy through the infinite Void.

If we could view the Universe from outside, It would look like an egg-shaped cloud with winds running in perpetual motion inside of It.

The energy is like those winds running at maximum speed and pushing out the borders of the Universe.

The Universe continues to expand as the waves that travel at the border of the Universe have never encountered, nor will ever encounter, any interference from the Void. These waves will forever expand the Space of the Universe they create and leave behind.

Wave-behavior relates to the medium in which the waves travel.

Thus, wave-behavior at the border of the Universe is different than wave-behavior within the Universe.

Inside the Universe, waves change their frequencies by colliding with other energy during their travel. These waves, because of the encountered interference, continue to transform part of their original energy in other forms. Waves travel gradually releasing heat, or amounts of energy, and their original short wavelengths, in time become longer and longer as they carry less and less energy than they did when they first started to travel. These waves lose energy releasing it in form of other waves with wavelengths longer than their own.

For example, the gamma rays, over time, diminish their energy level (and their frequency) to become X rays, from X rays they will become ultraviolet and so on. The original quantum is not lost but distributed into other forms of energy through "spontaneous symmetry breaking".

Once reached an almost flat longitude (and lower critical energy level) these waves solidify into hydrogen atoms breaking up their energy in opposite elements, like the split ends of a broken hair.

When the hydrogen atoms are reached by the heat of other incoming waves they fuse together to create more complex forms of energy.

The Creation of the Universe could be compared to a magnet hit by a hammer. Its energy is split in positive and negative charges like in a symmetry made of many fragmented elements.

As for the Law of Attraction (Quantum mechanics), the elements with opposite charges are attracted to form a new magnetic field while those with the same charges instead reject one another. In fact, for example, if we were to try to recompose a broken magnet we would notice how the interaction among the magnetic fields sets precise distances in between those fragments. This is also the same mechanism that keeps the stars apart from each other.

Imagine the Universe as a rolling ball expanding into the infinite Void and eternity of Time. From the inside, the ball seems to have crystal walls. They reflect everywhere the broken symmetries of all that is within. Elements of energy like many broken mirrors wandering suspended inside that crystal.

When two opposite and complementary elements recognize each other (as for example, hydrogen and oxygen) they join to manifest a more complex form of energy (like water). But also a complex form of energy, like water, becomes attracted by the opposite dry seed to create an even more complex form of energy, like a plant. So, all forms of energy are also the elements of other more complex forms of energy. The most complex forms, like Mankind, identify with their surroundings but can also perceive the distant reality reflected by the crystal walls.

The Universe is like one broken mirror formed by the symmetry of Its fragments.

http://www.wavevolution.org/en/freethinking.html

http://en.wikiversity.org/wiki/Wavevolution

Ettore Greco
30th October, 2013 @ 12:05 pm PDT

'Nuff said Ettore. :-)

About the null news. Bummer. I got really excited for a wee bit there. *sighs and waits*

Kimberly Kvw
30th October, 2013 @ 04:02 pm PDT

Ettore,

That is a of of claims, assertions, and metaphors without a hint of verifiable (or falsifiable) predictions. You have not cited a single piece of evidence (the wikiversity link has zero citations either), and the other link is merely to your blog. If you want your ideas to receive any attention or credence, step up and provide some sort of concrete evidence that they represent a more factually accurate description of the universe than existing theories. Until then, there is nothing to distinguish you from any (other) New Age pseudo-poet who dropped a little too much acid and "figured out the universe".

WinDrummer
30th October, 2013 @ 05:12 pm PDT

@WinDrummer, spot on. Ettore lost me at "Contrary to what science still believes...", which unfortunately was the first sentence. Total hand-wavey cobblers.

Synchro
31st October, 2013 @ 02:11 am PDT

@Ettore Greco

Science doesnt "believe" or disbelieve anything, science is a method used to understand what is happening around us, please do not advertise your unfounded theories and blog posts here again thanks.

FabianC
31st October, 2013 @ 03:03 am PDT

Sorry, FabianC, I do BELIEVE in a fractal structure of the cosmos, with particle sizes corresponding to an infinite succession of ever smaller / bigger scales with a dimensional ratio between any two consecutive scales corresponding to that of the solar system versus an atom / a galaxy respectively.

Hence, beyond the outer / inner space visible through the strongest telescopes / microscopes, there must be an infinity of ever bigger-scale macrocosms / smaller-scale microcosms.

From this it could be inferred that in an infinitely small portion of space an infinity of infinitely small particles are causing an infinite number of collisions within an infinitely short lapse of time.

Extrapolating from this, mathematicians might be able to calculate the cumulative particle mass... which might well account for almost all of the dark matter of which we will definitely never be able neither to see nor to measure any evidence...

And, sorry, but this kind of thinking is known to be the first stage of scientific thinking, i.e. a hypothesis, commonly called a belief -- like the Big Bang, which, to date, is indeed just a religion. Or like Higgs' boson for those willing to consider that the only peers who can confirm the positive results of the CERN are its very own members interested in receiving funds for their next accelerator.

Yet by claiming funds of merely 8 billion for their next rather short linear machine (barely more than the 6 billion price tag of the LHC), instead of, say, 600 billions for a new circular one with 300 km in diameter, doesn't this modesty betray that in fact they got it all wrong with the LHC in that its cost/benefit ratio was disastrous because of the huge part of the investment absorbed by the gigantic toroid magnets just to cope with centrifugal force? For null results?

euroflycars
1st November, 2013 @ 09:10 am PDT

@euroflycars your grasp of logical discussion is atrocious. Paragraph 1 has only a statement of personal belief, which is followed by Paragraph 2 which states an unavoidable conclusion based upon Paragraph 2, leading to the "inferences" and "extrapolation" of Paragraphs 3 and 4, respectively. Moving on in the statement does not equate to having properly discharged the burden of proof of earlier statements. Everything you say is a house of cards built upon quicksand: before extrapolating from your belief, you must give some evidence to support that your belief is an accurate reflection of reality. Your concluding statement of the Big Bang Theory being a religion, and lack of understanding regarding the difference between a linear and a circular accelerator only further shows your ignorance.

WinDrummer
5th November, 2013 @ 08:27 pm PST
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